Stock Tips

By J.G. Pasterjak
Jun 23, 2008 | BMW | Posted in Shop Work | Never miss an article

The SCCA’s Solo II Stock and Club Racing Showroom Stock classes are designed so folks in off-the-showroom floor cars can compete in racing with a reasonable amount of success-no trick engines and no full-race suspensions, just stock production cars. That’s the theory, anyway. In reality, the SCCA rules leave open a few areas where a smart gal or fella can pick up a few horsepower. And in a sport timed to the thousandth of a second, every horsepower counts.

We’ve had quite a few opportunities to sample a variety of Stock-class-legal modifications over the past few years as part of our testing on the chassis dynamometer facilities of Performance Dyno in Edgewater, Fla. Just about all manner of cars and parts have passed through our “Dyno Days,” and we have found that the dyno gives us real-world numbers that quickly separate fact from hype.

We know our readers are always eager to find out what works and what doesn’t, so we’ve decided to open the archives and give you the results of some of this testing. The power figures quoted here were extracted from tests performed on a Dynojet Chassis dynamometer. The figures are for horsepower at the drive wheels, which, due to parasitic losses, is typically 15 to 20 percent less than flywheel horsepower numbers.

Ignition Wires

Ignition wires are free in Solo’s Stock class: the rules state they may be replaced with pieces of any origin. From the data we’ve collected, it’s a worthwhile modification.

Replacing the brand-new stock wires on our BMW 318ti project car with a set from Magnecor gave us around two additional horsepower at the drive wheels, while our Neon got a three-horse bump when we replaced the stock wires with some hot ones. Installing a set of performance wires on our Honda CRX project car didn’t unleash any more power, but the new horsepower curve was both smoother and no longer fell away at the top end.

In testing with a variety of cars at our Dyno Day events, we’ve found that nearly every vehicle benefited from having the stock wires replaced with quality aftermarket pieces from companies like Magnecor or Ignition Equipment. We found consistent power jumps in the two- to three-percent range; that’s pretty significant. Plus, performance spark plug wires don’t cost that much. Figure $50 to $150 depending on the application.


This is another area that surprised us. We have seen gains similar to those we saw with wire changes-three to four percent-in a variety of cars, merely by switching to synthetic oil. The lower friction properties of synthetics really seems to be of some benefit, especially in “today’s high revving engines.”

While you can expect the numbers to vary a bit from run to run on the chassis dyno (usually less than one horsepower), when you see a consistent two-horsepower jump, you know you have something.

In our BMW 318ti-a car that we had a great deal of trouble extracting more power from-replacing the stock engine oil, tranny fluid and rear end lube with Red Line synthetic products did very little to increase the maximum power output, but it did get us more than five additional horsepower at 4200 rpm-right in the meat of the powerband.

As further proof that less friction is a good thing, we’ve also realized power gains by using friction-fighting additives. Although we couldn’t believe it at first, we did pick up measurable gains-sometimes as much as two or three percent-with a product called Oil Extreme. Although we have no reliable long-term data on these products, for many people, a few easy horsepower are worth the risk.

Oil additives cost around 10 to 15 bucks per application, while converting an entire car to synthetics should cost around $100.

Alternate Air Filters

Stock class rules allow for the substitution of air filters if the replacement is a direct, no-modifications-necessary replacement part. The only dyno testing we’ve ever done on a direct replacement filter was on our BMW 528i-not the kind of car you usually see in stock classes, but it is a 2.8-liter fuel injected inline six, so the results should be valid.

Replacing the stock air filter in our BMW with a K&N unit gained us nearly four horsepower (almost three percent). This isn’t a huge gain, but it is impressive for a two-minute modification. And since K&N filters are washable, and don’t need to be replaced, this is a cost-effective modification as well. Depending on what you drive, a K&N replacement filter should set you back between $30 and $100.


Stock class rules allow the exhaust to be replaced from the catalytic converter rearward, though the exhaust must exit near the stock location. Many competitors are now choosing to run straight pipes, taking the as-little-back-pressure-as-possible approach.

