Can somebody explain compression height?

[Norris W.]

I'm trying to figure some component combos for building up a project. I was looking at a set of pistons and the compression height was described as 1.095. I measured a set of LS7/454 pistons and they measure 1.095 from the top of the wristpin to the top of the piston. Logically I would have thought the measurement would have been from the center of the wristpin to the top. If that were true the deck height on the 454 pistons would be more like 1.595 and I can't figure a common stroke/rod length combination that would put the 1.095's in either a short or tall deck block. So the question is, how is compression height measured?

[Duke W.]

Re: Can somebody explain compression height?

The compression height is the distance from the center of the pin to the piston crown, exclusive of any dome or dish. On flattop pistons the crown is well defined. If the piston is domed or dished there should be a machined area around the periphery of the piston, which is the crown.

You might want to check of the compression ratio article co-authored by John McRae and me a couple of years ago in The Corvette Restorer. It describes how to achieve a specific compression ratio, including all the required measurements and reference to a Web-based compression ratio calculator.

Duke

[Norris W.]

Re: Can somebody explain compression height?

The compression height is the distance from the center of the pin to the piston crown, exclusive of any dome or dish. On flattop pistons the crown is well defined. If the piston is domed or dished there should be a machined area around the periphery of the piston, which is the crown.

You might want to check of the compression ratio article co-authored by John McRae and me a couple of years ago in The Corvette Restorer. It describes how to achieve a specific compression ratio, including all the required measurements and reference to a Web-based compression ratio calculator.

Duke

Thanks Duke. Although I've never ordered components that way, I figured it was the only logical way to measure. It seemed odd though that the compression height on a custom set of pistons would measure about the same (approx. measurement done with vernier caliper) as the top of the pin to the top of the piston (excluding the dome of course), on stock LS7/454 pistons, so I started second guessing my assumption on how they should be measured. This became especially true when I could not get the deck height to work out to zero with my calculator with commonly available stroke or rod length. I'm going to do the math again and see if I made a mathmatical error first go round.

I wish I had all the old restorers, but get about 30 magazines a month, so we keep 'em for two or three months and then to the recycle bin they go.

I don't know if you feel like venturing a rough guess but what I'm looking at is 4.375 stroke, 4.560 bore, 32CC dome (but unsure of volume of valve relief and whether or not it is considered in advertised dome volume) 118 CC heads, unknown deck height and head gasket thickness (to be determined). Is that enough information for some rough peramiters?

[Joe R.]

Re: Can somebody explain compression height?

Norris,

Here's a link to an online compression ratio calculator. I don't know if it's the one Duke prefers, or not:

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

Joe

[Norris W.]

Re: Can somebody explain compression height?

Thank you. That's very helpful.

[Duke W.]

Re: Can somebody explain compression height?

Yes, that's the calculator I use and included the link in the compression ratio article.

Norris - send me an email via the TDB email link, and I'll send you a pdf of the compression ratio article.
Since I work mostly with small blocks, I don't recall offhand some of the nominal data for big blocks, but I think the nominal deck height is 9.200". I just can't remember the nominal connecting rod center to center distance. Maybe someone like Joe Lucia could chime in and refresh our memories.

You need the crank throw radius, which is half the stroke, rod center to center distance, and piston compression height. This will allow the computation of nominal deck clearance, which goes into the calculator along with head chamber volume and piston volume. Then you can try different head gasket thicknesses to see what range of CRs you can achieve, but actual deck height measurements of all eight cylinders should be taken when the short block assembly is complete to make the final head gasket choice.

Piston dome volume is usually specified as net, which is net of any valve clearance notches, but as stated in the article, make sure you get the algebaic sign correct as different manufacturers use different sign conventions.

Duke