Maximum Compression Ratio

Rumble, thx for the nod

Robert
This subject is not well understood by us commoners yet. I am still compiling data from the threads here, and have not yet ,perhaps I never will, got a definitive answer. The best answer might not even be the best answer. Some guys are willing to walk the fine line, and modify things as they go. Some guys just want to slam something together and never think about it again. Some guys are racing, and don't want to tell, or for them it really doesn't matter, cuz they're not limited to pump gas. On the street, the every-day guy just cruises around and wants a nasty sounding engine and doesn't care if it's really nasty,sorta nasty, or incredibly nasty; as long as sounds nasty.
Then there are some guys like me, who are or were very curious, cuz our car's are long-term projects. We find it challenging to explore the limits. Fun to swap stuff. Exciting to be engineering on the new Dcr frontier.
I think you might be one of these guys. So Il tell you what I remember. These are all aluminum heads and all at under 1000ft.One guy here on FABO, I heard, is running a Dcr over 9.3 and 91 gas. Others are running right around 9.0 still on 91. I myself am running 8.6,but I run 87E10 exclusively. So if you look at the numbers, one might think that the spread is from 8.6 to 9.3, and from 87 to 91. A little math then says in the middle should be in the middle, or 8.95 should be 89, and that completes the aluminum range.
I have heard that for iron heads, both compressions need to be cranked back about a half a point.This also lines up with the scuttlebutt around the forum.
One thing that has to be remembered is that some physics never change. The maximum Static compression that can be run still has to be kept in mind. One cannot run the Dcr up to 9.3, with a big cam that requires a 12.0 Scr to get the 9.3 with. No! the static has been well established for iron for years,to be: safe at 10, maybe safe at 10.5 borderline unsafe at 11, probably will shatter stuff at 11.5. So, depending what the application is and what cam size is going in, the Dcr can be moot, cuz maybe 11 is your Scr limit!.
Another thing to remember is that Dcr is a tool,not a target,per se. We use it, to figure out how a particular combo will behave at low rpms to early midrange. Low Dcrs tend to be soggy off-the-line; and need extra work to get moving, like a hi-stall, a very low starter gear,and some creative timing work.Whereas a too-high Dcr leads to detonation and broken parts, same as too much Scr. This is very helpful stuff for a streeter to understand; not so much for a dragracer. Dragracers don't spend any time running around in traffic looking for gas stations. They, by and large, don't need to care about Dcr.
Another thing is that; adjusting the Scr to achieve a certain Dcr, gets expensive. Usually we are targeting Zero decks, tight Q, and small head-chambers. This all costs money, sometimes more than necessary.
And finally, the quest for a high compression ratio is,for most, kindof misunderstood. Rusty has driven this point home more than once. The difference going from 10/1 to 10.5/1 is around 1% in the horsepower department, but represents a TOTAL chamber volume difference of 5.2 cc on a 440. Sometimes these 5.2 cc can get very expensive to get. Especially if you factor that cost as dollars per horsepower. We are talking about 4.4 hp on a 1hp/cid 440, 5.28hp in a 1.2hp/cid 440.
So you can see how complicated the answer to your question can be.
Nobody, me included, want's to be responsible for your engines demise because you built it to a number that FABO said was safe.
All we can tell you is what worked for us.
But we are very eager to hear what worked for you, cuz one day that theoretically absolute number you wanted today, might just have been established; and your results may go down in history, right here on FABO..
So far I have heard of exactly one guy here on FABO,who wrecked his engine by detonation. And he did it by accident, using the wrong fuel, and ignoring the detonation. He just ran the wrong fuel.There may be others, I'm not here 24/7,lol

I think Moper said it best, in post #16. And IQ52 in #19
The builder is the guy who sees the traffic. He is probably the most qualified guy to say what breaks stuff. And when you know that, then you also know what doesn't.