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911st 911st is offline
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Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
re: primary and secondary sizing.

Originally Posted by RarlyL8 View Post
...This is one instance where applying N/A flow theory will hurt you. Size the headers too big and you lose velocity. It's a balancing act (litterally) between back pressure and velocity...


...Primary and secondary ID are critical to velocity. I use 1.5" primaries and 2" secondaries up to 450WHP and 1.625"/2.25" beyond that...

Spit balling again.

I am not suggesting we just use N/A flow quantities for the header sizing directly. That would not make sense. Just that we interpolate from the N/A numbers if we do not have any solid numbers for turbo'd motors.

For example, if we can flow say twice the air through the intake using pressure. Maybe the same applies to the exhaust side as, at least as a starting point.

As a point of reference 450whp might be compared to 225rwhp on a N/A motor. This seems to fit well.

I had no issue making 217whp through the stock 1.5" headers on my 3.2 Carrera and a sport muffler would have put me at near 220whp.

This worked well on my car but testing by SW confirmed that steping up a size would good for about 10 hp.

Not saying the relationship will be as direct but there may be a strong correlation as long as there is not something else that is going to create a restriction like the turbo hot side or compressor moving out of its efficiency range.

Before anyone making near 450whp runs off and buys a set of larger headers they might note that if the motor is still a 3.3 with SC cams how much might this hurt where the achieve full boost.

If one increases the size of the primary tube about 20% and the secondary tube about 28% velocity is going to slow down around 22% or so. Also the volume of the headers will increase about the same 22% or so.

This could push boost response back in as much as 600rpm (using 3000rpm as the starting point).

Now, bump the displacement from 3.3 to 3.5 and boost response should come in sooner by about 200rpm.

Also, I bet that if a 91/2 3.3 turbo comes on full boost by 3000rpm, a 93 3.6 that uses the same turbo will probably come in at about 2750rpm.

Going to a 1 7/5 from a 1.5" on a 3.3 and boost response could move back as much as 1200rpm or so. Great maybe for a race car if it has the heads & turbo to support it.

Thus. it might be better for most of us to stay with a smaller size to keep drive-ability even if 10-20 hp might be had with a larger size.

Just a thought.

Last edited by 911st; 01-17-2010 at 09:32 AM..
Old 01-17-2010, 09:28 AM
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