I have a set of aluminum L98 heads that I'm going to put on my L98. They've been decked, so I had the combustion chambers cc'd and they're 54.8 cc as opposed to the stock 58cc chambers. I'm runnning KB pistons which are stock replacements, but are hypereutectic. They sit .045" below the deck and the engine was bored .030". I calculated the compression and it came to 10.1:1. My question is how much compression can I run with the aluminum heads and be able to run 89 octane gas with no knock? I know that the LT1's have 10.1:1, but their computers detect the knock and adjust the timing tables and fuel curves accordingly. The stock L98 vettes run 10.9, but did they have problems with detonation? I just want to make sure I'm not going to have to run 93 all the time. Thx.

 

.045" is WAY too much IMO. You should have closer to .005"
Too much deck can make the engine prone to detonate.
If your deck was .005" you could probably run 10.5:1 with no problems on 89 octane.

 

I'm running about 10.35:1 with those heads on a 350, it will run on 87 but doesnt like it on hot days. Can notice a power difference too. Cam also plays a big role in detonation, wrong cam and it will knock like crazy.

 

Nasty Bird

Is that .045 under the bare deck of the block or is that also including your gasket? If that doesnt include the gasket you are going to have a very "octane sensitive" engine. A big thing that people miss is that from the factory a .015 steel shim is used as a head gasket, which is fine when running iron heads to an iron block. That is a no no for aluminum heads, you must run a "composition" head gasket. When building for performace most people run aluminum heads, and with that a performace gasket from felpro or similair. Now these gaskets are built alot different and range from .039-.041 for the gasket alone, they are designed for a "zero deck" height. So if you have .045 on bare deck and plan to run a performace gasket set you will possibly have serious problems getting to run on pump gas plus a drop in performance. Although the L98 heads are good they still dont have an extremely efficient combustion chamber. GM states .035 is the minimum clearance including both gasket and deck height, piston in the hole. Most performance engines dont see anything past .045-.050 total. Anything beyond .050 but less than .060 total can live on pump gas but questionably. Anything over .060 total and dire attention needs to be paid to the compression ratio and should not be used in a performace application.

Rule of thumb with aluminum heads on a performace build: Have your block square decked down to 9.005 and run a .041 gasket. Any piston that is flat top or dished will fit comfy in that space.

My .02.

 

Thanx for all the help guys. I made a trip to the machine shop today and got everything taken care of.