Hope you can help me straight out some questions I have when it comes to exhaust pipe diameter.
I heard about the 2.2 CFM per bhp rule. So that would mean that a 400chp 350 would need a system that flows 880cfm? Does this "rule" still apply?

Also I heard that a single 3 inch pipe flows around 820-840 cfm ( please correct me if im wrong ). So that would be around 50CFM short for a 400chp engine, right?
How much power loss will 50CFM be, approxamently? Is that even possible to calculate, or does that depend on to many other factors?

And what makes this even more confusing for me ,is that I see alot of cars with more then 400 hp use single 3inch exhaust pipe.

New skyline, 478hp.


( although i suspect there a diffrence between N/A engines and F/I setups? )

And I seen 11sec muscle cars run single 3inch exhaust setups. Many lsx camaros/firebirds etc.

Help me understand this

Thanks..

 

Just because 2.2 is the goal doesn't make it set in stone. If you're a few horses over the ideal, you'll have a little bit of restriction, and it will cost you a very little power. Another thought: If you have an engine that dyno'ed at 400 horses, by the time you add the alternator and power steering, you're at 390 horses. How many cfm do you need now? Most ricers just use the largest pipe that is readily available pre-bent to fit their car, and they really don't have space to go big. You, being a third-gen owner, can run a mufflex 3.5" or 4" single, or search on here for the custom dual exhaust systems that have been tried. There's been one test done that I'll tell you about. It was a pair of Bassani 3" mufflers, on an engine that made 700 horses, on the chassis dyno. With a 13% drivetrain loss (T56) that's 804 crank horses. 15% is 823. Does any muffler, except Borla, flow as well as 3" straight pipe? No. But with a loss that small, so what?

 

P.S. it was a trick question, you still need 880.

 

Good points about the alternator/power steering and HP.
For me I have to sacrifice ground clearance if I go dual 2.5inch vs my singel 3inch i use now. ( My car is really low.. )

Another thing that "doesn't compute" ( and the main reason for my questions ) for me is that it seems to differ between theory and what I can se on the local dragway. I seen many N/A LSx camaros/firebirds running 11s on the 1/4 mile, with headers and a singel 3inch exhaust setup. And those cars is no monsters, intake/head/cam "only". In my head that must mean the restriction of a 3inch exhaust must me minimal at those powerlevels ( around 400-430hp )?. Or would those car run low 11 all day long, if they used a dual 3 inch system?

Sorry, I just try to understand this

thanks

 

Owners of third and fourth gen F-cars are not that much different from the import guys, we start with the largest readily-available stuff, too. People are lazy. It's a fine approach, but once you get the new, more powerful combo fired up, you just need a way to check how much backpressure, usually in PSI, you have at the header collectors. This is easily done by temporarily replacing the exhaust oxygen sensor with a self-fabricated setup to use a boost gauge. 2 psi, at WOT at the rpm where you make peak HP, no big deal. 5 psi, way past time for a better-flowing exhaust. I learned about this with a SBC, Vortec 350 heads, dual springs, custom cam from COMP, 280/286 advertised, 230/236 at 50, 480/486, 112. Holley 300-36 under a 3310, 1.625" headers, but a dynomax 2.5" cat-back. Because it was already on the car. My 6000 rpm engine would NOT pull past 5000 rpm until I replaced the exhaust. The difference was night and day. As for the LSx crowd Dyno-ing 450 rwhp with a single 3", I don't KNOW, but I suspect their engines are ready for more. It seems they're all hitting a wall around that power level. I'll go ask about it on ls1tech.com and planetlsx.

 

OEM dyno HP is rated with the alternators/power steering pumps, stock exhausts, etc. This is Net Horsepower.

Independent HP engine dyno testing almost never includes these "factors", only occasionally includes mufflers. This is "Gross" Horsepower, what was used in the '60s. The two can be very different.

TA

 

Good tips, something I will try this summer for sure

 

Please do, im whery interested to hear what they say.

As for 450 rwhp with single 3 inch, that sounds crazy. Whay more power then I ever going to make..

 

You always want to go the next size larger if your at or near the max flow of a exhaust size. It will give you room for future mods and/or it will give you more of your power than the smaller pipe size.

