Torque and Power
Lose Weight, i.e., "Add Lightness"
Many motorcycles can out-accelerate the hottest performance cars, even though they have much less power because proportionally they weigh even less such that the ratio of their power to their weight is much higher than that of most cars. The easiest and cheapest way to make your Spitfire feel more powerful is to shed mass and improve the power-to-weight ratio. As mentioned earlier under handling, this is easy to do on the later Spitfires in the U.S. by removing all the extra bumper and bumper reinforcing hardware they were burdened with.
A good metric of performance is power per unit mass (or weight). Horsepower at the wheels divided by total test weight of the car is the ideal metric, but since wheel horsepower is typically not published information and has to be obtained on something like a rolling road that introduces additional variables, peak brake horsepower (bhp) is a good proxy for relative comparison purposes. Below is a plot of actual test data of Spitfires (from "Triumph Spitfire Gold Portfolio") and some assorted vehicles circa 2011 (from "Road & Track" test reports) spanning the range, from the 70 bhp Smart fortwo coupe to the 1001 bhp Bugatti Veyron Grand Sport supercar, plus the 119 bhp Ducati 848 EVO motorcycle for comparison. The trendline is a best-fit of the actual data (weights are as-tested and include driver weight), and given the mathematical relationship it is a straight line on a log-log plot. The point of the plot is to illustrate the relationship between acceleration and the power-to-weight ratio. The greater the power-to-weight ratio, the quicker the vehicle; each doubling of bhp/ton cuts acceleration time approximately in half:
"Adding lightness" is very high return for lower bhp/ton cars like factory-spec Spitfires. Note that driver weight is a larger proportion of total test weight for lighter cars. In the case of a Spitfire, the weight of the typical driver is around 10 percent of the car's weight so the driver's weight has a bigger effect on a Spitfire's performance than it does on a heavier car like a Ford Mustang. Having a passenger or some cargo aboard your Spitfire will noticeably reduce acceleration. Besides shedding vehicle weight, shedding driver weight is something to consider! This underscores why horse jockeys are small, and why minimum weight constraints exist for horse racing and auto racing. For a typical Spitfire, a reduction of 25 to 30 pounds is equivalent to adding one horsepower (not counting other benefits). Roughly speaking, increasing Spitfire engine peak power to 90 bhp and beyond (i.e., 1 bhp or more per cubic inch displacement) enables crossing the 100 bhp/ton, 10 second 0-60 threshold. Clearly, you can take your Spitfire's quickness from pathetic to acceptable by increasing bhp/ton by losing weight and adding power.
The following is basically about better breathing to get the fuel and air into and the exhaust gases out of the engine more effectively (carbs, exhaust, head work, cam), and increasing the engine's thermodynamic efficiency so as to better convert the potential chemical energy of the fuel+air charge into kinetic energy (ignition timing, higher compression).
Optimize the Ignition Timing
Ignition timing is critical to Otto cycle engine performance. Why? For best performance, maximum pressure developed by the burning mixture in a cylinder should occur a little after the piston has passed top dead center (TDC) and is on its way back down the cylinder to take advantage of the mechanical leverage of the position of the piston connecting rods and the crankshaft and optimally develop torque. Developing pressure too early or too late wastes performance potential, and in the extreme can lead to damage. But it takes a finite amount of time for the combustion, initiated by the spark, to propagate throughout the mixture in the cylinder. Pressure, temperature, motion of the mixture and fuel grade all affect combustion speed--pressure being the most important factor. To account for the finite amount of time it takes combustion to occur and for max pressure of combustion to be reached at the optimum position of the piston, the spark needs to be "advanced," typically to occur some number of degrees before the piston reaches TDC. As engine speed increases, the speed of combustion doesn't increase as fast as the engine, so progressive spark advance is needed to keep the max pressure of combustion occurring at the right time. This is true to first order up to a point, because as engine speed (RPM) increases, pre-combustion cylinder pressure (up to the limits of the engine's induction system to flow mixture), mixture motion and fuel atomization increase too, and these things speed-up combustion. Beyond around 3000 to 4000 RPM in most engines, these effects keep pace and so no further advance is needed. Finally, the amount of load on the engine affects optimum spark timing. Less advance is needed when accelerating, with the throttle wide-open and lots of air is cramming into the engine and raising absolute pressures inside it, as opposed to when cruising or decelerating and pressures are reduced. Therefore, at a given RPM, more advance is needed at low engine loads (i.e., low absolute manifold pressures).
As you progress with engine modifications, the best amount of total ignition advance for a given engine speed and load changes, so to fully reap the benefits of engine performance modifications, it pays to revisit your ignition timing when making performance changes. For example, as you raise an engine's compression, less advance is needed at a given engine speed, and you want to be careful not to have so much advance that you encourage detonation (a.k.a., knock, pinging, pinking). If you go with a "bigger" cam, volumetric efficiency and dynamic compression are reduced at low RPM but increased at high RPM, thereby reducing combustion speed at low RPM but increasing it at high RPM and requiring an ignition map with a different shape and a steeper "slope" (camshafts are discussed later in this article). So to get the most out of a given camshaft and overall intake setup, ignition timing is crucial to preserving low RPM behavior and maximizing high RPM performance. "More" is not "better" when it comes to advance. Don't be fooled by an ignition map with big advance numbers everywhere. Proper spark timing is about optimizing performance for a given engine configuration.
