Thread: compression ratios

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trying to calc. compression ratio on the 1971 460 with Dove c heads aprox 75.8 cc
bore 4.36 plus 30 over, and stroke 3.85. The pistons are dished .230. Any help would be appreciated.

the 460 with a stock crank with a 22cc dish would be 9.9

Thanks Pat. Was shooting between 9.5 to 10.0 so that will work just fine.

Your combo with L2404's (assuming) will get you at about 9.5:1 but with about 0.070" quench distance. Reduce that to about 0.040"-0.050" and detonation control will improve significantly.

Paul

Paul I talked with Casey in Wa. today. He said they bored it over.040 and got 200lbs compression on each cylinder, so I think it should be o.k. on 91 octane pump gas. Let me know what you think. Steve.

Originally Posted by stovens

Paul I talked with Casey in Wa. today. He said they bored it over.040 and got 200lbs compression on each cylinder, so I think it should be o.k. on 91 octane pump gas. Let me know what you think. Steve.

It's not the cylinder pressure he was talking about. It's the squish. (distance from the crown of the piston to the underside of the head with the piston at TDC)

The way I understand it, blocks before 1972 were cast and machined with a block deck height of 10.300". '72 and later blocks were cast and machined with a 10.320" block deck height.

With Silvolite 1143 pistons, your "stack" calculates to 10.290". If the block is 10.300, that leaves a piston deck height of 0.010". Combine that with a (guesstimate) 0.041" head gasket and you get a squish of 0.051".

The same stack, if used in a 10.320" block, would yield a squish of 0.071" which would be too wide to make an effective squish in the motor and it may have a tendency to detonate.

I think that's what Paul was getting at.

Hey Denny nice to see you back.
Tech thanks for the clarification and calculations. I'd never heard of squish before. I know the reason we went with the dished pistons was to lower the compression which I'd assumed lowers the chances of predetonation which would void the warranty(not to mention make a mess!)
Good to know it's right around .051 which falls very close to the numbers Paul gave.

Steve-o,

I agree with the two posts directly above. 200 cranking compression on pump gas is a bit much for my personal taste, and the quench distance in your combo could be minimized to improve detonation control/counter the high dynamic compression ratio's influence on detonation.

Don't mean to alarm you; it's very possible that your builder already addressed zero-decking the block, etc. Also, depending on exactly how this engine was machined/built, things may be just fine as they are. But now is the time to find all this out, not when it is running in the vehicle and you realize it has to come out again.

If the block indeed needs zero decking and the builder says you did not specify this and the work is extra, he is technically right.

(Parallel posting )

You ought to be okay with 0.051" quench distance.

Paul

The engine block is a D1ve(1971 lincoln) with the dove-c heads. So the deck height should work out to the .051 figure. Thanks Paul for all of your help and Tech too. Glad I didn't go for higher compression! I guess I could by octane booster to help too!

Originally Posted by stovens

The engine block is a D1ve(1971 lincoln) with the dove-c heads. So the deck height should work out to the .051 figure. Thanks Paul for all of your help and Tech too. Glad I didn't go for higher compression! I guess I could by octane booster to help too!

D1VE blocks were used in production for the 1971-1978 model years, so it can be machined anywhere relative to intended deck hieght for a given year. The actual casting date code is in the lifter valley and is alphanumeric, such as "3D4" or "5C27," the first number in the code being the year.

Paul