1974 318 duster. Can I port/polish these heads and get enough power out of them?
I don't know. I never have supported the "needs a little back pressure" theory. I think if he can afford the 3" system now, it'll be there when he upgrades later on. It's certainly not gonna make the difference between it having good power and not.
The 'problem' with large exhaust isn't that it reduces backpressure.
There is never a scenario where back pressure is a good thing. Tubing diameter and length affect more than scavenging. Cam timing will affect which exhaust sizing will affect combustion chamber scavenging much more than sizing. Get that wrong and the engine will be unaffected by whatever exhaust is bolted to it.
Scavaging might be the right term for it.
Rusty - picture it this way
When the exhaust valve opens and the pressurized gas is flowing out, the least pressure outside the port will let it out the fastest.
We don't want the exhaust gas hitting a wall of atmospheric pressure.
When the manifold or header and pipes are sized right, the flow from each cylinder into the system creates low pressure area behind it. The faster its flowing away, the stronger that will be. Also the faster its going the more momentum it will have. So the pulse or slug of exhaust gas is going to go further away from the ports before it slows alot.
When a system, or part of a system, is too big for the engine and rpm range the exhaust pulses slow and during overlap can get sucked back.
Yea. I tried a 3" X pipe on my Barracuda when it still had 340 manifolds on it. I do not have back to back times on it, but my conclusion was that it was contributor to the lack of torque and difficulty with getting a decent idle. Now that car has headers based in large part on Calvin Elston's recommendations.
Calvin Elston calls the gas flow "king" and explains it here:
"What is [exhaust] Blowdown Length?" p.2
He's been pretty generous in sharing his experiences. This is one about merging to a single tailpipe.
Flowmaster Outlaw muffler - anti-reversion? - Page 3 - Speed Talk
