Posted this to see if I get thoughts from forum members with much more technical know how on PDK. Hope get some details here.


Recently did PDK transmission change on the PTT. Before doing so, did research for some time, trying to get some technical details about the process.

Have done this on my CTT as well a couple times. To be honest, many folks in CTT world (non PDK), were doing without PIWIS, and I have not heard of anyone having any issues.

The general process is:

- car up off ground but level

- drain old fluid measuring how much came out

- put in new fluid

- run through gears few times while stationary

- bring transmission to 40 deg. Celsius

- top up until fluid starts leaking out fill/inspection hole

-close off all plugs and put things together. Done.



My curiosity is around how precise does the fill have to be?

Surely, the car cannot tell difference in mL. Putting in 1/4 of a quart too much is 250cc or 250mL (thats one glass).

What is the level of precision needed for these fills? I have been reading that overfilling transmission can lead to issues like over pressurizing system, foaming, etc.

However the more I read, I find contradictions in each point.



For example - foaming.

Foaming can occur if oil is picked up by gears in overfilled transmission. Causes fluid to overheat and lose its lubrication properties. Can help damage transmission.

I found that Pentosine FF3 and other trans. fluids, all have anti foaming agents in them, preventing that exact behavior.

So if fluid has anti foaming, surely small amount of overfill will not be something to lose sleep over, or is it?



Another example, fluid expands as temperature increases.

PDK is to be filled to max at 40C. That is 104 deg. F.

Anyone who has Panamera can tell that the car is hot at run time. Once parked overnight, engine is still at about 85 deg F or 30deg Celsius in the morning.

So if I do PDK change in summer time, and ambient temp is 85F, how much can this fluid expand in 20 deg. F? I am sure it wont be a lot. Read below.



Looking into this further, there are charts showing thermal expansion of petroleum products.

These values are between 0.0007 and 0.001 (this is a coefficient value 1/degree C).





Here is some mathematical stuff:



You cancalculate thermal expansion of liquid by formula: dV = Vo β (t1 - t0)

dV = change of volume
Vo = Original volume (initial volume amount)
β = coefficient of expansion, (m3/m3 oC, ft3/ft3 oF) - sample Chart Here
t1 = final temperature (C or F)
t0 = initial temperature (C or F)


Given above, we know for PDK:

Vo = 8.5 (quarts = 0.284071 cubic feet - used typical change time fill quantity, at volume converted to cubit feet per coefficient of expansion)
β = 0.001 (used worst case or most expanding coefficient - Petroleum...rage is from 0.0007 to 0.001 from chart - using values from the Celsius column since we use temp in that unit too)
t1 = 40 Celsius (temp at time of top PDK up)
t2 = 30 Celsius (cool engine summer time temp)


Mixing all this together, you get result of

0.284071 cu. ft ( (0.001)(40C-30C) ) = (0.284071 cu.ft.) (0.01) = 0.00284071 cu.ft = 0.08499994597403 quart (converting back to US quarts) = about 4cc (one fifth of a 20cc cup of cough medicine)

This cup shows you about 20cc of fluid, so 4cc is way less. Imagine adding that to your FFL3 fluid fill in PDK, sitting already at 8.5 quarts or 8044cc.













So looking at above, if you put in 8.5 quarts of FFL3 fluid into your PDK, while car is at cool temp,

once you heat the PDK to 40C, that 8.5 quarts will expand to be 8.50284071 quarts.

Meaning, the fluid will have expanded by about 0.99% or rounding off, to about 0.1%.

If that is true, then your 8.5quarts added, when heated, will become 8.5028 quarts, which is a minuscule amount of change, equivalent to about one hundredth of a glass, about 2cc worth.



If we assume that PDK transmission fluid expands at twice this rate (say coefficient is not accurate and we want to compensate for its possible error), doubling this bring expansion to about 4cc.

That is still a very small volume, about one fifth of a small cup you take your favorite cough medication with (20cc cup).



All that in mind, I find this very surprising, that PDK transmission would have this level of sensitivity, to be accurate to 2cc difference in expansion.

Is it possible that there is some bs mixed in the PDK fluid change technique, to help the company bottom line in terms of revenue from customers? Dealers charge about 750 from what I remember for doing this. Selling 10k cars brings in $7.5 million dollars revenue after 60k miles (or about 4-5 years, about $1.85 million per year). That would be for USA market only.



I looked up sales numbers for USA market. Took the 2014 model as it is more reflective of those who are more likely to do PDK oil sooner than later.

USA market sold some 4750 cars in 2014. Adding other markets in the World, you are looking well over 10000 units per year.





What am I getting at here? Is it possible PDK transmission fluid tolerance is so precise because of technical reasons that elude me? Do these technical reasons justify fluid volume sensitivity of one fifth of a medicine cup (4cc)? Put that 4 cc in a glass of water. Now picture 8.5 quarts of FFL3 in the transmission. Then add the 4cc to that volume. I bet transmission would not even detect that difference.



Please give me some info if you have it, so I can change my mind on this. I understand overfill/underfill of fluids is an important issue, just trying to justify how the modern PDK in Panamera, would detect 4cc (which is already overinflated number), and cause damage in its parts over time. I can see overfill of quart or 2 quarts can be an issue, but 4cc?


Please give me more insight if you have the details. Chime in with your thoughts. Thank you.

 

Ciaka,

Thank you for your thoughtful analysis. I am an engineer and I agreed with your reason. From my past experience with MB and BMW, I am more worry overfill than underfill. Overfill causes the leaking and it is bad since fluid expands due to temp, underfill is not too bad. if you measure the amount of fluid coming out from PDK, you would have some ideas of much fluid you put in. Overfill a little is no harm done, fluid will evaporate a little over time due to high temp.

From the $revenue for PDK service, it is a significant recurrent $revenue for every 60K miles or 6 yrs whichever come 1st. $750 is varied from every Porsche dealers. Some may charge up to $1k if it is PT. Please see an attachment for Pan maintenance checklist.