shock211

iTrader: (6)

I have recently had my whole exhaust replaced with 2.5" pipes on my 92 vg auto.
The y-pipe, cat conv., and b-pipe are all 2.5" with the stock muffler(only temp.)
My question is.....which would be better for overall performance a 2.25" setup or 2.5"? and when do you start to lose back pressure?
the reason i've asked is because since the exhaust has been replaced I haven't noticed a difference in power from 0-60mph. when i'm doin' about 70mph + it feels really powerful though.
hwy driving is incredible now! but how come i'm not noticing anything at lower speeds?
have i lost back pressure with the 2.5" piping?
or is this just the way it's supposed to work?

should i go back to my exhaust shop and have a 2.25 system built?

just a fews things that we're bothering me...hope someone can help.

Irish

iTrader: (3)

it could be ur muffler. i noticed lose of back pressure when i went up ti 2.75. now im moving down to 2.5

Blinocac200sx

Ok, to start, lets get something straight, backpressure is bad, period. Now, does that mean you should run a huge pipe to eliminate backpressure? Nope, too big a pipe will actually cause you to lose the scavaging affect you need to eliminate backpressure. It's all about flow. When your exhaust is flowing good, it creates a slight vacuum which scavanges exhaust away from the combustion chamber when the valve is opened. When your pipe is too big, it can't create this vacuum. Thats why, quite often, people say, "I ran too big a pipe and lost backpressure, and now my car runs like crap." Cuz they think they lost backpressure, and what they really lost was vacuum. The only time backpressure is useful is when your valvetrain is not set-up right, and backpressure keeps you from losing intake air out the exhaust valve. But the only people who set there valvtrains up like that, drive Chevy Novas etc. . . and have big glass pack mufflers, and set up the valvtrain such, because it sounds cool. I hope this clears some things up. As far as it goes, I have a good 2.75 exhaust on my car, but for a Maxima, I'd recomend 2.5, because of the drivetrain layout.

JC93SE

Basically (and more), what Blinocac200sx said is correct.

Where did the Y-pipe come from? My question is: if you had a new Y-pipe installed or fabricated, what size are the two downpipes from the manifolds before they merge together at the collector?

2.5" exhaust from the Y-pipe back may not be a bad thing.

shock211

iTrader: (6)

2.25" down to the collector.

JC93SE

I think that the 2.25" pipes from the manifold to the collector are too large. Warpspeed uses 2", and Cattman used to use 1.75". Without a lot of actual data to go on, it looks like the 2" pipe gains a few HP up at top end over the 1.75", but loses some torque down lower.

Your 2.25" probably is an improvement at high revs compared to the stock unit, but I think that the exhaust gas velocity suffers at lower engine speeds due to the size of the pipes. Going back to 2" on those pipes would improve things, and going back to 1.75" would probably give you more power where you use it most... under 4K... and especially with the VG. A smaller pipe might pump up the power low, but choke it at higher revs. We'll never know exactly without all kinds of testing on the dyno, but 1.75" is a pretty good compromise

shock211

iTrader: (6)

are you positive that the y-pipe is 1.75" from the downpipes to the collector?
i assumed that it was 2.0" all the way through.

JC93SE

Go here and take a look:

http://www.warpspeedperformance.com/vgside.jpg

The two downpipes on Warpspeed's unit are 2" each, and the end that bolts up to the converter is 2.5".

Cattman used 1.75" and 2.25" when their y-pipes were in production. Cattman then preferred to have 2.5" exhaust behind that, but I'm not sure what size converter he used.

In your case, I'd say that you need the 1.75" downpipes, and that a 2.25" collector is optional (if you already have 2.5"). The rest of the exhaust is fine as is at 2.5"... or you'd never know the difference without a dyno.

bonzelite

iTrader: (2)

you are correct. this also directly relates to valve overlap: at a point, the intake and exhaust valves are open at the same moment. in daily-driving conditions, you want this duration to exist, eliminating backpressure, yet you want it to be short to minimize compression loss. this saves your low-end torque. furthermore, the exhaust scavenging of the exhaust valve open *at the same time* as the intake valve, to a point, creates a vacuum effect whereby the outgoing exhaust helps to "suck" air IN to the open intake valve. but too much loss of compression within the combustion chamber due to prolonged overlap will work against this process, eliminating useful vacuum pressure. using too aggressive a cam with stock to mildly modded setups will do this.

this illustrates further the consequences of vacuum loss.

JC93SE

"Thinking" further: I wonder if a 2.25" (from collector back)wouldn't be better than a 2.5", especially in the case of a mechanically stock VG. The exhaust velocity could be better maintained past the converter, and the larger size would only really matter at WOT and higher RPM where the VG engine can't breathe anyway. The smaller size would be better for power under 4500RPM. But I can't prove it, and can't guess how much better it would be. Some at least, maybe not a lot. 2.25" is better than 2", and 2.5" is better than 2.75". A VE might make better use of 2.5", but only towards top end, otherwise 2.25" would still pull at low and mid-range better. You already have 2.5"... unless your exhaust shop is really cheap or you have money to spend, leave it alone... at least for now.

As for the muffler, I don't know (yet) how restrictive the stock muffler is, but in your case a larger inlet/outlet muffler (not necessarily louder) to go with the larger pipe that you have is probably a good idea. If you don't replace it, what was the purpose of you installing the bigger pipe in front of it? The exhaust system is only as big as it's smallest piece (excluding the down pipes). It's arguable that one might want to start smaller at the front and get bigger towards the rear, but not the other way around.

JC93SE

You are basically correct and on the right track. But, you should go back and study the chapter on camshafts and valve timing, and then retake the test.

bonzelite

iTrader: (2)

JC93SE, cool beans, man.