Build A Faster Car - helping drivers engineer

I've long had a theory that the reason there is reasonably "easy" power to be had with exhaust upgrades is because of the EPA 80dB driveby test. A new vehicle sold in the US is supposed to be able to pass this test, but if you look at the test spec (ok to post link?) the test is not replicateable.

This means that the average consumer can upgrade their exhaust, stay below noise local standards (law enforcement) and earn a small performance gain with little risk. But I have noticed a trend. Bigger vehicles, such as SUVs and pickups seem to become loud, but gain very little in power. Small vehicles seem to get a higher percentage gain in many cases.

A few performance vehicles seem to have some thought and engineering that goes into the factory exhaust. But for the rest of us the manufactures seem to use a simple formula. Big is better. In many applications this seems to work well. Lets just look at a 2005 F150 with a 5.4l (hardly a "faster car" ... I know). The vehicle is huge, and it weighs over 4000# stock. To get good flow and pass the EPA test mentioned you can just use a gigantic muffler. Assuming the muffler is big enough to avoid resonance in any frequency that the exhaust will hit, it will be quiet. And for what it is worth I suspect it will flow well too. Upgrades may start the hype about HP but often include the fact that the vehicle will weigh less too.

Smaller vehicles are more of a challenge. There simply isn't room for a 12 inch in diameter, 3 foot long muffler. The answers is to restrict it more with a smaller muffler. When that fails then add resonators or additional mufflers. I'm sure that huge amounts of engineering could produce a muffler that was smaller and would still allow the vehicle to pass the 80dB EPA driveby test ... or maybe not. Virtually every aftermarket exhaust will increase the sound level. No one in their right mind would attempt to replicate the driveby test in an open air environment to enforce sound level standards. So aftermarket exhaust system manufactures simply crank up the flow, and sound level to get an engine pumping efficiency boost. We have all seen dyno runs where the obnoxious exhaust has the same or less HP that a quieter one, but we are considering apples to oranges when considering the stock exhaust. There is a world of difference in the metered tests at many local racing events (or at the dyno lab) and the EPA test discussed here. The stock exhaust is simply held to a different and more stringent test.