Until recently and before the advent of electronics, spark timing on most Otto
cycle engines has historically been determined and controlled by the
distributor. Static advance is set by the position of the distributor such that
the rotor and plug wire electrodes in the distributor cap are aligned relative
to piston position at rest as desired (e.g., to set timing at idle). Dynamic
advance with a distributor is accomplished via two mechanisms--centrifugal and
vacuum. Centrifugal advance adjusts spark timing as a function of engine speed
and works this way: little weights constrained by little springs inside the
distributor fling outwards more and more as the distributor spins faster and
faster with increasing engine speed, up to a limit imposed by pins or some other
hard stop. The stiffness of the springs and the mass of the weights controls how
far the weights move for a given speed. The outward motion of the little weights
rotates a plate that holds the spark triggering mechanism (e.g., a cam actuating
contact points, or a multi-pole magnet interacting with a Hall effect sensor, or
a chopper wheel interrupting the light path between a light source and an
optical sensor, all of which function as switches to interrupt current flowing
in the primary winding of the coil and resulting in magnetic field collapse and
induction of high voltage in the secondary winding of the coil). Vacuum advance
(or sometimes retard) adjusts spark timing as a function of engine load, where
vacuum in the intake system is used as a measure of engine load (e.g., cruising,
accelerating or decelerating). Vacuum can be sensed via a tap or multiple taps
on the intake manifold, or a port near the throttle plate. A vacuum line goes
from the intake vacuum tap to a vacuum canister on the distributor containing a
diaphragm, and the diaphragm is attached to a link that is attached to the plate
holding the spark trigger mechanism, so motion of the diaphragm, in response to
intake vacuum, can advance (or on some distributors, retard) the spark.
Convert from Points to Electronically-Triggered Ignition, or to Fully-Electronic Ignition
Something to consider is making the switch from a mechanical spark trigger (points) to an electronic spark trigger (Hall effect (magnetic) or optical trigger), i.e., electronically-triggered ignition. The Pertronix series of products are easy, reliable drop-in replacements that take the place of points in your distributor. Switching from points to electronic ignition can enhance performance and will reduce maintenance and improve reliability.
A worthwhile modification to make, especially if you make other engine mods, is to eliminate the distributor and replace its function with a fully-electronic ignition control system like MegaJolt or the ignition portion of the MegaSquirt fuel management system. These systems, which sense engine position using a toothed "trigger" wheel attached to the crankshaft pulley rather than the shaft of the distributor, enable precise and fully-programmable ignition advance settings vs. engine speed and load. These are a bit more involved than simply replacing the points assembly in the distributor with a little electronics module, but they work well and have a large user community, so support is readily available. I installed MegaJolt on my modified '78 Spitfire 1500 and I'm thrilled with it.
So, why not stick with a distributor instead of going with a fully-electronic, programmable system for controlling ignition timing? Two main reasons. First, changing the advance map on a distributor is very difficult, time consuming and not really deterministic. It involves adding (welding on) or subtracting (grinding off) mass from the centrifugal weights, changing the centrifugal weight springs, and changing the range of motion that the vacuum/retard mechanism makes or imparts to the distributor. Being able to simply change spark timing incrementally at the click of a mouse, as with MegaJolt, is completely deterministic, much faster and allows results to be assessed immediately before environmental conditions change that can affect interpretation of results (like air temperature, humidity, etc.). Moreover, ignition timing maps can be implemented that simply are not physically possible to create with a mechanical system (if such a thing is warranted). Second, fully-electronic systems like MegaJolt eliminate several mechanical interfaces involved in communicating piston position and delivering the spark that add uncertainty and reduce precision in spark timing. In a traditional distributor-equipped engine, the crankshaft turns the camshaft through a chain or belt, and the camshaft incorporates a gear that engages and turns the distributor shaft, which actuates the centrifugal advance weights and springs that actuates a mechanism for interrupting primary current to the coil as well as turns a rotor that spins past contacts in the distributor cap to distribute spark to each spark plug. In the case of many fully-electronic ignition schemes, piston position is sensed directly off the crankshaft by the variable reluctance sensor "looking at" the toothed wheel and all spark triggering, switching and distributing is done purely electronically, and so the timing "slop" contributed by the rest of the mechanical interfaces in a conventional distributor-based ignition system are eliminated, thereby resulting in much more precise timing and virtually eliminating jitter. Lastly, all-electronic ignition systems sense engine load, via a measure of manifold pressure using a pressure transducer or throttle position via a potentiometer, more accurately than a vacuum diaphragm attached to a distributor plate carrying spark triggering components.
you want to retain a distributor and still have the advantage of
electronically-selectable alternative advance curves, a good aftermarket option
Standard 123Ignition distributors have 16 different preset curves to choose
from, and there is a line of programmable distributors that afford further
customization. 123Ignition is a nice, simple drop-in alternative to systems like
MegaJolt and MegaSquirt.