We tested three exhaust configurations on a D Stock twin-cam Neon: stock, straight pipe, and a Borla high-performance muffler. We found that both the Borla setup and the straight pipe produced nearly identical gains-about three horsepower at the wheels. However, the dyno graph on the muffled run was much smoother, not to mention the car sounded infinitely better.

On our BMW 318ti, however, the high-performance exhaust failed to produce much of a result. This was unusual since the car seems to have responded well to the synthetic oil and plug wires. We came to the conclusion that the stock exhaust was pretty good to begin with, plus the car’s OBD-II computer control system really wasn’t letting the car produce much more power.

Aftermarket exhaust systems and high-performance mufflers can range in price anywhere from around $100 to more than $1000. Generally, the more popular the car, the less expensive the system.

Spark Plugs

We also did quite a bit of testing with spark plugs on a different twin-cam Neon. At the end of one frustrating day, we learned that, on a Neon anyway, spark plugs don’t make a bit of difference. We ran the car first with the NGK plugs that had been installed since we took delivery of the Neon several years ago. We then tested the car with a variety of supposedly “better” plugs-hotter, colder, split electrode, pointy electrode. (No matter what your poison, good plugs cost less than a couple of bucks each.) In this case, all of them produced roughly the same power: within one horsepower, plus or minus. Unfortunately, this was two horsepower shy of where we started with the skanky factory plugs. The skanky factory plugs were quickly reinstalled. Our dyno shop swears by NGK plugs.


Blueprinted or factory-fresh fuel injectors can make a difference, says the dyno. In testing of these units on both a Neon and a Toyota MR2, we picked up a repeatable two- to three-percent more power over the existing injectors. The Neon injectors had seen around 60,000 miles, and the Toyota parts had covered more than 100,000 miles.

While new injectors can be quite costly, companies such as Marren Motorsports, RC Engineering and Performance Diesels can blueprint injectors, returning them to factory-spec performance for as little as $25 apiece.


During our dyno testing we’ve even tested a couple of “tricks” along the way. One popular staging line ploy is to ice down the fuel rail on a fuel-injected car. In the old days (and today), drag racers were fond of “cool cans” which would cool the gasoline before it entered the engine, thus making the fuel more dense. But that was more applicable in the days of carburetors, when engines were at the mercy of prevailing atmospheric conditions.

Today’s cars are harder to fool. We found that icing the fuel rail on a twin-cam Neon cooled the fuel to a point where the car thought the engine was still cold. The computer richened the mixture to a level that actually caused the car to lose significant power (on the order of three to four horsepower). These cars were designed to run with a very specific air/fuel ratio for maximum performance. It seems that artificially altering that ratio does more harm than good.

We’ve also tried the popular “blown alternator fuse” trick on a Toyota MR2. Supposedly, by removing the alternator fuse or replacing it with a blown one, the disabled alternator would “free up” some horsepower that would normally be lost to parasitic drag. Maybe this would work if you drove a three-cylinder Geo Metro with a 400-amp alternator, but it didn’t seem to affect our Toyota either way. (If you don’t believe us and have to try this trick yourself, make sure you have a good battery, okay?)

Caveat Emptor

Remember, these recipes are merely the result of our own observations, and results may vary slightly from car to car. However, all of the gains we did quote were obtained during same-day, similar-condition testing. Every piece may not work on every car, either-like the performance exhaust system, which worked on a Neon but barely affected the BMW 318.

Note also that the figures quoted were power gains for individual pieces only, not pieces used in combination. While a set of wires may get you three horsepower over a stock engine, and an exhaust will get you four, there’s no guarantee that the pieces used in combination will get seven.

There is easy power out there, though. Based on our experience and testing, we figure that most stock-class cars can gain eight to 10 horsepower over stock with the use of class-legal modifications. And we haven’t met anyone yet who didn’t want more power.

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View comments on the GRM forums
8/12/09 11:07 a.m.

Keep in mind that alternator fuse may power other things besides just the alternator. A good example is the Speedometer on an EK Honda Civic.

The SCCA expressly forbids running a blown fuse in stock classes, because that's not stock designed equipment.

chknhwk HalfDork
12/4/10 8:28 a.m.

Where does ecu reflashing come into this?

keadams26 None
6/5/11 9:30 a.m.

Sorry but the results of a Neon compared to those of the BMW made me laugh.

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