This was written with the intention of having it published in the magazine of the Vintage Triumph Register. Someday I'll make it look better in HTML, as well as fix all the typos, poorly constructed sentences, and so on.

Anatomy of an Autocrosser

Those of you who attended the autocross during the 1995 convention in Rockford may have noticed a somewhat ratty looking red Spitfire 1500, with fat tires and a strange looking exhaust pipe. And if you were keeping an eye on the times of the various drivers, you may have noticed that this Spitfire was one of the fastest cars at the event, getting beaten only by John Lye's very nice TR4 race car. That Spitfire is tuned for autocrossing, and competes regularly in Sports Car Club of America [SCCA] Solo II events. This article will describe the modifications made to the Spitfire, allowing it to hold its own against the 6 and 8 cylinder cars on the autocross circuit. While the details will be specific to the Spitfire and the SCCA preparation rules, the basic principles apply in many cases. Some things will be glossed over, with pointers given to further reading on the subject. If you have further questions, electronic mail is the best way to reach me, and I have been known to return phone calls to those who leave a message at 801-558-8817. And, donning my cap as proprietor of The Fat Chance Garage, I'll be happy to assist those of you who need help in tracking down some of the performance bits not readily available.

Autocrossing is not road racing. While some factors are similar, the demands on the car are different. In road racing, there is usually time between turns to get the car out of one turn, then positioned for another. The motors are tuned to rev high, and produce power at the upper end of the rev range. In an autocross, there is little time between turns for the car to settle down, quick transient response is crucial. And with a dearth of long straights where the car can make use of all the revs available, one needs to have as much low and mid-range grunt coming out of the turns as possible.

The car, known as "Junior," is built to SCCA D Street Prepared rules. A car for this class is capable of being street legal, with full interior, lights, wipers and such. The engine internals must be stock, but intake and exhaust systems, with the exception of forced induction, are pretty open. Suspension is quite free also, in terms of bushings, springs and shocks. Wheels are unrestricted, but tires used must be DOT rated. Several manufactures do make DOT tires for racing, such as the Yokohama 008R, the BF Goodrich Comp T/A R1, and the Hoosier Autocrosser. Not too good on treadwear, but do they stick!

The tires on this car are Hoosier Autocrossers, in a 225/45-13 size. Front tire pressure is 27, rear is 24. The wheels are lightweight aluminum, 8" wide. The Hoosiers are quite light, being bias-ply tires, so the wheel and tire combination weighs about 17 pounds. A 13 x 6 steel wheel might weigh this much alone! Reducing weight in the wheel and tire area is quite critical. First off, any weight removed from the car is weight that does not have to be accelerated or slowed down. So a lighter car will be faster than a heavier car with the same power. Secondly, the wheel and tire is unsprung weight, which means that the suspension has to work harder to control it during bumps and rough travel. This is a simplification, recommended reading on suspension dynamics and car preparation in general is the series of books by Carroll Smith, "Prepare to Win," "Tune to Win," "Engineer to Win," and his "Nuts, Bolts, Fasteners and Plumbing Handbook." Thirdly, the lighter tire and wheel combination is a benefit by reducing rotational inertia. Think of them as four little flywheels out at the corner of the car that have to be spun up to speed when accelerating, and slowed down when braking. The lighter they are, the less energy it takes to start and stop them. So lightweight wheels and tires save power in moving forward, going up and down, and going around.

The rest of the suspension is not that complicated. All four shocks are Spax adjustable units. When installing, make sure the adjuster is readily reached with the wheels on the car. For autocross runs, the shocks are set all the way up. For taking my wife to dinner up the canyon, I usually have them set softer. No additional sway bar or Z bar on the rear, and the front sway bar is the stock 7/8" Spitfire. Make sure the mounts are in good shape. The stock mounts were torn off on this car, so the front sway bar is mounted with some generic brackets and polyurethane bushings, with the brackets held in by bolts going through the front frame section. The front camber is about 1 degree negative, with zero to a touch of toe-out.