Better breathing is about making it easier for gases to get in and get out of the engine and it can cost some money, but the expense is not unreasonable for the corresponding improvement in performance and many of the following enhancements are relatively easy to make, so they fit the theme of this article.
Install a Better Exhaust Manifold or Header
The purpose of the exhaust system is to enable the evacuation and scavenging of
waste exhaust gases and even aid the intake of the next cycle's fresh air+fuel
mixture. When an exhaust valve opens, a compression wave of acoustic energy
rushes from the valve, out of the head and down the manifold or header tube at
the speed of sound, which is about 1700 feet per second (roughly 20 inches or 50
cm per millisecond) at such temperature. The actual exhaust gas follows behind
as a chunk of mass moving at a subsonic speed of about 300 feet per second
(roughly 3.6 inches or 9 cm per millisecond)--about five times slower than the
acoustic pressure wave ahead of it. The moving mass of exhaust gas leaves behind
a wake of low pressure as it moves along, and this low pressure helps suck gas
from the cylinder. This is inertial pumping. When the faster-moving acoustic
pressure wave reaches a transition where it experiences a sudden expansion, like
at a collector where exhaust tubes from other cylinders join together, part of
the acoustic compression wave continues down the exhaust system but part of it
is reflected back up the same manifold tube as an expansion or low pressure
wave. If this reflected low pressure wave reaches the exhaust valve late in its
cycle while it is still open and when the intake valve is also open early in its
cycle, i.e., during valve overlap, then it will reduce the pressure inside the
cylinder, which will not only extract remaining exhaust gas from the cylinder
but also suck fresh air+fuel mixture into the cylinder. This is acoustic
pumping. A performance exhaust system is designed to make use of both inertial
and acoustic pumping to improve the engine's volumetric efficiency and increase
An exhaust manifold or header is sort of like a tuned musical instrument. Pipe lengths and diameters determine its 'tune'. Longer pipes mean longer travel times, resulting in more torque at low engine speeds below the torque peak but less torque at high engine speeds above the torque peak. Larger diameter pipes mean slower gas flow (longer travel times) but less backpressure and reduced pressure loss with pipe length, shifting where peak torque occurs to higher engine speeds. Thermally-insulated pipes keep heat in the exhaust for higher exhaust gas temperatures, which means faster gas flow (but also a benefit in reduced intake temperatures for improved volumetric efficiency). Collector sizes and shapes are also important to header performance in that they influence the strength of the reflected low pressure acoustic wave and affect the way the pulses of exhaust gas blend and flow together.
The stock manifold on the later U.S. Spitfires has tubes so short that beneficial acoustic scavenging never really happens within the operating RPM range of the 1500 engine, although they are do allow gas flows to combine so as to at least not interfere with each other and enable some degree of inertial pumping. Almost any header can help a later Spitfire's performance simply by enabling smoother flow and delaying the timing of acoustic pumping, but a properly tuned header of good design and workmanship can significantly improve performance by optimally timing acoustic pumping to really harness it.
For four-cylinder engines like the Spitfire's, there are 4 into 1 headers in which all the pipes collect together at once (like the stock U.S. spec 1500 manifold, only much longer; see image on left below), and 4 into 2 into 1, or Tri-Y systems in which pairs of pipes for cylinders with exhaust strokes 360 crank degrees (180 distributor degrees) apart collect together first to form a single pair of pipes that later collect into one. Collecting pairs of pipes that aren't separated by 180 distributor degrees would degrade performance because their exhaust valve openings would overlap and gas flows would collide instead of meshing and one primary would pressurize the other, thus impeding flow. For the 1-3-4-2 firing order of the Spitfire and most other inline four-cylinder engines, #1 and #4 cylinders collect and #2 and #3 cylinders collect together. These first pipes leaving the head are called primaries, and the second ones after the merging of primaries are called secondaries.
For street use, the 4-2-1 systems are best because they provide improved performance in the middle range and over a broader range of RPMs, even though they typically don't produce as much peak power as 4-1 systems. 4-2-1 systems 'play a chord' and have broader torque curves with more mid-range performance than 4-1 systems, which 'play one note' and produce more high-end torque and peak power. Four-cylinder race cars often use 4-1 systems because their engines spend basically their whole time operating at high RPMs at or around the peak power point, and maximum power is more important than broad power or power in the low and middle range.
Note that for inline six-cylinder engines, like the GT6 2-liter, there are two ways to have a 'compound' header like the 4-2-1: the 6-2-1 (two sets of three primary pipes gathering into two secondary pipes that gather into one) and the 6-3-1 (three pairs of two primary pipes gathering into three secondary pipes gathering into one). By far the better performing configuration is the 6-3-1. It combines pairs of pipes from cylinders with exhaust strokes that are 360 crank degrees (180 distributor degrees) apart and thus one exhaust valve is closed while the other is open and the operation is bascially the same as that described above for the 4-2-1 configuration. The Triumph inline six firing order is 1-5-3-6-2-4, so the correct way to gather primaries is 1+6, 5+2 and 3+4. Contrast this with the usual 6-2-1 configuration where cylinders 1, 2 and 3 are gathered and 4, 5 and 6 are gathered, all cylinders that are only 120 distributor degrees apart so exhaust valve openings overlap and backpressure is an issue. In V8s, cylinders are usually combined like two separate four cylinder systems. This is easy to implement for single-plane crank V8s because each side of the engine is configured like a single inline 4 cylinder so all cylinders on the same side of the engine easily combine as 4-1 or even 4-2-1. However, correct pairing of cylinders on cross-plane crank V8s is not so easy because ideally two cylinders from each side need to combine, so a practical alternative is to configure like two 4-1 sets of pipes connected by a balance pipe.
