header primary size on a 496

Hi guys,
Whats everyones thoughts about this? 2-1/8, or 2-1/4?
Engine is a 496 with SCJ heads with a 780/720 lift cam, compression will be 13+, stock valves with a 30degree back-cut on the intake valves. and a light cleanup on the ports for now.
Also planning on step design headers with merge collectors. I don't plan on taking it over 7200 RPM
Thanks.
Al

72mav wrote:Hi guys,
Whats everyones thoughts about this? 2-1/8, or 2-1/4?
Engine is a 496 with SCJ heads with a 780/720 lift cam, compression will be 13+, stock valves with a 30degree back-cut on the intake valves. and a light cleanup on the ports for now.
Also planning on step design headers with merge collectors. I don't plan on taking it over 7200 RPM
Thanks.
Al

Header tube size is mostly reliant upon RPM level and the cylinder size.
That header is to big unless you intend to be above 7,000 RPM most of the time.

I'm with Randy. 1 7/8's are probably more than enough.

Too much cam, headers too big, too much rpm.jmo

bbf-falcon wrote:Too much cam, headers too big, too much rpm.jmo

Too much cam? The OP did not supply any duration figures.
13+ CR and almost 500" can support a reasonably "big" camshaft for 7000 rpm limit.

2" header primaries will certainly work fine.

Personally Al I like the 2 1/8 I figure it is always better to have to much header than not enough....

bbf-falcon wrote:Too much cam, headers too big, too much rpm.jmo

No cam yet. That 780lift is a guess,  somewhere 700 and 800 for sure though.

Another small motor member here was running a735 lift @7100 RPM. thought......

Btw, guys thanks for the info.
Regards.
Al

2" Primaries IMO.

bbf-falcon wrote:Too much cam, headers too big, too much rpm.jmo

Rick why you say to much RPM? I spun my stock offset crank 7800 for 10 + years, and still looked like brand new when it went East to Kentucky

Not saying it won't spin that far,just sayin imo that a pretty much stock scj head doesn't need that much cam or rpm's.

I think he has aluminum SCJs

That ain't no big cam..... and shifting at 7200 is about right for an engine peaking at 6700....


Now i know why you race dyno's instead of cars.

bbf-falcon wrote:That ain't no big cam.....  and shifting at 7200 is about right for an engine peaking at 6700....  


Now i know why you race dyno's instead of cars.

A step header with 2" to 2 1/8" primary tubes 31 inches long with a 3 1/2" diameter collector 17 inches long is the correct size for a 496 engine at 7,000 RPM ... on a dyno OR a drag race car.
USE a 2" straight tube header primary if the RPM level EVER drops below 5,000 RPM.

One thing I learned in 40yrs of racing is that just because my engine will rev to 8k, that's no reason to shift it there . and a dyno is not a racecar.