SHP big block compression ratio surprise!

[Duke W.]

I've run the numbers on several compression ratio calculators, and came up with some surprising, if distrubing, results. I used the following data for the nominal 4.25" OE bore

Nominal deck clearance, .020"

Piston dome volume, 36.3 cc (from the Speed Pro catalog for OE replacement pistons)

Head chamber volume 106.9cc (also from the Speed Pro catalog for '66 858 heads)

Head gasket .022" compressed thinkness, 4.38" bore opening. (Based on the 14015351 shim head gasket listed in the GMPP catalog for early Mark IVs)

This yields a lofty 11.83:1, but the OE spec is 11.0:1 and most SBs compute a quarter to a half point less than the advertised CR.

Even if I substitute a .038" thick compostion gasket the CR still remains very high at 11.33:1.

The '67 AMA specs also list total combustion chamber volume as 4.92 cubic inches, so (53.34 + 4.92)/4.92 = 11.84, which jibes with the CR calculators.

I'm just amazed that these engines could have an as-built CR of over 3/4 of a point higher than advertised, which may explain their propensity to detonate on current pump premium.

Is the data I'm using correct?

Has anybody taken all the measurments and computed the CR on a real SHP big block with OE equivalent pistons?

Duke

[Clem Z.]

Re: SHP big block compression ratio surprise!

anyone i ever build had to have the deck cut and the heads cut to get to the AMA/NHRA spec sheet numbers. they .008 deck,106.9 chamber,.350 high dome,36.34 dome volume,.028 gasket. i never calculated the CR all i did was follow the number required.

[Duke W.]

Re: SHP big block compression ratio surprise!

With those numbers I get 12:1!!! ...great for racing, but not so good for a restoration engine that you want to run on a diet of 91 PON Califunky pump premium.

I was initially targeting something near 10.25. ... may have to go with custom made pistons with a smaller dome or cut down the dome on the Speed Pros, if possible, without compromising crown stength.

Duke

[Clem Z.]

Re: SHP big block compression ratio surprise!

they ran great on sunoco 260 back then

[Clem Z.]

Re: SHP big block compression ratio surprise!

also don't forget the CC loss between the cylinder wall and the piston above the top ring.

[Duke W.]

Re: SHP big block compression ratio surprise!

I've never run across a CR calculator that accounts for that. On modern high land rings it's probably insignificant, but not on older designs with a lower top ring land.

...any idea how much volume it adds to either a vintage SB or BB with OE type low top ring lands?

Duke

[Clem Z.]

i should have it somewhere

easy to calculate.volume of the bore above the ring minus volume of the piston above the ring

[Duke W.]

Re: i should have it somewhere

I'd be curious.

Pi times the diameter at the crown times the distance the top of the first ring groove is below the crown is a very close approximation to the exact volume.

Anyone have an old piston laying around?

Duke

[Clem Z.]

stock bore piston,4.218 dia,.305 down *NM*

[Duke W.]

Volume equals...

Well, I screwed up verbalizing the formula in the earlier post, BUT:

.305(3.1416)(4.234)(4.250-4.218)/2 = .0649 cubic inch, which is just over 1 cc.

(4.234 is the mid point diameter of the gap, so its circumference times the gap width times the gap height is volume to a very close approximation)

Not much effect on a L-72 - about 0.13 point.

I figured about a tenth, and it provides a little margin if you push the number, but I might start taking this into account in my calculations by adding a cc to head volume.

Thanks for all the help today, Clem. You can go to bed now.

Duke

[Joe L.]

Re: SHP big block compression ratio surprise!

Duke-----

Most of the numbers that you used in your calculations look about right to me. However, I'm not so sure about the dome volume for the closed chamber L-72 piston. However, I have no better information. I don't have an original L-72 piston to measure.

It's hard to imagine that GM would have been that far off in their specification for the compression ratio of the L-72, especially that much to the LOW side of actual.

[Clem Z.]

as delivered i bet they were in the 11:1/11.5:1

range but GM wanted to have the racers have a "competition advantage so they speced the engine to get a higher CR. good thing the tracks did not have a "katech whistler" that checked the CR of assembled engine back in those days

[Richard F.]

Re: as delivered i bet they were in the 11:1/11.5:

In the spirit of good navel-gazing, I just measured the distance down to the first ring land on a TRW2239, .60 over piston. It is pretty close to .320 inches. These are 12:1 pistons. I took them out years ago when the gas was even crummier than it is today. I wish I'd checked all of these compression-related dimensions then, before I rebuilt and blueprinted the block and heads.