Header Primary Lengths



Preparing to build the exhaust system on my GT40. Everything I've read about header construction theory says it's important that primary tubes be equal length. In spite of that I've noticed that most existing GT40 headers I see have unequal length primaries.

Is there something I'm missing here?

Does the positive crossover effect overcome the negative unequal length primaries?

Are the unequal lengths of a certain percentage? For example with an optimum primary length of 32" would shorter primaries be 28"?

How much HP is lost with unequal length primaries?
the exhuast extractor effect is less critical than inlet tuning: it works over a wider rev range. Small differences in length will cause small differences in the range tuned revs for each cylinder, and unless you run a fully mapped EFI, the engine tune will also be slightly compromised to suit all of the cylinders all of the time.
What the varying lengths could possibly do is margiinally widen the torque curve, but reduce the peak: but these effects might only be a few percent, which may not be an issue for you



Like you I always thought that primaries need to be equal and I still think that. I suggest that practicalities of making them and fitting them in the engine bay etc spoils the ideal situation. Of course you could spend a fortune on a mega system that would look like F1 or CART systems and get equal lengths.


Talked to Vince at Burns Stainless (exhaust builders for Nascar etc.) says up to 5% loss with unequal length primaries. Planning on 400hp so that's 20hp.

Another thing I'm confused about is I thought the primaries were arranged in a circular pattern according to firing order. Looking at the original parts manual and a MarkV the left collector is correct but the right collector isn't.

with a 1-5-4-2-6-3-7-8 firing order

the left collector is 5-2-3-8 in a circle,
the right collector is 1-4-7-6 in a circle. when it should be 1-4-6-7

I also need to find out if the new 302 ('90) has a different firing order?


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1-5-4-2-6-3-7-8 (289/early 302)
1-3-7-2-6-5-4-8 (351/later 302)

What's your time frame for putting these together? I'm about to do the same.

409 is an alloy designed principally for the automotive exhaust industry, although it has been used successfully in other industrial applications. It combines good elevated temperature corrosion resistance with medium strength, good formability and overall cost.

Grade 409 resists atmospheric and exhaust gas corrosion. A light surface rust will form in most atmospheres; this rust retards further corrosion but makes the surface undesirable for decorative applications.


Neal, Just ordered some 304 stainless pieces yesterday (22nd) from Stainless Works(Ohio) and Burns Stainless(CA). Ordered less than I'm going to need, want to get started so I can get a better idea what's needed. Looks like about $700 to $800 just in parts and materials, just for the headers/collectors. No wonder stainless isn't commonly done.

And lets not even talk about 2 $275 racing cat.conv's and 2 $150 supertrapps.

And if you got it all from Burns Stainless you'd be looking at $1500, including "merge" collectors at $336 each. This is top line racing stuff though. I got some 4" radius bends from them 'cuz no one else has them.

Decided on stainless 'cuz it's so much work building headers, only want to do it once. And it's the nicest for maintainance, it can be left bare and although it turns colors like chrome, unlike chrome you can clean/polish it back to normal. Stainless also does not transfer heat as much as steel, so insulating coatings are not neccesary. More heat is retained thereby increasing gas velocity. And stainless is more heat resistant strength wise.

Of course you could step up to titanium or Inconel, with Ti starting at $25 per foot unbent. <grin>

top end racing stainless, best selection, GOOD TECH ARTICLES

best prices, good selection

http://www.holley.com/HiOctn/ProdLine/Hooker.html, limited stainless, lots of mild steel, including flanges, can be purchased through Summit

mild steel only, big selection

mild and very limited stainless

figured out the firing order with the later engine. Not much different from the original. Going to swap 6 and 7 in the pattern to get the rotational scavenging. The different firing order means the rotational direction is opposite. This is good, means you can pattern off the original headers (nice drawing in the original parts manual), changing 6 and 7 doesn't look too hard.

[ October 23, 2002: Message edited by: Kalun D ]

[ October 23, 2002: Message edited by: Kalun D ]


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Thanks for the info. I have a set of Shoenfeld 1 5/8" primary mild steel 180 degree headers in transit. These dump to the passenger side.

I'm planning on sectioning in material to bring the collectors to center. $300 in materials. I'll have them coated if I can pull this off. It's a bit of an experiement but, not being a welder, I wanted to minimize the amount of fabrication. Stainless would be nice...
Hmmm... Interesting. Just checked out the Burns Stainless page. CAV supplies stainless steel headers, but they do recommend having them coated. Mine are at HPC (only about 15 miles up the road) right now being done. Also, the CAV pipes must be 400 series (or not actually stainless in my case), as they are magnetic (though the monocoque is not)...

[ October 23, 2002: Message edited by: Steve Toner ]


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Here is a link to
header pictures. I think these are a good starting point. It will be a couple weeks before I can report results.

Shoenfeld has tried to maintain primary lengths on these. These are 1 5/8" primaries.