i wanted to know whats the highest compression ratio you can have when using 93 octane on ported L98 Iron heads... i was told 10:5:1 would be ok since im using 93 octane, but someone else told me that was too high, so i'm kinda lost here. thanks guys for your help

 

there's too many variables to put a hard number on it and say that's the limit, however 10.5:1 i think would be streaching it just a little

 

That would be the real world, practical, upper limit.
will require carefull tuning to pull it off.

The exact, exact limit of compression cannot be quoted
because every engine is going to be slightly different.
And operating conditions are slightly different.
But an engine that is detonating will not make power and will soon self destruct unless the compression is lowered , the timing is retarded, or the fuel's octane is increased.

10.5 :1 would be the limit with a big cam and cool plugs.

if you're using a mild cam use less compression.

The difference between 10:1 and 10.5:1 is very small.
The difference between a 10:1 motor that is making good usable power and a 10.5:1 motor that is detonating at WOT because of a lack of fuel octane is like night and day.

I would build for under 10.5:1 compression and concentrate on breathing improvements.

 

well, the pistons im using in my rebuilt have a cr of 10:3:1, but my heads are being ported and i had my mechanic raise it to 10:5:1 because i was under the assumption that that would be fine with 93 octane. my friend's uncle runs 14:1 compression on pump gas, and even my mechanic told me that i won't see any problems with 10:5:1... im just trying to make sure i dont see any problems, thats why i came to the board. is anyone here running a high cr with their iron heads?

 

your friends uncle doesn't run 14:1 on pump gas, not even 93 or 94. 14:1 is getting into the realm of pro stock engines. i'd worry more about optimoizing the squish than uping the compression a few tenths.

 

I agree with what has been said above. You can't just slap together a motor with regard to only compression ratio. There are so many other things that apply to the situation. With efficient cylinder heads, big a$$ cam and 93 octane it can be done. A motor with 10.5 to 1 compression and a little cam will build so much cylinder pressure that it would probably knock and clatter all the time. An engine with 11.0 to 1 compression, big cam, and efficient cylinder heads may not clatter at all. It just depends on the combination. IMO.

 

well, i just ordered myself a XR269HR (218 / 224, .495 / .502 112 LSA) cam from comp cams for my iroc. you think the 10:5:1 cr would be alright with that big of a cam?