Dcr

nm9stheham said
Hey Moper, just curious, what do you consider 'high DCR'?

That's the problem.. Every build will have it's own "problems" threshold based on everything from mechanical parts, machining accuracy, tuning, fuel type, and weather. Nevermind the differences in calculators and their assumptions... I use the KB site. I prefer having the numbers used at .050" rather than seat to seat. The figures below are based on using that site. Seat to seat or .006" numbers do not tell the whole story so I'd rather use later (and IMO more accurate) numbers, and fudge a little based on experience than let the calculator fudge for me.
In terms of pushing limits - with an engine built to my standards (sorry for the way that sounds but I'm beyond picky in terms of machining quality and how I do things) and assuming the typical 91 octane w/10% ethanol CT uses...
An open chambered head (no quench) - max dynamic ratio of 8:1
A closed chamber iron head (piston to head clearance no larger than .035") - max dynamic ratio of 8.25:1
A closed chamber aluminum head (piston to head clearance no larger than .035") - max dynamic ratio of 8.5:1

These do have some fudge factor. If you've read my old posts on Moparts arguing with Eddie Elvis about "pump gas" you'd understand I tend to rely on the fuel quality being crap at your local Quickie Mart. Pump fuel comes from the ground and is not consistently blended. Not out of a Rocket or VP drum that is perfectly consistent in every way. I've run DCRs as high as 8.5:1 with iron open chambers & no quench. But those ran on pump 93 and I tuned them and admittedly upon one teardown there were signs of detonation even though it was never heard. I used to be about 1/4 point higher for all of those but decided I wanted some cushion and realized that the power that might get me could be made in other ways.

DCR is a functional result of a lot of other things - not a specification. Keep it in mind - but don't build specifically for a certain level. There are some engines with DCRs down in the low 7s that make a ton of power safely on pump fuel.