450hp Exhaust Set Up

[84MonteCarla]

I'm coming up on the finishing touches of my LT1 swap and I am trying to decide on what exhaust system to run. According to other people with similar builds, I should put down 375rwhp (or ~460 @ the crank assuming 18% drivetrain loss).

I've got a pair of used Hooker Super Comps on the way and I plan on extending the collectors another 6-10" which will give me 30" primaries and ~20" secondaries. I think this will favor my 6000rpm red-line.

According to Vizard, I should be good running dual 2.5" exhaust. His rule of 2.2cfm per horsepower means I would need dual 506cfm mufflers. One of the best flowing mufflers that I can find are the 2.5 center/offset Pypes race pros, which flow @ 472cfm. Although these mufflers are short of what I should be shooting for, Pypes sells systems with X-pipes and these mufflers.

I guess my questions are....
Is 2.2cfm per horsepower a good rule of thumb to be using?
Will an X-Pipe or H-Pipe help increase the total flow (cfm) of an exhaust system?
Should I stop looking so hard into this and just run a 3" system?

 

[Blake442]

I don't think an H or X pipe will make a difference with overall flow in terms of cfm, what they do is help balance the back pressure. The balanced pressure helps evacuate the exhaust gasses more efficiently.
A full exhaust with X pipe will usually make more torque than open headers.

I'd go with 2.5" all the way back with an X pipe. It should be plenty big enough.
I went with 3" myself, but my 455 is pumping a little bit more air than a 350.

 

[84MonteCarla]

Thanks for the response. I'm not trying to be argumentative here, just trying to understand what is the best set up?

I'm not sure what all you've done to your 455 (your sig says Olds motor) but I think that my LE2 heads are going to outflow MOST stock 455 heads. I know this cause I am also a proud owner of a 71 Cutlass SX with a mildly built 455. 455s with stock heads would be LUCKY to put down 460hp (1hp per cube). I've read countless build ups (even did one myself ) and that is a tough number to hit without extensive head porting, an engine dyno and some trail & error.

Llyod Elliot claims that these heads flow 275/190cfms http://www.elliottsportworks.com/lt1.html. This flows more than ALL stock 455 heads http://nichibei.tripod.com/headflow.htm. A bigger cube motor doesn't always mean more airflow.

So along these lines, I should be getting a 3" system because my engine moves as much air as your Big Block???

 

[shotgun]

Here is a good site to poke around : http://www.headerdesign.com

You do have to "join" but I never have gotten so much as an email from them.

Threw research and actual builds I have found the following:

Dual 2 1/2" will support 260 to 460Hp
Dual 3" will support 350 to 600hp

You are right on the edge for a 2 1/2" system.

Here is a reply I made on another board.

"An H will add torque at low rpm and even out/smooth idle,reduces idle/low rpm by a few decibels . Good.

X does the same thing except it does it threwout the whole rpm range. Better.

Think about how fast the exhaust gas is going and how quickly it will not "want" to make the hard right into an H-pipe where as the x just kisses at the junction.

I have never had a car hurt by putting a x on.On some cars it adds a high rpm sound.

Also, even if you do not get a gain from the x-h you still can get a indirect gain from the decibel reduction.


How?

You can now run a "louder/lower restriction",ie HP gain, muffler and be at the same sound level."

In your case, run 3" to the x and 2 1/2" after that since the x will make the 2 1/2 "look" bigger..

If you are worried about hurting power, which I doubt this would do, run 3" into the mufflers and reduce after that to 2 1/2".

 

[84MonteCarla]

Thanks shotgun. Cool looking site, I'll definately dig into later.

Really, I am just trying to understand exhaust from an engineering perspective. I'm not a fan of the sledgehammer approach to solving problems, so I wasn't ready to throw on 3" exhaust if it is going to be overkill.

Everybody has been telling me go 3" on the F-body sites but the B-Body guys are all running 2.5". I posted here instead, to get a third opinion from people who are probably not running the same exact drivetrain as me. Even if the 2.5" pipes suite me now, what if I go 383? Or throw on a 125 shot? Doesn't look like I have much room to play with.

Looks like I am going to buy a full 3" Pypes system!