FE Primaries- How To Determine 1-3/4, 7/8, 2" 6"

[427HISS]

What size responds better for HP & TQ and why ?

I'm going to fab up a pair of new 304 stainless headers. My origional pair are 2"mild steel that came welded and ceramic coated.
The welds sucked and the coating lasted only 4 years. A friend and I built 304 highly polished stainless steel side pipes, but the product cost was too high, so the venture was not worth the time and labor.

My new stainlesd pair should last the rest of my life and the Cobra's. Should I stick with 2" ?

[scott foxwell]

What size responds better for HP & TQ and why ?

I'm going to fab up a pair of new 304 stainless headers. My origional pair are 2"mild steel that came welded and ceramic coated.
The welds sucked and the coating lasted only 4 years. A friend and I built 304 highly polished stainless steel side pipes, but the product cost was too high, so the venture was not worth the time and labor.

My new stainlesd pair should last the rest of my life and the Cobra's. Should I stick with 2" ?

How much HP are you dealing with?

[427HISS]

Somewhere between 500-550.

[hwoods]

https://www.racemufflers.com/Ford-FE-428-High-Rise-Custom-Header-Build-Kit-p/ford-fe428-na-hbk.htm

[WerbyFord]

Somewhere between 500-550.

There is no sure recipe that I know of although there are many "theoretical" equations. I say "theoretical" because the thing about theories is, they almost always assume a bunch of stuff and ignore other stuff. 2" seems about right. The Gonkulator says 1-7/8 might cost you 10 hp, but if it makes the bends less severe it could be less than that even.

Partly too it depends on the lengths, primary and secondary, and those in turn depend on the cam. Youre in the ballpark for sure at 2".

[TomP]

It could make more power with smaller but that looks funny having small tubes on a Cobra.
 For what it's worth I made 2" tubes 48" long on my ramp truck 428. Figured the long length would make up for the big diameter.

[plovett]

Agreeing with the Werb.   I dynoed a 517 hp 428 a long time ago and tried both 1-3/4" and 2" primary headers.  I remember the 2" headers being better throughout the rpm range.  I think about 10 hp just like Werby said.  I could probably find the dyno sheets.

I think the "conventional wisdom" is 2" primaries are too much for an engine like that, but I don't think it is necessarily so.

paulie

[plovett]

My 2" headers also have very long and very unequal length tubes compared to the 1-3/4" headers.

The 1-3/4" headers were Hookers with 3" collectors and the 2" headers are Crites with 3-1/2" collectors.

paulie

[andyf]

If you really want to know then buy a copy of PipeMax and input your numbers. PipeMax doesn't cost that much and it is very useful for all sorts of engine simulation questions.

Another way to look at the question is to ask how big your exhaust valve is and what size is the exhaust port? 2 inch is probably bigger than you need but it would work. Something slightly smaller would probably work even better but it sort of depends on how much time you want to mess with it. Are you going to make your own flanges or buy something off the shelf? Are you willing to build step headers or do you want one constant size?

[scott foxwell]

Somewhere between 500-550.

Wouldn't go any bigger than 1-7/8" x 3" collector. 1-3/4 are going to hurt you a little. If the choice is between 1-3/4 and 2", I'd go with the 2". The Cobra should have some fairly straight tubes off the heads, no real tight bends like a shock tower car.
These are headers I built for our Cobras...

[TimeWarpF100]

What size responds better for HP & TQ and why ?

I'm going to fab up a pair of new 304 stainless headers. My origional pair are 2"mild steel that came welded and ceramic coated.
The welds sucked and the coating lasted only 4 years. A friend and I built 304 highly polished stainless steel side pipes, but the product cost was too high, so the venture was not worth the time and labor.

My new stainlesd pair should last the rest of my life and the Cobra's. Should I stick with 2" ?

Ones I did for a Contemporary Cobra. We found 100 HP over what was on it.
A lot more in muffler than pipes if you use the typical issue Cobra mufflers.

[TimeWarpF100]

Cobra done with a original R code Comet & a Cammer 32 hiboy

On my 66 482 truck I used pipe max. Best combo was a step
Header 1 7/8 to 2” fairly long. Exactly 12” length on 1 7/8 rest in 2”.