Wednesday, April 24, 2013

The Dreaded Exhaust Back Pressure

I remember the first time I saw a tuner's huge tailpipe. I'm pretty sure it was on a Civic and was at least large enough to fit a small cantaloupe inside of it. It was intriguing and almost laughable at the same time. I quickly learned from someone that the idea behind having a huge exhaust system on a car is to allow for the freer flow of the exhaust gases from the engine block with the aim of increasing power output. 

Anyone who has ever been to a racetrack knows that race cars have extremely loud exhausts. Why? They lack any restrictions in them. Your street legal exhaust system comes with sound dampeners as well as a catalytic converter to help reduce emissions and in turn air pollution. If you take all of that away, you remove the things that are slowing down the flow of gases.

Since you cannot legally remove your catalytic converter, and in most areas it is illegal to remove all of the sound dampening devices from your car's exhaust if you drive on the road at all, some car tuners have tried to work around this by slapping on dramatically larger exhaust systems. A fair amount of large pickup trucks do the same thing. 

Anything that restricts the flow of exhausts from the engine to the muffler tip is called back pressure. Back pressure can be caused by a number of obstructions, including a clog in the catalytic converter or a collapsed section of a car's double walled exhaust pipe. Too much back pressure kills power output, fuel mileage and can even cause a car's engine to overheat. In cases of extremely high back pressure, a car's engine might stall out.

But there is such a thing as going too large, despite what some tuners think. Exhaust back pressure is a double-edged sword. Too much and it can rob performance, but too little and it can rob performance. Your exhaust system works with suction to remove the exhaust gases from the engine and move them out through the tailpipe. If you relieve all of the back pressure the exhaust gases could be sucked back into the cylinders through the exhaust valves. So outfitting a car with a 1.6-liter four cylinder engine with an exhaust the size of a semi truck's is probably a bad idea.

Measuring your exhaust's pressure is the only way to accurately determine back pressure. The easiest way to do this is with a low pressure gauge. You have to tap into the exhaust system to measure back pressure, which is best done by disconnecting the air pump check valve. For the best accuracy, the check valve needs to connect to the exhaust system before the catalytic converter.