exhaust diameter 101
 

My friend and i were recently discussing exhaust modifications and the question of increasing the diameter in the exhaust pipes were raised. If one were to replace the entire exhaust system from the headers back, and wished to increase the diameter of the exhaust to 3 in. Would this be possible?

Possible yes, but not comming off the manifolds. 2 1/4 is plenty big for a N/A 3.3 anyway.

what is the stock diameter?

I ment 2 1/2. I think stock is between 2 1/4 and 2 1/2. Of course I have duel 2 1/4 from the manifold back, that would be like running one 4 1/2 pipe! I think I need a S/C to make it worth it.

From the third cat back, the stock ID is about 60 mm which is about 2.35 inches. For a near stock SVX motor, going up to 2.5" inch ID from the third cat back is a good option. This increases the crosssectional area of the exhaust by 10%.

Yea, that is a good idea... going to 3 inches though on a N/A car just isn't really nessasary.... You'll probably get a big Hp increase up around Redline but you will loose so much torque down low it won't even be funny.....

3" is probably fine for the 3.3l

considering that 2.25 is optimal for the 2.2l that this is based off of

and 2.5 is optimal for the 2.5l engine

you might wanna try 2.75 tho everyone always jumps from 2.5 to 3

I would be more inclined to try to correlate exhaust diameter with horsepower which is going to be more closely related to the amount of exhaust gas produced than actual engine displacement.

Just going through some old posts here and noticed a flaw in your math. A dual 2 1/4 exhaust is not equivalent to a single 4 1/2 exhaust. A single 4 1/2 has twice the cross sectional area of a dual 2 1/4 exhaust.

A single pipe equivalent to a dual 2 1/4 exhaust would have a diameter of approx 3 3/16.

ive run open headers at hte track(uniongrove, w/o nitrous) and recieved a .1 second increase. I reset and let hte ecu learn to the gas station and back(15 miles) There is virtually no restriction in our exhuast, and it seems to be give here take there. I must say that our engine sounds quite agressive without exhuast, my friends said it sounded like it was gonna run tens!!(but it ran 14's...who are we kidding here? :) )
phil

I never was any good in math:o

Please tape record that the next time you try it. That must sound awesome. :D

[QUOTE]Originally posted by Phast SVX
[B]
ive run open headers at hte track(uniongrove, w/o nitrous) and recieved a .1 second increase. I reset and let hte ecu learn to the gas station and back(15 miles) There is virtually no restriction in our exhuast, and it seems to be give here take there.

Phil,

Possibly the 0.1 second resulted from practice gained as a result of the previous runs ! On second thoughts I am not sure if you by "increase" you mean an improvement or a longer elapsed time and therefore decrease in performance!?

In any event, the above should be shouted out loud from the roof tops, included in the How Too section as a "How Not Too" and posted in Mod Mania for good measure.

I have always been confident that the stock exhaust system is obviously well designed and is in fact a carefully tuned system and that anything fitted by an aftermarket exhaust shop will be an absolute downgrade.

Congratulations, Trevor:)

open headers is not as good as running a straight pipe. cut your exhaust off at the axle... then test it out...

with open headers you will loose the scavenging effect and all of the exhaust gasses will not be pulled from the engine in time for the intake stroke and exhaust gasses will be pulled into the combustion chamber... there is less oxygen in exhaust gasses than in air.. duh... this will hurt performance
back pressure is bad yes... but exhaust flow velocity for scavenging is very very important

I agree absoutely and what is explained illustrates why the original system works so well. It has been designed accordingly after a great deal of practical testing to an extent beyond the capability of any after market exhaust shop. What is more it is obvious that the original design has not been compromised on the basis of cost.

A properly tuned straight pipe system would result in improvement but as it is not possible to run one on the road, the comparison with open short pipes is valid.