Our old cars would have a hard time being more different. The odd couple?
I thought it would be fun to look at some of the engine design details that went into both the Spitfire and the 911 and see how the teams did what they did. 4 cylinder pushrod vs 6 cylinder overhead camshaft. Water cooled vs air cooled. 1920's carbs vs 1980's electronic fuel injection. Front engine vs rear engine.
In researching the cars, I found it interesting that both cars were introduced within a couple years of each other ('62 for the Spit, '65 for the 911) and were made in relatively similar numbers; Triumph made 314,000 Spitfires between 1962-1980. Porsche built 250,000 911's between 1965 and 1988. Compared to modern day "regular" cars, 250,000 units is more of what you would see as an annual figure, not the culmination of 20+ years of manufacture. These both belong to a bygone era of hand assembly. After the 1989 model year, Porsche would shutter Building 2 where they had assembled the 356 & later the 911 and moved to a new automated plant. They currently take 2 days to build a car, with 1,500 people touching it at some point. They made a bit over 38,000 of them last year; well over double the number in 1989.
I also tossed in my "grownup car", a 2014 Audi A5 just to see what 30+ years of auto engineering has done. Needless to say the impact of turbocharging and modern ECUs is pretty apparent. Its BMEP is roughly double what the 911's is. It can haul a car that is larger and significantly heavier nearly as fast with 2 fewer cylinders and 2/3 of the displacement. It also has fold down rear seats and a trunk. Maybe we have made progress? But if I could do anything, I would lose 500lbs. It feels great, but heavy. And that is despite high tech materials like aluminum body panels and plastic parts.
The comparison also shows just how small a classic G-body 911 was - it's only about 6" larger than the Spitfire in wheelbase and track, and the Spitfire, if it needs saying, is a very tiny car.
For general chassis related stats, I used the 911 as the 0 mark. +/- is relative to it.
BMEP (Brake Mean Effective Pressure) is a performance yardstick for engines. It is the average pressure pushing your pistons down to create the measured torque. More here: http://epi-eng.com/piston_engine_technology/bmep_performance_yardstick.htm
LBS/HP is a very rough performance yardstick for the car. Divide the weight of the car by the horsepower. The lower the number, the more responsive the car is likely to feel.
I have added HP/Ton which is another way of stating the same thing that may be more familiar.
The other stats should be self explanatory. Here are some abbreviations & terms I used just in case:
AWD = all wheel drive, RWD = rear wheel drive.
Open differential is the most common type, it drives one wheel.
DOHC = dual overhead camshaft. One cam for intake, one cam for exhaust. (Audi) The other cars have both functions on a single camshaft, although due to its horizontally opposed design, the 911 uses a pair of camshafts where the Triumph uses one.
CR = compression ratio. How much do we squish the fuel-air mixture before we ignite it? higher gives more power.
Powertrain | Location | Cyl | Block | Heads | Drive | Front | Rear |
A5 | Front | 4 | AL | Al | AWD | 255/35-19 | 255/35-19 |
Carrera | Rear | 6 | Al | Al | Rear | 205/60-16 | 225/55-16 |
Spitfire | Front | 4 | Iron | Iron | RWD | 175/70-13 | 175/70-13 |
Wheelbase | Inches | mm | Delta in inches |
A5 | 108.31 | 2751 | +18.86" |
Carrera | 89.45 | 2272 | 0 |