For more information about exhaust tuning theory and practice, see:
Exhaust Science Demystified, by David Vizard, from Popular Hot Rodding (February 2009)
How Headers Work, from Super Chevy (February 2009)
Four-Stroke Performance Tuning in Theory and Practice, by A. Graham Bell
Install Freer-Flowing Air Filters
K&N air filters are proven to flow more air than standard paper elements, and are sometimes even better than no filter. Some folks think they look nice too. You'll probably have to retune your carbs after installing K&Ns.
Install Short Air Horns on the Carbs
This will provide a smoother path for air to get into your carb(s). Short ones, called stub-stacks, will help performance a little yet still fit inside your air filters. Like the principle behind exhaust runner lengths above, air horns (sometimes called ram pipes) can help air enter the engine through tuning of inertial flow, and the longer the intake air horns are, the lower the RPM at which the ram effect occurs. Stub stacks are too short to tune the ram effect very much, but they do provide a radiused entry for air to go into the carb(s), which smooths airflow and does help performance a little.
Install Better Intake Manifolding and Carburetors
While the single Zenith-Stromberg (Z-S) carb that came on the 1500 engines for the US market isn't bad, the manifold between it and the cylinder head has two abrupt right-angle turns in it. This 'log' manifold works, but a more direct path from carb to head can improve flow. Moreover, the ports of the US market Z-S log manifold are smaller that the ports on the cylinder head, so they are not matched. But changing just the manifold isn't really an option for the Zenith-Stromberg setup, so a good alternative is to install twin Skinners Union (SU) HS2 or HS4 model carburetors and a matching ram-style intake manifold like what the 1500s for other markets got from the factory. The twin SU setup in which one carb feeds two cylinders allows for a nice straight-through flow of the air and fuel mixture into the cylinders. The earlier US market Spitfire 1300s have this twin carb arrangement using HS2s, and this is a simple bolt-on upgrade from the single Zenith-Stromberg configuration. HS2s will work fine on a 1500, while HS4s will perform better than HS2s at high RPM on modified engines. Used SU carb and manifold assemblies are available from many suppliers for not too much money. New SU carbs are available too (manufactured by Burlen Fuel Systems) but they are more expensive than used ones. You'll need to figure out what needles and dashpot springs to use too, which will depend on the engine's state of preparation and your altitude and general climate conditions. For suggested SU needles for various stages of tune, see the "stages of improvement" section near the end of this article.
You can also try other carbs, like Weber or Dellorto side-draught carbs instead of SUs. These are more expensive, but they are capable of terrific performance.
At this point, the way to improve breathing is to focus on modifying the cylinder head to get it to flow better. This can get expensive, but some minor mods involving simply matching the ports of the cylinder head and the manifold and removing any irregularities in the ports that might disturb airflow are easy and inexpensive to do and will help performance if done correctly.
"Port and Polish" and "Flow" the Head
The capability of the cylinder head to flow gases and promote good combustion has enormous bearing on the engine's ability to generate power. People with much experience at wringing power out of engines have empirically determined the modifications that improve flow in cylinder heads to significantly improve performance, and some of this knowledge has been captured in various publications (e.g., Bell, Vizard). You can pay a professional to make head mods or you can try some of the more pedestrian ones yourself.
On intakes, it's good to match the head ports and the manifold ports. You don't want to enlarge them, just match them so that there's no step and the transition from manifold to head is virtually seamless. On exhaust ports, almost the opposite is best for performance. Empirical evidence indicates that a step from the head exhaust port to a slightly larger manifold port/header pipe is good. It reduces exhaust backflow and helps acoustic scavenging. Therefore, don't try to match exhaust head and manifold ports. However, do make sure there are no locations where the manifold ports are inside of/smaller than the head ports so as to protrude into the flow leaving the head.
Care must be taken when modifying the port runners in the head between the mainfold ports and the valves. Don't significantly alter the shape or size of the port runners, but rather remove sharp edges and transitions within the port runners that can disrupt flow. Just some modest grinding and polishing to clean-up leftover casting and machining irregularities and eliminate sharp transition areas is good. Often there is a little sharp edge just before the valve seats left over from casting and/or installation of the valve seats, particularly on the short radius side. Grind away the sharp edge and make a nice smooth transition while keeping a nice radius. It's also good to smooth the areas around the valve guides, but you don't need to remove a bunch of material here. You want to have a little bit of a venturi shape behind the valve seats; ideally a valve port throat that is about 90% of the valve diameter is good. Be careful not to get carried away and grind too much. Do not simply enlarge the port runners everywhere or you'll make flow worse. Tapered valve guides help reduce obstruction to flow too.
