How much difference does exhaust pipe size really make?

[Edmond]

redbob,

to achieve performance gains, would the exhaust manifolds have to be taken into consideration... sorry, but the shift keys aren't working on this computer so i'll ask questions with'...'

aren't the gains advertised in the books gains if you change to headers and a free flow cat... gaining 30 horses is quite a jump.

and didn't you say that a bigger pipe doesn't necessarily mean more power because the flow of air also has to be smooth as well...

 

[Redbob]

redbob,

to achieve performance gains, would the exhaust manifolds have to be taken into consideration?

aren't the gains advertised in the books gains if you change to headers and a free flow cat? gaining 30 horses is quite a jump.

and didn't you say that a bigger pipe doesn't necessarily mean more power because the flow of air also has to be smooth as well?

1) Yesss; and the tube manifolds on my '89 are pretty good in this regard, although I do have a Hedman I'm thinking about installing. Unless you're "race-only", I'm not sure a header is worth the effort and cost.

2) If most aftermarket suppliers are like say TPIS, the HP gains they claim ("14 HP gain with a throttle venturi plate") are with a heavily modified engine. An L98 should gain 20 - 25 HP with good (not necessarily noisy!) mufflers on a "true-dual" system, WITHOUT cats since the cats alone contribute 40% - 45% of the exhaust restriction on these cars.

3) Yes again, Bwana! Bigger isn't necessarily better here, and too big a pipe will have a much worse sound quallity.
What you want is for the system piping to fit into the mufflers or resonators without having to step down OR UP. Note also that a muffler's internal tubes are usually much smaller in diameter than the inlet bushings.
Flow disturbance causes pronounced pressure drops in either case, while simple frictional losses in straight sections of pipe don't amount to hardly anything.
Most C4 cars will work just fine with 2-1/4" tubing until you're going near to 500 HP.
One final point is that I've seldom seen any need for a mandrel-bent pipe, except in the case of extremely tight-radius bends that are already going to be bad for flow.