98mm Piston to Head Geometry, deck height, squish, and compression ratio, oh my.

[WideRide 86]

Yep, that’s a lot to unpack! where to start?

Im working through a 3.2 -> 3.4 conversion and am having a heck of a time sorting out the deck height and piston to head clearance with the Mahle 98mm (Max Moritz style) pistons. The heads were refurbished by an experienced machinist including refacing the sealing surface and adding a chamfer transition to accomodate the large piston diameter.

While checking deck height using the solder method, it looks like the clearance at the very perimeter is greater than where the piston dome meets the head at/near the chamfer. This creates a potential pocket for detonation and/or unburned fuel to remain (poor performance and emissions). Common sense, the hive-mind, and even the Mahle installation guide states that piston to head clearance should never be less than the deck height (in other words, the clearance should always be opening up towards the chamber).


Note, the narrowest point on the solder is where the piston dome pass the upper chamfer edge on the head, not the end.


With the surfaced heads, I’ve lost some volume and pushing the bounds of reasonable compression ratio for pump gas (91oct). I was hoping to land between 9.8 and 10.0 but I’m right around 10.2 to 10.3 with a deck height of 1.1mm. At first I thought I could raise the deck height with thicker base shims, but I’ve learned through this process about squish/quench and realize it’s not advisable, and could also contribute to the detonation at the perimeter issue noted above.



I am able to get a reasonable deck height measurement directly with the piston at TDC and the head off (say 1.1mm avg piston crown to top of cylinder) with 0.50mm shims. But the actual thickness at using solder method (piston to bottom of head) is more like 1.35mm. And the min clearance happens just inboard of the perimeter where the piston dome begins and nears the head at the chamfer as mentioned above. Removing a 0.25mm shim will get the total deck height (top of piston edge to bottom of head) with the solder method down to 1.05mm, but then the piston dome slope is even closer to the head.

So I guess my question are:
Those who have run the 98mm Mahle MM pistons, did you chamfer your heads? (If I recall, KTL stated he has run them both with and without. Anyone else?)
If so how did the deck height measurement work out?
Did you have any detonation issues.
Do we know definitively that these heads should not be chamfered to accept the 98mm Mahle MM pistons?

Possible solutions that I can dream up:
Somehow recontour the piston dome side to ensure the clearance is always opening up (not sure how this could be done without CNC)
Weld the head and have them remachined to essentially replace the chamfer? (sounds less than ideal)
Use a higher than optimal deck height to minimize the tightening clearance after the piston dome starts rising? (I don't think this solves the potential detonation issue).

Ive been working on this issue for weeks, measuring, remeasuring, trying to glean what I can from the wisdom on this board and others… Frankly, I am stumped. None of the solutions I’ve imagined seems reasonable. So any input would be greatly appreciated. Im not too worried about the CR. Once I figure out the deck height and geometry issue, I’ll see what the CR is and mill the piston some if needed i guess.

Sorry for the long post, but its complicated (for me at least), so it took a while to explain. Thanks in advance for your input!

~Doug

[KTL]

Doug,

My heads were actually chamfered after I had the 98mm Mahle pistons. Mine were the Mahle motorsports pistons (not to be confused with your MM = Max Moritz wedge-style single-plug pistons) linked below. I replaced those Mahle pistons with typical JE domed pistons and that's when the heads were chamfered. And then I hosed the engine....

98mm Mahle Motorsport Pistons

I unfortunately did not check the piston to head clearance with the JE pistons. I just assumed that the "shelf" around the crown and the head chamfer provided an extra measure of assuring the necessary clearance.

Just so you know, my original application of the Mahle pistons is not a good example to reference because they were the wrong piston. The pistons were apparently a "cheater" style of using them with the typical SC 70.4mm crank and 127.8mm rod length. I believe it was someone's poor idea to use these pistons in a way that would mimic the 3.0L with 8.5:1 compression and the cheat was that the 98mm diameter made it a so-called 3.2L short stroke.

Why I say it's a bad example to follow is because my Mahle pistons were the for the 3.2 to 3.4 application and not for a 3.0 to 3.2 like should have been used for my SC crankshaft and rods. The 3.2 to 3.4 piston has a shorter compression height than the 3.0 to 3.2 piston- 32.8mm vs. 34.0mm, respectively. So that 1.2mm reduction compression height increased my deck height accordingly. I believe that was the intent by the previous engine builder because that increased deck height effectively decreases the static compression ratio. However, as you have mentioned, too much deck height creates a large void around the periphery of the combustion chamber and that invites detonation.

Take a look at this picture below and the tiny pock marks around the perimeter of the dome would lead me to believe those are signs of detonation. I suspect the saving grace for most of the engine's life is that it used 100+ octane race gas. When I got it, I didn't believe it needed race gas due to single plug and mild compression. Little did I know it had less than mild compression!



Also noteworthy in that piston picture is that the top of the piston not only has a flat "shelf" but it also has a bevel beyond the shelf to apparently address the piston to head clearance you are experiencing? For whatever that's worth........

Have you actually poured the volume of your head to calculate the static compression ratio? I would also suggest pouring the piston dome volume to confirm that. Reason being is that i'm not sure if Mahle's dome volume spec takes into account the gap between the cylinder and the side of the piston crown. That void volume reduces your static compression ration. I also think the poured volume is important because I would think the increased head volume from the head chamfer reduces some of the lost head volume from machining.

[WideRide 86]

Wow, that is a pretty wild combination of things going on in that (former) build! Thanks for clarifying Kevin.