Also, it's beneficial to have flow that enters each chamber with some curve or swirl to it so that as the compression stroke occurs and the mixture gets 'squished' it mixes rapidly and well, which is conducive to good propagation of the combustion flame front from the spark plug to all reaches of the mixture in the combustion chamber. There is often a noticeable step between the machining left over from port drilling and the cast portion of the intake port runners in Spitfire 1300 and 1500 heads. Grind this to seamlessly blend them together. Rounding-off the little sharp edges of the bevel near the spark plug hole can help gas enter and exit the cylinder but it can let the curl of the swirl open-up, so it's a balancing act there. Don't degrade swirl by grinding away on the inboard side wall of the port runners except to blend discontinuities, and beware the little bump in the outer wall of the exhaust port runner near the exit--it's close to a stud hole.
What matters is flow through the head as it will be operating--with valves installed and open to varying degrees. You can achieve big flow through a head with wantonly enlarged runners and no valves installed, but that's not the operating configuration of the head and such flow numbers are misleading and will not be representative of the head in actual operation. Besides, flow quantity is only part of the story--high flow alone doesn't directly translate into more torque and power. Proper, efficient flow into and out of the cylinders with the valves present is what counts. Flow around valves in heads that have rather asymmetrical port flowlike the Spitfire's will be improved by shaping the backsides of the valves to be less 'tulip' shaped and more 'nail head' shaped. A backside radius of about 0.2 times the valve diameter is about optimum for heads with asymmetrical port runners and 'bathtub' shaped combustion chambers like the Spitfire's (stock radius ratio is 0.25 to 0.3). Getting rid of that first ridge where the single stock valve grind meets the backside with a multi-angle grind helps flow around the valve, even if you choose not to tighten the radius on the backside. A multi-angle grind on the valve seats helps too, producing a nicely radiused venturi while ensuring adequate seat contact for good sealing and heat transfer (crucial for conducting heat from the exhaust valves to the head and coolant).
There are limits to what can be done to 'unshroud' the valves in Spitfire heads to further aid flow because the valves are very close to the walls of the combustion chamber recesses, and these recesses already reach out to and line up with the edges of the cylinders. But, if there is any difference (perhaps from boring the cylinders), then a little relief to the walls of the combustion chamber to unshroud the valves can help. As mentioned above, some rounding-over of the sharp edges of the beveled step near the spark plugs will serve to unshroud the valves some and improve flow into and out of the cylinders. Intake valves appear to benefit more by unshrouding than do exhaust valves.
Be sure to make the same mods from cylinder to cylinder and measure the volume of each of the combustion chamber recesses in the head. Perform any minor rework necessary to equalize all the volumes to within 0.2cc or less. Be sure to base any compression ratio calculations and planned adjustments (e.g., head shaving) on these measured values.
For more insight here, check out these references:
Four-Stroke Performance Tuning in Theory and Practice, by A. Graham Bell
Tuning Standard Triumphs Over 1300 cc, by David Vizard
Theory and Practice of Cylinder Head Modification, by David Vizard, et al.
How to Build, Modify & Power Tune Cylinder Heads, by Burgess and Gollan
Triumph Competition Preparation Manual
One last thing that fits under improved breathing is the camshaft. Changing the camshaft in most Spitfires can help, but only after other things have been done, like improving the exhaust and intake and head flow, and especially raising the static compression ratio of the engine, which is described next.
Raise the Compression Ratio
Increasing compression increases the efficiency of the thermodynamic cycle of the engine, which yields more torque and more power. Static compression ratio is simply the ratio of the volume above a piston when the piston is all the way down at bottom of its stroke (bottom dead center, or BDC) to the volume above a piston when the piston is all the way at the top of its stroke (top dead center, or TDC). If you think of the volume left when the piston is at TDC as Vcc (combustion chamber volume) and the volume swept by the piston as Vd (cylinder displacement volume), then the volume above the piston at BDC is Vd+Vcc and the compression ratio CR = (Vd+Vcc)/Vcc.
Vcc includes not only the volume of the recess in the cylinder head, but also the volume of the gap determined by the head gasket's thickness, the volume of the cavity in the top of a dished piston (or the negative volume of the space taken by a crowned piston) and what little other volume that might remain between the piston and the top surface of the block at TDC ("deck volume") or in the little gap between the piston and the cylinder wall and above the top ring (negligible for practical purposes). For Spitfire 1300 and 1500 engines, gasket volumes and deck volumes are all the same, so compression ratio is determined by the head volume and the piston type (dished or flat). On a Spitfire 1500 with 7.5:1 static compression ratio, as all North American models were (except 1976 models, which had 9:1 static compression), changing the pistons from the stock dished ones to flat-top ones will reduce Vcc by the volume of the dished space, which is about 6.7 cubic centimeters per cylinder, and thus raise compression to nearly 8.4:1. This helps performance some. You can raise compression further by substituting a compatible head with smaller combustion chambers and/or shaving/milling material off of the head surface to shrink combustion chambers. However, increasing compression increases combustion pressures and temperatures and elevates the likelihood of detonation and detonation-induced pre-ignition, which can damage or destroy pistons. A static compression ratio of 9.5:1 or 9.75:1 is the practical limit for a Spitfire street engine with a reasonable cam profile running on contemporary pump gasoline. More modern cars sometimes have higher compression ratios, but that's because they employ knock-sensing systems that dynamically adjust (retard) the ignition to avoid prolonged detonation. Race engines typically have very high static compression ratios, but they also have more aggressive camshafts with very long valve opening duration and overlap, which reduces the effective or real compression inside an engine while it is operating. Racing competition and the overarching objective of winning a race typically favors peak power over long-term reliability and low or moderate-speed driveability and justifies spending more for special, tougher materials (e.g., forged instead of cast pistons), frequent engine rebuild or swaps, and other expensive and time-consuming things.
