I think it's supposed to be like 8.8:1 with 68ccs. I think I remember the stock 340 pistons being in the ballpark of the higher 318 pistons below the deck. I'd be lying if I said I remembered the exact, but while not absurd it's not small enough to be a challenge to measure either.
308s and 587s should be right around the same ccs as the 915s.
The 1.78 intake closed chamber 318 head they advertised as less HP than 1.88 valve smog head cop car motors? Or the 308- the 1.88 intake open chamber swirl port head they sold as the dream child from its introduction in '89 until after the Magnum's introduction?
Yeah, close to stock in a 340 or 360 set up to run on pump is quite an adequate driver engine.
It's much less against the norm than you think. Many who rebuild 340s put smaller cams in them, worse intakes, smaller carbs, and that all combined with higher compression pistons makes a dog that's unpleasant to drive too. Swapping to unworked inferior stock parts will not be increasing performance.
o a leakdown rent/ beg borrow a b oroscope ,or yank the heads. Sounds like the heads you have ,are better than what you have on it. Run a thicker head gasket, if needed. Grinding those chambers down: Get some dykem, scribe it against the head gaskets you decide to use. Cam timing comes into effect,about bleeding off compression. Some of the slower ramp camshafts ,( budget grinds,most likely..)work really well here.
