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. (The current version is pre-loaded with 924 2.0L parameters, but should be usable for 924S/944/968 applications provided you know the critical dimensions).
Ideola's Compression Ratio Calculator

Features

• Includes 14 user-selectable profiles based on known-good OEM and custom configurations (for 924 2.0L)
• 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!

Grandpa#3

A Friendly Note of Interest to you since you are not a paid Member nor Sponsor.

Please read the below posting.

https://rennlist.com/forums/924-931-...e-posting.html

ideola

I have been a long time member and sponsor, as well as an annual sponsor for 944Cup and Spec44 for the past three seasons. I have also contributed multiple times to Rennwish.

The post was not an ad, product announcement or solicitation. I thought it was a cool tool that would be useful for RLers.

Historical context.
https://rennlist.com/forums/964-foru...-cv-bolts.html
https://rennlist.com/forums/924-931-...as-garage.html
https://rennlist.com/forums/924-931-...n-starter.html
https://rennlist.com/forums/924-931-...ip-report.html
https://rennlist.com/forums/924-931-...il-2011-a.html
https://rennlist.com/forums/944-turb...round-3-a.html

Cris Brady

This is a cool idea, thanks for posting it. It's actually very topical as we've been having a big discussion over in 944Cup over our engine spec rules for 2013, and it looks like compression ratio will be a big part of it.

ideola

If someone can send me (or point me to) the critical dimensions for the 2.5L derived motors, I'd be happy to add them to the profiles.

Paulyy

Very neat!

I've got a turbo calculator program floating around here somewhere on the turbo forums!

If you want 944 specs, post on the turbo forum, a lot of frankenstein engines happen there. 2.5 with 16v head, 3L with 2.7 head. 3L with 2.5 8v head. There's now custom "budget" stroker builds which are very neat.

Grandpa#3

Ideola, I owe you my most humble of apologies. I have rotten egg smeared all over my face again. An apology is better late than never given at all. I should have kept up with this posting.

Cheers,
Larry

ideola

It's OK, and not your fault...they've been having trouble with renewals and reflecting users' proper status...didn't notice mine until after you posted, but it's fixed now. Apology accepted and no harm done

ideola

Does anyone know where to find the critical dimensions for the 2.5L series motors? In particular, I need block height, volume in head, and volume in pistons. It would be great to get these figures for all of the various permutations. I think I can get the rest of what I need in the tech specs booklets, but those don't publish the block height or combustion chamber volume figures...

milodorr

The deck height of a 2.5L block is 230mm. I believe that combustion chamber volumes range from about 55cc to 62cc. For '86 turbo pistons, the diameter is 99.3mm and the compression height is 40.7mm. In case you're taking into account such details, the distance between the top of an '86 turbo piston and the top ring land is 8.5mm.

ideola

Thanks for that, I'll play with those numbers and see if they add up!

ideola

Using the numbers provided by milodorr, I've just added a profile for the 86 951. Check it out: http://garage.ideola.com/top-DL-Comp...tioCalc.html?8

Since I don't have a breakdown of the volume in the head vs in the piston, I put all 55cc in the head for now. Will try to get better definition and some other profiles now that we have a starting point.