BackPressure?

[Guest EnthuZiast]

Okay, I am just wondering if anyone out there can explain it to me so that I fully understand what it does.

 

Thanks

[auxilary]

turn your car on, hold up your hand to the exhaust pipe before it gets hot. feel the pressure? now imagine that in the exhaust pipe right off the manifold, because gas cannot escape quickly enough, it builds up pressure.

 

common causes are catalytic converters, mufflers, severe bends in piping, and small exhaust diameter.

 

Some cars benefit from back pressure as it gives them a little more low end grunt, those with free flowing exhaust system will gain a little better response in high end

[Michael]

Alternative discussion: when the exhaust valve is fully open, the static pressure just downstream of the valve is some number larger than atmospheric pressure. At the exit of the exhaust pipe, the static pressure is atmospheric. The difference between the two numbers is the back pressure. That difference results from restrictions and friction losses in the cylinder head exhaust port, the headers, the exhaust pipe, muffler, etc.

 

The issue of whether “some small amount of back pressure actually helps low-end torque” keeps going back and forth. David Vizard makes a convincing argument in his books that zero back pressure is the best back pressure. What helps low end torque is exhaust scavenging (pressure pulses in certain portions of the exhaust system dynamically assisting the emptying of some cylinders) – something that’s lacking in open headers. In other words, an exhaust tract with zero backpressure also has zero opportunity for exhaust scavenging, so tolerating some back pressure is a necessary evil in order to allow for exhaust scavenging.