Secondary cat delete

[jmanous]

Can anybody comment on why loosing backpressure is bad?

I would imagine that going with a larger pipe will result into lower gas speed
at low RPM, with that been the cause of loosing some power at this band.

[tmontague]

Originally Posted by jmanous

Can anybody comment on why loosing backpressure is bad?

I would imagine that going with a larger pipe will result into lower gas speed
at low RPM, with that been the cause of loosing some power at this band.

Here's my take on it after a ton of reading from various viewpoints:

You don't want back pressure as this would slow down exhaust scavenging. Back pressure is often a misused term. You want exhaust velocity, this is what creates negative pressure and "pulls" or scavenges exhaust gases out of the engine.

Velocity is changed with diameter shape/size. Too narrow a pipe and back pressure increases which leads to slow exhaust velocity and poor scavenging and therefore a hindrance on performance. Too large a diameter and you also lower exhaust velocity even though there is little resistance to to exhaust gases to exit.

Think of a large hose with water flowing through like a garden hose. Assuming that the power of the water being pushed through the hose is always the same the only change in speed of the water comes from the diameter of the hose. The same amount of water will exit (unless the hose becomes way too small) but too large a hose and the water slows down. Too small and you reduce the amount of water that can exit. The right size is one that causes the water to increase it's speed (velocity) without being restrictive.

The reason why at high revs this is less of an issue is because the engine is pushing exhaust gases out at a much higher rate. In the water example above that would be equivalent to turning up the power of water being pushed through the hose. In that case a large diameter hose is less of an issue, but a low water power the velocity slows and it becomes problematic.

This is why Ferrari and a few others have variable exhaust so at low rpm's the exhaust is switched to a smaller diameter to keep velocity high. At high rpm's the exhaust is switched to a larger diameter to allow the increase of exhaust gases to escape but also keep the velocity high.

These variable systems are expensive so if you are going with a custom exhaust a middle ground is ideal. If you are building a track car a larger diameter straight pipe type would be good since you spend most of the time at high rpm's. For a higher performing DD you still want some low end torque so a larger than stock pipe that is still small enough to create a high velocity at lower rpm's is ideal.

I'm sure other's have varying opinions on this but the above is what I've come to conclude after reading through various experienced individuals opinions.