Spitfire | 83.0 | 2108 | -6.456" |
Track - Front | Inches | mm | Delta in Inches |
A5 | 62.6" | 1590 | +7.56" |
Carrera | 55.04 | 1398 | 0 |
Spitfire | 49" | 1244 | -6.04" |
Track - Rear | Inches | mm | Delta in Inches |
A5 | 62.01 | 1575 | +6.7" |
Carrera | 55.31 | 1405 | 0 |
Spitfire | 50 | 1270 | -5.31" |
Weight | Lbs | Kg | Delta lbs |
A5 | 3494 | 1585 | +744 |
Carrera | 2750 | 1250 | 0 |
Spitfire | 1750 | 794 | -1000 |
Turning Circle
A5: 37 feet, 5" / 11.4 meters + 1' 4"
Carrera: 36 ft 1 in (11.00m)
Spitfire: 24 ft.2in. (7.3 m) *winner :) -11 ft 11"
Engine Data
A5: 220HP @ 4,450rpm / 258 ft/lb at 1,500rpm
Straight 4, turbo charged, water cooled, DOHC, 16 valve, Bore 3.25" Stroke 3.65" Disp 121.1 cu in / 1984cc CR: 10:1 Boost: (7-10psi est)
LBS/HP: 15.88
HP/Ton = 125.93
BMEP = 150.8 * 258 / 121.1 = 321.275 (roughly double the 3.2l Porsche)
HP ((torque x rpm/5252) = 258 x 4450 / 5252 = 218.60
Mean Piston Speed (6000 rpm): = 3.65 x 6000 /6 = 3650 fpm (A5) = 41.448 MPH
Max Piston Speed: (6000rpm): = 3.65 x pi /12 x6000 = 5,733.4 fpm = 65.15 MPH
HP/Liter = 110.89
Quoted 0-60 (mfr): 6.5 sec (6.4sec in the press)
Spitfire:
US Spec - 53 HP Torque? (was not quoted)
Mine dyno'd at: 63HP (@5,000 rpm) and 85 ft/lbs of torque (@3,000rpm) with a 4:1 header and a weber 32/36 carb, 9:1 pistons. shaved head for 9.5:1 net CR (Head was milled .063")
With current SUs, better tuning and better headers we should be well over 70HP (EU spec is 71HP). Using 6,000rpm redline for calcs.
Straight 4, Firing order 1,3,4,2, water cooled, pushrods, Bore 2.9 in. (73.77- mm), Stroke 3". Disp 91.3 cu in / 1,493cc CR 7.5:1 stock, 9.5:1 as configured.
LBS/HP: 33 (US Spec)
LBS/HP: 24.7 in EU spec (25% increase)
HP/Ton = 81.14 (EU Spec)
BMEP = 150.8 x 85 / 91.108 = 140.69 (as currently configured).
HP Est ((torque x rpm/5252) = 85 x 5000/5252 = 80.92 HP (as currently configured, est)
Mean Piston Speed: 3.44 x 6000 /6 = 3,440 feet per min. = 39.09MPH
Max Piston Speed: = (3.44" x PI /12) x 6000rpm = 5,403fpm, = 61.40 MPH
HP/liter = 42.11
Quoted 0-60 (mfr) US Spec was 15.4 sec, EU spec was 11.3 sec. As configured I think we're close to the EU spec.
911 3.2l: 231 HP @6,250 rpm / 209.5 ft lbs @4,800rpm (EU Spec)
Flat six, firing order 1,6,2,4,3,5, air & oil cooled, OHC, Bore 3.74 in/95 mm x Stroke 2.93 in/74.4 mm, Disp 193.00 cu-in/3,164 cc, CR 9.5:1
LBS/HP: 11.90 *Winner
HP/Ton = 168
BMEP = 150.8 x 209 / 193.08 = 163.23
HP Est (torque x rpm/5252) = 209 x 5900 / 5252 = 234.79 HP
Mean Piston Speed: 2.93 x 6250 /6 = 3,052 feet per min = 34.68MPH
Max Piston Speed: (6250rpm) = (2.93 x pi /12) x6250 = 479,412 feet per min = 54.48mph
HP/Liter = 72.19
Quoted 0-60 (mfr): 6.1 sec (best press quoted is 5.5sec)
Internal combustion engines are air pumps at the end of the day. More air going in and out means more power can be made. The rpm where maximum torque is stated is where your engine has the most volumetric efficiency. When you want to beef up power you can raise the compression and you can raise the displacement. Compression is limited by fuel type and the specifics of your engine. To increase your displacement, you can make the cylinders bigger, and/or you can increase the stroke - how far the pistons move. There is a practical limit to any engine's displacement just based on its physical size. You will run out of room. After that you get to supercharge it or turbocharge it or replace it if you still need more power. The physical size and layout of the engine also affects the car's packaging and styling.
