What size primaries (exhaust) ?

[GregP]

I'm starting to line up the stuff to fab the headers and side pipes. Since the block is in (and not much else) thought I'd do them while I have room to work around stuff. QUestion is how big of primaries to go with. Most Shoenfeld racing headers are using 1 5/8 dia. I can get collectors for 1-5/8, 1-3/4, and 1-7/8. I know 1 7/8 require either "squaring" the pipe or running double flanges to get the bolts to clear. I'm "planning" to end up with something around 500HP and 396 cu in (small block). I run 2 1/8 primaries on the big blocks (460 cu in at ~475 HP), so am thinking I should go with the extra work and setup for the big 1 7/8 tubes. What think you all or what size are you using for high HP mice? -Greg

[427Hotrod]

If I was going to all the trouble to make headers, I'd use the 1 7/8". 500 hp isn't into the radical stage yet as far as HP but 396" is getting up there.

What RPM are you planning? If it's a big deal as far as clearance I don't think the 1 3/4"'s would kill you at that hp level. In fact, if you don't plan on upping Hp or spraying they might be the best choice. I just know me, and I ALWAYS end up looking for more HP. If you decide later to add a blower or whatever, it would be a shame to be choked up.


Jim

[gcrouse]

A good compromise might be 1-3/4"; the physical fit is easier though they are squared and you can still barely get allen head bolts in there. The size keeps exhaust velocity up for broader torque. I run Hooker SuperComp 1-3/4" on my supercharged 383 at similar power levels to yours.

But 1-7/8" is better if you want to shift the powerband upward- to reduce low-end torque induced wheelspin, and make max top-end horsepower. Since you're fabbing them you could do a nice tri-Y and get the primary lengths alot more even than off the shelf stuff.

Be sure your heads have big valves and great flowing exhaust ports though; or those giant headers are a waste.