What is my compression ratio?

[Paul J.]

Can anybody tell me the com. ratio of my '61 230hp? It has .040" over bore and I will be using FelPro gasket from gasket set, Ibelieve it is .039"thick.
Duke if you see this I will be getting my 283" head flow specs posted soon I hope. Thanks in advance.

[Joe C.]

Re: What is my compression ratio?

You'll need accurate measurements of your cyl. head volume, head gasket cylinder bore diameter, piston-to-deck clearance, and piston crown volume/or valve relief volume. Also need the obvious bore and stroke measurements. Do not use published values (except for piston crown data), but measure those of your engine.
Here is a link to one of the better calculators that I have found. It takes everything into account, including rarely included head gasket bore diameter. It only neglects "ring land volume" which is negligible:

Joe

http://www.csgnetwork.com/compcalc.html

[Timothy B.]

Re: What is my compression ratio?

Paul,

Try this calculator, with the correct intake valve closing point data you cam get an idea of dynamic compression which is what the engine really see's.

http://www.kb-silvolite.com/calc.php?action=comp

[Paul J.]

What is my compression ratio?

Thanks Joe and Timothy.

[Duke W.]

Re: What is my compression ratio?

The OE spec for your engine is 9.5:1, but that was with a thin shim gasket. With that thick gasket it will probably be less than 9:1, and the engine will probably tolerate regular fuel, but you will be leaving performance on the table.

For base camshaft engines, I recommend a maximum true CR of 9:75:1, and as Joe said the only way to determine this is to have accurate dimensions/volumes and run them through the calculator.

Measure your deck clearance and at least use nominal published volumes for the piston and head chamber. Then use the calculator to select an suitable gasket. If your block and head mating surfaces are flat a coated shim gasket should seal.

Those thick composition gaskets are okay for quicky commercial rebuilders since they will tolerate more warp than a shim gasket, and the low CR precludes detonation, but they are not useful for precision engine restorations.

The dynamic CR calculator on the KB piston web site is useless!

Duke

[Timothy B.]

Re: What is my compression ratio?

I don't understand why the intake valve closing point @ .050 plus 15 degrees is asked for on the calculator. The way I understand things, intake closing point at a checking clearance about .010 (tappet) for hydraulic which should yield approx. .015 valve lift is a good figure to use. I think solid lifters use .020 checking clearance. The closest I can find for the hydraulic 300HP 929 cam is 59*. Where am I wrong here, and why won't the calculator work with these figures? Sorry for the long post and getting off subject. Tim

[Paul J.]

Re: What is my compression ratio?

My heads are not flat so I was accepting the use of composition gasket because even if I did get them trued it seems the shim gaskets will not safely seal because of the block. I guess there is nothing to do about raising c/r cost effectively. I was just curious if the overbore would equal the thicker gasket.

[Duke W.]

Re: What is my compression ratio?

The DCR is the ratio of total volume to clearance volume at the crankshaft postion where the inlet valve closes.

The question is: When does the inlet valve "close". SAE defines it as .006" valve lift. The KB web site defines it as an ARBITRARY 15 degrees after the .050" lift point. The SAE method yields a lower number than the KB method.

You've given a couple of other "definitions". Which one is "right'? Which one is used when someone "quotes" a DCR?

That's the trouble with DCR. It is not consistently calculated and depending on the SCR and cam you may get a number less than 8:1, which is "okay", but may not work. For example a 327 with a 12.5:1 CR and a BIG cam may have a DCR of less than 8:1. So will it run on unleaded premium?

NO WAY!

An engine with a mild cam like the OE 300 HP cam may be okay with a 8:1 DCR, but that doesn't apply to all cases such as the one I mentioned above.

DCR is just a guideline that has to be taken with a grain of salt. Experience indicates the maximum true SCR, which is a precisely calculated unambiguous number, and I have listed the maximum true SCR I recommend for all OE engine configuration on several occasions.

DCR gives some indication of whether the engine will detonate at low revs, and low rev detonation can usually be tuned out with ignition map changes. DCR is of little value in determining whether an engine will detonate in the mid range near the torque peak when VE is highest. That can only be determined by experience.

Duke

[Duke W.]

Re: What is my compression ratio?

Why? Is the block deck not flat?

If the block is flat you could get the heads skimmed, but then you will have to measure the chamber volumes.

If the head and block pass the .0015" feeler gage and machinists bar flatness test, a coated shim gasket should seal.

The thinnest compostion gasket I know of is a the .026" Victor Reinz 5746, available at NAPA.

Duke