Shaving the heads of an M117 engine to increas CR? - PeachParts Mercedes-Benz Forum

GGR · #1 01-20-2010, 12:32 PM

Hi all,

I have an early euro 5.0 M117 engine out of a 450 SLC 5.0 that has retained its original tri-Y exhaust system and am thinking of improving its performance in different ways. I would like to keep it wihin a reasonable budget (no overboing, replacing pistons etc.). My fist step which is currently under process is to convert it to Megasquirt fuel & spark management. My next steps would be to fit some more aggressive camshafts with solid lifters and increase the compression ratio.

Some research on the net showed that a combination of the two would yield more results than any of each separately. This is due to the fact that increasing the compression ratio not only increases the power of the explosion but also favors a better filling of the cylinders (by a greater differential of pressure when volume suddenly increases ten times with a 10:1 CR rather than nine times with a 9:1 CR) making a bigger/longer opening of the inlet valve more relevant. Of course CR and cam profiles should be matched. It is therefore important to determine the maximum CR an engine can run safely before determining the cam profiles.

I read that the 93 octane grade I find here in the DC area is good for up to 10:1 CR. I also see that later euro M117 engines such as the 250 hp W126 500 or the 300 hp 560 also have a 10:1 CR. I could run more given that Megasquirt gives me the possibility to integrate a knock sensor in the engine management system. But aiming at 10:1, may be 10.5:1 would keep me on the safe side as the above examples show the rest of the engine components can handle safely the additional stress caused by an inceased CR at least up to 10:1.

I also read that the most efficient way to increase the CR is by decreasing the squish area. This can be done with higher deck pistons and this is the way MB achieves it within the M117 engine range. However, this would mean purchasing a set of new pistons, when the same result can be reached much more cheaply by shaving the heads by the right amount.

So, did anybody here already increase the CR of an engine in general, or of an M117 engine in particular, by shaving the heads? By how much? With what results? Are there any negative side effects?

Thank you very much in advance for any contribution!

JayRash · #2 01-20-2010, 12:47 PM

shaving the heads by enough to get to 10:1 CR will cause u overheating issues. And this over heat cant be done with by just increasing cooling. As far as i know

GGR · #3 01-20-2010, 01:09 PM

Is shaving the head that will cause the overheating issue, or increasing the CR?

In the latter case, I'm not too worried as some Euro M117s came with 10:1 CR (different pistons) with the same heads references.

JayRash · #4 01-20-2010, 02:39 PM

as i know its shaving the head that will cause the over heating. I hope some other members will shed more light on this.

Lander91 · #5 01-21-2010, 07:07 PM

Im unsure how the method of taking metal off the heads will cause overheating, the increase in CR will cause a temp increase as you will have increased the supercritical gas pressure. You will however have other issues i.e. timing chain length, intake manifold interference (machine the intake manifold with the heads) The best way to increase comp on the 117s is to go to an engine shop that has a good ally welder and get them to fill in the depression next to the spark plug bores, after this you can remove/smooth the weld down till the desired comp volume is achieved. I know for a fact that these engines can withstand 12:1 with appropriate cooling and retaining a reliable RPM limit i.e. 6000rpm.