Steve, if you are moving from the 273 920 heads to the 302 heads, and keeping the same 273 2 BBL pistons, then you are not changing the compression ratio if using the same pistons in the 273, and the same low torque problems at low RPM's will still be there.
So, I am assuming that you are moving on to the 318; is that right? You have the opportunity to improve things... a LOT. With the best flat top cast pistons that you have, you will still not have a great compression engine for low end torque. The Sealed Power 526P flat topped cast pistons are still going to end up .080" or so in the hole, and with a 1121G head gasket (the thinnest out there now), your SCR will be in the 8.6 range, and your DCR in the 7.1 range.
I'd need to work a bit more to figure up the 273 SCR and DCR that you had, but you really need to be heading to the range of mid 9's for SCR and upper 7's to low 8's for DCR if you want to cure that low RPM torque problem. Since you want to keep the 3.23 rear gear (for reasons I would agree with), then it is even more critical to move up in SCR/DCR. Low RPM torque is where you need to focus; you will gain HP for mid and high RPM range but CR is what extends your torque band down as low as it can be.
BTW, know that if your 318 comes with the 4 valve relief pistons, then you are even further down in the CR toilet. Don't go there LOL Check how far the pistons are down in the hole with the heads off; the best you are gonna find is in the .075" to .080" range. Some piston pix would be appreciated.
Ways to get this SCR/DCR up are :
1. Block decking to reduce in-block (See RAMM's Cheepy 360 thread)
2. Head milling to reduce chamber volume
3. Raising piston compression height to reduce in-block volume
4. Going to a smaller cam to maintain higher DCR
You CAN keep that cam and get where you want to go; you just need to implement 1. or 2. or 3. above. You just need to decide which is the cheapest: some combination of 1. and/or 2., or 3.
- Head milling with the small 302 head chambers is going to take .100" off the heads to get up to around 7.9 DCR with the 526P cast pistons, and I would serious expect problems trying that.
- Decking .050" will get you there with the 526P pistons. Now you probably have to change the pushrods.
- Pistons for getting close to zero deck: The KB's are the most easily found and least expensive that I know of. (Try Campbell enterprises.) The KB167's will put the piston tops just .012" in the hole, and with the 1121G head gaskets, your DCR will be around 7.9 with no other machining. Easy..... And the nice part is you will have some quench effect with those 302 heads with the gap around .040". You can advance the cam and ignition timing more and get away with it; so some more torque can be 'tuned' in.
Now just one issue for you to deal with on the KB167's; they weigh around 85 grams lighter than the stock 318 pistons, so that is a balance issue. BUT, use those 273 2 BBL piston pins (around 220 grams each.. .heavy!) and you are within 20 grams of the stock piston+pin weight and bobweight will be approximately just 26 grams light. A machine shop can make that level of bobweight change on the crank with a few small holes in the 2 large counterweights; it is not a big change.
And of course, changing cams will be the other way to get that DCR back up. But it is never going to be very high at all with a stock 318 LA bottom end, and even with the 526P flat top cast pistons. (Those 120 psi compression readings tell the story there...) I don't want to blow your budget, but really I want to see you get out of your low torque hole.