Raising compression on a Spitfire, by swapping and/or shaving heads and switching pistons, will help performance. Changing pistons isn't too difficult but new pistons are moderately expensive. Swapping or shaving a cylinder head is easier and can be less expensive. Removing the cylinder head isn't too tough a task, and once you have it out of the car, a good machine shop can quickly remove the amount you desire for a very modest fee. I've constructed a compression ratio calculator spreadsheet that you can use to figure out how much you need to shave off a given head to reach a desired static compression ratio. I've also compiled information about interchangeable cylinder heads for Triumph 1300 and 1500 four-cylinder engines (see table below--still a work in progress) that can be used in conjunction with the compression calculator to generate various engine configurations and compression ratios.
Triumph 1300 and 1500 heads are interchangeable, and evidently were made in three castings (identified by different cast-in numbers) that in turn were machined different amounts to yield different combustion chamber sizes (stamped with different part numbers) for various car models for different markets around the world. All three castings accommodated valve seats for the same size exhaust valves, but different size inlet valves. Note that the hot ticket for the North American market "low compression" 7.5:1 1500 with dished pistons is to fit one of the "big inlet valve" heads, use either dished or flat pistons, and perhaps do some shaving of the head. The bigger inlet valves will support greater airflow and permit higher performance to be extracted from more extensive mods (like a bigger cam). For example, nearly 9:1 compression can be achieved on a "low compression" dished piston short block by simply installing a stock 218142 head from a world market (non-North American) late Spitfire mkIV (FH25001 and later). A little shaving of this head (about 0.040 inches or 1mm) before you install it and you'll have a 9.5:1 engine with big inlet valves and without taking anything apart in the bottom end. Or, take a head from a Toledo 1300 (part number 218141) and put it on a 1500 with flat-top pistons and get about 9.6:1. Some "big inlet valve" heads came in North American market cars, and these can be shaved and used with flat-top pistons to get the same result. Another option for the "low compression" 1500 is to replace the dished pistons with flat-top ones and shave about 0.080" off the "low compression" head it came with to produce an engine with a static compression ratio of about 9.5:1. Note that merely changing from dished to flat-top pistons in this instance will not result in 9:1 but rather 8.4:1 because the 9:1 1500 configuration also uses different heads (e.g., TKC1155 and TKC2748) that have smaller combustion chambers. 1500's that started out as 9:1 engines can benefit from "big inlet valve" heads and/or shaving to raise compression too. 1300 engines, particularly the more robust "small journal" ones popular with racers, can be similarly upgraded by substituting shaved versions of one of the "medium" or "big inlet valve" heads. All of the above heads are interchangeable on 1300 and 1500 blocks, so beware of certain "bad" head, block and piston combinations that either raise compression too much for typical street use or result in a downsizing of inlet valves.
Change the Camshaft
If you have an engine with raised static compression and improved flow characteristics, then it's possible to take advantage of a more aggressive camshaft (i.e., more valve lift and longer valve opening duration and overlap). The air+fuel mixture has inertia of course, so opening valves earlier and closing them later (i.e., increasing duration) enables more mixture to get into the cylinders at higher engine speeds, thus improving volumetric efficiency and increasing power at high RPM. The trade-off is that longer duration and overlap (principally later intake valve closing) dilutes pressure at low engine speeds and lowers the effective compression ratio, reducing torque and power at low RPM. So, choosing a camshaft for a fixed valve timing engine is a compromise between increasing performance at high RPM and decreasing performance at low RPM.
Reground cams, which are ordinary/stock cams machined to have a new profile, are less expensive than virgin ones, but be aware of material and surface finish choices and compatibility. You'll need resurfaced or new tappets and maybe shorter valve guides and different valve springs too, depending on cam profile particulars. There are many aftermarket Spitfire camshafts to choose from, and manufacturers/vendors only reveal certain data about their cams (exact profiles are usually closely-guarded), so choosing among them can be difficult. To first order, don't get too aggressive with duration or overlap or your engine will not perform well at low RPMs and you may be disappointed in the way it behaves driving it around in normal use. Furthermore, increased valve lift beyond a certain amount will not add to performance because of the limitations of the head to flow gases. According to Dmitri Elgin, valve lift greater than about 0.380" in a Triumph engine will yield diminishing returns unless some flow improvements have been made to the head and valves. However, this is not to say that a cam that generates peak lift greater than 0.380" is wasteful. This is because valve velocity and acceleration are important too. A cam that has a fast 'ramp' will open the valves more quickly and allow the valves to be more open more often and thus capable of more flow. But, if the change in cam ramp is too abrupt, then valve acceleration will be too high, generating excessive forces on the valve train. A way to get sooner and quicker valve openings and later and quicker valve closings without excessive acceleration is to have a cam with a higher peak lift than the optimal max opening. Obviously, cams and valve operation are more complicated than just duration, overlap and peak lift.
