Quote:
Originally Posted by BRUTAL64
Ok, what do you guys think Back Pressure is??
Hint, have you ever watched the Weather Channel? 
Ok, so you know we are working with 14.7 pounds of air pressure--sea level.
I could address this Back Pressure crap in the head porting class. A picture is worth 10 million words here. |
Without looking up the actual definition I'm going to think about what I think it means and answer as follows:
If at the end of the combustion stroke, when the exhuast valve opens, and the pressure in the exhaust manifold/header is greater than in the cylinder, that's back pressure and will exhibit as a tendency to:
- create resistance against the piston as it evacuates the cylinder, clearly robbing power from the engine
- if there's overlap, and both intake and exhaust valves are open simultaneously, it could interfere with filling the cylinder with the A/F charge
The part that I struggle with is that it could also lead to somewhat higher cylinder pressures after intake (if carefully designed as OEM engines should be) but would depend on the interplay of several complicated variables (int.exh duration, overlap, valve and port size, . . ). That is, the pressure inside the cylinder at the end of intake before compression could be greater than atmospheric pressure. This would equal higher combustion pressure and therefore higher torque under some conditions. However as engine speed increases the negative effects of this "back pressure" become greater than the positive effects at lower speed. In support of this hypothesis consider that given an engine like the stock LS1, if you remove the stock manifolds and put on a nice set of equal length tuned LT headers that flow vastly better, you will LOSE some low end torque, but the increase in breathing efficiency at high rpm results in considerably higher peak power.