I have cc'd the heads, and seem to be ~ 1.5cc under the 90cc stock size, this includes the chamfered area. I did try to cc the assembled chamber and also the dome volume but neither provided reasonable results. I should probably try again. Interesting point about volume around the piston crown and cylinder annulus. When I did pour the assembled chamber volume, I greased that area to prevent leaking, so honestly, Im not sure I would have captured that volume either...

[WideRide 86]

I put together a sketch to try and better communicate the geometry that I am observing. Please forgive my poor pdf editing skills. This image was based and altered from the Mahle Motorsports Aircooled Porsche Application Guide.



So obviously the flat portion of the piston crown is much narrower than they show, but you get the idea.

The conundrum is:
if I lower the deck height, the pinch point gets worse
I already have a pocket there
If I raise the deck height I will have less of a pinch point but a larger pocket, and less squish.

[HaroldMHedge]

To provide some more guidance, Competition Engineering recommends a deck height of 1.25mm to 1.5mm for most stock engines.

Easy Deck Height

[WideRide 86]

Thanks for that reference Harold. Seems to be the original source from Wayne’s book. Interesting that they reference the larger deck height measurement. I think Wayne’s book mentions 1.25-1.0mm min by memory. I wonder if their recommendation still stands for slightly higher compression ratios.

Originally Posted by HaroldMHedge

To provide some more guidance, Competition Engineering recommends a deck height of 1.25mm to 1.5mm for most stock engines.

Easy Deck Height

[David]

Might get more input on the the engine forum, but I would be worried about the squish band getting smaller before it opens back up. My 930 heads didn't have a chamfer like your pics, but rather a slight radius. I don't know what original design is.

[WideRide 86]

Thanks David... I after I posted here I realized I maybe should have posted in the engine building forum. Maybe I should start a new thread there.

Im glad you chimed in. I also reviewed your posts pretty closely as well. I was wondering how yours turned out in the end. Hope you got many years of fun out of that build. Is it still running strong?

[David]

It's still running strong after over 12 years. It was down a little on power last time I dyno'd it a couple years ago but it was a different dyno, weather, etc so I'm not too worried about it. I had the engine out last year to clean it up so I adjusted the valves and changed the plugs.





Here's a pic of my head when I was building it before machining for twin plug.

[stanglife]

Interesting thread - I have somewhat related concerns that maybe you guys can help with. I did post a thread over in the engine section but it has not had any replies just yet.

I'm building a 3.6 964 engine as a 3.8. The cylinders and pistons are 104mm and the heads were purchased at a different time...and I guess they are setup as a 3.6? The reason I say that is because the chamfer you guys are referring to only extends out to about 100mm, leaving the flat of the cylinder head exposed inside the 104mm cylinder. Here is a close up of what I'm referring to.


You can clearly see the point where the chamfer stops and the flat of the head (this is looking from the bottom of the bare cylinder with the head resting on top).

I was planning to use JE pistons and they do have a flat area on the piston probably close to exactly the width of the combined chamfer and exposed head. Assuming tight but appropriate deck height clearance, is this OK? If I "open up? the head so that the chamfer extends to the full width of the 104mm cylinder, will I be creating a situation similar to yours, maybe?

[stanglife]

Here is the area that I'm talking about. It's the area where the cylinder and piston have a larger diameter than the chamfer in the head.

[David]

I'd reach out to an experienced Porsche engine shop. Perhaps EBS or whomever you bought the parts from.

Have you measured the compression ratio and done a solder check of the piston to head clearance? This would help in making a decision.

[christian977]

Hello Sirs,
What was the outcome here, i am in a similar predicament?
Thanks
Christian

[r lane]

Chamfer the unused portion of the cylinder to head mating surface. Match the angle of the piston dome if you can. This will lower the compression ratio somewhat. You can also soften the outer step on the piston perimeter. I am thinking I understand your delima, maybe I don't.

[stanglife]

still struggling with this in my mind as I gather parts... Not so much the squish - but the potential small pocket created by the bevel on the combustion chamer. Seems like it would have been better if that was simply flat, as it would close that little gap. The piston is flat there for @2mm on the edges before the dome starts.
Like this:

[WideRide 86]

Apologies for the radio silence here. I had notifications off and haven't noticed the activity here.

Update on my end, completed my build last spring and all went well. I ended up with about 10.1 compression and used thicker shims to get there. The squish shape retained the odd geometry shown in my pdf sketch in Post #4 above with reasonable deck height (I'd have to look at my notes at this point).

Start up went fine, the motor runs smooth and makes good power. No knocking, pre-ignition or detonation issues even with higher temps and track abuse. Borescope inspection shows no signs of trauma on the piston top or cylinder walls.

[WideRide 86]

The geometry you are depicting in your sketch makes total sense to me with the following caveats.

Maintain a reasonable/safe deck height at the flat perimeter edge while still maintaining the CR you are looking for.

If I could do mine over again, I would dry assemble the top end before the heads were chamfered, set the deck height, check the CR. If the CR was too low at a safe deck height, you could try decking (shaving) the head face to reduce the compression volume in the dome IF the piston and head geometry allows it. Its possible the edge of the head dome will get closer to the bevel of the piston dome, creating the weird geometry again.

In then end, the chamfered edge with MM style pitons have been used for decades, and it seems to be working in my case just fine, despite all my efforts to avoid the strange squish geometry.

Feel free to PM me if you'd like to discuss more. Im happy to discuss here as well if its helpful.

Good luck with your build!

Originally Posted by stanglife

Here is the area that I'm talking about. It's the area where the cylinder and piston have a larger diameter than the chamfer in the head.