924Board.org

[ideola]

Have you ever asked any of these questions?

• If I resurface my block by .xxx", how much CR will I gain?
  
• What Cometic gasket should I use to achieve x.x:1 CR?
  
• What combination of NA head and 931 pistons will yield a usable result?
  
• What displacement and CR can I expect with a stroker setup?

I've just completed version 1.0 of an online interactive compression ratio calculator, which will provide answers to these and many other related questions.

I've been wanting to build one of these for some time. Finally got some down-time this holiday season to work on it.
Ideola's Compression Ratio Calculator

Features

• Includes 14 user-selectable profiles based on known-good OEM and custom configurations
  
• Ability to switch from metric to standard (for those of us with machinists who prefer working in thousandths rather than mm).
  
• Definitions for all of the parameters
  
• The three most commonly (and easily) modified parameters are highlighted in bright blue: deck height (changed by milling the block); cylinder bore (changed by going to oversized pistons); and head gasket thickness (changed by using thicker or thinner Cometic MLS gaskets)

Notes

• Where possible, pre-loaded values are based on OEM or manufacturer specs.
  
• Several specs are not published by Porsche, so I have done my best to "back into" those numbers based on my own observations, measurements, info distilled from this forum, comparisons to known aftermarket configurations, and a whole lot of number crunching.
  
• Block deck height is not published anywhere, but is an absolutely critical number for determining CR. Most blocks I've measured are between 230mm and 232mm. My presumption is that the factory original spec is 232mm.
  
• Head volume of the 931 head is presumed to be 19.5mm based on recent measurements by RC and by my head machinist.
  
• Piston dish volumes are "backed-into" calculations I've made based on OEM published CR, known compression height of the piston, and the presumed values for deck height and head volume listed above.
  
• OEM head gasket thickness is presumed to be .046". The calculator is pre-loaded with all of the available thicknesses from Cometic.
  
• The calculator is currently hard-coded for 4 cylinder engines, which really only impacts the calculated displacement. I am considering a future expansion of the tool to include profiles for 911, 944, 928, etc., at which time I will make the number of cylinders a definable parameter.

Take a look and let me know what you think. Suggestions and constructive feedback are welcomed!
_________________
erstwhile owner of just about every 924 variant ever made

[Fifty50Plus]

Based on these calcs, I've given my blocks and pistons to the machinist! Looking forward to kicking some SCCA ass in the spring....
_________________
1979 924 NA race car H-Prod SCCA
1982 924 NA race car - Sold
1981 924 Turbo sold
1982 924 Turbo sold
1972 911 E race car - traded for Cayenne Diesel
1975 914 1.8 Building for H-Prod SCCA

[peterld]

That's cool Dan! Nice work.
_________________
80/81 932/8 ROW

[Carrera RSR]

Hi Dan. I've noticed your garage website has been in and out of action recently. Any chance your very useful CR calculator can be reinstated. Thank you!
_________________
1980 931 - forged pistons, Piper cam, K27/26 3257 6.10 hybrid turbo, 951 FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife
Now www.924board.org/viewtopic.php?t=34690
Then www.924board.org/viewtopic.php?t=31252

[ideola]

[peterld]

Thanks Dan. Nice to be able to use it again.
_________________
80/81 932/8 ROW

[Carrera RSR]

Thank you Dan. Most appreciated.
_________________
1980 931 - forged pistons, Piper cam, K27/26 3257 6.10 hybrid turbo, 951 FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife
Now www.924board.org/viewtopic.php?t=34690
Then www.924board.org/viewtopic.php?t=31252