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compression vs timing

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what are your thoughts on compression ratio vs timing? e.g going from 11:1 to 12:1 is there a point where the extra comp isnt helping from having to retard timing to compensate. I know there are a lot of different variables but in this case we are wanting to raise the comp on a 383 ls1 running 98 octane fuel from 10.8:1 up to 12:1.

LS1 I assume NA, regardless of that I would only dare to approach your CR with using good e85 or C16 or another fuel with an octane rating of 110 + to be on the safe side.

curious to hear your reasoning behind that?

Can you reach MBT with 11:1 or are you knock limited ?

From my experience with the LS1/2/3 on our local 98 octane fuel I'd suggest also that 12:1 will be detrimental. In stock form on pump gas these engines are typically knock limited although the situation does improve somewhat once you start letting the engine breathe a little.

12:1 would suit a fuel like E85 quite nicely but I would suspect that on pump fuel you would be heavily knock limited and would do better with a lower CR.

I've just had a 7.0l LS7 on the dyno two weeks ago.

Heavily modified....ported heads, cams, solid valvetrain with bigger valves, ITB intake, and whatnot (about 11.5:1 CR)...

I couldn't induce knock on good quality pump gas. I honestly couln't hear very well above 5000rpm, but even running the recoreded audio file through Goldwave, I couldn't find anything that would indicate knock.

Made me scratch my head, as I always was under the impression that those kind of engines are heavily knock limited.

I'll make a detailed post soon about my findings on that particular tune ;)

I haven't tuned an LS7 personally but I would assume they respond in a similar way to an LS3. In stock form these are heavily knock limited and you will have knock retard constantly with timing in the 23-25 degree range at WOT and 5000+ rpm. With a decent cam, intake, headers and a good exhaust though the situation changes dramatically. In most instances they will still be knock limited but you might be able to squeeze 30+ degrees timing into the top end and generally you can get to MBT or very close.

Bare in mind though that the above is at a stock CR. 12:1 is quite a jump.

Very Interesting Thread!

Andre, that corresponds exactly to what I've been experiencing on that particular car (which actually features a big bore sleeved LS2 block ;)).

With the exhaust bypassing the muffler (just 3" exhaust piping for each bank), I actually needed 2 deg. less advance to squeeze out the same power (630whp) than with the cutout closed.

On both runs (cutout open and closed), 2-3 deg additional advance actually dropped the power output a bit. So I left it were it was, and took an additional 1-2 degrees out for safety as it'll be driven hard anytime it runs.

Andy