How To Calculate Compression Ratio and Piston Clearance 356/912

[joe cogbill]

Here is a little video I made when I rebuilt a 356 engine recently. This is an instructional video how to calculate your Porsche 356 or 912 compression ratio and how to measure the distance from the piston to the cylinder head to decide how many copper shims should go under the cylinders to build a proper Porsche 356 or 912 Motor. This works for Porsche 911 also with different math. Ok I am not a video guy. The racing pictures are jerky and the sound is sometimes compromised but the information is good. It was recorded just using an I phone 8. I hope it helps you. The 356 racing community has always helped each other with Vic Skirmants, John Osteen, Robert Overby, and George Bryan and many others sharing the information. Here is the link.

https://www.youtube.com/watch?v=vL3v80iN-LM

[Wes Bender]

OK Ron. Your cue.......

[Ron LaDow]

OK Ron. Your cue.......

Wes, how can I pass up that invitation?
Joe, suffice to say we are not in agreement, especially regarding piston clearance. I've never built 616 race engines, so my advice is limited to 616 engines running to 6KRPM, but...
(added by edit) I've certainly built other race engines, SBC and Cosworth DFV among them.

Here's the "Why":
"QUENCH AREA INFLUENCE ON COMPRESSION RATIO, IGNITION LEAD. AND POWER.
(or WHY THE LAST 15% OF AN ENGINE’S POWER TAKES 85% OF THE TIME)"
https://www.912bbs.org/forum/threads/wh ... ers.56252/

And here's the "How":
https://precisionmatters.biz/pdf/compression-ratio.pdf

[Harlan Halsey]

1. I used Joe's method for years, sans the solder. I measure the clay directly. Chuck Forge did the same. While Joe's direct measurement is less subject to error, I went to bench measuring the head volume and the crown volume and calculating the head space and CR simply because it is much less messy. Manhandling the whole engine and spilling the ATF/solvent fluid on the floor got old. I used to check TDC by rocking the crank a bit with the chamber full.
2. Engines are not rigid. Therefore a minimum piston to head clearance at rest is needed to insure clearance at the red line. Years ago Dema Elgin suggested .050". Occasionally I have shaded that a bit, to .040".
3. Deck height is easy to measure with a linear digital caliper.
4. Quench, and its effect on power, is a separate topic. Ron's posts, while long on nice illustrations, are short on either race results or dyno proof.

[Ron LaDow]

They are the result of dyno tests. 'Nuff said.

[Steve Hatfield]

Most everything I’ve read or listened to over the past ten years has said anything over .040” squish is unnecessary (and inefficient) in anything other the a Briggs lawnmower. Please. What’s the real answer guys?