Physical Size & Configuration:
The Triumph SC power plant started life in the 1950s as a 800cc engine; it got heavily bored and stroked to get to 1496cc, and that had implications. Being physically small made tapering the nose of the car relatively simple, and still left plenty of room for all the other bits. The GT6 of the era needed a "power bulge" in the hood to fit the straight 6 engine, and wound up with a tiny radiator.
The flat 6 in the Porsche has a unique challenge to stroking the engine - the engine gets physically wider when you do that. (by 2X the change in stroke) but it is much simpler to increase the bore because the cylinders are discrete units. We're talking inches though. The engine's wide, flat design would not work in the front; it would interfere with the steering & suspension. So you either give up the rear seats and mount it in the middle, or you hang it out back, as Porsche did here. This is great for traction but can be tricky in some cornering situations. They were also able to mount it very low in the car thanks to its dry sump oil system since there is no sump beneath the engine. There is no traditional cooling system to deal with, no radiator to place near airflow. This helped keep the front of the car sleek. There are oil lines running into the front right fender to an oil cooler, but it is well hidden. One of the downsides of a horizontally opposed engine is the need for 2 heads rather than one. This can increase costs unless you can use the same part on both sides - which they did. All 6 cylinders are the same casting (there are actually 6 heads!) so there is only 1 part number. There is also a camshaft/rocker housing that shares a single part for both sides, since each side has its own camshaft. It is a very different approach from other auto manufacturers.
Audi uses a turbo in our comparison to extract quantities of power and torque you would expect from a much larger engine. Since the engine only has 4 cylinders and is not very large, it gave them styling and packaging options like a relatively short front hood and room for the all wheel drive system, and a trunk. The engine uses a contemporary approach to its design, aluminum block and heads to save weight, dual overhead cams and 4 valves per cylinder for good breathing.
Bore & Stroke:
Bore vs Stroke dimensions give us our two basic families of engines - over-square, where the piston diameter is larger than the stroke, and under-square, where the opposite is true. It's really 3 families - there are cars with square engines (bore = stroke) but the list is short. Funny enough, the 3.2 S50 engine in my old (US Spec) E36 M3 was within 1/100th of a mm of being square.
Over-square engines tend to favor higher RPMs and better power: the pistons don't have to move as far (and deal with friction). The piston has to move up, stop, move down, stop, and move back up again really quickly. The farther it has to move, the more friction there is and the more stress it is under at any given engine speed because it has to go farther, which means faster all else equal. All that starting, accelerating and stopping stress also goes into the rods, rod bearings and crankshaft. This limits the maximum RPM or "redline". (Maximum RPM is limited by a number of other things, of course.)
Under-square engines tend to favor more torque and lower RPMs. (insert tractor joke here)
Tying it together
The Triumph's engine having been bored and stroked to get from 800cc to 1496cc is just barely under-square its final form here. You can see the piston speed differences are significant, despite what looks like a minor difference in measurements. The Spitfire 1500 in stock form made about 35HP/liter. It uses push-rod activated valves and the intake and exhaust ports are on the same side of the head. The manifolds are fairly restrictive. My Spitfire's engine, which has had performance modifications, is running around 53HP/liter. Still well behind the stock Porsche unit, but in a much better place. In my own dyno test (below), we had a lot more torque than horsepower. This was likely due to a poor state of tune combined with some other shortfall, but being under-square could affect this as well. The Spitfire suffers a bit from its older engine design; cylinders 3 & 4 are very close and have lead to head gasket failures in my case (see pic, below). The engine also only has 3 main bearings which was probably fine for a 800cc making 26HP, but is not great for the larger more powerful 1500 version. The engine displacement grew 187% over time, and power grew to roughly 71hp. (a 273% increase) The camshaft does not run in bearings (it uses journals) and the stock intake manifold is pretty bad, forcing 90 degree turns for cylinders 1 & 4 in US trim. The SU HS4 manifold for contrast is nearly a straight shot for each port. Straight is better. The headline is it has been stretched thin in every direction getting to this point.
