Just so I'm able to kind of walk on my own when trying to figure out something like this,

What factors do you take into consideration when trying to figure out header size? And then how do you know what size tubes, etc to choose?

Thanks..

If you go to www.headerdesign.com (http://www.headerdesign.com) you can see what data they require for a header design. You need to sign in before you can use the calculator.

I have a program called Pipemax that calculates header designs and it asks for quite a bit of info like:

bore, stroke, rod length, valve configuration and size, peak power rpm, cam timing and lift.

Unless you are building your own, which I assume that you are doing, the only factor is what is available.

We just bought a set of Patriot headers for our 1969 sbc. The biggest (maybe the only size) that we could find was 1 5/8." I would have preferred 1 3/4" but the smaller are probably fine. For a big block you will need larger.

Taylor,

There is a wealth of information on this website that should answer your questions

http://headersbyed.com/

Enjoy,

Gaz

Headers like carburetors,,,smaller is usually better graemlins/thumbsup.gif

Heres a answer racer1320 gave on another board. I would say his answer is pretty close.
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Most everybody chooses headers that are too big right along with cams and carbs.

Smaller is better!

I also don't agree with the notion of sizing header primary tubes to cubic inches.

Primary tube size should be sized based on the HP a motor is making which in turn dictates where the powerband is/peak torque is achieved.

When you increase primary tube diameter you raise the RPM at which peak torque occurs!

500HP is 500HP regardless of whether it's made by a V6 or V8, 350 or 454 because it takes X amount of hot air to produce 500HP!

So do the math. My motor is making about 650HP at the flywheel and peak torque occurs about 4700 RPM.

Established trends tell us that for every 1/8" increase in primary tube size, we raise the torque peak 500 to 1000 RPM. Same trends reveal that for every 1/2" increase in collector size we raise the torque peak 250 to 500 RPM. So as you can see primary tube size is everything.

Length also comes into play but not to the extent of primary pipe diameter.


Here's my guidelines:

500HP and < use 1 5/8"
525 - 625HP use 1 3/4"
625 - 700HP use 1 7/8"
700 - 775HP use 2"
775 - 850HP use 2 1/8"
850 - 925HP use 2 1/4"
925 - 1000HP use 2 3/8"

So you're saying ol' Ed would assume that my 500 hp one lunger needs a 1-5/8" tube? I don't think so. I agree with the assumption that most headers are too large in diameter, but there's a whole lot more to it than picking a number from a chart.

We're really not talking about diameter, we're talking about cross sectional area, and differences in area are quite large. The difference in CSA between a 1-5/8" tube and a 1-3/4" tube is about 17%. The difference between a 350" motor and a 410" motor is 17%. The difference between 9:1 compression and 10.5:1 compression is 17%. 17% is a lot of difference. Ed Henniman has some charts showing the CSA differences between tube sizes and the cost of making what he calls a 'two tube size error'.

Length is also more important than he makes it out to be. Would the headers from an 8500 rpm 283" Super Stock engine be the same for a 421" bracket motor making the same power at 6000 rpm? Probably not. The diameter would be similar, but the length would be quite different.

Would your engine be better off with an interference design or an independence design?

Since we are putting in our .02 worth, you also get to choose between shorties and long tubes. We went with shorties because of the ground clearance issue.

onovakind67...have you tried the smaller tubes?