1974 318 duster. Can I port/polish these heads and get enough power out of them?

RustyRatRod said:

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.

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The 'problem' with large exhaust isn't that it reduces backpressure.

448Scamp said:

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.

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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

Step 1...... figure out your total budget.

Step 2...... determine how much the essentials are going to cost.

Then you can see how much is left for hot rod stuff.

After you figure out step 1........ see if that dollar amount realistically coincides with your goals.

Re: your 300hp goal......
30 or so years ago I re-ringed a previously rebuilt 318. I used mildly ported 318 open chamber heads, a std performer manifold, 340 cam, and small tube headers.
It made just under 300hp.

67Dart273 said:

I may have recollected wrong no. I was thinking of the small cc heads

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302 heads. You got it right the first time.

When It comes to low powered (low cr "compression ratio" /2bbl) engines there's usually a decent amount of power improvement with a a few basic mods eg 4bbl, mild cam, headers/exhaust and tune. Basically this 318 Long Block Bolt Ons - Tech Articles - Mopar Muscle Magazine 282hp @ 5000 rpm, what I consider easily attainable and fairly cheap above this hp becomes harder and more costly per hp. This engine is about 20 hp shy of your goal, cheapest option would be change goal to 282hp, another way would be a bigger cam but will be less streetable even with decent cr and with low cr not the best way to go. So leaves either better flowing heads and or more cr, a bump up to 9:1 cr would probably get ya there with no porting. Above cam/headers/4bbl most 318 guys have to decide if they want to invest $$$$ in better heads piston etc.. for 318 or go the 360 route, either way even with your low goals I'd rather have either LA or magnum 360 ported or not over ported 318 heads and go 9:1 cr 4bbl headers xe262h or even xe268h or similar on either bottom end. But if your looking for bang for your dollar a running 5.9l magnum makes 300 hp with just 4bbl add mild cam and headers for 350hp ish, I imagine 5.2l would be similar and or not too far behind.

Save your money find a set of [ X or J ] heads someone posted a set recently on here cheap go forward with them and u will be better off !!!

toolmanmike said:

Way back when, I put a set of Mickey THompson Try Y's on my 340 in my 73 Challenger and off to the strip I went. With no further tuning or modifications it went slower by a few hundredths. The stock manifolds and exhaust must have been pretty good back them. Good thing I kept all the old parts to put back on. A awful lot of work for no gain.

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The stock manifolds were pretty good but the Tri Y’s were probably better and being better and flowing more air through the engine that caused the engine to lean out. A bit of carb tuning is typically needed after making a change that effects air flow.

The biggest two things besides bolt ons you can do for low compression stuff is advance the camshaft timing and get a good hot curve in the distributor. Those two things together can add an honest to goodness 50HP. And that's no joke.