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  #1  
Old 12-06-2009, 08:34 PM
lxrubin lxrubin is offline
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Default Exhaust size for a 425 small block

Were going to go with the 425 stroker kit form ohio crank and expecting about 45ohp. What's a good header and pipe size? The largest one's I've seen for a 340/360 are 1 3/4.

Currently the car has 1 5/8 headers and a 2.5" x pipe but I'm worried that wont be enough.
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Old 12-06-2009, 08:50 PM
cudabob496 cudabob496 is offline
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1 and 3/4 would be better, but I don't think the 1 and 5/8, with the 2.5" X pipe will hurt you that much at all, if its a street car. May make you lose 10 -15 hp at 6000 rpm. Make sure the header gasket is not interfering with exhaust flow, and port match the header to the head. I had 1 and 7/8 hooker supercomps on a 500+ horse 440 and they did the job.

The quality and design of the header is equally important to the diameter of the tubing. One of the reasons Hookers are popular (among other reasons).

http://headerdesign.com/extras/design.asp
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Old 12-06-2009, 11:17 PM
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Ray Bell Ray Bell is offline
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What a great site, cudabob... thank you!
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Old 12-07-2009, 12:19 PM
peg leg peg leg is offline
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Default look at

headers that have a larger diameter after the port tubing. In other words, 1 3/4 to 1 7/8" . It helps scavenging.
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Old 12-07-2009, 02:17 PM
DartGT66 DartGT66 is offline
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We tried two different headers on a stroekr SB making 450+ hp, 2" primary fenderwells and 1 5/8" El Cheapos that had a couple of primaries smashed to the pavement. The difference in peak power was around 10 hp, however the 2" were totally worthless below 3500 rpm while the engine pulled cleanly from 2000 with the smaller tubes. Both were used with full 3" exhaust with mufflers. I would say that it's about a tie between the 1 3/4" and 1 7/8" primary. If you want better lowe end go with tie 1 3/4" and for better top end 1 7/8"
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Old 12-07-2009, 07:32 PM
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Ray Bell Ray Bell is offline
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There's an awful lot of explanatory information on that site cudabob cited that many could learn from...

It goes into the reasons why smaller pipes will help mid-range and so on. Anyone contemplating getting or making headers should look at it.
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Old 12-08-2009, 03:45 AM
DartGT66 DartGT66 is offline
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Didn't look at the pages, but if I have built headers myself, the theory has always been to size the headers according to the RPM where it drops ahen you shift upwards. That's teh RPM where the engine stays the longest time so I've used it with decent success, instead of shooting for peak power.
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Old 12-08-2009, 08:48 AM
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There's a calculator there that enables you to work out the diameters and lengths that work best for all sorts of applications...

You should have a look.
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