i have a set of pacesetter 1 3/4" ceramic coated headers running into TSP true duals, my car is stock ride height and the turn downs scrape some but you can have the turn downs rotated out ward and the hanger shortened for very cheap which is what i plan on doing to mine. it will give me at least another 1-2" to play with. thinking of lowering my car 1". IMO slammed maro's don't look right.

http://videos.streetfire.net/video/l...cam_704402.htm

^my cammed ls1 SS. can't beat that sound i have yet to hear a y-pipe set up that i like better.

edit: small cam and exhaust only, got me 373 rwhp and needless to say i have a lot of growing room yet to fill. once the car has full bolt-ons i should be making just shy of 400 to the ground.

 

I would like to see where you got your numbers from and if they accounted for thermal contraction as the exhaust cools down. I would also argue that you can't use a linear scalar for your calculations because the internal surface area on the pipe is going to have a drag on the gas moving through it. I will admit there is a reasonable difference, but it's unlikely he's going to outflow the single 3" soon. As I stated before, a linear scalar can give us an idea of what the piping may be able to flow, but it is not a true representation of the flow of the gasses.
The entire basis of an exhaust system is to get the gasses out and away from the cylinders as fast as possible. The basis behind a headers is to utilize a tuned sinusoidal wave generated (mostly) from the collector to help pull gasses out of the cylinder faster, thus improving torque. An optimized primary is mostly dependent on the cylinder volume (remember, this is to a point) rather than flow in order to obtain the correct resonant frequency for the primary tubes, and thus a good frequency for the collector. If you know you're going to outflow the 1 3/4" primary (this is where my previous statement about 650HP and 7000 rpm came from) you can step up the size, but once again, I doubt we're going to be making those numbers unless the OP has some big plans.

 

Ill post up the links I used. It was just as a rule of thumb.

http://www3.sympatico.ca/big-d/polytek.htm
(funny he talks about the testing etc, and granted the only testing I have is on ls1s with stock headers, i run a 3" Y, and we ran 2 exhausts on my pals GTO, a twin H 2" and a twin X 2.5". You guys might have had more but I was just trying to give some useful input)

http://www.installuniversity.com/ins...n11.122001.htm (2.2 cfm from David Vizzard - how to build HP)

http://ls1tech.com/forums/advanced-e...p-roughly.html (whole thread on this thing and yes I used the race engine upper estimate 2.2 cfm per hp)

http://www.pontiacstreetperformance....p/exhaust.html (real tests running true dual, and why I ran a 2.5 system on my pals GTO)

http://www.j-body.org/forums/read.ph...40299&t=439986 (quick summary)

http://www.superchevy.com/technical/...exh/index.html (the full write up here that is useful, think it may already be a sticky somewhere on the forum)

Im a physicist and know that I cant calculate it without concidering the flow velocity and more detailed calculations. The pipe walls and viscous forces only apply usually for liquids in pipes, but dont quote me on it) All I was trying to say was at some point, for a given velocity the larger diameter will be better. Its the same issue with porting heads, if you make them bigger area acts to make the CFM increase, however the velocity is slower so the CFM decreases. So for exhausts too big is bad on a small/low hp car and you will lose power. The size will also be optimized for a give rpm range where your engine and optimum exhaust flow are matched. More low down torque for smaller pipe but loses out at high rpm when the car is spitting out as much gas as possible. On the other hand a larger pipe is optimized for high rpm peak hp cars as the velocity at low rpms is slower and mans flow (CFM) if reduced.

I agree with you about the whole scavanging effect of tuned headers, the wave created by one pistons stroke acts to 'suck' exhaust gases out of the other primaries due to the equal length setup, however tuned is not going to make much difference as superchevy says. Only know a little about this here but like you said earlier they will not be outflowed.

I was just trying to help out and im sure everyones posts are apprichiated, but at the end of the day you would hit a limit with the Y pipe before a dual set up at a given power, and trues sound awesome

 

Cool, thanks for the links. I don't have time to read them right now, but I'll get to them in the near future. The main reason I brought up considering drag etc. (in all reality we should probably be more concerned about calculating the potential flow of a pipe with a compressible fluid) from the pipes was to illustrate that you can't just blindly throw out numbers that someone gives you and use that as a bible for flow. I'm not suggesting you take this as the bible of flow, but some may. I completely agree that he would outflow the Y-pipe and cat back before true duals, but is he ever going to actually benefit from the extra money he puts down on true duals? I would argue no, unless he has some serious engine modifications down the road I think he would be better off spending less on the Y-pipe and putting the rest of his money elsewhere. Just my $0.02.

 

Thanks for the mountain of information on this topic, guys. 95slvr, I planned on a complete engine overhaul. A 396 with about 450 RWHP.

 

Yep, it all comes down to the question of cash. Either you have surpluss to spend on duals, or you can make your own. I prefere the latter

 

I prefer that latter also but, one, I do not know how to or have the tools to bend pipes. Secondly, my friend has been talking to me about oval pipes instead of the standard circular pipe. Better ground clearance due to me already lowering the car.

 

My father is an engineer so he can fabricate the exhaust with me, but the bender we have kinks the pipe. Usually a steel shop will sell the parts, im sure there is plenty of places in the US where you can get elbows etc. It takes a bit of time to hold up the exhuast and spot weld it together to check the fit. the pics of the GTO about are is an exhaust we made and it was a full days work.

If you make duals and dump them before the axle or go under the axle there is not as much work involved

The oval pipes will give better clearance, however the x-sec area of a 3" pipe in the shape of an oval is slightly less than that of a circle.

 

I don't have a engineer at my bidding like you do, Maudy. However, I will just have my exhaust shop do it for me. I'll just watch and take notes. Thanks for the help, Maudy.