340 exhaust manifold vs stock 318

Before I knew anything, I was pretty sure that on a performance engine, the output needed to be at least the same as the input, times some unknown to me factor, to allow for the increased volume of the hot expanded gases. Otherwise the pistons were gonna have to pump the exhaust out thru the logs. Ok I get that the oxygen has been reacted but the atoms still have to go somewhere and as steam, they are many many times the volume that they started out as. And they are gonna stay as steam for most of the trip thru the exhaust system.
Since an 850Holley, at WOT, has a throttle area of 9.6 sq inches, I reasoned that the logs should have at least that, times the unknown factor. So half to each side is 4.8 sq inches or about 2.5" outlet before adding the factor.
A 600 has a throat area of 7.67 sq inches so a pair of 2.25 pipes would meet that... times the expansion factor.
So when the factory made my 340 to breath thru a pair of 2.25 pipes, it was no surprise to me, when the exhaust coming out of there at just 4000 rpm was hot as Hades and plumed like a rocket engine. I knew right then, in 1970, that a pair of 2.25s on a 340 were too small.

But as I learned more, I realized that on every revolution, only half the plugs are gonna fire and half of the half, exhaust on each side..... mostly. But then I realized that two 340 cylinders are gonna ingest about 340/8x2=85 cubic inches, per side on every revolution, and just how many will this be when it is coming out the logs at 850degrees? I didn't know then, and I still don't. But I reasoned that it was probably more than 4times that 85, and less than 10times, so say about 7 times as big. I mean, I was 16 what did I know? So 7x85=595 cubic inches. In a 2.25" pipe that would be 149 inches long, or 12 feet, about the total length of the pipe. So on every revolution, the engine, at WOT and with a VE of 100%, the engine is ejecting a column of steam 12 ft long.... down each pipe. No wonder the exhaust was perturbing the road surface 4 feet away from the bumper.

