Max compression on pump 93 gas
 

Hi All,
I am an experienced Porsche enthusiast and an engine builder. I have a 3.8l 964 motor in parts and I will be building it for my 77s. It has Mahle motor sport 3.6 to 3.8 liners and pistons. The pistons are 12.5:1. I am guessing this is too much for 93 oct. Has anyone got experience off skimming the top off the high dome pistons? It would be great to use them but I am concerned about the piston thickness and if I would be taking any sort of coating off the top of the piston. Also what is a nice sweet spot of compression for 93oct. I unfortunately have not figured out the cam specification yet as that can have an effect of the working compression ratio.
Any thoughts would be great.
Cheers
Lee

The real question is "what is the dynamic compression for safe running on 93 octane fuel"
Generally the number for an aluminum head is around 8.5:1.
You will need to decide on your cam specifications before you can calculate dynamic compression.

If your pistons have too much compression and you want to trim them, as long as you have .180" at the thinnest point, you're good to go. I shoot for .220" on turbo pistons. Under-crown milling will be the challenge.

I came up with a formula for trimming that seems pretty consistent.

Formula For Milling Pistons
For 4032 material
Piston dome cc's to gram conversion: 1cc (volume) = 2.8 grams (weight)

This is a good way to remove excess dome without having to re-cc piston after each cut.: Mill a small amount and re-weight piston until total desired reduction is reached.
Example: A piston has 12.5cc effective dome volume. The desired effective dome volume is 10.5cc.
To remove 2.0cc, cut 5.6 grams (2 X 2.8) from the piston dome.

Piston modification calculations

Basic number for weight vs. volume for forged aluminum pistons is 2.8 grams / cc.

Starting dome volume 43.5 cc 10.5:1 compression ratio
Desired dome volume 29.5 cc 8.5:1 compression ratio
Desired dome change 14.0 cc x 2.8 = 39.2
Starting weight 463.41 grams - 39.2 = 424.21
Actual weight change 14.65 x 2.8 = 41.02
Actual dome change 463.41 – 41.02 = 422.39
Actual dome volume 28.85 8.42:1 compression ratio (measured)

Awesome response. As mentioned I have to figure the camshaft I have for the dynamic compression. The cc to gram calculation is awesome. I will be able to measure the Mahle pistons dome depth pretty easy. On a 964 is it possible to shim under the liner like you can with an older engine? That could also be another way.
Many thanks

Maintaining engine dimensions is always important but it is possible to shim the barrels slightly. Remember that base shims will increase deck dimension and with a 102mm bore .060" is generally considered maximum. I believe LN engineering makes the proper shims. Anyone with a laser or water jet can also make shims pretty easily if you want to make a bunch. A reasonable mill can make shims if you only want six.

Henry,

What do the ROW 930/10 with 9.8:1 requiring RON 98 do in North America?

It sounds like a liability, which is weird because they are coveted by some.
I can only find 91 without ethanol around here. Do you then dial back the advance / live in fear / mix their own fuel? Ouch to most of that.

Phil

It is a challenge but there is a timing adjustment on the side of the DME (small plastic triangle drive) that can retard the timing 3 degrees (IMSMC).
Of course you can also have a new chip burned. Steven Wong might be the guy for that.

Keep in mind that the Research Octane Number "RON", is higher than the US method of measuring octane, (R+M)/2. 98 RON in Europe is approximately equivalent to 93 in the USA.

Hi Pete,

Is your 93 stuffed with ethanol too?

Phil

Just ran some quick numbers. with head 89cc, 102mm bore, 76.4 stroke, 42.8 cc dome volume with deck height of 0.040" you get 12.5 CR. If you go up to 0.060" deck height you get 11.7

Another question on this dynamic compression. At what level do you need 100 and 110?

Phil, I thought I answered you, but I don't see it in the thread, so maybe I didn't punch "post".

Anyway, yes, the 92-93 where I am is ethanol contaminated.

I run 93 octane exclusively in a turbo 3.4. I have ran up to 1.5 bar and the engine static CR is 7.5:1.

Take it FWIW, but I have pulled this motor down twice and didn’t see any signs of detonation.

Apples to oranges, I know. Just another data point.