Pmccooey

Continuing my build and looking into the future and planning everything so I can do it now while the body is off. I've been reading and searching about the best exhaust for big blocks and keep getting different answers from different sites. So I'll throw it out there to some people that know way more about exhausts or have been down this road. It doesn't need to be model correct, not crazy loud where I need to wear ear plugs but something you can cruise with and still hear yourself think!

My 69 is a convertible. 454 Big block, Winters aluminum heads, Victor Jr intake, EFI, aggressive cam. It has no power steering as of now (might be putting it in if it fits), and putting in vintage air and electric fans. Motor should pull 500-550hp.

First which is better flowing straight back or side pipes? What size primaries? If side pipes are better I was looking at STS baffles. If straight back is better the largest exhaust I see is 2.5" without going full custom.

I'll thank everyone know for any help. Project is going great thanks to the help I've been getting!

69ttop502

Pat, I have been down this road with both under chassis and sidemount headers. Have spent a great deal of time testing on both an engine and chassis dyno. At your projected horsepower level, you could run either with little restriction. The problem becomes how much noise you can put up with. When you are running a muffled system, it is much easier to keep horsepower up and sound levels down with under chassis exhaust.

Under chassis headers have much more ideal (shorter) primary lengths than sidemounts. The sidemounts also have the large bend which limits flow. I had sidemounts with STS baffles cut in half, and uncapped, and at a little over 600 horsepower only lost 12 horsepower or so. The baffles are great, and tuneable. The problem is that it was way too loud for me. I then had a custom under chassis exhaust built including headers and a 3 inch system. Car ran much better and sounded great but not nearly as loud. My vote would be undercar exhaust for sure. A well designed, mandrel bent 2 1/2 inch system would run and sound great on your engine. I will admit the side exhaust looks great, but so does a nicely built under chassis system.

I would be glad to give you some dyno results if you are interested.

Bill

Pmccooey

Thanks Bill! Did you end up with true dual exhaust or use an x or h crossover?

69ttop502

My system has an h pipe just behind the transmission crossmember.

gkull

The truth is: side pipes are not a tuned length header. The primary length and diameters determine the range of rpm that the pipes actually increase hp

Under car 3 or more inch under car is easy to accomplish when you get rid of the stock pass through tranny cross member

69427

I'm running a 3" system with an H-pipe right at the header reducers, with a set of Dynomax turbo mufflers for good flow and a pretty quiet exhaust note. My pressure gauge shows 9" H2O backpressure @ WOT with this system.

Alex66

Does a 3inch System fit thru a 71's stock welded in crossmember?

3X2

I see a lot of you are advocating under car headers. If they are better why did all the AP and BP Corvettes use side exits? Just curious to the reasoning, not start a header war.

69ttop502

Sidemounts are lighter and the hp is there without the mufflers which the race cars did without. But the primaries on the Hooker type headers are 45 inches long, collectors are 4 inches in diameter and have a huge 90 degree bend. All of those specs are far from optimal. The best sidemount header for making horsepower would be the Stahl design, which many of the race cars ran. Those primaries are 26 inches I believe and the collector is smaller although still far too large.

And yes 3 inch will fit through the crossmember holes. I had my exhaust system built by a guy who has done a lot of race car systems and he built my system for a 650-700 horsepower motor with a 2 1/2 inch choke merge collector which he said would be good into the 800 hp range. So on mine he ran some 2.75 inch pipe from the collector through the holes to 3 inch out the back. Definitely learned that the bigger the better mentality prevalent on the internet regarding exhaust sizing does not make a faster car.

mrichi

I stuck with standard cast manifolds , gives more room in the engine bay, unless you are using the engine high up in the rev range not much to gain in the lower rpm's

69427

It fit in my '69, but ya gotta get the pipe centered so it doesn't bang around with small engine movements. I eventually replaced the stock crossmember with an aluminum version (to pull out weight) and made the pass through holes 4 1/2 inch diameter for additional pipe clearance.