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Discussion Starter · #1 ·
I have a spare JDM exhaust header on my bench and it looks like a bit of grinding here and there might improve it. The welds at the port flange are on the inside and protrude into the pipe a bit.

At the other end of the primary, where the primary pipes join from 4 to 2, the pipes have been formed into "D" sections with the flat side of each pipe against it's pair. They are welded together on the outside.

This shaping of the pipes reduces their cross-sectional area by about 1/3rd.

Looking for opinions on what effect it would have if I ground out the partition formed by the 2 "D" sections. That would restore the full area to each pipe but their flows would intersect slightly rather than flowing parallel into the collector.

Wish I could post pics :(

Opinions?
 

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if it is the same as mine, then the secondaires r very short, widen them to about the firewall, they r a good piece and only a few more hp would the hotshot register over it if so much
 

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Bitter Old Man
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The flows won't really intersect; due to our slow chemical internal clock, exhaust flow appears to be continuous while in fact it's pulsing. No two primary pipes have pressure pulses at the same time, so there isn't a fluid dynamics merge issue, per se. I don't think you can go wrong with your plan other than comprimising weld strength.
 

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Discussion Starter · #4 · (Edited)
Dave Deen said:
if it is the same as mine, then the secondaires r very short, widen them to about the firewall, they r a good piece and only a few more hp would the hotshot register over it if so much
Thanks Dave,

Yes the secondary pipes are very short before merging. What effect does lengthening them have?

It's not so easy to do for my car since it is AWD and the secondary in it follows a different path to clear the transfer case. However, my wife's wagon is FWD and I'll keep it in mind if I use the spare header to replace her cast iron manifold.

I like the factory piece too. For one thing, I have never heard of one cracking - something that cannot be said for any of the aftermarket headers, except maybe the recent stainless HS copy for which time will tell.
 

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Discussion Starter · #5 ·
Slartibartfast said:
The flows won't really intersect; due to our slow chemical internal clock, exhaust flow appears to be continuous while in fact it's pulsing. No two primary pipes have pressure pulses at the same time, so there isn't a fluid dynamics merge issue, per se. I don't think you can go wrong with your plan other than comprimising weld strength.
Very nicely worded :D

If I grind the partitions out, the pulses will then flow slightly across the exit of the other pipe for each pair. I was wondering what effect this would have on scavenging. Also, whether the effective length of the pipe shortened if it ends sooner on one side and not the other.

Hopefully the weld strength wil be ok - I plan to stop grinding well before I reack the weld.

Thanks,

Sandy
 

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alot of the torque is in the secondaries and i just copied what the aftermkt makes and sells,
i used 1 7/8" secondaires which is what approx comes out from the primaries and damn right about its strength
 

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i too am working on making a jdm header work on my g20. i have the flange made and was curious about the secondary size and you recommend 1 7/8".... holy crap! and would you guys know what angle the pipes need to be bent to to go under the oil pan and to a collector? you guys think a 2 1/2" collector is a good size to go with? TIA, matt
 

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yup that would work with the rest of the exhaust being 2 1/4", my next upgrade
mandrel would be the way to go but they weren't available for me at the time so i used muffler shop bends and because of the angle involved, they weren't crushed that bad so it all worked out well.
 
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