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Old 09-30-2019, 05:53 AM
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Default Headers size

Hi: What size headers would be required to support a 750 hp engine? Round port.



Looking at a set of Doug's 1-7/8" for a 68 Firebird.


Also wondering if they will fit a 68 Firebird?



Thanks


https://butlerperformance.com/i-2445...tegory:1234810

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Old 09-30-2019, 06:05 AM
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Here's some base line info for you.
The main concern before Exh flow numbers though is that the inlet to the header tube is bigger then the Exh port outlet flange size by atleast 1/16" and preferably 1/8", especially on the floor of the port which is not a issue if the round ports are the better working D shaped version .

How many cid is the motor?
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Wernher Von Braun warned before his retirement from NASA back in 1972, that the next world war would be against the ETs!
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Old 09-30-2019, 08:44 AM
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That graph os useless in my opinion. If you follow that graph then ci and rpm jave nothing to do with exhaust size. According to that graph our GTO should be running roughly 1 3/4" headers, the car was 2tenths slower with that size header vs 2".

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Old 09-30-2019, 09:02 AM
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Chuck I am running 1 7/8 on my pos

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2017, 74 firebird -3600 lbs (all bests) 1.33 60 ft, 6.314@108.39 9.950@134.32
M/T 275/60 ET SS Drag Radial

2023,(Pontiac 505) 1.27 60 ft, 5.97@112.86, 9.48@139.31.... 275/60 Radial Pro's
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Old 09-30-2019, 09:07 AM
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Quote:
Originally Posted by grandville455 View Post
Chuck I am running 1 7/8 on my pos
That's probably the problem

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Old 09-30-2019, 09:31 AM
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Personally I think 1 7/8" headers are going to be marginal for 750 HP. A few years back I dyno'd a ~ 550 HP engine that was 461". We initial used 1 5/8" headers because he was going to use Ram Air manifolds on it in the car and we thought they would be similar in flow. After getting a base number, we swapped to 1 3/4" headers and the engine picked up 18 HP. I think at 750 HP you would be giving up some power versus 2" tubes. Just a fyi, I recently built a 535" that made 872 HP and we used 2" headers. Myself, I run stepped headers 2 1/8' to 2 1/4". Engine made 1039 HP

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Old 09-30-2019, 09:33 AM
Steve C. Steve C. is offline
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I don't know how useless it is but that formula posted above was published by David Vizard. As many know much of his work has been centered around Chevy engines, especially the 350cid. Therefore I've often wondered if it might be canted in that direction.

That said I've taken note of it and worked the formula around my Edelbrock heads and with porting the exhaust port ended up towards 250 cfm at the upper lifts. As noted David's formula is based on the INSIDE diameter. I run 2-inch headers (outside diameter) on my 505 that makes peak power at 6000 rpm.

Here is another one below that tends to back that up.

How Header Primary-Tube Diameter Affects Performance ?
Tech article from Hot Rod magazine:

https://www.hotrod.com/articles/head...s-performance/

2-inch outside diameter less a typical wall thickness.

Example- 16 guage = .062" x 2 = .124
or 1.876" inside



.

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Last edited by Steve C.; 09-30-2019 at 10:17 AM.
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Old 09-30-2019, 10:57 AM
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You reply that way every time I post up that info slowbird so why don't you post up your guide lines and be more helpful.

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Wernher Von Braun warned before his retirement from NASA back in 1972, that the next world war would be against the ETs!
And he was not talking about 1/8 or 1/4 mile ETs!

1) 1940s 100% silver 4 cup tea server set.

Two dry rotted 14 x 10 Micky Thompson slicks.

1) un-mailed in gift coupon from a 1972 box of corn flakes.
Two pairs of brown leather flip flops, never seen more then 2 mph.

Education is what your left with once you forget things!
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Old 09-30-2019, 11:17 AM
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Quote:
Originally Posted by steve25 View Post
You reply that way every time I post up that info slowbird so why don't you post up your guide lines and be more helpful.
Buy pipemax, very cheap software, tons of info, and will not lead you down the wrong path. No header builder that i've talked to has ever asked me for exhaust flow numbers.

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Old 10-01-2019, 06:11 AM
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Quote:
Originally Posted by PAUL K View Post
That's probably the problem
Paul, LMAO!

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74 Grandville 2Dr 455 c.i 4550#
2011 1.60 60 ft,7.33@94.55-11.502@117.74


2017, 74 firebird -3600 lbs (all bests) 1.33 60 ft, 6.314@108.39 9.950@134.32
M/T 275/60 ET SS Drag Radial

2023,(Pontiac 505) 1.27 60 ft, 5.97@112.86, 9.48@139.31.... 275/60 Radial Pro's
  #11  
Old 10-01-2019, 07:08 AM
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Thanks guys..

Charles

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Old 10-01-2019, 08:24 AM
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Quote:
Originally Posted by slowbird View Post
Buy pipemax, very cheap software, tons of info, and will not lead you down the wrong path. No header builder that i've talked to has ever asked me for exhaust flow numbers.
Correct. EXH travel length & ID for best "CFM" (try CFS) is thermal-dependant.

The thermal dependancy is CSA & Length dependent. These tings show up as EXH velocity vs travel length.

Id be keen to see a plot of EXH dia vs RPM for best ET. Could be an eye-opener.

  #13  
Old 10-02-2019, 06:49 PM
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I like the Wallace racing calculators.

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