what is back pressure
 

what exactly is that. because i was talking to this guy and i told him my o/r h pipe came in and he was like ohh talk about back pressure.. what does that mean???? was he jokin??

Restrictions in the exhaust system that slow the exit of exhaust gases from the combustion chamber.

since you have an o/r pipe you will have less back pressure

Having less back pressure is not always a good thing.. for example when I put my LTs on and my O/R x-pipe I lost some downlow power, but I gained a lot more up high. This is part of the reason why people don't recommend dual exhaust on V6's

0 backpressure would be great for everyone all the time accross all parts of the RPM range if not for one thing.Exhaust waves.These are waves created when the exhaust valve opens and the exhaust gasses start down the pipe.Since exhaust gases have mass and therefore inertia they compress the gasses ahead of them like a spring.If there are no restrictions they pass out the end of the pipe and then have no effect on the engine.Unfortunately there are always restrictions.Bends,welds,joints,etc.all cause sonic waves to be reflected back toward the valve.If this wave gets there after the valve is closed then fine,but if it arrives before it closes then some of the spent gas gets shoved back into the combustion chamber.This is bad.A perfect exhaust pipe would have about the same volume as the cylinder that it is bolted to.This would allow the slug of high speed exhaust gas to scavenge the cylinder completly every exhaust cycle.This is why long tubes generally make higher HP numbers than shorties.This perfect length head pipe would only work at a certain RPM level though.The same inertia that drags clean fuel-air mix into the cylinder also creates a vacuum in the exhaust and can cause the back flow of a second but weaker slug of air to come in from the outside of the exhaust.This is easily heard on drag pipe equipped Harleys.At around 3500 RPM they will stumble or backfire.At the very least they will have a dip in HP numbers on a dyno sheet.It's just the nature of the engine and cam timing.That why tey call them "Drag Pipes",they're for drag racing.Back to Mustangs.To keep these unwanted pulses from getting back to the exhaust valves in any engine they must be damped.This is accomplished by slowing the exhaust down to minimize the effect of reflected pulses.Voila!Backpressure!!!It's not needed actually but it's an unwanted byproduct of having an exhaust that works from idle to redline. :eek: