Use Numbers, Not Emotions, to Select Your Next Autocross Car

Facing Change

Examples of these situations are happening right now, as the SCCA Solo Events Board has just completed a reorganization involving four classes: Super Stock, A Stock, B Stock and C Stock. The existing B Stock and C Stock classes were combined into one single group. That made room for a new “tweener” class that encompasses the slower cars from Super Stock. Then the existing A Stock cars were simply moved to B Stock.

For the newly combined C Stock, there is plenty of data out there from the existing results to suggest that the top dogs from either the old B Stock or C Stock can win on any given day. After that, it just depends on the course.

But what about that tweener class? No one has developed and run those cars for years, and they are about as different from each other as can be. The standouts appear to be the non-Z06 C5 Corvette, third-gen Mazda RX-7 and Porsche Cayman S. Since these eligible options are so diverse, how do you choose which one to campaign?

An even harder choice is currently being made by folks wishing to compete in another brand-new class, Street Touring Roadster. This one takes the highly popular Street Touring formula and applies it to a number of higher-powered, better-handling sports cars like the Honda S2000 and Mazda MX-5. Other legal cars include the Toyota MR2 Spyder and Nissan 350Z plus the BMW Z3 and Z4. 

Unlike the Stock category, where factory performance can be a good indicator of autocross potential, Street Touring allows bolt-on mods that can affect different cars in different ways. The result: A factory dud can sometimes be turned into an autocross rocket. The 1989 Honda Civic Si is a great example. It fell out of contention in Solo’s slowest Stock class, but the modifications allowed in the Street Touring ranks have made it pretty much unbeatable.

Two Approaches

There are two basic approaches to the selection process, and ideally drivers should employ both. Getting the same answer after following each thought pattern is a good sign that the car is a strong contender. As a comparison, these two methods are not unlike how valuations are done in the real estate business.

The first method—we’ll call it the empirical method—simply compares cars that are already prepped to similar standards. For example, to develop a car for the new STR class, it’s often a good starting point to look at results from recent Stock or Street Prepared contests. This data can then be altered when the allowable modifications are figured in. 

This approach has the benefit of using actual developed cars in a real competition environment. It’s also relatively quick and easy to do.

Let’s call the second method the theoretical approach. This one’s a bit more involved: Build the best example of each car on paper and compare the field on several key points. The devil is in the details, though, and the paper model won’t hold up to reality if the comparisons aren’t performed accurately.

Practical Examples

Using the empirical approach for the new Street Touring R class, we see that in Stock form the Honda S2000 is a front-running car for A Stock, while the MX-5 is a leader for the slower C Stock class. The Toyota MR2 Spyder is only mid-pack in C Stock. This same progression plays out in the Street Prepared ranks, where the S2000 is a mid-packer in B Street Prepared, while the MX-5 and MR2 Spyder run up front in the slightly slower C Street Prepared category. 

In Stock and Street Prepared, though, the extra weight of the S2000 is offset by its ability to put its extra power down through larger wheels and tires. Take those away with the Street Touring R tire width limit and you have a whole new ball game. The same thing happens to the 350Z and BMW roadsters, but to an even more significant degree. Seems like there are no clear winners—yet.

Now it’s time to apply the theoretical approach. Generally speaking, our first step is to identify the best year, model and option package combination for each car in question. 

This is pretty straightforward, as the sporty model is typically the golden ticket. These cars have the best suspensions, make the most power, and eschew many of the heavier luxury items found on the upscale versions. Also, the first year of a model generation is typically the lightest. Manufacturers tend to bloat out their cars in succeeding years, making them “better” by adding more gizmos that have nothing to do with performance. 

Wheel width is a critical factor when selecting an autocross car. Wider is almost always better, but the rules in many classes—including Street Touring—limit the maximum width of a wheel-and-tire package. Photography credit: Alex Groves

Occasionally, however, power gains, gearing changes and even special models pop up near the end of a run. Porsche is famous for this, and it drives the rule makers nuts.

As we do our virtual builds, we keep track of all relevant data in a spreadsheet for easy comparison. We’ll need the published specs for curb weight, engine power, torque and rev limits plus OE wheel and tire sizes. It’s also important to have the ratios for the final drive as well as the lower gears. Finally, we’ll also note things like alignment adjustment ranges and any tire clearance issues. (You can view the sheets below.)

Next, we “prepare” our cars. For machines in the Stock ranks, this prep is limited and its effect is fairly consistent across all cars. The easiest approach is simply to list the tire choices that will fit the OE wheels. 

