compression increase

[Burke Kilgour]

I am running a 1983 MERC 150 and have a spare set of heads .Does anyone know how much I can shave off these heads and still run premium pump gas ? I've heard .030" a few times but would like to hear from people that have learned the hard way so I don't have to . What kind of jetting will I have to do , if any ?

[Jay Smith]

Burke,
Sorry to say it ain't that easy. I just cut a set of heads this afternoon with the same part number off the same motor and had to cut .015 more off one head to equal the cc numbers off the other ! Have someone cut your heads to a cc not a thousands of an inch you'll be much happier. Also it make a world of difference what head gaskets your planning on using !( thickness vary as some are .041 to .065 different in the same package from mercury )Not to say its rocket science but it ain't as easy as cutting off X amount of material !

Good luck and good boating !
http://www.jaysmithracing.com
Jay @ JSRE

[4Speed]

I have a 1993 150 HP 2.0L. I had G Force do them about 5 years ago. They were all cc'd to 32cc (32 - 34 is stock) as I told Gene I want to be able to run 87 octane (which I have never had to do)in the event of a emergency. 32cc will allow for that... Better safe than sorry... Gene also supplied the head gaskets.

[Burke Kilgour]

Thanks for the replies . I will cc them in that case .
I will ask one more thing then......... what cc can I go down to and still run premium pump gas ? You said you went down to the lower stock cc , can you go lower than that ?
These heads are just collecting dust on the shelf and I'm eager to give them a try !!

[4Speed]

I heard when looking for the same data you are the following: 122 Cubic Inch motor, 93 or higher octane => 26cc. 100 octane or higher => 21cc. Depending on your casting number for every .005 removed the cc can change .297 to .338 cc.
This info was for one piece heads, add 1 cc to two piece heads. My guess is that there is no margin of safety in these numbers.

[Burke Kilgour]

I have the two piece heads and apparently the stock cc's are 32-34 , is this wrong ? If it is correct then I have lots of room to the 26 cc range .Which is probably the safer bet for me. I don't want to push the performance envelope and blow up my motor !!
THANKS !!