Fast forward to 1999. There was no way I was putting logs or 2.25s on my 292*/108 equipped 367.Which is 8% bigger than a 340. And 2.5s I reasoned were 11% bigger. When the sales girl at TTI asked if I wanted 2.5s ; I said no; then THREEs ! popped out of my mouth, and the rest is history. Yes she gave me the "you need back-pressure, else you will burn your valves, speech". Sweet girl.
And I soon discovered that at 11.3Scr, the dual 3s with modest TTI turn-downs, were still raising dust-clouds on the road in the range of 3 to 4 ft away........ parked and idling,lol. So I cut a hole in my hood and got rid of the hot-air induction,lol. And then raised my minimum coolant temperature to 205*F...lol.
Boy I'll tell ya; cutting a hole in a pristine 68 Barracuda hood, just about killed me.
And contemplating a minimum coolant temperature of 205*F, was pretty scary too.
I saved the hood cut-out,lol.
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But I gotta tell you;
With logs ,and modest throttle openings, you are always gonna have pressure in them. So then with a performance cam, at low rpm, that pressure, during the overlap period, is gonna be forcing hot expanding exhaust gasses into the intake manifold. This occurs from idle to about 2000rpm or a lil higher depending on the cam. You can prove this is happening by removing the intake and seeing the baked on carbon in the intake ports and runners. This is EGR.
And you took your super tiny ported Egr valve off the intake because you had a poor opinion of EGR, right? Well, that factory EGR was controlled to only occur under one tiny operating circumstance, while here your big cam is doing it 100% of the time that it is operating under about 2000 rpm. If you, a streeter, have a stick car, yur pretty much in it all the time. This EGR is partly responsible for why your big-overlap engine idles the way it does.
So with logs, the trick is to reduce the overlap period.... But that drives the intake duration late into the compression cycle, and so your cylinder pressure drops about 1.5psi per 1 degree of lost compression duration, and pretty soon you are on the death-slide of poor low-rpm performance, due to lack of pressure.
Here I'll show ya:
Say you had performance cam like
268/276/108/56 overlap, installed on the split, so 56* of Effective overlap; 28* each to intake and exhaust. So then you would have 120* of compression and 112* of exhaust. Your cylinder pressure in a smoggerteen would be about 124 psi on the 60* Ica.
Now lets take the same cam and change the LSA to 114*
268/276/114/44 overlap, again installed on split overlap, giving 22* each to intake and exhaust, so Effective overlap is all that it can be, namely 44*. So then you would have 114* of compression and 106* of power, and the Ica moves to 66* and the pressure drops to 116.
Did you catch that ? The Wider LSA lost 12* of overlap. But also lost 8psi of cylinder pressure. This is bad and good.
124 psi was already very low. Now 116 is crippling. (the bad)
But the minus 12* of overlap is just what you need with log-manifolds.(the good)
So what do you do to get the pressure back. Well, there is only one thing you can do, namely raise the Scr. And IMO if yur gonna install hi-compression pistons, then you might as well raise the Scr to the top of what your gas can support. This is usually 165psi with iron heads but not always. So say we target 160psi for 89 gas, and the Wallace spits out about 10.25Scr @700 ft elevation.
Now you have a log-friendly, gas-friendly, low-rpm as friendly as it gets, 318LA combo, with a power-peak up around 5100 . So all you gotta do is gear your combo to put that 5100 where you need it to be.
Unfortunately this 114* LSA combo comes with a price. The power extraction duration is only106*, and as any factory 340 owner can tell you, those 340s sucked gas pretty bad, and a 318 won't be any better with that 114*LSA cam.
Now, more bad news.Don't be thinking of picking up a bunch of power with headers on this combo, even if the Scr was jumped to 10.25;
and the reason is ; the mediocre 44* of overlap. Yes you will still pick up power from the loss of manifold pressure, but the reduced overlap cycle sorta kills the power boost up top, giving you instead a broad flat curve, which with an automatic is actually a good thing. You just don't get the big absolute number you may have been hoping for.
With a stick car, this is a waste ; get the tightest LSA you can find, cuz your powerband requirement is quite narrow. With a 6000 shift, the Rs will fall to .73 or ~4400 so a band of 1600. Whereas the auto falls to .59 on the 1-2, and .69 on the 2-3, requiring powerbands of 2460 and 1860, on the same 6000 shift.. This is why, in the old days, Stick-cars were faster, cuz 3 drops of 1600 beats 2460 and 1860, delivering more average power over a measured distance.
In the street realm the measured distance is about 330 feet, and it will take less than 6 seconds to get there for most any warmed up combo. How you get there with your combo, determines the smile factor. I for instance, get there with two gears and magnificent smoke,thunder,and screaming tires. And it never fails to make me smile.

Now getting a 318LA to 180psi is quite a trick. and getting it to burn pumpgas at WOT is a bigger trick. I suppose it could be done, but on the street, you're gonna spend waaaaay too much money to make it happen. Sure I could enjoy an HO 318 with a power peak of near 6000. But I sure wouldn't build one, knowing the same money on a 360 would result in a much more street-friendly combo.
The biggest obstacle will be to make the pistons fit in the little tiny chambers that this will require. I think it would be hundreds and hundreds maybe into the thousand(s) of dollars cheaper to start with a 360 which falls together at about 10.6 or better with aluminum heads. It didn't take me much to hit 11.3.

And today, there are small-chamber heads available for the Magnums.So, Magnums are now a natural. I used to say "no-brainer" but got kicked a couple of times for it, so I changed to "natural", lol.
And because of the steady march of technology for the Magnums, HO 5.2s are now so much easier and cheaper to build, because high Scr is so much cheaper to achieve, with the really small chamber head offerings, and the already, as delivered, higher in the bores, pistons.
As already said by others, It's really hard to beat starting with a Magnum. Even A stock one. Even with a retimed stock cam. It's rated at 252/264/110@ .008 tappet rise. How does that compare to a Comp at .006 tappet rise? IDK but I bet it could be;
256/270....................... or more,lol. Yeah, I know, it ain't much at .050....... but it comes with the engine, so basically free!... lol. Now, looking at the specs, I see at .050 it might actually be smaller that an LA 318 cam; OOps.