Compression Ratio

[Garysriv]

Just a question for you guys while I have the opportunity.  I believe the factory comp ratio on my '65 401 is 10.25:1.  Since the shop is boring the block .030, that will go up a bit (have a calculator-but for now a general question).  I will be using Best head gaskets which I believe are battle thicker than the stock flat steel ones.  Has anyone purposely reduced the compression on their 401 by milling the flat part of the piston?

 

When I built my GTO I calculated it down to 9.5:1.  It actually runs well on 89 octane and still has plenty of power (455 +.030 with aluminum heads).

 

Thanks

 

[telriv]

   The best STOCK compression I've EVER measured was 9.75-1 with the piston in the hole at .040" which is unusual. Most times on average they are in the hole .055" which now equals around 9.45-1. A .030" overbore WILL NOT raise compression a considerable amount. Add to that the .035" compressed thickness of the best gasket & the LARGER bore diameter of around 4.425" as opposed to a stock gasket of about .015"-.016" & bore of around 4.350" & you'll end up with about 8.8-1. Because of the combustion chamber design you can run a REAL 10.0-1 using today's available fuel.  Another thing to consider is MOST cast replacement pistons have to fit ALL applications with differing deck heights which can be up to as much .020" difference. On average they will sit in the hole somewhere between .060"-.080". Now your compression will be closer to 8.3-8.4-1. ANOTHER thing to consider is the quench/squish area. Our "Nails" run best at .025"-.050" which helps detonation resistance even more, ups available HP, runs more efficiently & other factors I'm forgetting at the moment.  From the factory they averaged out at .055"-.070". Now your talking around .100" +.

   There's LOTS of things MANY don't even think about when rebuilding their engines. You don't know how many calls & inquiry's I get about how the new/rebuilt engine doesn't have as much power/performance as the old worn out engine they just had rebuilt. Doesn't get as much fuel mileage as before, etc.

   Just some MORE things to consider when rebuilding your "Nail".

 

 

Tom T.

 

 

[Garysriv]

5 hours ago, telriv said:

......

5 hours ago, telriv said:

.....Just some MORE things to consider when rebuilding your "Nail".

 

Tom T.

 

 

Wow, great info Tom, sounds like ill be okay with the component mix I have.  Also gives me some good change dimensions to measure when I get everything close to assembly.

[telriv]

Highly doubtful. Start measuring & you'll see how far off you are in reality.