Is this a street engine?
Then coming from being happy with a 292/108, anything else is gonna seem doggy on the top. I've been there.
With that 292 cam in at 104, the ICA was 70*, and
Static compression ratio of 12.5:1.
Effective stroke is 2.43 inches.
Your dynamic compression ratio is 9.43:1 .
Your dynamic cranking pressure is 197.62 PSI.
V/P (Volume to Pressure Index) is 149
This is not suited to a streeter at all. But
check out the 149VP; that's a pretty good number. If you managed to figure out a way to keep her out of detonation, firstly congratulations, and secondly 149VP is a great street number. So you wanna shoot for that with whatever cam and compression you choose, even tho it means giving up some of the 292 topend.
Problem #1;Going to a smaller cam is only gonna drive the pressure higher.
Problem #2; Since you are sticking to iron heads,this is gonna be a tough project, Generally with a tight-Q, 165psi is considered the max for pumpgas. The only way for you to run a tight-Q is with no gasket at all,lol.
Problem #3; If you can't get into the Q-zone,then you might want to reduce your pressure target to 160 max psi.Unless this is not for street and/or you are OK with running your race-gas..... forever.
> the 268 is a pretty good size for a streeter..... at the proper compression ratio for the head material you are running. It puts the power peak at a more street-friendly rpm of say 5000/5200. Good heads will stretch the operating rpm out, if you have the right springs.
I had an old Hughes 270 (HE2430AL) that was still pulling well into the 6s and I usually let her buck to 7000. That combo in my 10.9 Scr-
367 made 185psi (aluminum heads), and VP'd at 162. You can imagine my delight with that. And I won't tell you I pulled over 30mpgsUS out of her with a double overdrive. No, it didn't pull like the 292 did,at 110mph; but it didn't need to; she was a streeter.
Your engine with pop-up pistons would really like aluminum heads so you can continue to run very high cylinder pressure, but now on pump-gas. But the 12.5/1 is still gonna have to be reduced, for the street.
I see 11.6 Scr doable with an ICA of 66*, giving you a cylinder pressure of 188psi and a VP of......146.
That's about the same VP as your 292/108 cam, and a pressure loss of just about 10psi, and you are now running on pumpgas.
So then it remains to find a cam with the 66* ICA.
For a stick like mine, I like the 108Lsa,
so here's one:
276/282/108 in at 108 for an ICA of 66*, and overlap of 63* this will be about
[email protected]. I run a 230 and can attest that this is a far better cam in my combo, than the 292 ever was.
Here's one at 106Lsa;
280/284/106 in at 106 for an ICA of 66*, and overlap of 70* This will be about
[email protected],
Or you could go to a mechanical cam and get almost all the 292 power back. The 280 above could then be a 241 @.050. If you had advanced the 292, to in at 100, which some guys do in an effort to get some bottom-end back, then this 280 mechanical cam could run right along side of it....... but on pump-gas.
All cams I listed are theoretical, you may not find a match in a catalog.
So then what remains is how to get your 340 down to 11.6Scr.
IMO, the cheapest way to do this is to cut the tops of the pistons down as YR mentioned, to a pop-up of .005, then run closed 58cc aluminum heads and the .039 gaskets(8.8cc). This will get you a total chamber volume of about 68.7cc after you put 3cc eyebrows back into the piston crowns. My math is 58+8.8+3-1.1=68.7. Now with your 340 at 4.06 bore the swept works out to 703.28, and the Scr becomes 11.24,close enough. The Q will then be .039 less .005=.034 and I have been running that since the early 2000s.
With the compression now at 11.24, the 276/282/108 works; in at 106 for an
ICA of 64* , now at 185psi/147VP. If you make this a mechanical cam, you will have about 237*@.050. This is about a size and a half smaller than the 292. So the power peak moves down about 300 to 350 rpm.
IMO, aluminum heads, by themselves, may not be the best thing since sliced bread, but the cylinder pressure that you can run with them is waaay better.
BTW
your 268 cam, in at 110 would have an
ICA of 64*............see above,lol. It's a hydro tho so the .050 is probably just about 222*, and the overlap is just 56*...... but at 112* of power extraction, it can make pretty good fuel mileage under steady-state cruising at over about 2000rpm. That will take the edge off those 3.91s when hitting the hiway.
Parting shots
>You can do this with an open chamber Eddies, and leaving some pop-up on your crowns, but,that's gonna be a lotta work to engineer, and really;it's not the best idea.
