What is the highest compression you can run on 91 (not 93) octane for a 427 LS7?

244/256 .629"/.604" 114+3 cam.

I was told 12:1 is no problem with good tuning.

My current motor is a 346 ci LS1 with 11.4:1 compression with MAMO TFS heads and a 228/228 .525/.525 112 LSA cam. No issues tuning it at all.

 

🤔 Best I got is
Ask Pat G or contact Cam Motion. Either or has unlimited amounts of data and or references for something like (example California octane ratings.
Mamo is based out of Cali if I'm not mistaking

 

Mamo Motorsports
28415 West Industry Drive Suite 510
Valencia, CA 91355
(661) 714-1317

 

This is an interesting subject I too would like to know more about. A few conversations in with Darth about my next build had me thinking. It seems he uses DCR and cam specs to truly calculate the limits of your octane. Iirc, it was 8.5:1 max DCR, and around 12:1 static CR. So many people build n play it safe these days, it just seems everything lands at 11.5:1. I often wondered why we cant aim for 11.6-11.8 kinda thing. All conversations i should have had with my builder last go around. Hopefully Darth n others chime in with real world experiances.

 

Yup. Darth said 12:1 should be fine because of the size of the cam.

 

Lots of variables here guys....
camshaft
head material being aluminum or iron
block material being aluminum or iron
quench
chamber design
piston design
coatings used and where?
elevation above sea level
tuning and particularly ignition timing

I could build a 14:1 engine and run it on 87 octane with the right parameters while giving it very little ignition timing. Of course it wouldn’t run to its full potential and would be down likely 70-80+ hp and a bunch of torque, compared to running it on VP116 and giving it 24-25 degrees total timing.

 

Tony told me 93 or boostane for my 12:1 ls3 427 with [size=16px] a 243 / 254 on a 114 + 3 cam that he put together for me awhile back. Look up 427 small bore in the dyno section to see the results[/size]

 

Turns out I was asked in a PM about this situation and after seeing this thread I thought I would share my response with the community and expand on it even further

I quoted Chevelle's response above because its essentially what I am trying to say in the first sentence of my PM response which is directly below in italics

Depends on many other variables.....if I was spec'ing your whole combo and knew all or most of the "variables", the question would be simpler to answer
Off the cuff and more as a generality with the size cam you describe I would say 11.5 to 1 is safe......maybe a touch more with a good tuner but 11.5 would be a number I was comfy with

-Tony

LOTS of variables including the talent of the tuner as well which is actually a very large variable so Im not just speaking of mechanical variables. Also how much room for error are you willing to be OK with??

11.5 is a "safe" number.....you have a little more room for error....room for some of those variables that could help an unoptimized situation still potentially fly

Can you run 12 to 1 with a very optimized combination and a very optimized tune.....sure.....buts its close to or right at the ragged edge. Could you run that situation in Houston TX or Florida on a 100 degree day with 90% humidity and elevated engine coolant temps? Not really....your going to detonate in that situation but likely not enough to hurt parts.....just hurt power output but lets face it detonation is something we all should strive to avoid

So we are back to square one.....the answer for one persons combo is not the right answer for another person's combo.....there are countless variables that have to be looked at if one plans on pushing the static compression while still running borderline fuel quality. It can be done under certain conditions but not all conditions and there is a minefield of things that have to be avoided to properly pull it off.

If in doubt.....11.5 to 1 and call it good.....even that could detonate with a poor tune or crappy quench or other obvious screw ups but its just not as likely to happen there, especially with that large a camshaft which bleeds off some of the lower RPM cylinder pressure.

Hope some of you find this helpful

-Tony

 

I’ve done 11.6:1 with aluminum heads on aluminum block, .036” quench on a 5.7 without any detonation using 91 octane. The larger bore of a 7L might have something to say about it though.