header suggestions

gestalt1 · 05-22-2007, 07:21 PM

hi all, i built but have not started yet a 3.2 short stroke. i'm trying to decide on an exhaust system. the engine has 9.8:1 CR, DC62 cams, heads ported to 40mm and 46mm individual TB injection, etc. the car will be a street legal track car so high rpm power is more important than low end torque. i have a set of 1 5/8 (1 1/2" ID) full length headers and a set of george headers with an ID of 1 3/4". with the full length headers i was going to run a 2 in 2 out muffler, with the short headers i could run some type of glass pack 24" long individual mufflers. are the shorter george headers too big? my primary rpm range will be 4500 - 7500rpm.
please give me some opinions.
thanks, matt

Steve@Rennsport · 05-22-2007, 08:13 PM

Matt:

The real decision about what headers to use involves your gearing as that defines where your torque spread will be,...

Got close-ratio gears?????

Offhand, I'd use the 1 5/8" ones.

gestalt1 · 05-23-2007, 05:43 AM

the gearbox is stock 72 ratios with a 7:31 R&P. I find it odd to make the effort porting the heads and then bolt on headers that are 5mm smaller than the port size. the gaskets are 39-40mm, why wouldn't primary tubes of the same or greater diameter be better?
thanks

euro911sc · euro911sc's Garage

Steve can certainly tell us more on this, but I think I remember reading there is some need for back pressure in a normally aspirated engine to help scavenge the combustion chamber. How this works I'm not sure on. A reason no one had 3" ID primary headers I suspect... This is the opposite in a turbo application where the differential in pressures across the turbo blades directly affects the speed of spool.

Not hijacking the thread here, but your choices beg a question I have been wanting to ask lately as well... When I look at domestic V6/V8s (don't shoot me) they join their exhausts in an H or X and 911s do it in the muffler or pre-cat for some. I also have a set of short headers and long headers for a 3.0 I'm building that I must choose from. If Matt and I were to go with the short headers and glass packs/megaphones/other straight pipe setup we will not be joining the streams. Does it matter? Good/bad/better high end/low end...??

Best regards,

Michael

gestalt1 · 05-23-2007, 12:25 PM

in my research (using the search function on this bbs) i've found that the larger the diameter and shorter the length of the primaries the better the high end horsepower. the longer smaller primaries help low to mid torque. this is assuming the primaries are appropriate for the engine size and cam profile to begin with. a 3.2 short stroke with hot cams an 1 5/8" OD (38mm ID) headers are recommended, 1 3/4" OD (41mm ID) not unheard of. i have the 1 7/8" OD (44.5mm ID) and this is larger than optimal but 1. i have them and B. i'm not sure how negative the effect of having primaries slightly oversized will be. i think it may negatively effect the low end torque, but the high end power may be the same, if not better. i am sure having a 40mm port meeting up to 35mm header primaries would likely make the exhaust porting pointless because of the turbulance from the step at that connection.

As far as cross overs and mufflers it is my understanding the crossover pipe should be 80% of the collector diameter and its placement in the exhaust system must be right for the crossover to benefit power. just having a crossover pipe is not the whole story. the typical 911 muffler does not act as a crossover, all will have some droning at some rpm range. having separate mufflers or megaphones supposedly eliminates this droning.

all of this info is based on what i've learned on this board, i'm no expert beyond that and have no dyno results to back this up. so, if it were your engine, what headers would you use?
thanks

euro911sc · euro911sc's Garage

Great info. I'm using the short headers, because:

1) Like you, I have them.
2) They are bigger, 1 5/8" vs. the 1 1/2" longs, and this is a track car so more on the high end is desirable for me.
3) I got them for virtually nothing and can sell the long ones to facilitate buying the mufflers/super traps/glass packs/etc.

I do have the extensions as well so I may dyno both and see what happens... but that is a long way off as my motor is still torn down completely.

Best regards,

Michael