Few pay much attention to head gaskets, but the head gasket is an important engine performance part. In particular, head gasket thickness is critical to achieve desired compression ratio.

It should be as high as possible, but not over the detonation threshold, and the target CR range is a function of installed camshaft. The later the inlet valve closes the higher the target range can be. Good maximum targets are 9.75 with the 300 HP cam, 10.25 for the Duntov and L-79 cams, and 10.5 for the LT-1 and 30-30 cams. The bottom end of the target range should be no less than the above maximums less 0.5. These are actual measured CRs, not spec book pipe dreams and represent achieveable target maximums/ranges that can be obtained with OE replacement pistons, typical OE machined deck heights, and proper head gasket compressed thickness selection.

The proper measurements to determine CR have been discussed many times before, and it starts with measuring deck clearance of each cylinder prior to block teardown. Online CR calculators are also readily available, and I've posted a link to one many times.

To illustrate the role that head gaskets play, a change in compressed thickness of only .010" will change the CR of a typical small block by about 0.25 point. Judicious head gasket selection is critical to a good final rebuild product, and different head gaskets on each side can be used to equalize the CR and quench height when deck height/clearance is different side to side.

The big players in replacement head gaskets are Felpro (Federal Mogul), Victor Reintz (Dana), McCord, and ROL manufacturing.

My recent survey of head gaskets indicates that compressed thicknesses are available from .015" to any reasonable value in .005" increments either with single gaskets or by doubling gaskets, including gasket of different types.

If your block and head surfaces are dead level, then an OE shim type gasket can be used, and many have organic coatings, which aid sealing. Composition gaskets are thicker and more forgiving of slight warp, and most will seal with up to about .003" warp. Shim type gaskets can be doubled (as GM did on SHP/FI engines in the sixties) or they can be mated with a composition gasket to achieve desired final compressed thickness, but the shim type gasket must bear on a surface that is dead level. Also, Felpro offers a .020" flat copper shim that is designed to be used as a "spacer" with their composition gasket to lower CR, but I expect it can also be used with a shim gasket, and place the copper shim above the raised portions of the shim gasket so they crush against the copper when the gasket is compressed.

Compression ratio calculations should take into account both compressed thickness and gasket bore ID. If ID is not available use 4.100" as a representative average. ROL Manufacturing has the best online catalog, which includes all relevent tech data and a good cross reference listing.

Some SB gaskets have smaller diameter than is acceptable for 4" bore (and overbored) blocks, so pay attention to bore diameter. Some have larger than necessary diameter and "steam holes", which are necessary on 400 blocks, and these will usually work okay on 4" bore blocks. Also, be aware that cross references mean that a gasket will work in a given application, but thickness may not be exactly the same, so always verify compressed thicknesses and bore diameter with the manufacturer. If the data is not available in online or paper catalogs, call the manufacturer's tech support line for verification of dimensions.

So if you have an engine rebuild in progress or plan one in the future, pay attention to head gasket technology and thickness to achieve the best possible results.

Duke

 

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

this site has an easy to use tool to calculate your actual CR based on measurements

 

Thanks for the head gasket primer.

Another good link for a tool to calculate CR and the dynamic CR (DCR) is :

http://members.uia.net/pkelley2/DynamicCR.html

This calculator can be downloaded, eliminating the need to log into the internet to model a combination.

The hard part with the DCR calculation is finding accurate cam timing information.

 

Another cut & paste for my folder. Thanks for the info.

Bud.

 

Duke:

What is your take on the issue of quench? Put another way, when do you stop stacking gaskets to lower compression and say it’s time to modify the piston or head cc’s?

Also, is there any connection between the quench area and the quench height, that is, does a bigger quench area tolerate a larger quench height?

 

You cannot beat Duke for plain good info.

Thank you Duke.

 

Some guys make a big deal out of quench clearance. On a race engine it's important when you are striving for every last iota of compression and power on the available fuel octane, but I don't think it's that important on a road engine, especially on a restoration project where you are not willing to deck the block to achieve minimum quench clearance.

The smaller the quench clearance and the larger the quench area, the higher CR an engine will tolerate on a given fuel octane, but Taylor says the effects of detonation resistance are mostly lost once quench clearance goes beyond .005 of bore size, which is .020" on a 4" bore, which is less than Chevrolet's minimum recommended .035-.040" clearance.

On a restoration project if quench clearance has to go beyond .060" to keep under the target maximum CR, consider a different piston, or if the pistons are domed, consider having the dome partially or completely turned down to bring CR in under the target maximum.

For example, if you have a SHP small block and the deck height/clearance is right near the 9.025"/.025" nominal spec, the heads are about 64 cc, and the surfaces are dead level, turning down most of the piston dome to net zero piston volume will yield about 10.25 CR and .040" quench clearance with a .015" shim gasket, which is a nice outcome that should allow an aggressive centrifugal advance curve that will maximize low end torque without detonation.

With the standard 5.3 cc dome volume the CR with this setup would be right at the 11:1 spec, so as you can see, a few cc of piston or head chamber volume can make a signficant difference. That's why having all the data and measurments to compute CR is so important.

Duke

 

Thanks for the info Duke.