I was thinking about building my motor with flat tops, .005" out of the hole, with 71 Stg1 heads (73cc after chamber work), for a compression ratio of 12.17 static. I will be using a Poston GS112 cam with an advertised duration of 320, 322. This should give me a dynamic compression ratio of about 7.5-7.6 when installed straight up. Do you think it will run on premium pump gas?
well i have 11to1,polished combustion chambers, 113a cam,witch has 13 degrees of overlap, i can drive it on pump gas as long as its under 80 degrees and i dont hit a hill, i mix to 94 octane and its fine, no knock, but when i race go to 96 octane just to be safe.so i think you could start it, and rev it up in your driveway on pump gas. also i think that the loss of commpression due to bleed off from cam overlap dosnt help detonation. just my opion
You have a true 11:1 with a GS113A? Wow! That seems like it is really pushing it. It's not so much the overlap that gets you, it's the intake duration that counts, from what I understand. The GS112 has a lot more duration than the 113A, so maybe I could get away with it?
I believe the hypers are a 10:1 or 10.25:1 piston with a zero deck. This would correspond well with what Phil Sedlon said about his 10:1 motor in his convertible (he also runs the 113A, and had detonation before he polished the pistons). I could be wrong, of course. They do have quite a lot of valve relief. Flat tops with a 3cc valve relief would come in at 12:1, so maybe you're right. Take a look at this link for dynamic compression. It explains why it's the intake closing time that matters. http://members.uia.net/pkelley2/DynamicCR.html
Thanks for the example. It always is worth more to hear about a real combination than to just do the math and hope for the best. It doesn't always work out like you think it will.