The front springs have a rate of 480 pounds, and a fitted length of 7 inches. The car could stand to be lowered another inch in the front, which would require flaring the front fenders to clear the Hoosier tires. Maybe before next season. The rear spring is a stock 1500 swing spring with an extra leaf in the lower section clamped to the diff. This extra leaf was free, taken from a discarded spring with a broken eye. The rear spring is only slightly decambered, and another 1/2" of lowering is gotten by making up an aluminum block spacer between the diff and the spring. When the front ride height is finalized, the rear spring will be decambered more, to give the proper ride height without the spacer.

On the squaretail body, the wider rear fenders clear the fat little Hoosiers once the inner fender lip has been hammered flatter. This assumes that the wheels have 4" of backspace, the distance from the rear edge of the wheel to the mounting face of the center section.

The differential was changed from the stock 1500 unit. It is a Mark 3 unit, with the 4.11 to 1 gears bolted to a Quaife torque-sensing limited slip. The Quaife was one of the bigger ticket items, but made quite a noticeable difference in the way the car could launch off the line and come out of the corners under power. With the 4.11 gears, and the 20" tall Hoosiers, the 6300 rev limit gives the Spitfire a top speed of about 95 miles per hour. Not exactly a Salt Flats record breaker!

The heart of the car is the 1500 motor. Many road racers prefer the early 1296 version for production car racing, citing the lower rev capabilities, higher piston speeds and subsequently larger stresses of the 1500 motor. But for autocrossing, and especially under SCCA Street Prepared rules, the torque of the 1500 is hard to beat. The stated torque rating for USA market 1500s was 73 lb-ft at 3,000 rpm. The home market car was blessed with 82 lb-ft, pretty good for a car that can end up weighing in at around 1650 pounds. The latter torque figure can be surpassed with improved breathing and some tuning. With most runs only lasting 30 to 60 seconds, a well built engine can give several seasons of performance with little loss of power. And naturally, you'll want to build a motor to the 1976 specs, the year with the flat top 9:1 pistons, rather than the 7.5 pistons of other years. Unfortunately, the Fat Chance Garage is not blessed with a dyno, so no real power figures are currently available.

As stated earlier, the car is built to SCCA Solo II rules for the Street Prepared class. The VTR Modified class corresponds fairly close to this, with more liberties available to the VTR competitor. Since Junior is built for SCCA racing, the stricter rules apply. First off, no internal engine modifications are allowed, other than a slight overbore of the cylinders, and a maximum of 0.010" removed from the cylinder head. The valvetrain, camshaft, pushrods, rockers, and springs must all be stock, no fancy race cams allowed. And in many cases, a performance cam in an autocross car moves the power band too far up in the rev range, where it isn't really usable on course. And if your "stock" 1500 motor is consistently turning 7 or 8 grand, your fellow SCCA competitors may be looking to chat with you. But the VTR rules do allow for other cams in Modified, so maybe some research is in order...

With the only internal engine mod being balancing, the quest for more power is focused on the intake and exhaust. Opening up the breathing of the 1500 induction makes a great improvement in overall power and throttle response. Junior is set up with a pair of Weber 40 DCOE carbs on TWM manifolds for the intake, and a common 4 into 1 header for the exhaust. The header feeds a 2" diameter pipe, which goes into a Walker Turbo-Tube glass-pack muffler under the trunk floor, and then a Supertrapp muffler. The Supertrapp is adjustable to some degree, with the exit of the gas restricted by a number of plates bolted to the rear of the muffler. With more plates, there are more gaps allowing the flow to increase, and making it louder. Fewer plates restrict the flow, quieting things down, relatively speaking, as well as increasing the low end torque a bit. The car has two mufflers because neither one was quiet enough by itself for street use. Any decent speed shop muffler can be used, such as a Sonic Turbo. Just make sure the shop installing the pipes doesn't let it hang too low, just waiting to get torn off on a bump.

The Webers on the car are a great improvement over the single Stromberg which afflicts the USA Spitfire 1500. The throttle response is excellent, the carbs flow as much air as the engine can use, and really put a dent in the mileage. So it goes. For two years the car was run with a single Weber, and the difference to be had in putting on the second carb is not that noticeable. If you need to keep an eye on the budget, a single DCOE is fine, or a pair of the 1.5" SU carbs as fitted to the home market cars, and quite common on Spitfire road racers, will provide adequate airflow for an autocrosser. Don't try to go too big on the chokes, you want to keep the air velocity high at lower revs, improving throttle response.

