cooler on Porsche 928 3 speed 722.00?

[jp928]

On the 928 installation the tranny cooler is up front in radiator side tank, while box is in the rear , so the cooler hoses are pretty long. One guy has installed a rear mounted cooler with much shorter hoses, and direct in/out connections instead of banjo bolts. he reports it feels more responsive and crisp than before, and connects this to better oil flow through the cooler connections.
Anybody have any comments on the impact of reducing restriction of cooler pipes on operation of the box please? I tend to think its more to do with reducing the length of the hoses than the restrictions of the banjos, but I am interested in any others experiences here....
TIA,
jp 83 928

[C32AMG]

Here are my comments; they are the same I give to porken last year.


Steve Cattaneo
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Rennlist Member

Join Date: May 2001
Location: new york
Posts: 748

Proken,

The oil feed thru the cooler lines is not line pressure oil (working pressure); the oil used to operate the inner workings of a transmission, like bands and clutches. Cooler line oil is super heated oil from the torque converter and gear train lubrication oil. Since the torque converter generates 95% of the heat in the transmission it must be cooled before being introduced into the transmission and gear train lube oil circuit.

There could be an explanation for your performance claim of “The car feels 'heavier' at take off”

Here is a simplified description on how a torque converter works. The engine drives the impeller; it puts the transmission fluid into a circular motion creating a fluid force, that fluid force turns the turbine blades. The turbine turns the transmission planetary gears; driving the wheels. The medium used to transfer the engine torque to the wheels, is transmission fluid.

By lowering the transmission fluid temperature the torque converter is more efficient at multiplying engine torque. The colder THICKER transmission fluid with its higher viscosity has more potential energy than hot THIN transmission fluid. That greater potential energy when from, mechanical (engine) to fluid (oil) to mechanical (transmission)

Cons

You do not want to over cool, especially in colder climate areas were the temperatures can drop below 20 degrees. The flow of the fluid thru the cooler can be restricted; if it happens there is a possibility for planetary damage due to no or low lubrication oil. For this reason some cars have a cooler bypass valve. This is when synthetic oil would be extremely beneficial for its low temperature lubrication protection.

By having it mounted closer to the Trans even with its larger surface area with extreme use or on a hot summer day there is not enough time for the cooler to extract the heat from the oil. So the transmission is circulating hot oil.

Pros

The stand alone cooler isolates the radiator coolant from the transmission fluid eliminating a potential transmission failure when the radiator transmission cooler fails allowing water and oil to mix.

A bigger cooler extends the life of a transmission by lowering the transmissions operating temperature. The worst enemy of a transmission is heat. At 220 degrees oil starts to break down, at 260 degrees the rubber parts start to harden.

There are duel function coolers available, were you can cool transmission and engine oil, and you might what to consider one mounted in the front of the car as a stand alone system.

Nordskog makes a nice digital Trans temp gauge, I installed one on the ash tray cover of my car since I don’t smoke I have no use for it.

You did a great job, thanks for the AN fitting number, I was looking for them


__________________
Steve C
928S 83
The Great White928
85 S2 EURO
02 MB AMG

[jp928]

Steve, sorry to have missed the earlier explanation. In summary then, any noticed improvment would be due to cooler oil in the given circumstances? Which my be different in hot weather. Nothing due to reduced pump loads with lower restriction connections?
jp 83 S