Hi everyone.I need help with choosing header primary size for my 73 javelin that will be set up for recreational road course use. The engine will be a 10.6:1 360 with ported edelbrocks with 2.08 intakes flowing 300cfm. I dont have the engine dynoed yet, so not sure what the power numbers will be. Camshaft is roughly 600 lift and 260duration @50. I think rpm range will be 3800-7000. Im looking at the thorley headers.1-3/4 or 1-7/8.

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I dont have a ton of experience with amc 360's regarding what they tend to like. I run a 1-7/8 header with my ported w2 small block mopar, which it seems to like. Any input would be helpful.

Woah there willbur! -your question is filled with too many other dubious assumptions! The potential is there but it takes a lot of know-how to build any '66-'91 AMV8 to turn 7000 rpm dependably; it's not merely a matter of buying bolt on parts!

(the question puts the apple cart before the ox) But to satisfy your header tube size question with a direct answer: Take an approximately ten inch long piece of relatively thin wire and shape it/bend it to match the exhaust port shape exactly & snip off the ends where the wire overlaps. Unfold the wire, to make it straight again, and measure the length. That measurement is the circumference which tells the diameter of the correct tube size that should be used. Here is a link on engineering exhaust systems. A bit techy but if you can get passed the calculation standpoints, there is a bit of interesting stuff to take in account when having to design or pick which type of header you need.

Sounds like you are having a dual purpose role with the car. Mine will be done that way as well. For my build I plan on a stepped flange with side exhaust. May incorporate T/A cross over to allow better torque while keeping a wide curve.

This article dose not explain the odd even exhaust crossover to even out pulses within the header exchange into the collector for scavenging. It is a bit complex and requires room to fit. It is trick and makes you v8 sound like a mad v6. But it is not required, just noted for now.

Second, a bit of simplified theory. Holding engine size constant (and assuming you are at least in the ball park on primary size), a smaller primary will give better low-end and mid-range power and torque, a larger primary will give better top end numbers. The whole goal is to get mass (exhaust gas) out of the engine. Velocity and flow are the ways to do that. For a given engine speed, a smaller primary will have a higher exhaust velocity (same mass of air thru a smaller area) and that velocity can be thought of as an inertial force.

Meaning the exhaust gas will have more momentum behind it to help evacuate the cylinder. So a smaller primary helps when engine speeds (and exhaust volume) are lower since they keep velocities higher. As engine speeds get higher, this smaller primary may not have the area needed to support the needed flow to evacuate the exhaust gasses effectively and will pose a restriction (like trying to breathe thru a straw) and hurt top end power. Conversely, a larger primary will pose less restriction at higher rpm, giving better flow and better top end power, but will be a bit lackluster in the low and mid rpm due to the exhaust gas velocity being lower since the primary is larger. This will kill that 'inertial momentum' that helps to clear the cylinder of exhaust gas. Third, any change in the cross sectional area of the exhaust path causes pressure waves (good or bad ones).

I'd recommend the article in 304-dude's post as a more in depth primer to much more nasty math. But basically, any time there is a change in the cross sectional area (port size to primary size, stepped header, primary into collector, etc), there is a corresponding pressure wave (positive or negative depending on if the size change is bigger or smaller). This pressure wave can either help or hurt power at specific rpm points and those with really big brains and wallets will 'tune' their exhaust using this theory to help increase power at specific rpm points they deem most useful to the type of racing they do.

Fourth, Ken_Parkman is the only one to offer a data point (so far). And he gained 15 peak hp by switching to a 1 7/8 primary but lost some in the mid-range because of it. His motor was also 50 cubic inches larger than yours (1 7/8' may be too big for a 360). And we have no idea if the exhaust was the limiting factor. Meaning he could have gained more if his intake was completely optimized, or it could have hurt him if it was the reverse.