The dual Webers as run on Junior currently have the following:
Chokes 28mm
Main fuel jets 100
Air correction jets 135
Emlsion tubes F11
Idle jets 40F11
These jets reflect the 4500 foot elevation of the Salt Lake City area, and at Rockford the main jets were upped to 120s after a quick bit of tuning. Close, but they still weren't quite right at Rockford. And do put in a fuel pressure regulator, one of the more expensive types like a Holley if you are serious, though one of the cheaper dial types will work, but check it often. Use a guage known to be good when checking the pressure, don't trust the marks on the dial completely. Junior's carbs are fed with a Facet electric pump and a high-flow fuel filter mounted in the trunk. Don't be careless with the plumbing here! Set the regulator to give about 2.5 to 3 psi at the carbs.

Other items done to the engine on this particular Spitfire were an oil cooler and remote filter setup. A TR6 oil pressure guage was plumbed into the system, mounted on a plate where the radio used to be. The ignition system consists of a Mallory dual point distributor, no vacuum advance, and a Crane HI-6 ignition box. The Crane is a good solid unit, offering the advantages of a modern capacitive discharge system, as well as a built in, easily adjustable rev limiter.

While it is true that autocrossing is one of the least expensive forms of motorsports, it is still easy to spend as money as you'd like on car preparation. Below is a table of approximate costs for getting the car to where it is now. The total listed below does not include the price of the car to begin with, regular maintenence items, labor, or the $500 to $1,000 dollars of machine work and parts for a motor rebuild. I assume you are starting with a good, sound Spitfire. If you were to simply call up the Fat Chance Garage and say "I'd like a duplicate of Junior" it could easily end up costing around $7,500. I could be persuaded to sell Junior for about half that, since we are working on another car. If the price seems too high, compare it to the cost of a new Miata or Neon ACR, cars that Junior can outperform.

Junior was not built all at once, but came about as a daily driver was modified here and there for competition. And in 1993, Junior blew yet another $200 junkyard motor and we took a '76 Spitfire to the Seattle convention, with only some of the Trick Racing, uh, Stuff attached, and still managed to get within a tenth or two of Fast Time of Day. I keep the rear thrust washer from that motor on the garage wall as a reminder to check the details.

The single most important factor in a car's autocrossing performance is driver skill. Attend as many events as possible, pay attention to car control and placement in daily driving, and worry about getting the car perfect later. As a matter of fact, Junior is really short for Killer Junior, as it is only a temporary racer, while we work on the Killer Spit, an earlier roundtail car. There are still things we have not done, like completely lowered the suspension, polyurethane bushings, perhaps programmable fuel injection. Albany is a long haul from Salt Lake, but maybe Texas in '97...

Item Cost
450 pound Front springs $100
stiffer rear spring $25
front shocks $180
Spax Rear shocks $150
Poly Sway bar mounts $10
3 piece alloy Wheels $500
Autocross tires $450
Quaife limited slip diff carrier
(not including installation)
Exhaust Header $100
Supertrapp $85
oil cooler, hoses, etc. $200
oil filter mount and adapter $60
Weber DCOE carb setup $800
Electric fuel pump, fittings $50
fuel filter $30
Mallory distributor $180
Crane HI-6 ignition box $160
racing seat $200
racing harness $75
Total $4,185

Recommended reading: [All available through Classic Motorbooks?]

Prepare to Win, (no ISBN in my copy) Aero Publishers inc, 1975.

Tune to Win, (again, no ISBN) Aero Publishers, Fallbrook CA, 1978.

Engineer to Win (ISBN 0-87938-186-8) Motorbooks Intl, 1984

How to Make Your Car Handle, Fred Puhn, ISBN 0-912656-46-8, HP Books, 1981.

Secrets of Solo Racing, Henry A. Watts, ISBN 0-9620573-1-2, Loki Publishing, Sunnyvale, CA. 1989.

Winning Autocrss Solo II Competition, Richard H. Turner & J.B. Miles. ISBN 0-932522-01-7. No publisher information, 1977. Contact: National Academy for Professional Driving, Inc. (214) 742-3471.