Ok guys, what can you tell me about back pressure, whats so important about it? i know i need a certain amount...but what is it with guys saying less back pressure is good too...what do i need to know about this before i build my exhaust. I'm planning to build my own dual exhaust for my 1965 F-350 with the 352...I'm planning on 2" (maybe?) pipes and glass packs...what do you guys think of that?

 

Backpressure is the backing up of exhaust gasses into the combustion chamber due to restrictions in the system. Pumping losses of as much as 8 hp have been measured in overcoming these restrictions. Burned exhaust gasses mixing with the incoming fuel charge is not good.

When carburetors were still in use, if you opened up the exhuast system to better flow, it would sometimes cause the engine to run lean which in turn would cause the burning of valves and pistons, detonation, pre-ignition, etc. With todays engine management systems, this is not a problem.

I think many confuse backpressure with velocity. As the gasses move thru the piping, a low pressure area is formed behind the moving gasses. This low pressure allows the high combustion pressures in the cylinder to exit the cylinder more efficiently as the flowing gasses have a tendency to "pull" the spent exhaust out of the cylinder when the exhaust valve opens. With both intake and exhaust valves being open at the same time (valve overlap), as the exhaust is being pulled out of the system, it is at the same time pulling in the fresh fuel/air charge. This has two effects. It evacuates the burned gasses out of the system, and when the intake valve closes the intake charge rams into the cylinder due to the high speed of the flowing gasses. This makes for a mini supercharging effect.

Pipe sizing effects the velocity of the moving gasses. A smaller pipe increases the velocity of the moving gasses. The result is that at lower engine speeds the higher velocity will add more torque to the engine due to the speed of the moving gasses entering the combustion chamber.

When you increase the pipe size, the same thing occurs, only at a higher engine rpm. The torque increase will occur higher in the engine operating band, and will suffer in the lower rpm ranges.

This is a simplistic explanation. This subject can be very complicated when getting into all the parameters involved. jd

 

Sorry to cut in here, but if I go from say a 3" outlet to a 4" outlet am I really going to notcie a difference or is it all .0001% here and there (like fuel mileage gains!).

 

Hi, thats very informative indeed. Where i live i do not have really any noise restrictions. What would happen if i take the muffler out completely and replace it with a straight pipe? Would that increase power output?

 

Usually you won't see any more power with the muffler off. If you do you will not notice it while driving.

 

Here again, it depends on the factors involved. There is no hard, set, rule as to what changes will produce certain results. Usually, if you make a drastic change, such as you have outlined, you will definately feel the results in the lower rpm ranges.

In many cases, this is true. Removing the muffler completely, and not adding a pipe in many cases has a detrimental affect as it kills the flow velocity.

In adding the pipe, if there is no change, the system may be as efficeint as it can get without further tuning (headers, pipe sizing, etc.) Any exhaust system will flow no greater than it's smallest restriction. jd

 

Usually the greatest restriction on a stock engine is not the muffler, it is the heads or the exhaust manifolds.
Thought I would just add that in there.