Cylinders 3 & 4 are maybe a bit too close in the Spitfire. My engine during a 2012 repair. (noted below)
The 911's engine, not surprisingly, is over-square. It loves to rev. With it's more modern cross flow heads, overhead cam & electronic fuel injection makes about 72HP/Liter (it was de-tuned for the US market a little, this is the ROW spec) The 3.2l engine is legendary for being robust, and stock examples can run for hundreds of thousands of miles. The engine case and crankshaft are shared with the more powerful 930 turbo, and the design is essentially the final state of an engine that Porsche had been refining for 24 years. Over that time it grew from 2.0 liters and 130HP in the 1965 model to the 3.2 liter and 231HP seen here. (160% and 177% resp) Less displacement growth percent-wise than the Triumph SC and easier to accomplish thanks to the modular nature of the engine. It is using a stronger foundation with better materials that are under less stress than the Triumph power-plant. It doesn't even use a head gasket. Triumph was pushing their old mill as far as they could to save money; they were in rough shape by the 1970s (parent British Leyland declared bankruptcy in 1975) and Triumph built their last car in 1980.
The Audi A5 (B8 generation) has an even more under-square engine than the Triumph and a ton of torque way down the RPM range despite having a turbo. Its piston speeds are also the highest. You can feel it run out of oomph as you approach the redline, where the 911 really wants to keep going. Despite being the heaviest car, the A5 is easy to move off the line thanks to that low end torque. The A5 is using lots of "modern" stuff. Modern is in quotes because dual overhead cams and 4 valves per cylinder date to the 1912 Peugot L76 Grand Prix car and turbos were patented back in the 19th century. GM gets to claim the first production turbocharged passenger cars with the 1962 Corvair Turbo and the Olds F85 Jetfire. Everything old is new again! But combining 100 years of refinement, materials tech and computer controlled electronic fuel injection allows some amazing efficiencies for the Audi. The TFSI 2.0 liter engine appeared as the base engine across the VW and Audi ranges in various states of tune. The stock turbo can flow enough air to generate about 280HP (per performance websites) so they could raise or lower the HP more or less at will with a software tweak. The A5 also has the best all wheel drive system on the planet providing unmatched grip, and finally it has the largest wheels & widest tires. So despite being the largest car, slightly down on power (but up on torque) and almost 750lbs heavier, it is not far behind the 911 in the 0-60 dash. The 911, thanks to its light weight and rear biased weight distribution is able to win the dash with comparatively small tires and without so much as a limited slip. Net net the Audi is using all 4 giant tires and electronic traction control while the 911 is using a single rear tire and no electronic aids. The Porsche figures are also conservative. Auto magazines had no trouble getting them into the mid 5 second range. Each doubling of HP/ton cuts acceleration times roughly in half as a rule of thumb. You can see this in the 911 vs Spitfire 0-60 pretty well. The 911 has double the figure and half the time. It's not precise, but close enough.
To show how far we've come, the current 911 GT3 RS makes 130HP per liter while still being naturally aspirated, which is more than the Audi manages with its turbo charger here, and there are even more extreme examples out there.
Actual Spitfire Dyno Results (2013)
These were taken with an older Weber 32/36 carb and 4:1 headers into an old Abarth GT6 exhaust system. The car ran ok, but could have been tuned a lot better. Nevertheless, it is a yardstick. And 85 ftlbs of torque is not too shabby.
Those bits have since been replaced by a pair of re-built SU HS4s and a Moss Europe 4-2-1 header into a performance Spitfire exhaust. The stock Delco distributor has since been replaced by a custom unit from Advanced Distributors. The SUs are set up with ABT needles currently. Hopefully I canget it tuned and dyno'd again this summer and wind up somewhere near my theoretical 80HP bogey.
The engine was bored .040 over, & the head was shaved in conjunction with installing 9:1 pistons which results in a 9.5:1 CR. The head uses EU spec dual valve springs. The distributor and ignition system were stock at the time except for a Lucas sport coil. Not sure if the drop in power after 5k is due to ignition, breathing, mixture or all of the above. We should peak around 5,500 rpm.
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