Name those mystery pistons

[Jeff V.]

CR is calculated based on multiple variables such as rod length, compression chamber volume and head gasket thickness.

DRP did give me the piston dish volume, which I need for calculating the ratio.

When someone like 3SX sells you "8:1 pistons" that's assuming everything else in the engine is stock. Use custom heads that have had the chamber enlarged, and suddenly those pistons aren't 8:1 any more.

[Jeff V.]

Here's what I've learned. Anyone care to add their $0.02?


Does anyone disagree with these numbers (assuming an otherwise stock 6G72)?

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

Bore: 92mm (looks to have been slightly over bored)
Stroke: 76mm
Rod Length: 140mm
Deck height: 210mm
Chamber volume: 48cc
Compression height: 32mm
Crank throw: 38mm
Head gasket bore: 92mm
Compressed HG thickness: 1.0mm
DRP piston volume: -16.8cc

CR: 8.07:1

I think I may be a little off on the compressed HG thickness, and the only number I've heard for the stock piston volume is 10cc, which comes out way too low. Using all the other numbers above, that'd put the stock engine at 8.5:1. So either the 10cc number is crap (which I suspect) or John's measurement of the HG is way off. (Which he admitted to being possible, but it works with my other numbers)

[jackhammer]

I cc'd the stock pistons and the combustion chambers at one point. It would take a lot of digging to find my notes since ive moved twice since then, if i even still have the notes. Ive tossed a lot of stuff since i quit working on cars

[Hans_GZP]

I believe the stock head gasket height is a little greater then 1mm... something like 1.35mm sounds about right (about 0.054 inches which I believe is what cometic gaskets are).

[Jeff V.]

John Monin came up with .042" (1.03mm)

Helmet has some brand new ones coming in. I'll measure them when they show up.

[sublime_whatigo]

~1.35mm on new fel-pro gasket...

[Jeff V.]

I got anywhere between 1.15 and 1.25 on a new OEM HG that Jeremy C had laying around. Mind you, that's compressed in the caliper. I bet it squishes a bit more once the head is torqued down.