What is the CR of a stock motor

[turbowrenchhead]

Will be rebuilding soon and would like to know what the stock compresion ratio is. What would be a good cr to go with 8 to 1 or 8.5 to 1. I will use mostly on the street but drag race on the weekends. I like to run high boost and race gas at the track and pump gas and lower boost on the street. Give me some insight please.

 

[cool 84]

Stock is 8:1. I've run 7.8:1 with my TRW pistons in the past. Since I get to design my own pistons for my 4.1, I'm trying 8.5:1. Should get a little better mileage and better spoolup with very little difference in how much boost I can run for a certain octane. My main purpose in increasing it is so I can get away with a 3,000 stall or less with my T-64 turbo with no downsides. Only time will tell but I think it will work. Coming from Ken D himself, you don't want to go over 8.5 on a street car.

 

[turbowrenchhead]

Where are you getting pistons from? I called the local machine shop and asked them if you bored to 4.000 could you use chevy 350 pistons. They said highly likely.

 

[FJM568]

You can't just use off the shelf Chevy pistons...you need custom forged pistons with the correct pin height, ring location on the piston, etc...You are just using the stock diameter piston...Now what you can use off the shelf are the rings...There are many, many more choices for piston rings by going with the standard Chevy 4.000 bore than be using a custom turbo piston built for the Buick 4.1 by going to another size other than the Chevy 4.000...

Piston ring selection IS the main advantage to boring to a 4.000 bore...

 

[turbowrenchhead]

I asked those same questions. The tech said with the many various combonations with the chevy 350's there sould not be a problem finding the right pistons.

 

[TType84]

86/87's are 8.0:1.. hotairs have a little higher compression stock, due to deck height differences and steel shim HG's.. just a lil fyi

 

[FJM568]

84/5s have a taller deck height but a thinner headgasket...it all works out to be the same..

 

[TType84]

ever since i got into buicks everyones said hotairs are a bit higher on compression and a little bigger cam... and by a bit higher it was like 8.1 or so.. but maybe they were wrong

 

[cool 84]

The problem with Chevy pistons is most of the "blower" pistons are ~9 to 1 compression. On top of that you have to bush the small end of the rod since Buicks use a bigger pin. No one could find me an off the shelf Chevy piston with the right specs. It seems everyone either has a piston for the stock short Chevy rods or the 6.0" but nothing for the Buick length.

I'm having them make my pistons an actual part # so maybe there will be off the shelf forged pistons for the 4.1. BTW, the slug they're starting off with is a 351W forging. I guess I will have two Ford parts in my car now.