"It's very simple physics," says Butler. "The trick is there is a tremendous amount of energy that we waste, that we throw away. We vent it to the atmosphere through the exhaust system. We've simply found some tricks where we try to capture and reuse some of that energy before it's wasted and gone, to reduce the pressure, or self-pump, the exhaust system down. The other name for that that's commonly used is to 'scavenge.'"
Rules To Race ByHere are a few tips from Butler and other exhaust professionals that can be applied to all levels of stock car racing.
Chris Kaufmann, motorsports director for Borla Performance Industries, says one common mistake is that racers sometimes make abrupt turns in their exhaust systems, coming straight back from the headers and building maximum velocity, for example, then abruptly pitching the exhaust out one side.
"They have probably cost themselves 10 to 15 percent of flow just in making that hard turn," says Kaufmann. "Or they'll find themselves constrained because of different chassis modifications or the way their car is built, so they end up putting sharp bends into their exhaust system, and those are killers. Anything they can do to make the turns in the exhaust system as smooth and as gentle as possible, the better."
A common mistake many racers make is that exhaust systems are developed as an afterthought, according to Dale Dotson, Flowmaster's motorsports manager.
"For optimum performance, the exhaust system, either with or without mufflers, needs to be figured into the package at the beginning of the R&D stages," says Dotson.
He recommends caution when installing mufflers, taking heed not to place the exhaust tubing too far into the muffler and cutting off flow. Plus, when building an H crossover, the H pipe should be butted against the exhaust pipe, not into it, to avoid creating a blockage.
Crossovers, according to Butler, are crucial to the performance of a finely-tuned exhaust system, and they're a key to the success of Butler's Dr. Gas system.
"If a racer goes with two pipes that never see each other, he should at the very least put an H pipe in there," says Butler. "An H pipe is not very efficient, but it's better than nothing. But then the best things are the X pipes and the Y pipes, the Y pipe just being a simple 2 into 1. The H pipes help to reduce the popping (associated with 90-degree crankshaft engines). Each pipe will actually take some noise out, but they're not as efficient as an X pipe or a Y pipe."
Kaufmann points out that exhaust systems built by Borla are tuned specifically for a particular engine, including what the engine builder is designing the engine to do, what the rpm range will be, where the torque level is maximized, etc. Nonetheless, certain techniques can be used to pull performance from any exhaust system.
"For the weekend warrior, he actually can do a lot of little things to find out, let's say, where he would want to put his crossover," says Kaufmann. "He would want to put his crossover in a place where the system started to cool down. In other words, depending on how rich he ran the engine, how it tested on the dyno, there's going to be a point where it's going to cool down.
"He could find it the old simple backyard way, which is to get in there with a can of spray paint and start spraying back on his exhaust system under load and find out where the paint quits blistering off, where the paint actually starts to look like paint. At that point is where he really would want to put his crossover. While the fuel is still burning in the exhaust pipe, you don't want to interfere with flow. Simple things like that can give the backyard guy a performance advantage, sometimes as much as 5 percent, by merely getting that crossover tube in the right spot."