Higher compression octane requierments

Lefty71 said
Everything was measured, and everything is known. The worksheets are not right here in front of me (i'll grab them), but I personally made all calculations, checked all clearances and verified all cam timing with a wheel, CC'ed chambers with a burette, and .500 down fill for dome volume, gaskets are very thin Cometec units, etc, etc. I mocked it up several different ways to see which is the proper direction to go, and what is too much for iron heads, in all practicality. The static is over 11.5 to one with the block decked and the 915 closed chambered heads shaved, pretty close to what the ancient catalog said they would be, and the dynamic is a true 10.0 to one, as according to the cam timing.
But you make my point in asking the question in the first place. So many motors are built with relative known parts, and it works out just fine. But, and this is a big one, there are large numbers of folks that are running by the seat of their pants, and don't know for a fact what the dome is, the chamber volume is, the critical valve clearances, the actual minimum gas requirements, yada, yada, yada, then want to comment but are kinda scratching their heads. I'm trying to learn where the edge is, NO GUESSING. I had hoped cranking pressure would tell us something, as an added layer of dimension to the puzzle. Thx again to all who have jumped in on this thread, I appreciate the voice of experience!! Lefty71

Cool. You are ahead of 99.875% of the people doing this. That's the ONLY way to know your CR.

I'm a firm believer that the engine doesn't know or even care what materiel the cylinder heads are made of as far as it relates to detonation resistance. Neither do many people I know. Some guys continue to spew that, but I've never seen that as the case. When I pick a CR, i don't even ask what the heads are made of.

Running that CR requires good tuning, a damn good ignition and camshaft selection is by far the the biggest factor in detonation resistance verses CR.

And that doesn't mean buy a cam that's the biggest in the book.

It means calling a cam grinder on the phone and telling them what you are doing. If they don't make you confident in what you are doing, call another until you find one who agrees with what you are doing and makes you comfortable.

It sounds like you have the vast majority of the hard work done. A cam change my be in order.

I also forgot to mention that spark plug selection become much more critical as you try and run higher CR's on pump gas. You may need to drop another heat range to get detonation under control. Which went with my saying a damn good ignition is essential. That eliminates the HEI, Chrysler, Pertronix and probably a couple of others I'm missing.

Edit: I forgot to mention that the edge is an ever moving target. There are just too many variables to nail that edge down with any sort of accuracy.

I don't think you have too much compression. You may have the wrong cam, and maybe another small thing or two wrong but you are very close to get this combination to make power and not rattle it's guts out.