Louie928

Rick,
You are trying to stuff 366rwhp down a single 3" pipe?? Get an X pipe and 2.5" duals.

Rick Carter

I found this:
So a single 3" is good for 369 hp and dual 2.5" is good for513 hp according to this formula.I'm guessing this is crank hp so I should "upgrade"; edit or a single 3.5" is good for 502 hp and a single 4" is good for 657 hp.

Is there a system that will fit the 85 with the big battery box?

hacker-pschorr

Rick, an exhaust bung costs less then $5. A pressure gauge maybe $20. 10 feet of vacuum line is about $10.

Weld in the bung, I can post pictures of the device Mike Schmidt made to screw into the bung with a vacuum fitting attached.
Run the vacuum line into the cabin, attach the gauge and go for a drive. If you see over 1-2psi....it's not flowing enough.

PorKen

Single exhausts are much more difficult to control noise with.


So the factory 52mm ID pipes will flow only ~293 rwhp comfortably? (My best dyno runs flatten out at the top, like they hit a wall. 52mm ID pipe after the 59.5mm ID X-pipe.)

Dual 2½" OD (59.5mm ID) = ~383 rwhp


52mm = 2.047" = 3.291"² * 115cfm = 378.553cfm * 2 = 757.106cfm / 2.2 = 344.139chp - 15% = 292.518rwhp
59.5mm = 2.342" = 4.309"² * 115cfm = 495.626cfm * 2 = 991.252cfm / 2.2 = 450.569chp - 15% = 382.983rwhp

Fabio421

Notice that the book excerpt says "straight pipe". You have restriction to flow every time the pipe makes a bend. Keep that in mind when building your exhaust.

hacker-pschorr

+1
So unless you are planning on installing an exhaust that looks like this:

Louie928

I remember those back in the good times. Excellent. Thanks for the pic.

Louie928

Rick,
It's my personal preference to use dual pipes on a car you'll be driving on the street. IMHO, they sound better and you can muffle them better. Even if loud they still sound nice if there is a suitable crossover. The large (>3.5" OD) pipes have advantages for a track car being lighter and somewhat easier to route, but it's really difficult to muffle well. I did some dyno tests on single 3.5" vs dual 2.5" and the duals gave better mid range torque although top end was the same. Not sure what effect, if any, there would be on a SC car.

There is no problem in getting a 2.5" pipe between the diff housing and battery box. I built a dual pipe system for my neighbors '85 several years ago.

123quattro

Not to doubt Louie, but my Audi is making ~400whp through a single 3" (which is 500 crank hp for a quattro). It is possible. It's loud though.

Rick Carter

OK,
I will start looking for an )( and a shop that can fabricate mandrel exhaust. Thanks Louie.

Hacker,
I should be able to use my WB bung to check back pressure, I would like to see pics of the set up. Thanks.

hacker-pschorr

I'll see if he has it with him when he gets here in a few hours.

Basically it's an O2 plug with a hole in the middle and a vacuum port screwed in.

Louie928

I'd say your Audi needs a bigger pipe then. Heh, heh.

123quattro

I'm lazy and don't feel like building another exhaust for it...

Louie928

Rick,
Supercharged engines need to be a little careful about exhaust size. This is especially true when the cams are not designed for supercharging as in the 928s. You can easily get into a situation where during the valve overlap period some of the incoming (pressurized above atmosheric) charge gets blown out the exhaust valve. You are going to need some backpressure to minimize that. How much is the question. You want to end up with a situation that puts the greatest incoming charge in the cylinder when the intake valve closes. The N/A rules for backpressure and exhaust pipe size don't hold up when supercharging. The 400 hp Audi with 3" pipe that John has is an example. I'm not saying the 3" exhaust is good for a 400hp 928, and maybe it isn't even the best for the Audi. It's an example of the exhaust being smaller than it seems like it should be, yet does work. Turbo charged engines are entirely different in exhaust requirements.

In your case, the 3" single seems like it's small and would be by N/A standards. However, it may not be for your engine. When I was working up Jim Hurtt's SC installation, Jim wanted 500 rwhp as a goal. He bought a set of headers for it, and I was going to build a nice dual 3" system using them. I worked up some exhaust system mods on Engine Analyzer simulator. I was surprised to see that there would be no big difference between a dual 2.5" system using stock exhaust manifold and the headers with dual 3" pipes. I stayed with the stock manifolds, X pipe, and dual 2.5" system and he got his 500 rwhp at 9 lbs boost. I did some other work to the intake system too which did help.

Before doing a lot of work to change your exhaust I'd do one test. Take your car to a dyno. Do a few baseline runs to establish power on that day. Then remove your exhaust behind the Y. Do a couple more runs to see if there is much difference. That'll tell you if the length of 3" pipe is having a big effect. If you see more than 10-15hp, then either get a 3.5" pipe, or go to dual 2.5" pipe. If no 20hp gain, then the 3" single is probably a good compromise. The 3" full exhaust may give more mid range torque than without. You may prefer the fatter mid range even if the top end is down some. Without the rear exhaust, it'll be loud and the dyno guy will probably whine and grumble. They'll live through a couple runs. The dyno result removes speculation and can save money in the long run.

Rick Carter

I started by pricing new exhaust, )( for the 928 is about $500.00 then there ae the pipes and mufflers maybe body work depending on how it is routed since my car wasn't designed for duals. Carl F has a dual exhaust set up (diameter not listed I'm guessing 2.5") for my car for $1200.00 not counting an X that doesn't require body modification. I'll follow your suggestion and oncork it on the dyno first. I always appreciate your advice.