For our Street Touring example, we’ll need to extrapolate further; this is where the art comes into play. For example, while each Street Touring class has a limit on tire and wheel width, not all cars can fit the max due to fender clearance. There’s a chance the hot tire will be unavailable in the optimal size, meaning a compromise would have to be made between width and gearing. 

We’ll also need to make some educated guesses as to how much power the allowed mods will add over the baselines. Not all of the legal modifications can be applied to every car, so careful research within the existing enthusiast community for the particular model can be beneficial. We’ll also look to see if fixes can be found for any basic OE deficiencies, such as alignment limitations. 

And finally, we’ll do some weight loss analysis. Does the car have a particularly heavy stock exhaust? Heavy power seats? Potentially significant savings should be noted in our build sheet.

Comparing Weapons

Now that we’ve done our virtual prep, we want to compare performance potential. The first data most people look at are the power-to-weight ratios, as carmakers have ingrained these numbers in our heads for years. 

More power equals better performance, right? Maybe at the drag strip, but not so much for autocross. Sure, it’s important data, but not nearly as much as handling characteristics. 

Unfortunately, handling is much harder to quantify. Magazine tests can be helpful, as they contain skidpad and slalom data on consistent courses, but variances in OE tire choices skew those results drastically. So, where to start?

A key predictor for autocross handling can be made by comparing weight to the wheel or tire width. Given an optimized suspension, it all comes down to how much rubber is on the road and how much load it has to support at each corner. 

Furthermore, the wheel width is a controlling factor in how much tire width can effectively be used. For street tires, here’s a good rule of thumb: Additional tire tread width provides gains up until it matches the wheel width. For example, anything more than 8 inches of tire tread on an 8-inch-wide wheel is mostly extra weight to rotate. That said, thanks to the super-stiff sidewalls of the R-compound tires allowed in the Stock classes, “overtiring” a rim can often provide a handling improvement, albeit with some loss of responsiveness and potentially a gearing hit. 

Class revisions and additions can bring once-unpopular cars to the fore. The 1989-'91 Honda Civic Si was a dud in Stock class competition, but these days Street Touring is jokingly referred to as Spec Civic. Photography credit: Alex Groves

For our Street Touring R spreadsheet, we’ll want a column that divides vehicle weight by tire tread width for all tire sizes under consideration. We normalized those numbers as a percentage of the best, then put it in a column labeled “grip factor.” 

Sometimes it pays to take this analysis a bit further. For example, how do you compare a front-wheel-drive car to a rear-drive car? Here, you’ll want to know the weight distribution of the vehicle so that you can compare tire loading on the heaviest end of the car. After all, that will be the limiting factor in lateral grip, assuming equal tires front and rear. 

In the case of the Street Touring Sports class, for example, both the Honda CRX and Mazda Miata wear the same tire size. However, the Miata is a full 100 pounds heavier. Advantage goes to the CRX, right? Not really. The Miata has a 50/50 weight distribution, while the CRX has 65 percent of its weight on the front tires—the same ones used for turning, braking and accelerating. Advantage: Miata.

Now that we have a decent read on basic handling, let’s return to forward motivation and add a column representing acceleration in our spreadsheet. For these figures, we’ll take the weight of the vehicle and divide it by the actual torque applied to the wheels on a typical autocross course. 

The latter piece of data is computed by multiplying the engine torque by the second gear ratio and final drive before dividing it by the tire diameter. These numbers have also been normalized to a scale of 100 and listed under the “thrust factor” heading. We also want to compute how fast this combination goes at redline—that info goes under the “speed” heading—as short gearing for better thrust may require too much shifting to be effective.

Another tip we’ve learned through the years: Slalom performance is primarily related to car width. Even street tires can spike amazing lateral g-loads, and putting them on a narrow Civic will have it outrunning a portly R-compound-equipped Z06 Corvette through a steady-state slalom. 

Wheelbase also plays a factor here, as a longer distance between the front and rear wheels can add stability in big offsets or fast 90-degree turns. Too much wheelbase, however, can also make the car harder to rotate in a slalom or sweeper.

A column should be reserved for notes on special circumstances. This is especially important in the Stock classes, but some of the items that cannot be fixed under the Street Touring rules should be included. Issues include limited camber adjustment, major toe or bumpsteer issues, geometry limitations on lowering, fender clearances, weird gearing, low rev limits and even parts failure issues.

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