Sizing headers

[eastyorker]

About to get new headers for my '69 Road Runner with a 440. After looking over the various choices i am faced with what size I should get. 1-3/4,1-7/8 or 2. Think anything over 2 would be too big.

Car is stock with I think 3.23 rear, 4 speed. Engine is a 750 Holley, aluminum intake. Mopar Electronic ignition. Thats all I know about the engine.



Thanks
Darryl

 

[WileERobby]

For what you stated, 1 3/4 .

 

[coloradodave]

X2...

 

[eastyorker]

What determines size? Is bigger better or worse ?

Thanks

 

[WileERobby]

What determines size? Is bigger better or worse ?

Thanks

Depends if you're a female or a car. Seriously, though... without getting into the engineering formulas, primary tube diameter and length, as well as collector diameter and length, must be tuned to the engine. Scavenging and resonance must be matched to the ability of the engine to move air. Greatly simplifying, the 1 3/4 header will provide velocity and scavenging over the stock manifolds on a stock engine for street use mostly under 5500 rpm.

 

[Propwash]

For what you stated, 1 3/4 .

X2...

X3

 

[Meep-Meep]

A good way to size the primary tube is 1/8" larger than the exhaust valve. Reason is the boundary layer inside the tube effectively makes it smaller. 1.74" valves go with 1-7/8" primaries.