If you are not sure what to pick, be conservative and don't over-cam. The objective is to achieve a compatible match between compression ratio, cam duration, overlap and lift, and the flow characteristics of the head, intake and exhaust, while keeping drivetrain forces within design limits. As a rough guide for sporty street use, keep duration under 280 degrees or so and valve overlap under 60 degrees or thereabouts so that adequate performance at low RPMs is preserved and good performance in the mid-range is achievable. These guidelines are not hard and fast rules because many factors determine camshaft performance, and it is assumed that you have raised the static compression ratio and have installed good-flowing exhaust and intake parts, the specifics of which affect camshaft selection.
Actually, the Triumph factory 270 degree duration (25-65-65-25) cam that was stock in many of the 1300 engines is a really nice street cam. Be advised that this cam and the other early "small journal" cams ran in bearings installed in the block, while the later 1300 and all 1500 engines had "large journal" cams that ran right in the block without camshaft bearings. You can use a small journal cam in a late block as long as you install cam bearings.
For more on camshaft terminology, theory and practice, check out these references:
Four-Stroke Performance Tuning in Theory and Practice, by A. Graham Bell
Camshaft Glossary (Elgin Cams)
Camshaft Theory (Second Chance Garage)
Performance Camshafts (Dimitri Elgin of Elgin Cams)
Camshaft Selection (Newman Cams)
Cam and Valve Train Questions (Crane Cams)
Camshafts and Valve Train Basics (Street Racers Online)
"Blueprint" the Engine
The engine, like any manufactured thing, is imperfect and has certain machining specifications and assembly tolerances. If you are asking more from your engine, it's good to reduce variance and tighten the dispersion about the specs. There is no such thing as perfection in machining and assembly, but with extremely precise machining and meticulous parts screening guided by excellent metrology, it is possible to minimize variance about an ideal spec to be within the resolution of the measuring equipment itself and be so tiny as to be inconsequential, and thereby achieve "practical" perfection. The process of rebuilding an engine to exact specifications is "blueprinting." Get all the moving and rotating parts matched and balanced. Have a machine shop align bore your block (i.e., precision machine the main bearings to be in alignment and the cylinder bores to be more parallel to each other and more perpendicular to the crank), resurface the top of the block to ensure flatness and orthogonality to the cylinders, grind and/or polish the crank to be more straight. Some moving parts, like connecting rods, can be lightened before they are matched to reduce acceleration forces, but lightening of such critical parts must be done carefully and should be performed by someone who knows what they are doing. Note that lightening of the flywheel, particularly on the 1500, will let the engine rev-up quicker, but it won't change the output torque or power and can make for rougher idling, so be forewarned. A good machine shop will check for cracks (magnafluxing, die penetrant) to assure integrity of parts. Blueprinting will enable the engine to run smoother with less self-induced stress and enable it to be run at higher RPMs and thus generate more peak power. If you are doing more than just bolting on some better intake and exhaust parts, then have the engine blueprinted.
Fix Some Bottom-End Weaknesses
There are a few specific enhancements that ought to be made to Spitfire engines to fix some intrinsic weaknesses. One is to enlarge the passageway that feeds oil to the center main bearing and subsequently the big end bearings and connecting rods for pistons 2 and 3. Enlarging it to 5/16 inches will aid in the delivery of adequate amounts of oil here. This task should be performed by a competent machinist. Baffling the oil pan to prevent oil surge and starvation of the oil pickup, particularly during left-hand turns, is a really good idea. You'll appreciate this if you've ever watched an oil pressure gauge on a Spitfire with an unbaffled sump during a long left-hand turn at speed. Lastly, pinning the thrust washers to the mains to keep them from falling-out after they wear-down is a good idea that can provide peace-of-mind and prevent costly damage to the block.
