Re: [Chevelle-list] Exhaust System

[Craig Ellis]

Just my two cents but the old axiom "you drive torque and race horsepower" is true. Unless you actually bet folding money on street races or a drag strip regular, I'd choose the options that produce torque. I've got a BBC 402 that's .060 over - that's only 413 CI. Not to offend anyone out there with a BBC but that is a smallish motor and does not justify 3" exhaust or 2" headers. All that stuff seems pretty cool but even the calculations argue against those sizes. I know that my motor will spin to 6500 but rare will be the day that it does. I routinely shift at 5500 and once in a blue moon spool it out to 6000. 1 and 3/4 headers and 2.5 exhaust produce the ideal torque and HP for a .550 lift cam. Note how many 400+ CI small blocks with 1 and 5/8 headers and 2" exhaust are written up in HotRod and other mags. This subject has been studied for years and every time the results are the same. Once in a while one of the big motor builders will "reveal" that they put a 1000 CFM Dominator on a SBC with an RV cam and it ran like a scalded cat. You know as well as I do that somebody is zommin' somebody. I don't know about you, but every engine builder I've ever known would BS their mother if it won them more business or won races. If a motor combination sounds too good to be true...it probably is. Stick to the calculations and run the recommendations and you won't regret it.   Speaking from experience - the only "mistake" I made is to put a 770 CFM Holley Street Avenger on my motor. It runs perfect but I am sure that a 670 CFM would have been the better choice. That motor just can not pump that much air. It's like octane - any more than is needed to eliminate preignition is wasted - and more carb volume and exhaust volume than needed to balance the pumping action of the motor is detrimental to performance. I was just another one of those guys that didn't want to stand around the car show and tell people "it's a 600 CFM Holley Wimpalator". But the fact is, the calculator said, as I recall, my engine should have a 600 - I just couldn't buy one.   Just my opinion.   ----- Original Message ----- From: Larry Shouse To: The Chevelle Mailing List Sent: Tuesday, November 29, 2005 8:48 AM Subject: Re: [Chevelle-list] Exhaust System Thanks to everyone for your input. I spent a few hours going to all suggested websites, trying to learn what I can. Personal experience based input is very important to me.   It looks like we are pretty evenly split between 2.5 and 3 inch exhaust systems. I've heard (and please correct me if I'm wrong) that having too much pipe could be detrimental to your torque numbers, because you need a certain amount of back pressure to scavenge (pull) the exhaust out...   I guess the real question then becomes does my 396 with a Performer RPM Airgap intake and L78 heads redlining at 6500 - 6800 rpm move enough air to warrant 3 inch pipes. Is there a formula out there to figure that kind of thing out? Of course I'm willing to pay for the 3 inch system if it sounds better and has no detrimental effect on my numbers <grin> I found Clint's comments interesting, using 2" head pipes and reducing down to 2.5" after the muffler...   Thanks for putting up with my persistent stream of questions here. As I dump all available dollars into this project while dodging my wife's broom, I want to keep my future regrets to a minimum.   Thanks, Larry Shouse  ----- Original Message ----- From: Larry Shouse To: Chevelle List Sent: Monday, November 28, 2005 4:06 PM Subject: [Chevelle-list] Exhaust System It's time to buy me an exhaust system. The system will go on a fairly healthy normally aspirated 396 running L78 heads and Sanderson intermediate headers with 1 7/8" primaries and 3" collectors. Those of you in the know, please help me decide whether I should go with a 2.5" system or a 3" system. Also, brand suggestions would be great.   Thanks, Larry Shouse