would one be better than the other ? or is it than u run one or the other depending on fit lowered vs stock height vs drag spring ?

 

better yet what power range constiutes 3 inch over 2.5?

 

Someone was discussing exhaust flow volume and i thin that 2.5 true duals were not restrictive unless you were making >800hp

 

can anyone back this up ?

 

2.5" duals good past 800 hp? Don't think so.

I'd switch to 3" once you get to ~425+ at the flywheel. 3" should close to 600 fwhp. At the level most of these guys seem to be shooting for (~500 fwhp) then you're going to see a significant hp loss with 2.5".

If you look at dual exhaust cars that GM has built you can see a few trends.

For instance, the GTO ~340 fwhp uses a 2.5" dual exhaust system. It does vary along the way and is 2" in some areas.

The new Z06 at 505 fwhp uses a 3" dual system. Probably could support close to 600hp with that system but a 2.5 incher would definitely cost some hp... even for a stock car.

If you start getting real serious at 600+ hp then oval exhaust is the only way to fly if you want ground clearance. That's what I'm running on my car right now but it wasn't the easiest thing to put together.

-Mindgame

 

Got any pics, of the oval exhaust???

 

At what point do the 2.5" duals become a significant hinderance? Does the design of the system have any influence on this 'cut-off' point? Muffler design too? IMO, it seems like a few variables need to be defined, for each application, before a determination can be made.

 

SS,

Those are really good questions and I meant to say something about that in my post but lost the thought before I made it to the end. Old age I guess.

Absolutely right about the mufflers. Put a bad muffler in the system and you have a new restriction point. I would only use straight through mufflers like the Borla XR1, Magnaflow, Dynomax Ultra or Bullet, Straightline Performance, etc..

The closer the muffler is to the headers, the more it's design will influence the power losses. The smoother the bends in the system, of course, the better. Line 1 of this paragraph applies to the bendy sections too.

As for cutoff points, there aren't any hardcore #'s. The losses will be proportional to the engine's output. I did see a 66 Nova gain ~5 rwhp, 8 lbs-ft torque with a change from 2.5" x-pipe setup w/bullet mufflers to a 3" x-pipe with new 3" bullets and nothing else. All the piping was from Summit and everything looked, to my eyes anyways, to be the same. Car made ~390 hp at the rear wheels. I would have liked to look at the average #'s but this was a dyno day car and I didn't know the owner well enough to ask for his printout.

edit: My #'s were off on the peak hp & tq

-Mindgame

 

My 3" true dual setup with 16" Dynomax bullets.

http://www.auburn.edu/~doddsam/td1.JPG
http://www.auburn.edu/~doddsam/td2.JPG
http://www.auburn.edu/~doddsam/td3.JPG
http://www.auburn.edu/~doddsam/td4.JPG

 

Based on a design discussion I had previously with Flowmaster, I was told 3" duals become a beneficial addition over 450hp (flywheel). Below this 2.5" is generally enough on a street car.