For more details on Triumph engine rebuilding, see the following:
Building a Reliable Spitfire Engine for High Performance, by Calum Douglas
A Guide to Racing your Triumph Spitfire or GT6, by Jon Wolfe
Triumph Competition Preparation Manual
the writings of Kas Kastner
More Work but Bigger Results
Swap Triumph Engines to Create a "Spit-6"
One engine swap that I'll mention in this article since it is still "keeping it Triumph" is to put a Triumph in-line 6 cylinder lump into a Spitfire. The resulting creation is commonly referred to as a "Spit-6." Many people have tried this mod and there are some important nuances to note. First, adding the extra mass of the 6 pot lump and distributing it differently will significantly change the handling characteristics of the Spitfire. To safely deal with it, the brakes should be upgraded to GT6 type all around and the front springs should be uprated. One way to do this is to take a GT6 chassis and put a Spitfire body on it, which is more precisely referred to as a "convertible GT6" or CGT6. But there's more. Mounting the engine the way it is in the GT6 puts the two additional cylinders forward and tilts and raises the engine, necessitating the substitution of a bulged bonnet, like the GT6 bonnet, and a forward-mounted GT6 radiator. Also, the front suspension towers on the GT6 are slightly different than the Spitfire's, so GT6 towers should be used when mounting the 6 cylinder the way it's mounted in the Spitfire's small chassis cousin. Also, Spitfire and GT6 transmissions and bellhousings are different and not interchangeable. If a 2.5 liter engine from a TR5/TR250 or TR6 is used instead of the 2 liter from a GT6, be advised that the oil pan is deeper (to accommodate the longer stroke) and it will interfere with the steering rack and frame. A remedy is to swap the 2.5 liter pan out for a 2 liter oil pan that has been locally 'modified' to provide clearance for the 2.5 liter crank. Also, the TR6 transmission sits higher and requires modifications to the transmission tunnel cover and the tub's propshaft tunnel to accommodate it. You get more torque and power with the 6 cylinder swap, and the same kind of enhancements already described can be applied to enhance performance (download David Vizard's "Tuning Standard Triumphs over 1300cc" for more detail and specifics), but the car will handle quite differently than a Spitfire due to the extra few hundred pounds and its distribution so far forward and a little bit up.
An innovation that significantly enhances the Spit-6 is to substitute a Spitfire front engine plate for the normal 6 cylinder one and mount the engine like a Spitfire engine is mounted. This puts the front of the 6 cylinder in the same location as the front of the 4 cylinder and shoves the entire engine and transmission aft by 6 inches from the normal GT6 configuration. This absolutely transforms the way the car feels and handles for the better. However, this is more work as it requires fairly involved trimming and modification of portions of the frame (some localized notching and reshaping to accommodate the bellhousing and exhaust, and a new mount for the transmission 6 inches farther aft), the tub (cutting and refabricating parts of the firewall) the gear shifter (removing 6 inches from the GT6 or TR6 cantilevered gear shift extension, and rewelding) and the propshaft (shortening), among other things. It also requires some relocation of items depending on whether the car is right or left hand drive. For example, left hand drive (e.g., North America, Continental Europe) requires implementing a take-off plate at the stock 6 cylinder oil filter location and mounting the filter remotely to accommodate the accelerator pedal, and right hand drive may require the brake master cylinder and pedal to be relocated slightly to accommodate the intake (depending on what it is). Having driven GT6s and "ordinary" Spit-6's myself, as well a Spit-6 with such an "aft mounted" configuration, it's my humble opinion that aft mounting is well worth the extra work. However, it's somewhat involved and more difficult than anything mentioned so far in this article. I am in the process myself of putting together an early-bodied Spit-6 (more properly, a CGT6) with an aft-mounted engine and fully independent rear suspension. Having seen Stephen Attenborough's aft-mount modified GT6 in person and having driven Paul Tegler's aft-mount, fuel-injected Spit-6 called "FIS6" (pronounced "physics"), I'm excited to have my own version.
Comparison of engine positions--normal GT6 mounting and innovative aft-mounting (photos c/o Paul Tegler):
Stephen Attenborough's right hand drive GT6 and Paul Tegler's left hand drive Spit-6 "FIS6" (both are fuel injected):
Switch to Fuel Injection
Fuel injection is intrinsically more efficient than carburetion, and modern
technology fuel injection control systems like MegaSquirt allow closed-loop
feedback control over fuel
injection and ignition and enable superior fuel efficiency and performance. I
have driven a few fuel-injected Spitfires, and I can testify first-hand that
fuel injection on a Spitfire or GT6 can work very well! Paul
Tegler's Spit-6 "FIS6" is a modified Spit-6 with a MegaSquirt fuel-injected 2.0
liter 6 cylinder GT6 engine installed in the "aft-mounted" configuration
described above. Paul's car is a fine piece of engineering--a "21st century"
Triumph and a real blast to drive! Paul lists some other folks
who have fuel-injected a Triumph.
Use Forced Induction
Stages of Improvement
Spend More to Get Less
Above and Beyond
Beyond the aforementioned things, getting more power out of a Spitfire engine or other Triumph lump can be a matter of diminishing returns, and is likely to require large sums of money and sometimes will lead to reduced reliability and increased hassles and operating costs. Special, ultra-high performance internal engine parts (e.g., forged pistons, special steel cranks, special connecting rods) are very expensive, add durability and will make the engine capable of higher performance, but are they warranted for a street Spitfire? Extreme levels of power (e.g., in the realm of 100hp per liter of displacement and up) often come at the expense of durability and reliability and can require significant modifications to strengthen the block as well as special ultra-high performance parts. Moreover, at some point the drivetrain cannot handle the added stress, so even if you have a terrific, reliable, high-powered engine, you won't be able to apply that power without breaking the transmission, differential or other drivetrain parts, or replacing them with more robust units from unrelated cars. Besides, at this point, your Spitfire may not be very well mannered or suited for street use, or would not really be a Triumph anymore and would be beyond the theme of this article. The important thing is to enjoy your Spitfire, and what constitutes "enjoyment" is a matter of personal taste and choice. Enjoy!