rgs944

I have been doing a lot of reading on different car forums trying to decide on exhaust options. I know there are some good vendors here that sell kits but I do not want to spend $2500 on exhaust to update the entire factory exhaust. I am thinking of going to a single 3 in. mandrel bent exhaust coming off of a Y just after the cats. I will be using pre- bent 8 in. offset pipes from flowmaster to do the routing. My local shop will weld the pieces together after I get the correct layout. The car will remain mostly stock except for the exhaust. A big advantage in going with the single is weight and cost savings, but will it hurt performance? I have learned it should flow as well as 2.5 duels so I do not think it will. So here is the big question, will a single 3 in. exhaust have any negative effect on the x-pipe gains upstream? Below is a link to a flowmaster Y that I would use to go to the single 3 in. pipe after the cats. The Flowmaster guy told me that the scavenging effects of this Y have proven to be remarkable. Who knows but it sounded good. After the Y" I will be installing one flowmaster muffler in the original resonator location. Any thoughts?

http://www.flowmastermufflers.com/sh...=&orig=Y250300

Rick Carter

Ott's Performance Engineering Single vs. Dual & Pipe Size

Landseer

Great link!

Mongo

I would never say that dual exhaust is overrated. There are single exhaust systems that yield the same gains as dual.

I can compare a car that does have a single 3" exhaust that makes over 370 HP and that is a 208 chassis CLK55 AMG. It has a 5.4L V8 and a single pipe exhaust that still makes more power than our newest 928, the GTS. Of course the piping is all mandrel bent, which would significantly help flow.

According to Louie Ott's research and judging by his dyno graphs, there is only a 7-10 HP difference between a single and dual setup all depending on pipe size with the 3.5" single pipe losing torque and a 3" pipe maintaining the factory curve but less HP than a dual 2.5" exhaust (only 10HP which could be picked up somewhere else like Roger's EIS intake.) Had I known about Louie's page sooner, I probably would have opted for a 2.5" to 3" merge collector and kept it single all the way back. I also keep getting burned by the passenger side muffler when checking the ATF. A 10 horsepower difference cannot be felt anyways in your butt dyno.

If you decide to go 3" single pipe, get rid of your factory's dual cats. Adding a Y-pipe afterwards will introduce too much scavenging and you will experience horsepower loss. Other than that, stick with an x-pipe and 2.5" dual pipes.

Below is also a chart that will give you a general idea of how pipe sizes relate to engine displacement and horsepower:

Aspkiller

Why stop at 3 inch? A 4" Flowmaster 50 will fit, as will the 4" continuation pipe.

Mongo

4" is way too big on a street 928 with a 5.0L unless it's supercharged. I would say 3.5" is better if a large pipe is needed.

The trick is to keep the exhaust gasses hot so that they are passed through the pipes faster. If the pipe is too big, the exhaust will cool becoming more dense and may result in loss of torque in low and mid-range. It's all about balance.

Aspkiller

Good Point. My reference is my '82 Auto, where I put in an LS6 that is making about 420 flywheel hp. With the 3 speed auto, it is really a great street car. Just need to put some stickier tires on now.

Also, where's the chart you mentioned?

rgs944

One big reason I would go with 3 in. is cost. I really like that new Flowmaster Y- pipe but I do not think they have it in 3.5 or 4. If I could find the pre-bent 3.5 in., (and a good Y in 3.5 in) I would look into going that route. There is just no one around here that does mandrel bending, so for best value for the buck I think I will need to stay with the 3 in. Flowmaster line.

Mongo

If you can't see it, click this link. It is in my Rennlist album.

https://rennlist.com/forums/members/...eter-table.jpg

Rick Carter

Burns Stainless and Stainless Works

mickster

Seems the 3.5" Single pipe into a magnaflow muffler is the best solution--seems to have the most torque based on Ott's dyno chart.

I guess the backpressure helps.

Michael

Mongo

Your Flowmaster part is Y250350. This is for a 2.5" to 3.5" Y-pipe merge. Good luck on finding mandrel bent 3.5". It is not cheap. I do have a good source for mandrel bent 2.5" and 3" though.

What is your displacement though? If you are not running a stock S4 or GTS motor, you need at least 3.5".

Mongo

I remember a thread a while back where Louie said a 3.5" pipe would reduce the torque output though, but yield more HP on the top end. It's a trade off depending on where and how you drive your car.

James Bailey

The typical V-8 hotrod sound is the result of having two seperate exhaust systems, one for each bank. The uneven firing order where two cylinders on one bank fire one after the other is part of the rythmic pattern our ears and brain seem to LIKE. So I would say Yes dual exhausts are over rated BECAUSE we like how they SOUND.....and two exhaust tips look better than a single

Mongo

Yup. I am getting tired of mine. I love the loudness, but I want that scream of a single pipe!