OK, I've heard that a 3" O2 housing/DP will allow the turbo to spool more quickly than a 2.5" setup. But I've also heard that a 3" setup allow a larger power gain, but at the price of decreased low end torque. These theories seem contradictory to me. Which is correct? Do any DP/O2 housing manufacturers have dyno charts comparing a 2.5" with a 3"?

Also, would a 2.5" from the turbo to cat and 3" from the cat-back be a good compromise between sound (not as loud) and performance. I know this setup won't flow as well as a full 3" system, but will it be better than a full 2.5" system?

We should talk exhausts further when you check out my car...... fyi... the new ISC's housing broke, so we're going to transfer housings between the dead isc and the new ISC today..... so car should be running tonight long as nothing ELSE goes wrong.

well, i don't know how much power u plan on making but, on one of the friends cars, with a ported 1g o2 and a 2.5 turbo back, we made 370 something, we un-bolted it from the dp and made only like 6 more so, that tells us that the 2.5 will perform pretty well in higher hp cars, i run 2.5 also, but i don't make quite that much.

Jim,

One thing to consider, is whether or not you will be running a cat. I went 3" because I knew that:

1. A 2.5" cat will be too restrictive
2. A 3" system with a cat will be pretty quiet

If I were going to run catless, I think I'd just go with the 2.5, depending on how far you plan on going with your setup. I built my own system, so I can mix-and-match mufflers and resonators as I please. If you're about make a decision on an "off the shelf" system, I would decide right now how far you plan to go with your mods. If you plan on making 400+, go with the 3".

Mike C.

coorect me if im wrong, but i heard that with a relly good flowing exhaust system, the car onyl looses top end torqe, rather then lowend

You're wrong.

With a turbo car, less backpressure=more flow. More flow=
more power. NT's get complicated with tuning pulses, savaging,
etc. A guy in our local club gained .3 just by disconnecting his
catback. This is with a 3" turbo-back.

Mike C.

How restrictive do you think a 2.5" cat will be? Do you think it would "hold back" a maxed out FPbig28 or big 16G?

14B Wonderboy: was the 370 HP car with the 2.5" turbo back running with or without a cat? What turbo was it running?

Would there be any point in running a 2.5" downpipe flanged to 3" at the cat, 3" high flow cat and catback? (in terms of noise reduction)

Thanks

the turbo was a mutt lvl3 or ete42 i believe, and it was running a test pipe

The greater the flow on the output side of the turbine the faster the turbo will spool, and the more power it will make (both peak and power under the curve)

The high pressure/low pressure zones created on either side of an exhuast turbine accually help to spool the turbo, as the low pressure (exhuast side) will pull the high pressure exhuast (manifold side)through the turbo.

Theres no need for savaging or pulse tuning on a turbo car becuase as soon as tehe valve opens the turbo is taking care of all this for you, forcing in air as hard as possiable.

Turbo Drag cars run the exhuast right out the side fender well for a reason, the shorter the tubing the lower the pressure.

Originally posted by Dyre


Theres no need for savaging or pulse tuning on a turbo car becuase as soon as tehe valve opens the turbo is taking care of all this for you, forcing in air as hard as possiable.

Click to expand...

What about before the turbo spools? Shouldnt there be some scavaging effects while the turbo is essentially at idle at about 2000 to 2800 rpm.

How often do spend time BELOW 3500rpms when racing though?
Your rpms might drop down around there as you slip the clutch on luanch, but besides that, it doesn't really happen.

On an auto with a lower stall you might be down in that range, but i think the shorter spool time greatly out weights any slight low end torque increases.

The turbine wheel provids plenty of backpressure for the car to operate properly. There ARE reasons to go for a smaller piping sometimes, but the purpose is not to increase backpressure... Sometimes smaller diametr piping can reduce backpressure. If you go too large on a long pipe you can allow the gas to cool before it reaches the end on the pipe... As it cools, it loses engery, and speed and can accually create pressure. Anything bigger than a full 80mm (3.15") on a streetable DSM would be overkill IMHO as you can't flow enough gas to fill up something like a long, full 4" system...

This thread in the Advanced forum of DSMTalk has been one of my favorites for months. If you can wade through some of the chatter, there's some good info on 2.5" vs. 3" in there.