915 oil cooling locations

[gliding_serpent]

Just wondering if my understanding of the 915 anatomy is allowing me to understand where these oil cooler return lines are going.

1. oil hitting the leading half of the pinion prior to connecting the ring (opposite to the side of where it contacts the ring).


The oil hits the ring, at the half-way mark before connecting with the pinion.


This looks to be hitting the leading edge of the R+P where it connects the ring, which may not be good for hydrostatic pressure in the gears.


Thanks for any thoughts. The first one is mine.

[gliding_serpent]

bump

[RedCoupe]

The heat that is generated in the differential is due to the large difference between the centerlines of the ring and the pinion (extreme hypoid angle), so there is a fair amount of sliding on the teeth which causes the heat. It would seem that directing the cooler returns to the gears would tend to help cool them right where they are the hottest. The actual lubrication typically occurs from the ring gear dipping in the lubricant right before meshing into the pinion.

[gliding_serpent]

So maybe I understood this incorrectly. Based on other posts, some folks felt that directly spraying cooling fluid on the R+P contact point would increase hydrostatic pressure between the gears (due to the extra oil), and thus increase wear.

If the transmission fluid level is such that the ring gear is submerged in oil before the point the R+P connect, then this kind of makes the entire hydrostatic pressure argument moot, and thus debate on the best return line location a bit pointless, and my location (first picture above) or the final picture posted, would be the best locations as they are closest to the pinion (the hottest part).

Or does the ring become submerged in oil only after it meshes with the pinion?

[Jon B]

Originally Posted by RedCoupe

The heat that is generated in the differential is due to the large difference between the centerlines of the ring and the pinion (extreme hypoid angle), so there is a fair amount of sliding on the teeth which causes the heat.

The 915 ring & pinion gears are not hypoid, as the rotating axis (centerline) of the two gears intersect.

[Eagledriver]

IMO the reason to cool the gearbox is to keep the overall temp under control. If the tranny gets hot the case expands and the gear alignment gets out of whack increasing the wear and causing shift problems. So where the oil goes in and out doesn't matter much.

[Jon B]

Originally Posted by gliding_serpent

Or does the ring become submerged in oil only after it meshes with the pinion?

The ring gear is situated to the left side of the pinion, and rotates in the same direction as the rear wheels.
If you are using oil as a cooling medium, then the best place to direct it is immediately after the gears mesh. In a 911, that would mean under the pinion gear.
The oil at the bottom of the transmission will obviously be hotter than oil that has just passed through an external cooler.

[Jon B]

Originally Posted by Eagledriver

If the tranny gets hot the case expands and the gear alignment gets out of whack increasing the wear and causing shift problems.

The gears, shafts and differential will expand in kind, so there will not be a significant misalignment issue.
The operating temperatures of the steel gears and differential will be much greater than the alloy case, so will offset the different expansion rates of their materials.

[gliding_serpent]

Below is what I am talking about hydrostatic pressures. Hayden weighs in. Also note that the factory oil cool enters in a location after the r+p contact/unmesh.

To answer my own question, the transmission oil fill line is about mid case,or mid pinion. Now, with all the turbulence of fluid, who knows the real world fluid level, but at full song, it is a safe bet to be between fill level and just touching the base of the pinion teeth as they unmesh with the ring gears. The faster the transmission spins, the lower the effective level. Thus haydens concerns below are probably real if not deal breakers, especially at very high rpm. My oil entry location to the passangers side of the pinion from the top (see a above) should be fine.

Originally Posted by wevoid

Good information from Bill as usual.

I will offer one subtlety regarding the cooler return oil into the leading edge of the CWP mesh area. The highest temp is immediately after the mesh on both parts, also the introduction of excess oil as the gears lead into mesh can create very high hydraulic forces, enough to reduce the life of the gears, cause surface errosion. Cool oil would be best added to the trailing side as the gears come out of mesh.

As Bill says the "Virtual Hurricane" or as I put it in my favorite term "a whirling dervish of oil mousse" kind of defeats any real effort to have an effective oil spray bar system. I used a new analogy in a discussion the other day - "like trying to piss down the drain hole while your standing under the shower." It is unlikely that the oil from a spray bar nozzle makes it to the intended target. Unlike the piston oil squirters in a 911 motor, that operate in a relatively oil free environment inside the dry sump crankcase of the motor.

