ok, need to streighten something out.
Refering to Non turbo cars
backpressure is never needed, nor is it desired. backpressure is created by having the proper sized exhaust tubing size. having the proper exhaust tubing size creates the proper exhaust gas velocity. the scavenging effect you are refering too is because of the high velocity of the exhaust in the pipe.
having a smaller diameter pipes makes air travel faster or at a higher velocity, when it travels faster its momentum increases. if the air moving threw a pipe has alot of momentum it has a high resistance to slowing down or stoping. since it doesnt not want to stop or slow down, it will actually create a vaccume after the once open exhaust valve closes. when the next exhaust valve opens there will be a slight vaccum in the exhaust system. that vaccume sucks or scavenges the exhaust out of that cylinder. then that exhaust has the same effect on the next and so on.
backpressure is the undesired effect of this process. too small of a pipe will cause excessive backpressure. such excessive backpressure cancels out the benifit of the added scavenging. having too large of a exhaust pipe, will not create enoguh velocity untill higher rpm thus resulting in lost low rpm torque.
SO after all of that, to determine the right size exhaust pipe diameter ou need to compromise on top end flow and low end velocity. using a pipe just small enough to provide the needed velocity and scavenging effect for low RPM but just big enough to maintain the right velocity with minimum backpressure at high rpm
NOW, for Turbo cars, its slightly diffrent.
on a turbo car you want least backpressure possible. so you want the biggest exhaust you can fit within reason before the point of diminishing returns. for instance if your stock turbo 7m and theres 1 horsepower diffrence between 3 inch and 4 inch exhaust but the 4 inch is 500 bucks more....stick with the 3 inch.
velocity on a turbo charged car isnt as important as the pressure ratio before and after the turbine wheel on the turbo. the benifit of the exhaust scavenging in the low to mid rpm is minimal compared to having the highest pressure ratio between the turbine wheel of the turbo.
having the highest pressure ratio actoss the turbine wheel results in faster spool. also backpressure in an exhaust on a turbo car has a much more drastic effect on the power output.
FAQ Thread On SF said:
Here's a worked example (simplified) of how larger exhausts help turbo cars:
Say you have a turbo operating at a turbine pressure ratio (aka expansion ratio) of 1.8:1. You have a small turboback exhaust that contributes, say, 10 psig backpressure at the turbine discharge at redline. The total backpressure seen by the engine (upstream of the turbine) in this case is:
(14.5 +10)*1.8 = 44.1 psia = 29.6 psig total backpressure
So here, the turbine contributed 19.6 psig of backpressure to the total.
Now you slap on a proper low-backpressure, big turboback exhaust. Same turbo, same boost, etc. You measure 3 psig backpressure at the turbine discharge. In this case the engine sees just 17 psig total backpressure! And the turbine's contribution to the total backpressure is reduced to 14 psig (note: this is 5.6 psig lower than its contribution in the "small turboback" case).
So in the end, the engine saw a reduction in backpressure of 12.6 psig when you swapped turbobacks in this example. This reduction in backpressure is where all the engine's VE gains come from.
as you can see that backpressure in a turbo back exhaust is a substaintail reduction in power
this is my reduced version of the thread on supraforums. the one on there is much longer and more technical.
that thread can be found here:
http://supraforums.com/forum/showthread.php?t=227843
So for a Non turbo supra the exhaust diameter of choice would depend on your RPM range and power level. i personally would run about a 2.5 inch exhaust. that shoudl be good for a stock n/a with stock redline.
