Running slow.. exhaust too small?

[SkylineDriven]

Hi. I just got my SR installed. New plugs, no a/c, no p/s, downpipe, testpipe, 2.5" exhaust, smic, greddy bov (non recirc). Timing is supposedly right. walbro fuel pump as well.

My friend has a near identical setup except he has a 3" exhaust and an fmic. He is set at 10 psi, I am set at 9 psi.... when I drive his car, there is a big punch at around 3k or 3.5k. and it pulls strong till redline.

Also, his car gets to 3.5k pretty quickly... my car doesn't punch until like 5.5k and it doesnt punch as hard and takes longer to rev up there than his does.

He thinks that it is because my exhaust is only 2.5". What do you guys think? Is this true? Does exhaust make that big of a difference? His car and mine are night and day.... the only thing we have not checked is engine compression and we will check that whenever he is able to get the compression tool.

The motor is faster than my KA but it just doesn't feel like it has the power his SR has.

[180crafter]

Yes it will make a huge difference...

A 3inch exhaust to an SR is like breathing normally.

A stock exhaust, with its crappy bends and archaic muffler design, is like tring to breath thru a bent coffee stirrer.

Hope this analogy will help to clear up what you really need to do... Replace everything after the turbo, and then go spank your friend.

[SkylineDriven]

hehe. Thanks. I took off the ehxuast today and ran with downpipe/testpipe only.. just to test this theory.. It was kinda rough. I guess not enough backpressure since it just drops off after the test pipe, but it defnitely revved faster. I wasn't able to get it to spool because a) not enough pressure and b) it was getting WAY too loud.... but I did notice alot less restriction. I'll definitely invest in a 3" exhaust now. Thanks for confirming that for me.

[msaskin]

hehe. Thanks. I took off the ehxuast today and ran with downpipe/testpipe only.. just to test this theory.. It was kinda rough. I guess not enough backpressure since it just drops off after the test pipe, but it defnitely revved faster. I wasn't able to get it to spool because a) not enough pressure and b) it was getting WAY too loud.... but I did notice alot less restriction. I'll definitely invest in a 3" exhaust now. Thanks for confirming that for me.

That doesn't make any sense. Running open downpipe will mean your car spools fast as hell, trust me, I've done it to try out on just about every turbocharged car I've owned (this is my 3rd).

Are you sure your car is running well in other aspects? From your symptoms (no power until 5.5K) I'd be willing to bet your timing is off. I had the same problem and my timing was back 5 degrees (20BTDC instead of 15BTDC).

~matt

[SkylineDriven]

they checked it and said it was normal but ill have another person check it for me on monday.

[SET240SX]

Take off the down pipe and test pipe also. Should make a huge difference. it nets me .5sec in the 1320 with just turbo extension.

[BaliLover]

ARe you sure that your turbo is still in good condition? I notice that you say you're only running 9psi but your friend is running 10. With my blown turbo it would only boost 9psi max and it didn't nearly have the punch that my friends did at the same RPM. And from what I remember, turbos really don't need back pressure, they create their own.

[MainEvent212]

eh, turbos dont want backpressure cuz they want the exhaust flow to be going away from the turbine blades, as to not push against the natural flow....that would be bad

[SkylineDriven]

Take off the down pipe and test pipe also. Should make a huge difference. it nets me .5sec in the 1320 with just turbo extension.

I think it is in good condition. I bought it from a guy here on the board.. there was no shaft play on it. No idea.. if it is in bad condition I guess i'll look for his screen name again (has the word goat in it i think) and u guys can beat him up for me.

not we are gonna check some things on monday and my exhaust is coming in this week sometime. so we will narrow the possibilities down.