Shaving (and saving) the pistons ALH/AHU...thoughts,comments

[RabbitGTDguy]

Not that I am considering it at the moment even (though the thought has crossed my mind in an effort to lower compression slightly to allow for more fuel and boost in my particular application), but I was wondering if anyone has tried this and had it be successful. If so, how much did you shave off and what were your particulars about it. Its alot better way to reduce CR in my opinion than stacking head gaskets, etc. and with the meatier pistons it seems like it would be a feasible venture.
The other reason I ask....

The set of AHU/ALH pistons that I received as a set are in pretty good shape. Number1 was toasted thanks to a bad t belt job but the remaining three are in good shape with no valve impressions, etc. on them. However...there is some what I'd call "cross contamination" i guess....evident especially on number 4. I should take pics and post specifically but its like a few little splinters of metal got impeded in the piston. I picked them out but the metal "had" slightly chipped the surface of the piston. I'm trying to figure out whether this would be ok to use in my motor I'm working on...and if it is...i may run it this way (after coating) or I may also consider shaving the pistons very slightly with the benefit of lower the CR a bit as well as cleaning these surfaces up...

I'm open to any ideas, opinions, etc. that anyone might have on the subject.

Joe

p.s. I don't know whether this 'cross contamination' is from the toasted number 1...or possibly a bit of a glow plug...its no more than a shavings in size...

And again...thanks to Mikeyworks for helping out with the pistons...now its just figuring out "how" to use them....

[RabbitGTDguy]

here are some pics...

[RabbitGTDguy]

Well, after doing some digging and pulling from fred's, I've found that the pitting is minor and to a certain degree normal on the TDI pistons... Shouldn't be a problem...

However, in the interests of lower CR slightly to enable me to run more boost/fuel I am looking at the possibility of shaving the lip of the piston bowl to decrease the CR to around 17:1 or so... VWMS CR opti was up around 18.5 or something close to that if I remember correctly...so maybe a little less in on order. Kerma did this mod on TDI club and has had good results with only a little increase smoke on cold start. He also noted that shaving the lip of the piston bowl benefited the injection spray pattern he thought has he noticed the spray pattern to a certain degree hitting some of the lip area of the bowl instead of going down into the combustion chamber itself. Removal of the lip bowl and ceramic coating IMO would also reduce the single potential hot spot on the TDI pistons...that is...the area of "the lip" of the bowl. The thinnest, and the hottest part of the piston.
Now its a question of how much to remove and who to have do it and for how much. I don't trust the machine shop around here to do it!

Joe

[RabbitGTDguy]

The piston that Kerma modded....
Simply...shave the lip off the bowl of the piston...very simple..



Now....

Was just looking on the internet, etc. quickly for some various other DI applications and their respective pistons and bowl/combustion chamber designs, etc. and found some interesting pics. I'm sure there are more out there but I thought I'd share a few...what do you notice  

Ford Powerstroke


Perkins Clevite


Misc. Kolben


and I believe this is a cummins unit...not sure though...like i said..."quick" search just to see what I could find



The combustion chamber/bowl design of the various DI engines out there is so cool to look at. Notice anything? None of them have the bowl lip like the VW units...interesting. Also. The various depths of the bowls, the curve of the bottom portion, etc. ... All very interesting. Just thought I'd share.

Also...thinking about Kerma's mod more and more ...wouldn't remove of the lip also benefit the burn and downstroke after the fuel is combusted ? I was just thinking of the "path" of explosion....with the lip its inject, combustion and the return power wave is up around the lip ....whereas...no lip would result in a more direct path on the "power" stroke of combustion... maybe my thought is off.

Thoughts....opinions?

Joe

[fspGTD]

I remember a very early SAE paper detailing the TDI motor development that tested different combustion chamber shapes - IE: the depth of the toroid, flat bottom versus raised center, and possibly had a test with different "lip" areas.  I could be remembering wrong and it is a little fuzzy, but I will take a look and see if I can find this section.  It was probably in the SAE paper that described the original TDI engine development.

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Edit: yes, just plucked up my copy the relevant SAE paper I was thinking of (number 92038), and it tests various inlet swirl ducts, piston recesses, and "injection apparatus".  The piston recess section says:

"On the basis of previous research, the combustion chamber was further developed with a view to reduced dead space and a better compression ratio for emissions and noise.  The piston recess designs investigated for these improvements are shown in Figure 3 [figure 3 shows 6 different cross-section piston recess shapes; 4 "lipped" with cone shape at the bottom, 1 "lipped" with flat bottom, one non-lipped with flat bottom, and one non-lipped with cone bottom.]  Recess diameter, depth, shape, head cone, and turbulence ring were varied.  Recess 2a, with a compression ratio of 19.5, offers the best compromise between torque response, maximum power, specific fuel consumption, exhaust emissions, noise, cold starting behavior and resistance to cracking."

[RabbitGTDguy]

Jake...i believe I have that piece too...only the page with the different combustion chamber shapes... where did you find the complete piece? that is...the SAE paper for the TDI?

Also...Kerma did this mod with no ill effects, and also as I mentioned he observed that with the lip in place there was a hot spot on the piston as well as fuel that was being sprayed from the injector but not all making it to the bowl area where it could best be used...

There were 8 bowl configurations I think...can't remember for sure...i have it here somewhere...

Joe

[DigitalK]

I sense cooooool christmas ornaments in the making...

[fspGTD]

where did you find the complete piece? that is...the SAE paper for the TDI?

Back when I was in undergrad, I special-ordered it through my university's engineering library.

[caddy]

The piston that Kerma modded....
Simply...shave the lip off the bowl of the piston...very simple..

do you check CR with this piston?

[RabbitGTDguy]

compression ratio is 17.4:1  on his application... no ill effects... and he is modded heavly to warrant CR decrease...
VWMS recognizes I believe around 18:1 to be optimal... 18.6:1 if i remember exactly...

Joe