318 with big valved or 340/360 heads

If you can get those flat tops up close to a zero deck, and use the thinnest headgaskets, it wont be too bad. But the big thing is to be conservative with the intake closing point. You can get the StaticCR up to 8.5 or so, but to keep the DynamicCR up takes an early closing intake. I should think in this case the 1.88 valves would be sufficient.But if you already have the 2.02s, no biggie. If you go too big on the cam, the bottom end gets sluggish.Gears and a higher stall TC can get you through that. But if youre running a stickshift, its a PITA, cause you always have to slip the clutch so much to get moving.The soft Dcr also makes sub-2000rpm soft, so you will be spending more time in the lower gears, which gets somewhat tiresome.Best is then to go automatic, and a slightly higher stall TC.
Or get a custom cam ground with an early closing intake but a bunch of lift. That would probably require a fair bit of valvespring to keep the parts together, so the rpm might have to be limited.
Its always easier to get the Dcr up on a teener with a smaller chambered head.
With a teener and flat-tops,it seems a guy is forever making sacrifices.
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Rosco, its not the big valves so much that are a problem, but rather its the big chambers that come along with those valves.
A .030over teener has a swept volume of 661CC. Pistons down in the hole.067 are 13.4CC. The gasket @.039 is 8.5CC.Eyebrows in the pistons are about 5CC.So thats 26.9CC so far. Add 70CC for the big chambers and doing the math thats a Static C/R of 7.7. And thats were a sluggish bottom end lives.
Now with a teener head of 60CC, the math comes to 8.6 now we are getting somewhere.
But if you machine the decks to get zero decks, that throws those 13.4CC away, so now the small heads make 9.86, and the big heads still make 8.9. With these higher Scr numbers a larger than stock cam can be introduced and the Dynamic C/R doesnt fall into the basement quite so fast. Its the low Dcr that makes for a sluggish off-idle response. The Scr is merely the target to build to, to get the Dcr you need.
Machining the decks leads to fitment issues.
As for me I would spend the money on a custom set of pistons instead. I would get pop-ups and fit them to the big chambers/big valves that you already have, and try and build a tight quench into there.Then you can target more serious compression and snappy bottom end response would be the result of that. And how much time in your driving experience is spent getting off the line? And how many of us dont love frying the tyres? Yup,we are kindofaddicted.
Just so I dont mislead you, a high Dcr is not mandatory.It seems guys have had good success with Dcrs as low as 8/1, and occasionaly less.I myself am running just under 9/1 DCr. 9 is snappy. 7 is soft. In between you have to pay close attention to the intake closing point.
So again, run what you brung. If most of your driving is sub 4000rpm, run the teener stuff including the cam. If you want to foray into 6000regularly, the X heads can make good power up there. Between 4000 and 5000 could be either, and it really amounts to how much softness you are willing to suffer. But then, if you dont like the big-chamber softness, you can always slam the teener heads back on. (I have successfully reused the .039 blue Felpros a couple of times, and the intakes as well.)