I am redoing the exhaust system on my max, and wish to maximize my performance. I'm not quite sure why backpressure is needed for low end torque, as it was my understanding that the less backpressure the better (i.e.: running open headers). Any answers or links appreciated.

Thanks,
Tyler

 

It's not backpresure that you need. You need a system that is tuned to have low backpressure and maintains high exhaust stream velocity. Velocity is the key...not backpressure.

 

Ive always wondered that too. So its not really back pressure, why exhaust streal velocity? And how is this acheived?

 

super charger is the answer to everything.

 

I shall quote directly from the latest issue of SCC:

"An old hot-rodder's tale: Engines need some backpressure to work properly and make torque. That is not true. What engines need is low backpressure, but high exhaust stream velocity. A fast-moving but free-flowing gas cloumn in the exhaust helps create a rarefaction or a negative pressure wave behind the exhaust valve as it opens. This vacuum helps scavenge the cylinder of of exhaust gas faster and more thoroughly with less pumping losses. An exhaust pipe that is too big in diameter has low backpressure but lower velocity. The low velocity reduces the effectiveness of the scavenging effect, which has the greates impact on low-end torque.

Low backpressure and high exhaust stream velocity can be achieved by running straight-through free-flowing and small pipe diameters..."


That pretty much says it all.

 

..but now how to find the optimal pipe size for a particular exhaust setup? Is there such a thing as a velocity meter for exhaust? You probaly have to measure air pressure instead like an aircraft speed guage?? From this board seems 2.5" dia. is the largest for our NA engines but who knows if a straighter smaller dia. pipe gives a higher HP gain?.

 

Thanks for that. So I guess there is an optimal size (pipe) for the amount of exhaust from an engine, not nessesarily in relationship to its capacity (engine) though

 

Actually it is directly related to the size of the engine. SCC has a little chart that recommended 2.5" for 3.0 liter engines.

 

For N/A engines... what about forced engines.

 

For forced induction and nitrous...as big as possible.

 

I've been misusing the term backpressure instead of calling it exhaust velocity. Thanks for clearing it up. I never knew the term before.

So in essence, you still want to run a smaller diameter piping to get a higher exhaust velocity. But, at what point do is it beneficial to have a larger opening? In the case of the variable flowing 5th gen muffler, is this efficient? I always thought of the exhaust system and piping to be like a garden hose. The more restrictive you make the hose, the greater the velocity the water comes out yet at a slower rate. In this sense, if you make the restrictions in the beginning or middle of the hose (the Y-pipe precats and B-pipe crimps) you mess up the flow and the water still comes out slow. But if the restriction is just at the end (muffler) with say a nozzle, the velocity is much greater. Of course when you have the hose on full blast and the water just wants to be released (ie at a higher RPM) it'd be more favorable to have less restrictions on the hose entirely.

 

There is no real one answer for your question. Larger piping might move the hp/torque peak out to a higher rpm but at the loss of low rpm pulling ability. Smaller dia. piping might have better low end power at the expense of higher rpm breathing ability. It depends on what you want. The 5-gen muffler design, is the first gen of mufflers(from the skyline) that feature good exhaust velocity at low rpms and good flow at high rpms. All this and good(relative) noise levels. It depends on what you have and what you want outa of your engine.

 

I knew someone would respond with an informed response. Much appreciated.

Tyler

 

hmm... what about the people that say 2.5inch piping is better than 3 inch piping and have the dynos to back it up? now im confused.

 

Because 3" pipe doesn't mean high exhaust velocity.