how do you know if your getting enough back pressure and when its to the point that its helping or hurting you?

 

Backpressure never helps, period. A good performing exhaust will have the lowest possible backpressure, but will use the correct pipe diameter to keep exhaust gas velocities up. Velocity is the key, not backpressure.

 

Why do so man people think that you need back pressure. I've posted this question before and no one has ever given a good answer. The most common answer is that it needs some backpressure.

 

i heard that backpressure is what gives torque.

 

backpressure is what gives you low end power, but at WOT the less back pressure the better, thats why drag cars run open headers because it gives more power at full throttle..

 

Back pressure is NEVER good.

What you need is an exhaust system which maintains sufficient exhaust gas velocity to promote scavenging at the RPM at which you want to operate. Smaller diameter tubes will give higher velocities so they will be more efficient at lower RPMs than large tubes. Unfortunately smaller tubes also have greater backpressure than large ones. This has created the myth that backpressure increases low RPM performance. It does not. The smaller diameter tubes give the greater velocities which increase the low RPM performance. The backpressure is just an unfortunate by-product. As RPMs increase, the backpressure overwhelms the velocity gainsin the smaller tubes and larger ones become more optimal.

 

No, actually, the combustion inside the cylinders gives torque and power in general.

Backpressure = not good. Like what StrangeRanger noted, you need good exhaust flow, rather than oversized pipes that allow the exhaust gas to sit relatively stagnant in the manifold......

 

Your seem to contradict yourself in your post. Your explanation of what is good in an exhaust system for a gas engine and why describes backpressure. If you think I'm wrong please clarify.

 

Some backpressure is good, though.

Look at the new 6.0's. Alot of people that are blowing out there Turbo's..what do they have in common? Free-flowing 4 inch exhaust that isn't providing enough backpressure..so their turbo overboosts and blows itself up.

When people say that some backpressure is good, they mean that the exhaust gas velocity. It's nice to correct them, but not to an overextent.

You need to match pipe diameter to horsepower - or the level that you want to get to.

Don't dragsters run a pipe out from each individual cylinder? I thought they had pipes off of each cylinder going down...which is why they have trouble blowing out their spark over the nice 6 second runs.

 

Saurian, your comment about diesels is off base. There is a wastegate that limits boost in all PSD's since 1999 so overboost is not possible. Also, a modern diesel doesn't require backpressure like a gasser. Completely different applications. The turbo provides all the backpressure that the PSD needs. The turbo problems with the 6.0 have to do with other problems.

 

is it bad for the engine when you rev it up and let off, that it pops or crackles?

 

^As long as you are not over reving it, it wont hurt it..

 

I thought dragsters ran open headers because thier compression is so damn high that at that level of an engine backpressure or no back pressure doesnt amount to a hill of beans.

 

I agree.
And the reason race cars dont have full exhaust systems, is because 1) they are not mandated for most, and 2) low-end torque is not needed when the motor is always at the upper end of its powerband. A properly designed system can help build low end torque, but backpressure has absolutely notyhing to do with it. Backpressure is NOT your friend.

 

Back pressure is NEVER desirable. The concept that a normally aspirated gasoline engine needs an exhaust restriction for maximum performance is a myth. For performance purposes, backpressure in an exhaust system increases engine pumping-losses, restricts air/fuel flow rates, and leaves spent exhaust gasses in the cylinder. None of which can possibly add anything useful to available engine power.

Maximum power is always achieved with the lowest possible backpressure. An engine needs around 240 feet per second of exhaust flow velocity (more is better) for best throttle response and torque. This velocity provides optimal scavenging and moves the gases out through the exhaust system before they begin to cool and become denser.

The problem stems from the fact that exhaust flows are dramatically different across the RPM range and one exhaust size will not work perfectly over the entire range. For example, a small diameter exhaust pipe gives you the optimal gas velocity at low RPM’s. This combination gives you great off-idle acceleration, but as the RPM’s climb the exhaust gas volume quickly exceeds the amount that can flow unimpeded through the pipe and your engine needs to literally pump the exhaust out the end of the pipe resulting in a seriously inhibited mid-to-high RPM performance. A larger diameter exhaust pipe has enough pipe volume to see the optimal flow rates at mid-to-high RPM, but at lower RPM’s the gas volume is not large enough to maintain the flow velocity in the larger pipe and low-end throttle response suffers.

Exhaust systems are a compromise, you can maximize for low-range and mid-range performance or mid-range and high-end performance but you cannot simultaneously maximize for all three because of the differences in flow volume and velocity.

Backpressure is neither necessary nor desirable, it is simply a by product of a exhaust system optimized for low RPM performance and driven at a higher RPM.