What's the highest compression I can run on 93 octane gas?

Right now I have 8.5:1 compression heads and I want to at least go to 10:1 compression, is this possible with these heads? How about higher on pump gas, no 94 octane though.

What would I have to do to get at least 10:1 compression and put in some 2.02/1.6 valves, right now I have 1.94/1.5. Oh and I mean by no internal work on the motor, just head work.

And how much might this cost?

 

So I take it a super charger is not in your future?

Brian

 

There's really not any way to get it that high with what you've got, except go with 305 heads. 3 steps backwards (maybe 4) to get the one forward.

You can at least get it up around 9½, maybe a little more, with some iron L98 heads.

 

Dang it, yeah I'm not going to get 305 heads.

Guess I'll just port and polish these, what about putting on a thicker gasket that should help some right?

A SC is planned in a few yrs once college is done, I figure I could get new heads by then.

 

A thicker head gasket will reduce compression, not raise it.

I would not mess with a head gasket just for that, in any case. The thinner head gaskets are notorious for blowing, because they are affected by so much smaller of a change in the size/shape of the metal parts. For all the added risk, you'll get about half of a tenth of a point at most, at the CR you're at. IMHO, too much maze for nowhere near enough cheese.

Keep your eyes on the classifieds and elsewhere for a set of worked-up iron L98 heads. They'll outflow your "Hecho en Mexico" 993s, and will bump your CR up a good half a point, and you can sometimes get them that way for a whole lot cheaper than starting from scratch with a set of junkyard castings.

 

LMAO. The valve covers were off the other day and I saw that writing on the heads.

 

lol... yep... got the same fine merchandise from across the border under my hood too.

 

Just to let you know I'm running about 10.7:1 compression on 93 octane without a problem.

 

you could get a set of vortecs and mill them down. For a 305 you go .045" and that gives you a 55cc chamber and 10:1. Don't know what it would be for a 350, maybe .020"?

 

my heads with that same motor gave me 9.6:1 compression with 64cc chambers

 

Look for some 58 cc Aluminum L98 heads. They can be ported very easily and give decent flow and the D port exhaust.

That is what I installed on my 91 L98. Switching from the 64 cc iron to the 58cc aluminum bumped my CR from 9.75 to 10.15. Part of the reason the my increase in CR is not as much as you'd expect with 58cc heads is that I used a thicker head gasket.

In 1991, L98s came with .018" with the iron heads. That's one of hte reasons they have 9.75:1. With Aluminum L98 heads, GM doesn't recommend the .018" shim gasket. With .039", you would get JUST under 10:1. If you use the thinner .029" like I used (the thinnest recommended for the Aluminum L98 heads), you will get 10.15:1. The only way to get the CR higher is to mill the heads.

I chose not to mill my Al L98 heads because I may go to a 383 when I rebuild my engine and I would like to be able to reuse the Al L98 heads. With a thicker .051 gasket, I would get just under 10.5:1 (unless I have to deck my block).

PS: Make sure you clean up the combustion chamber, whatever you do. I usually run 94 octane anyway, but I got no knock retard whatsoever (my old heads were prone) and I have advance my spark advance further than with the iron heads.

The only thing I wish I had done was install larger 2.02/1.60 valves. Oh well, something for "rebuild time".