worf928

No. Maybe. But, that wasn’t what drove the 2-tray concept.

I think the exhaust ‘sticks down’ enough that an undertray that clears the exhaust will be a mulching pavement mower.

I could be wrong though. Cardboard modeling required.

worf928

Also, ‘additional’ underbody devices would need to be no lower to the ground than the stock pieces else unwanted aero stuff happens.

ptuomov

I've converged to the view that the sensible next step is to run it "as is" and measure how much the oil temperature drops over the heat exchanger (we've got thermocouples for that). If the drop isn't enough as is, then the next step is to add a puller hand above the heat exchanger. "Puller fan above" because I don't want to fight the 3.5 kW gross Meredith effect.

Cheburator

Jesus...

Just get on with it...😜

Some empirical evidence - the GTS box with external Setrab cooler mounted in front of the main radiator is running too cold! I have a proper Stack analogue gauge which is fed by a sender mounted inside the diff drain plug. With a 500+ chp and 600Nm running through it during 40min sprint races flat out I can’t get the oil temperature to rise above 75C!!! That’s a problem since the Motul 300v 75w90 LS likes to run hotter...

By the way, the Setrab cooler is very similar size to yours.

ptuomov

Why get on with it when there's an to opportunity dwell? ;-)

John's system has a pump activated at one (lower) temperature and then a thermostat diverting oil to heat exchanger at another (higher) temperature. So the heat exchanger doesn't see oil before oil makes it to 180F. Measuring the temperature of the gears and the oil there accurately is pretty difficult, though, so we've got to make sure we're not picking up the case temp.

I am very surprised about your temp readings, if they are the oil temp from a sensor that isolated from the case and submerged in oil. They sound low, given the use you're describing.

Cheburator

I was equally surprised - the sensor is definitely submerged in oil at the bottom of the diff. Also, I have drained hot oil in a cup straight after qualifying and my IR gun has shown oil temps inside the cup to be within a degree or two from the Stack gauge on the dash.

My uneducated guess is that the Setrab is a lot more efficient than I think - also I am exhausting all the air that hits the front air dam and radiators through a massive bonnet vent. That maybe helps too with cooling efficiency?

ptuomov

Have you rigged it to the stock GTS five-speed oil pump in the differential end or are you running an electric pump? Do you have a thermostat control? Also, are you using the stock GTS spray bar on the gears?

Actually let me just first confirm that you’ve got a five speed not auto? Because the above questions only make sense if you have GTS manual.

Cheburator

The car is a former GT, which is running a heavily reworked GTS engine, brakes and a stock GTS transaxle. I am using the stock GTS pump which is driven by the diff. There is no thermostat - the Street GTS did not bother with one - the pump will always pump oil as long as the wheels are turning - the thermostat would add complexity. The original spraybar is still there. The only difference with a stock GTS 5-spd is that I am using a much better radiator, which is positioned lower.

I also wonder if the GTS PSD being effectively an open diff on the straights helps with keeping oil cooler vs a 220 LSD? The GT PSD system still works, thus I see proper lock in corners etc, but unlike the 1989 S4 5-spd with LSD that I also have, it reverts to open when conditions allow it.

I have two late rebuild LSDs sitting in the garage and once I am done with restoring a E46 M3 i am planning to swap one of them in place of the LSD. Will obviously continue to monitor temps.

Wisconsin Joe

If you want another 5 speed trans temp data point...

When I got my car (85 Euro S2) the clock surround was gone. To fill the hole, I put in a couple of gauges. Oil temp & trans temp. Both have the temp sensors installed in the drain plugs.

After having it in for a while, I realized the trans temp was pretty foolish. -
Normal, steady 70 - 80 mph travel gives me a steady temp below 150F. A long steady run at 90 mph with outside temps in the 80 - 85F range made it rise a bit. To just under 200F.

