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There are alternatives but none of them will be less expensive than waiting for an oversized piston.
You're kind of at a standstill.
I found a set of iron sleeves for a couple of 5.0 blocks I have. That opens up a lot possibilities. But, not cheaper than waiting. I'm lucky I have a couple of shops nearby that have the experience and CNC machines to sleeve my blocks.
I just can't pay them enough to change over their sunnen machines from steel to alusil.
The issue with steel sleeves is that the engines sound like Detroit Diesels, especially when cold, because of the required piston to wall clearances.
Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
I've got an incredible connection to Mahle.
I've inquired about them making a run of S4 pistons, with the proper iron/tin plating.
That's a complete non-starter, because they can't make anything they supplied to Porsche for anyone else.
And then there's some sort of "issue" with the plating....like they do not own the patent, although Mercedes uses the same stuff (I think.)
Perhaps that is "licensed" technology? Or perhaps so toxic that they hate doing it. (997.2 pistons were "Ferroprinted, instead of plated.)
There was an exception that Mahle did for Andial. They made pistons for Andial to build their 2.8 liter 951 engines, with the proper plating.
Of course, that was Germans making special stuff for other very well connected Germans.
Even the Nicosil engines we sometimes do are noisy.
It's really tough to beat that Alusil to Porsche piston to wall clearance of .0007".
That's why I try to build everything I can, with that technology.
Try not to forget that Porsche changed the design of the S4 (and later) pistons (compared to all the other 928 engines...even the '85/'86 engines) so that the piston offset was not different from one side of the engine to the other.
(Which was incredibly dumb, for all those previous years....and continuing today, with other Porsche engines. Note that the Ferroprinted 911 engines only ever have the 4-6 side pistons fail....because they are upside down and slap the coating off, when super cold.)
This piston change was all about piston slap fooling the knock sensor system, in a 928 engine.
And an '85/'86 engine is virtually silent, compared to a S4 928 engine with steel sleeves.
We struggled with piston slap in 951 engines that got steel sleeves.....for about a year, in several test engines.
The knock sensor system (which I considered super important in a 951 engine) was completely "perplexed" by the piston slap. Timing would be retarded, without any need. (Give up 9 degrees of timing, when it is not necessary, and you just lost a bunch of power.)
We kept reducing the piston to wall clearance, down to the point where the engines would not make it through the break-in process, without "sticking" the pistons in the bores.
Magical coatings, different alloy pistons, special break-in procedures, nothing worked. We had to give up on getting these engines quiet enough to not confuse the knock sensors.
We either had to tune around the "fooled" knock sensors or run Alusil bores (which limited what we could build.)
Even the Nicosil engines we sometimes do are noisy.
It's really tough to beat that Alusil to Porsche piston to wall clearance of .0007".
That's why I try to build everything I can, with that technology.
Try not to forget that Porsche changed the design of the S4 (and later) pistons (compared to all the other 928 engines...even the '85/'86 engines) so that the piston offset was not different from one side of the engine to the other.
(Which was incredibly dumb, for all those previous years....and continuing today, with other Porsche engines. Note that the Ferroprinted 911 engines only ever have the 4-6 side pistons fail....because they are upside down and slap the coating off, when super cold.)
This piston change was all about piston slap fooling the knock sensor system, in a 928 engine.
And an '85/'86 engine is virtually silent, compared to a S4 928 engine with steel sleeves.
We struggled with piston slap in 951 engines that got steel sleeves.....for about a year, in several test engines.
The knock sensor system (which I considered super important in a 951 engine) was completely "perplexed" by the piston slap. Timing would be retarded, without any need. (Give up 9 degrees of timing, when it is not necessary, and you just lost a bunch of power.)
We kept reducing the piston to wall clearance, down to the point where the engines would not make it through the break-in process, without "sticking" the pistons in the bores.
Magical coatings, different alloy pistons, special break-in procedures, nothing worked. We had to give up on getting these engines quiet enough to not confuse the knock sensors.
We either had to tune around the "fooled" knock sensors or run Alusil bores (which limited what we could build.)
This is what makes a Rennlist membership so worth it!
Thanks for the above explanation. Am I to understand that the Alusil to Porsche piston-to-wall clearance of .0007" (0.0178mm) necessitated the iron coating? Otherwise, uncoated pistons need more clearance and then would slap?
Is this why there is no aftermarket pistons?
What about the 944S 2.5L 16v piston? I see the skirt length is different. But can these be used or adapted, or is your point about changes for symmetry in V8 engines, want makes these unusable?
I know the devil is in the details, but Mike is right, here's a PET excerpt for the M44.07 engine:
The tolerance group 1 and 2 number are just right there. Looking at PET more closely, I don't understand what we saw in the motor teardown. The connecting rods clearly have 944 part numbers, but PET only shows 928 part numbers for the S4 engines. These things confuse me.
As with all of these things, I really look forward to learning as much as I can.
Cheers
Last edited by Zirconocene; 09-21-2023 at 10:00 PM.
Thanks for the above explanation. Am I to understand that the Alusil to Porsche piston-to-wall clearance of .0007" (0.0178mm) necessitated the iron coating? Otherwise, uncoated pistons need more clearance and then would slap?
Is this why there is no aftermarket pistons?
What about the 944S 2.5L 16v piston? I see the skirt length is different. But can these be used or adapted, or is your point about changes for symmetry in V8 engines, want makes these unusable?
The iron coating is for compatibility with the Alusil bore. Running a raw aluminum piston in whst is basically an aluminum bore doesn't work. The dissimilar iron plating makes this work.
I believe any Porsche piston, iron plated and made for an Alusil bore, would work fine in a 928 engine. However, they will be enough different that you would need an entire set. And if they required a custom rod (which I'm not certain of), one might as well buy 968 pistons and benefit from the increased displacement.
I know the devil is in the details, but Mike is right, here's a PET excerpt for the M44.07 engine:
The tolerance group 1 and 2 number are just right there. Looking at PET more closely, I don't understand what we saw in the motor teardown. The connecting rods clearly have 944 part numbers, but PET only shows 928 part numbers for the S4 engines. These things confuse me.
As with all of these things, I really look forward to learning as much as I can.
Cheers
This is certanly worth checking out, further.
928 5.0 rods are essentially 944 2.5 liter rods, machined down thinner, since they share a crank journal, in a 928 engine.
Same casting/forging/metal squishing.
Cast number in part like conrod is not actual final part number when part is machined. Therefore it doesn't tell anything about in what setup part works.
There are three basic problems in using Alusil pistons meant for 944, 968 etc. in 928.
1. Compression height
- Different rods so cost escalates easily
- Every 944 and 968 piston type has this problem AFAIK
2. Piston top cc including valve pockets
- Easy to make cc larger and add pockets
3. Piston weight
- Doable by adding heavy metal to crank, lightening piston etc.
So easy solution would be if only there were modern aftermarket coating which works on Alusil bore. Wössner is only manufacturer I know who offers such stuff.
If we are "kinda done" with the whole block/piston/connecting rod diagnostics and discussion, anyone want/brave enough to continue on with the bearings and crankshaft?
If we are "kinda done" with the whole block/piston/connecting rod diagnostics and discussion, anyone want/brave enough to continue on with the bearings and crankshaft?
Yes please, share what you see. I see some wear on the bottom side of the bearings but not much on the top side.
Cast number in part like conrod is not actual final part number when part is machined. Therefore it doesn't tell anything about in what setup part works.
There are three basic problems in using Alusil pistons meant for 944, 968 etc. in 928.
1. Compression height
- Different rods so cost escalates easily
- Every 944 and 968 piston type has this problem AFAIK
2. Piston top cc including valve pockets
- Easy to make cc larger and add pockets
3. Piston weight
- Doable by adding heavy metal to crank, lightening piston etc.
So easy solution would be if only there were modern aftermarket coating which works on Alusil bore. Wössner is only manufacturer I know who offers such stuff.
Piston weight going from 928 pistons to 968 pistons has never been an issue. (I can not speak for the 944S pistons, however.)
A 1st oversize 928 piston tips the scale at 775 grams. (Rings, pin, circlips.)
A 1st oversize GTS piston is less, at 725 grams. (Rings, pin, circlips.
A 104mm 968 piston is 675 grams. (Rings, pin, circlips.)
09-21-2023, 05:23 PM