However the spray bar is a good way to fully distribute cool oil to many locations - think of it more as a multi location distribution manifold, rather than a discreet oil squirting device.

In terms of the black fitting on the transmission on the photo, I suspect that is a custom made oil return fitting. To return in that position is probably intended to socket directly onto a spray bar system, as that is where the spray bar will make contact with the front cover, if that axis is extended.

On of our current 2005 projects is a 915 cooling system. It will be somewhat modular and fill as many 915 cooling requirements as possible, including a no-disassembly-required kit, that will allow a 915 cooler system to be installed with the transmission in the car. No cutting, drilling tapping or welding required.

One of the flexible options incorperated is to have a spray bar option - also installed without disassembly of the transmission - just the front cover. To do this we had to come up with a machining operation to provide the port and bolt pattern for the oil supply to the spray bar. It looks similar to the black fitting in the photo.

Jerry Woods and I compared notes and agreed on what we will share as a universal port location, size and hole pattern.

It seems that the more 3.6l/915 transplant cars built, the more demand there is for 915 cooling systems. A system like Thom's is very effective and depending on what stage of your project you are at (transmission disassembled??) probably the most straight forwards option.

Regards

Hayden

[stownsen914]

I've read different opinions on hydrostatic pressure and gear meshing. I looked into this a few years ago when I added oil cooling to my 901 box, to determine placement of the oil jets I set up as oil returns after the cooler. I read a few warnings about hydrostatic pressure wearing the gear faces. Then I read another opinion that made sense to me - that hydrostatic pressure occurs when you force oil into a confined space where pressure builds, like the gears in 911 oil pumps. Tranny gears don't have this situation because the oil can always squeeze out the sides of the gears where the teeth mesh. In the end, I couldn't find enough information about gear wear fromhydrostatic pressure, so I wound up not worrying about it. I'd be curious to hear from any tranny experts hear if they've observed it though.

Scott

[Matt Monson]

Originally Posted by stownsen914

I've read different opinions on hydrostatic pressure and gear meshing. I looked into this a few years ago when I added oil cooling to my 901 box, to determine placement of the oil jets I set up as oil returns after the cooler. I read a few warnings about hydrostatic pressure wearing the gear faces. Then I read another opinion that made sense to me - that hydrostatic pressure occurs when you force oil into a confined space where pressure builds, like the gears in 911 oil pumps. Tranny gears don't have this situation because the oil can always squeeze out the sides of the gears where the teeth mesh. In the end, I couldn't find enough information about gear wear fromhydrostatic pressure, so I wound up not worrying about it. I'd be curious to hear from any tranny experts hear if they've observed it though.

Scott

This. And especially since this car is a track toy and not heading off to run the 24 Hours of Anything I wouldn't lose any sleep over it.

Look at how the factory did it on the 3.2 Carrera. It returns from the side. Cooler oil is the important part. Location is splitting hairs IMO.

Also keep in mind on a 911 it is the gears that can get starved, esoecially 5th. The ring and pinion has a ton of oil on it due to orientation and acceleration forces. Look at a GT3 Cup. The cooler return is in the Tailcone to feed the spraybar and hollow shafts. The ring and pinion doesn't get sprayed at all. The pickup is in the main housing but the return is out back.

[gliding_serpent]

I appreciate the input guys. As I go through my build I am working to "understand" design choices, thus helping to make better design choices. I very much enjoy splitting hairs, and learning something in the meantime. It never hurts to debunk dogma along the way.

[Eagledriver]

Originally Posted by Jon B

The gears, shafts and differential will expand in kind, so there will not be a significant misalignment issue.
The operating temperatures of the steel gears and differential will be much greater than the alloy case, so will offset the different expansion rates of their materials.

You may be right about the gears being hotter but the shift rods and forks should be about the same temperature as the case. Most of us who race have experienced the difficult shifts of a hot transmission.

The transmission does a great job of circulating it's oil. The ring gear is set up like an oil pump in front of a port in the case. If you cool the oil,you cool the whole transmission.