Example: I have a 2 1/2" y-pipe into a 3" with a flowmaster super 44 out the rear and i've been running this with my 383 with 11.1, .600 vodoo mech roller, air gap, 750 speed demon, hooker comp headers. It dyno's at 319rwhp. Plus I keep blowing out collector gaskets (very annoying). Its because my exhaust is to small (to much back pressure). But thats what i had so i'm running it. When I unbolt the exhausts it jumped to 390rwhp. I shouldn't have jumped that much hp by just unbolting my exhaust. As you can see VERY RESTRICTIVE. I really need bigger exhaust.

Point is you can run a single 3" but it can be restrictive. If you want max performance then you might want to step up to a 3.5" or try duals system.

 

I have not dyno my car. But its a miniram 355, canfield 197 heads, lt4 hotcam.
My exhaust looks like this. Hooker longtube headers going in to hooker front y-pipe to a single 3inch pipe ( to replace the stock cat ) then to a rear 2,5 y-pipe and dual mufflers.
Its a -91 vette.
I hade a plan to use a 4inch pipe instead of the 3inch pipe.

Thanks

 

Im running a Miniram'ed 383, my old exhaust set-up was SLP 1 3/4 shorty headers and a Flowmaster 3" cat back system, (no cat) I put down 375 rwhp.

I then upgraded to Hooker longtubes and a Mufflex 4" cat back system with the Spintech muffler, (custom 3" Y-pipe)with NO other changes I put down 413 rwhp, that includes NO tuning changes either.

Now I realize most of the 38 HP gain came from the longtubes, but if what people say about the Spintech muffler being restrictive is true, with tuning and a muffler change Im looking to gain another 20 rwhp or so. So for me, 4" is much better!

 

I wounder if the power loss, due to restriced exhaust diameter, is exponential with chp and c.i.d?. A 600hp bigblock may se a power loss off 20% with a 3inch pipe, and a 400chp SB perhaps only 5% ?

 

It probably is, I think most people here would agree that a SLP 1 3/4 shorty headers and a 3" cat back exhaust system (no cat) on a thirdgen would be considered a "very good" flowing exhaust system. To thoes that say 3" is all you need, and that 4" is "too big" is just not true for everyone. In my case I left 38 rwhp on the table with that 3" system. After some tuning, (adding some more fuel to go with my additional airflow) I should pick up 5 to 10 more rwhp. Hell, throw in a straight thru muffler like a Dynomax ultra-flow and maybe another 20 rwhp. SO, for me, im looking like I might be picking up an extra 60 to 70 more rwhp just from the "too big" exhaust system? But, did it cost me some $$$, oh hell yea it did. LOL Just my .02c

 

The 3" y-pipe into a 4" would probably be good way to go. My 4" Mufflex with spintech race muffler should be here tomorrow. I've been waiting two in a half weeks for it. But I called and they said its on the way. Can't wait to see how it sounds and to see the power gains on the dyno. But i am also in the process of a T56 swap so it will be hard to tell what the exhaust will do for power.

I've also read in here that the Dynomax ultra-flow will give you alot more power and flow better than the spintech or the flowmaster. It would be nice if they'd offer the 4" catback with the Dynomax ultra-flow.

 

I just got an e-mail from Magnaflow, the guy gave me 2 paragraphs about the 2.2 rule, but didn't say anything we didn't already know, or give a more practical number, like maybe 2.0 or 1.8 or anything. The guy then spent another paragraph playing dumb and beating around the bush about the power level a single 3" can support before needing 3.5" or duals. In my initial query, I stated that I would be sharing their answer with an entire forum, who would judge their company by how polite and concise their response was. To me, there's no excuse for vagueness when they should have found our answers through prior testing. So I'm now anti-Magnaflow. Before this I was neutral towards their company.

 

attilla that is the first time i hae ever seen you angry. well i guess its the hun and not the fun now.
post that letter when you get the chance i would like to see it.

 

When I ran my 3 inch setup with the cut out closed I ran a 12.67, with it open I ran a 12.03 both with tranny problems. The general rule if your making or will make 370 or less horsepower, a good 3 inch catback will work great. Anything under 503 horse a 3.5 inch will work. I think a 4 inch is to big unless your going to pull some major horsepower. And a good set of 1 3/4 headers with a straight through ultra flo muffler will take advantage of maximum power. I will try this 3.5 inch setup along with a 3.5 ultra flo muffler sometime this summer

 

do you have any suggestions on a single long tube exhaust that is still quiet. im looking into running a 3 inch single in the stock position by the drive shaft but then it dawned on me that i might be able to run a long muiffler and then run one pipe over the rear end. do you know of a muffler that might handle that but not be too loud?