The results of measuring my heads, clearances, and compression has given me a lot of anxiety. In consulting with my machinist, his advice has been to use thick shims to avoid interference and run with it.
Uncomfortable with some conflicting info and not knowing how to proceed, I used a metal contour gauge to try to get an accurate profile for the shape of the head and piston. The contours are subtle, so It’s not perfect but a decent reference I think. It has been a long time since I used a protractor…
The head has at least four angles (measuring cross section perpendicular to wrist pin). They range from 13-degree on the squish angle to 49 in the deepest part of the cavity. They are not identical in each head cavity.
New AA Pistion appears spot on 22-degree as you would hope
Since my forum posts, I consulted with a long-time 356 Registry Technical contributor and a well know Porsche mechanic for another opinion. Both expressed concern with the angle of the head. The head angle should be parallel to piston crown, or worst case a little greater than, so as to not create pockets for the combustible mix during ignition. Most of my head measurements of the squish area are coming up several degrees less than the piston crown (22-degree).
In addition, neither were comfortable accepting a compression ratio of 9.31 with a deck height of 1.67mm I was getting with .75mm cylinder base gaskets.
Targets
You need to know what you’re targeting if you want a clear path to success. Even here there are some differing opinions on what is acceptable. Best I can surmise:
Deck Height
- If you’re a pro, or very confident in your measurements, machine shop, and assembly skills, it appears the target is 1mm (.04 inch). If you get any tighter you risk exhaust valves hitting things when oil gets hotter / thinner and the engine begins to wear.
- A safe desirable range seems to be somewhere 1 – 1.54mm (.04 – .060 inch).
- Heading up into 1.575 – 1.6 mm (.062-.063 inch) the engine will probably run without problem, but is getting a little “out there”
Compression Ratio
- A Super 90 stock compression ratio is 9:1
- Everything else being equal the AA Big Bore Kit should land you at 9.2:1
- Folks seem comfortable getting up into the 9.3x:1, staying below 9.4:1 for a driver
