340 HP's vs headers

toolmanmike said
I have been following this old thread you revived. I just bought a pair of early HP 340 manifolds. I would also like to liven up my 318 in my Swinger. I have a LD4B and a assortment of 600-650 cfm carbs. I would like to follow up with TTI exhaust or at least get the front pipes from them and finish out the exhaust. This combo seems to help the teen's performance. I may put in a little cam while I'm at it.
I think with a stroker it would be best with headers of some kind. The engine is just a big air pump and a stroker uses quite a volume of it. It has to get out as fast as it came in. Keep us posted on what you do and the outcome. tmm

Headers are more about tuning the exhaust pulses than overall flow capacity. There are some magazine builds out there where they added a set of longtube headers to a stock 300-HP 5.9L Magnum crate engine and it picked up like 3 HP. Put the same headers on the 380-HP crate with the big cam and it's more like a 20-30 HP boost even though the displacement is the same. I liken it to tuning a 2-stroke; when you have both the intake and exhaust passages open to the cylinder at the same time (like in a 2-stroke or a 4-stroke engine with a high-overlap cam) the ability of the high-inertia exhaust gases to move quickly out of the cylinders and help pull more fresh intake charge into the cylinder is critical. A 408 stroker will work fine with manifolds, you just have to build the engine with that excessive high-RPM backpressure in mind so it will (in my mind) need a short-duration split-pattern cam with extra lift and duration on the exhaust side compared to intake. Also check out the episode of Engine Masters where they bash a set of headers to oblivion and it didn't affect HP at all, showing that the length and overall diameter of the tubes is way more important than the actual cross-sectional area of the tubes which is what everyone intuitively points at for making more power (bigger tube=flows more air).

Comparing it to intake restrictor plates isn't quite the same because in reality those NASCAR engines take advantage of the massive negative pressure exhaust pulses produced by the tuned headers to pull air through those tiny intake holes as hard as possible. When you mess with the exhaust side and take away that tuned scavenging affect it throws everything out of whack and kills massive amounts of HP. Granted most of what I'm saying comes from literature (David Vizard's How to Build Horsepower) not from my own testing unfortunately.