Wanting to try out my new $18 ebay burette, I cc'ed some pistons I had laying around. Shown in the pic from L to R is the '70 superstock Sealed-Power 2362P at a shade over 23 cc, the same 2362P with the centering boss removed at just under 24 cc, a stock GM 1237454F from '76 at just over 27 cc and a stock GM 1237454B from '73 at just over 35 cc. My burette reads in .2 cc increments but I'm not sure I trust my methodolgy enough to quote numbers that accurate. The depth of the dish (from L to R) is .157", .157", .225", .265" . It's interesting that the 76 has a smaller volume than the 73.
That's very interesting about the '73 dish being larger than the '76. I thought the '76 had less compression. Now that I'm thinking about it I remember hearing something about the '76 heads being open chambers. Some different year heads cc'd would be very interesting as well. I'm thinking I should get a cc kit as well!!!! :Brow: Good Work -STEVE
75 and 76 have the same extra large, open combustion chambers. i've seen it noted before that the 75/6 pistons have a smaller dish than the 71/4 pistons but it's nice to see someone doing a cc job on em. the 70 pistons should have the smallest dish when stock.
I was also wondering how the late piston pin height compares to the 70 piston or the forged piston. I have a 76 motor here and I have wondered if putting 70 heads on it would make it about the same compression as the 70 motor was.
Very interesting results. Both the Sealed Power 70 and the GM 73 measured 1.975" pin (compression) heigth. The GM 76 measured a lower 1.935". So, the 76 sits 0.040" further down in the hole relative to the 73. But the dish on the 73 is 0.040" deeper than the 76. The means that the "floor" (if you will) of the dish is the same distance from the head for both the 73 and 76. So the 73 and 76 piston will give nearly the same CR in a particular engine. I said nearly, but the 73 has higher dish "walls", taken up slightly more volume, giving slightly higher CR.
I had wondered about that for a long time, So the late pistons would be the worst ones to use since they are so far away from the flat surface of the heads if early heads were used. Would be good stroker pistons for offset ground crank though, for street use I mean lol.
Yes, the earlier pistons have better quench, since the flat edge of the piston will be closer to any flat part of the head.
thanks, i was wondering about that. guess there's no point in ganking the 76 pistons and putting them in the decked block.
If you zero decked a block, ran stock 76 pistons, .040" gasket, and had a 67cc head, you would have 10:1 SCR !
I heard I "heard" that the reason the 76 has better pistons is that the compression loss is from the heads and that if you put 74 or earlier heads on a 75-75 motor you get about 9 to 1 compression. Not the best but the difference is from the 23cc 70 piston to the 27cc 76 piston. As I said I "heard" this so definentally don't take it as fact.