I realize that 'racy stuff' (lots of acceleration & deceleration) would generate more temp, but it's a data point for you.

ptuomov

I agree that there's a chance that this is all a fools errand. There are two thermostat mechanisms in the system, so the cooling system shouldn't harm things. That said, the idea is to put 3x stock S4 torque thru the transmission so there's also a chance that it will want to run a little hotter.

hacker-pschorr

It really depends on how the car is going to be used. 100% street doing a blast or two up the gears (keeping it under 3k though, don't want to wear out those bearings.... ) might not have much ill effect and a simple cooler should be enough to cool things down until the next stop light.

Continued use on a track? Not sure a simple cooler will be enough (see below).

One thing to note I'm not sure has been discussed. One drawback of our transaxle design is shared fluid between the trans & differential. In the Corvette world it's far more important to have a big cooler on the differential than the transmission (most do both anyway). We are at a disadvantage not being able to separate the two. At one time Todd and I were studying the early 5-speed to see if there was a way to seal off the passageway between them. With Todd anything is possible just not sure it's worth the effort. So that brings up another question, do you want to draw from the differential where the heat will be greatest or feed the differential the coolest oil?
If it were me, I'd at least put a squinter on the diff.


This may not apply since these are track cars (DE not racing).
Years ago when Jim Page started to track my 79, towards the end of every session it became very difficult to shift. He experimented around with various coolers, nothing really made a big difference until he went with squirters.

Fast forward a few years he upgraded to a low mileage S4 and even running it bone stock with street tires, every session the trans would start to act up since it was obviously getting hot. So again experimented with coolers and in the end, went with squirters again.

By the time I tracked down Jim's former 79 and purchased it, the previous owner had removed all of the coolers (oil and trans) since he was planning on restoring it to a street car. I ran it "as is" since I was eager to get on the track with this car. Well, I was having the same issues with the 79 as Jim experienced back in the day, even on a cool day in the 60's. At some point in a 20-25 minute session I could not find 4th or 5th. So I parked the car from track duty until I get a chance to put all the coolers & squirters back on. I don't plan on totally replicating that setup. The coolers & accusump were in front of the radiator along with an oil cooler. I'm moving everything to the back like Jim's S4.

These were also posted in this thread covering a similar topic:
https://rennlist.com/forums/928-foru...track-use.html

79 & S4 pics:

ptuomov

Nice photos.

I think that not being able to separate the differential and the gearbox is more of an issue for the type of fluid one can use. The differential wants a little different oil properties than a synchro transmission.

For the 928 S4 5-speed, the gears break easier than the rear differential. Therefore, I think it makes sense to spray the gearbox and generally worry about it more than the rear differential. A theory, not a fact.

Another thing to note is that this spray system currently in my car is designed to prioritize cooling. I've also got Rob Edward's old box rebuilt by Greg Brown to prioritize lubrication with the oil jets. That'll go on after this box breaks.

SwayBar

Another data point:

My 5-speed S4 at a steady 80 mph, my trans temp is 165, and with good, spirted driving, it'll go up to 185, and the ambient temp was 85.

Double-clutching/rev-matching into 4th at 80 mph for passing at WOT brings the trans temp up from 165.

hacker-pschorr

Correct, the initial reason to separate the two was to run different fluids. Who knows, I may still look into it the next time my trans comes apart, which should be this winter, to rebuild it with Todd.

True the gearbox is more fragile than our differentials, point is the differential is doing more to raise fluid temps than the trans gears are.

Kind of a catch-22. Cooling of the diff a lot means the base fluid is rising in temperature along with it. A thought....have two pumps and two coolers. Even though they are shared fluids, have one sucking from the rear drain plug, going to a cooler then back into the diff. Then have a second cooling system off the front drain plug feeding back into the upper center of the case.

Rob Edwards

I don't remember the exact numbers, but Mark A had issues with shifting in the Zombie when trans temps would get over (I want to say) 275. He put a setrab cooler, with a fan and cooling air from a duct where the right turn signal used to be: