Backpressure is a common misconception. Backpressure is never good for ANY car, EVER. It will only hurt efficiency.
Exhaust velocity is actually what you are trying to maximize. The highest exhaust velocity with respect to volume of flow will net the highest efficiency. Higher velocity will increase the scavenging effect of exhaust gas out of the head. That's why if you take a stock naturally aspirated car, chop off the exhaust, or put an insanely large exhaust on it, you will lose torque especially at the low RPM range, because exhaust velocity is very low, even if the potential volume of gas exiting the engine is high, there will be no scavenging effect and that makes the engine work harder. Look at a typical Honda factory exhaust. It's very small, like 1.5" or so, since most Honda engines make maximum HP at high rpm's, they increase velocity at low RPM's to keep the torque up. When a typical Honda enthusiast buys a stock Civic and slaps a bigger exhaust on there, it becomes a slug at lower RPM's but gains a few HP on the top and they think they have done something clever. Only by modifying intake air through a new manifold, throttle body, cams, etc... can they get the velocity back up to match the exhaust and actually gain power throughout the RPM range.
Turbo cars are different. Having a turbo bolted to the manifold is like having a brick in the exhaust- turbos create a lot of backpressure in the manifold. Remember what I said earlier? Backpressure is BAD. That's why turbos are not 100% efficient. So exhaust velocity makes little to no difference in a turbo car after it exits the turbine- in fact, the turbo pretty much has to "push" the gas out of the turbine section, so anything you can do to help the exhaust move away from the turbo will increase efficiency some- so the bigger the exhaust the better, up to a certain point. Some new downpipe designs have a cone-shape as it exits the turbine, the theory is, the exhaust gasses, which are still very hot and at some times, still burning, expand as they exit the turbine, the cone-shape helps expel the gasses away from the turbine which can increase velocity and increase efficiency.
Supercharged cars sort of fall in the middle of this problem. They are capable of moving a lot more exhaust gas volume, since they are being fed additional air from the charger, so they typically require larger exhausts, since they will be expelling more gasses. But there is still probably a limit to how big you can go before once again, velocity goes down, which would make the engine (and the supercharger) work harder.
With a naturally-aspirated Diesel, I would imagine it can tolerate a larger exhaust proportionally to a gasser, mainly because the ratio of intake valve size to exhaust valve size is closer in a VW diesel as opposed to a gasser- that is, the diesels have smaller intake valves but larger exhaust valves when compared to a counter-flow VW 8v gas car. The only real way to know what effect a larger exhaust would be is to take it to the dyno and measure the differences.
Brendan
Topic: Exhaust Upgrade (Read 3569 times)
