Mahle Max Moritz 98 mm pistons

[pooder]

Ok, here is a question based on a little bit of knowledge. A 3.4 conversion with Max Moritz type pistons is near completion in my '87 911 and I have a question about timing. Is the design of this piston considered to be one that allows for a faster burn rate in the combustion chamber than in the standard 3.2 pistons? I ask because if true, wouldn't this design then allow for less timing and thus be less prone to detonation?

[gliding_serpent]

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[Lyle O]

Yes, hence the higher compression ratio (9.8:1) for single plug Motronic.

[Josh D]

I have a set of the 98 mm Max Moritz wedge pistons with the 22 mm wrist pin for the 3.0 motor that I plan to use when I rebuild my motor. I've often wondered why Porsche didn't use this style of piston from the factory. I realize the factory pistons are shaped to create a swirl and good mix of incoming A/F, but the shape really lends itself to detonation concerns with poor fuel and too much ignition timing. It really must have been an economy/emmissions thing because stock CIS pistons just don't lend themselves to performance.

[Jcslocum]

Originally Posted by Josh D

It really must have been an economy/emmissions thing because stock CIS pistons just don't lend themselves to performance.

Any OEM makes HUGE compromises in engineering and design to meet the rules that apply as well as costs, warranty, ease of assembly, etc.... It is what it is, that is why there are so many aftermarket suppliers.

[78SCRSMAN]

Of topic a bit but where are folks sourcing these MM (type) pistons? I can't seem to find them available without cylinders. (Have my own to bore and plate) I assume we're talking Mahle?

It'd be great if someone can PM me where they can be had and maybe a price range.

Thanks,
Derek

[gliding_serpent]

Ebs racing sells them without cylinders. They quoted me about 1800$ for a set with rings, pins, etc

[Lyle O]

This is a picture of the pistons...

[78SCRSMAN]

Awesome guys!
Added these to the build list!
Thanks a bunch for the info and photo Lyle.

[Josh D]

I bought this set off a Pelican for $500. After cleaning them, I honestly couldn't tell if they'd ever been used! There were no wear marks on the skirts at all.
I picked up a set of 3.2 P&C's from another Pelican for $400. So after I send the cylinders to get bored and plated and add rings, I'll be into the set for around $2K.



Another interesting tidbit: I held a cast 95 mm 9.5:1 3.2 Carrera piston in one hand and the forged 98 mm MM Mahle in the other, and then switched, and you can feel how much lighter the bigger forged piston is over the smaller cast piston. That in itself will take some load off of the rod bolts.

[pooder]

I'd like to re-ask the original question to any experts out there as to whether the optimal timing of these MM pistons would be less than the stock 95 mm piston because of this piston's profile.

[78SCRSMAN]

Pooder,
You may want to ask this question on the engine forum but I'd guess, if stock 3.2 setup, no... but a call to Steve Wong (911 chips) might be in order (again if stock Motronic) to get a chip burned for your application. Your engine might be on the lean side with increased displacement.

Sorry for the diversion...

[Josh D]

Originally Posted by pooder

I'd like to re-ask the original question to any experts out there as to whether the optimal timing of these MM pistons would be less than the stock 95 mm piston because of this piston's profile.

It would be my opinion that these would be a faster burn than a piston with a crown in the middle. My plan when I build mine will be to start at 30* total and go from there.

[pooder]

Thank you forum for the input.

[KTL]

You are doing the conversion and retaining single plug ignition, correct? If so, i'd be careful with too much advance. In my limited experience, your fuel octane (or lack thereof) and the lack of knock control is going to dictate the ignition timing more so than the piston crown shape.

[Josh D]

Originally Posted by KTL

your fuel octane (or lack thereof) and the lack of knock control is going to dictate the ignition timing more so than the piston crown shape.

Yes, as well as ambient temperature, elevation, A/F, etc. But flame travel should have an effect. How much in this circumstance would be anybodys guess. Err on the cautious side and adjust, in small increments, from there.