Because we have so many Alloy head choices now, It's starting to be an adventure. All I can tell you is that some FABO members have stated that they are running 200 psi on pumpgas. I'm assuming 91 or better. Allowing 5 psi per grade gasoline (with a tight-Q), that allows up to guessing 190 on 87E10 with a correct power-timing curve. I have run 185 on 87E10 no problem. For a short time, I was running close to 200
The point is this; I don't think we should be paying any attention at all to the Scr when using alloy heads, because in all likelyhood you will leave some performance on the table.
Do you need to run at the pressure ceiling with a streeter?
Well no you don't, and there is such a thing as way to much VP. IMO, a VP of over 150 is pointless unless yur running Caltracs or something, because they don't make decent 15" tires to withstand any more VP, so I'm always either spinning, or on the verge of spinning.
Yaknow, spinning in a straightline is ok, but when the back lets go around a turn when yur not expecting it, that's a recipe for disaster. I had to fit 295s in the back before I could corner with a 750DP carb,lol.
So really, we should be looking at the Dcr and pressure, and never mind the Scr, let the drag-racers worry about that.
So then, because of the flexibility in chamber sizes in alloy, if you have not yet bought heads, then with zero-deck flat tops, you should probably pick the cam first, so you can know the installed Ica. Then figure out what Dcr and pressure you wanna target, and let that guide your chamber size selection.
_________________
And BTW, the 268/276/114 cam was designed in an age where that's what it took to make the 340 the engine that it was, with iron heads. But, the power was up around 5000rpm.
Your modern streeter running ~185 psi, no longer needs that cam, because;
a power peak around 5000 leads to a shift point with an automatic and alloy heads, of around 5800, which in first gear with 3.23s, is about 53 mph, and the tires will still be smoking; on a good build. And second gear will hit ~90 mph with those same 3.23s.
So, IMO, the 3.23s are totally the wrong gear or
that 268 cam is way too big.
But, going to a the right gear, is gonna make the tires even easier to spin, so WTH do you do?
Well IMO, you gotta at least think about bringing the powerpeak down, or even downsizing the engine. The 340 being a bigbore, is a great sized engine to downsize the cam on.
Yakno, people are always saying the 318 is only a lil smaller than a 340, but often overlooked is that the 5.2M is gonna go back together at stock bore, whereas the the 340 will almost always need to go to 4.07 to clean up. Now yur at 160 thou difference. That doesn't sound like much but convert that to area; the 3.91 bore is 12.007 sq inches, versus 13.020 sq inches for the 340, or 8.35% bigger. That is no small number.
Anyway; the point is this, those alloy heads are gonna be a life-saver, making plenty of power due to the small chambers. So lets say the pressure jumps from 165 on a borderline detonating open iron chamber X head, to 185 on a barely even breathing hard alloy head still burning 87E10. That alone is gonna be worth a lotta cam duration, so already that 268 is "obsolete" in a streeter. IMO, you can easily scub off 7 to 10 degrees intake duration, and throw away that 114 LSA and end up with a much sweeter engine. So what if the power is down; yur still spinning the tires at the top of first gear, and the power at 65 mph is likely to be at least as high as it was with the 268 iron headed 340, with the same gears. You won't notice the power loss until you are fighting wind resistance, say 85mph, well into speeding territory.
So that opens the door to a 262* cam or even less. A 256* cam, for example, will bring the power-peak down about 400 rpm (as compared to the 268); and bring the bottom end down about half that so 200 rpm. Between that and the increased pressure, maybe you can use a regular stall and 3.23s or a lil more stall and a lil less gear. Or some combination.
I can tell you this;
I have had iron-headed hi-compression 340s on and off since my first one in 1970.
When I was in the engine selection stage for my Barracuda, I had 5 340 engines sitting around and one 360.
I chose the 360 and put alloy heads on it from day one. I have never been sorry about that.
I was sorry I ever put a 292/292/108 cam into it, yes.
But I soon put a 270/276/110 cam into it (and later a 276/276/110) and was never sorry about that either. In both cases and the third one,lol, the Scr was readjusted to maintain the 180psi or so pressure.
After I installed the 276 cam, I realized I shouldda gone the other way. Whereas; the 292* cam hated 3.55s, the 270*cam liked 3.23s and the 276* cam was very unhappy about the 3.55s until I jumped from a 2.66 low gear to a 3.09 low gear. ( plus 12.8%). This is the same as from 3.55s to 4.00s. That was with just a 7* intake duration increase! You can't really get a sense of that from perusing the catalog.
Bottom line is this; When it comes to cams, a 7 degree change is very significant.
If you design your pressure around a certain cam, you can always go bigger. But to go smaller often results in failure. Then the cam gets blamed for what is really a pressure problem. Don't be that guy, lol.
BTW; all this talk is particularly important for a streeter with a manual trans...... because the engine is sorta married to the tires. If the Dcr is low, (with street gears), taking off will usually require a lot of rpm to get moving. There is no Hi-stall convertor to help.
and of course;
for a dragracer with an automatic, this is of almost no importance, because his engine is never required to work at the rpms where optimum Dcr or VP matter.
