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GT40 Tech - Powertrain/Transaxles Transaxles and driveline - don't dare post engine things here!

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Old 21st September 2009, 10:38 PM   #1 (permalink)
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930 inverted box ---- halfshaft angle issue

I've see inverted 930 boxes being used from time to time, but after tonight's mock-up, my fears were confirmed. I've got a question about half-shaft geometry with an inverted box.

Using a 26" tire, the axle centerline (perfectly horizontal) is obviously about 13" above the ground. This puts the crank centerline approximately 10" above the ground, which means that unless I use a dry-sump system with the LS block, my pan is going to be in the weeds.
In order to get the pan up to a 5" ground clearance (that is the clearance of the tube frame at the rear), I've got to raise the entire assembly up by about 3", which puts the half-shafts at about a 10 angle. The 930 CV is capable of 28 (I believe) under torque, but the 10 appearance seems to be a lot more than any view I've ever seen of this configuration.

Options?
1) Tilt the engine a few degrees just as has been done for decades on RWD cars, but then I'm concerned about oil pooling at the back of the 930 case.
2) Lower the engine an inch or so to help improve the geometry, but risk damage to an aluminum oil pan,
3) Dry sump (probably my last option)
4) Combination of 1 & 2
5) Find some really huge tires for the rear so I can lower the car, and re-align the half-shafts.

So what kind of oil pan clearance is had on the 930 inverted boxes out there, and what size tires and chassis ground clearance is being run?
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Old 22nd September 2009, 01:26 AM   #2 (permalink)
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Re: 930 inverted box ---- halfshaft angle issue

Hi, I understand that option 3 is your least preferred, but it will also pay significant dividends in CG height and therefore ultimate handling.

Cheers, Andrew
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Old 22nd September 2009, 01:39 AM   #3 (permalink)
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Re: 930 inverted box ---- halfshaft angle issue

Hi Terry!

Dry sumping is going to be a lot of work and complication. There's a reason Shelby removed the dry sump systems from the GT40s he got from England in the day.

I believe a 295/50/15 wheel/tire is more like 26.6 inches - a small difference.

On my 911 the CVs look to be sitting at about 11-12 degrees off centerline at rest, and the car is set at stock ride height. I've heard from some of the porsche powertrain specialists that the CVs actually work better (more reliably) at something off centerline in this degree range.

Of course, you'll want to figure out what's the maximum degree you'll see at full droop - not that you typically have full power-on at full droop. And, yes, 28 degrees is more than you want at full power but less than say 23-24 degrees should be OK I would think. You might want to check with some of the porsche powertrain specialists on this subject - there's a lot of smart people that have looked at this before and developed some nice CV-based solutions.
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Old 22nd September 2009, 08:22 AM   #4 (permalink)
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Re: 930 inverted box ---- halfshaft angle issue

Quote:
Originally Posted by blueovalz View Post
Options?
1) Tilt the engine a few degrees just as has been done for decades on RWD cars, but then I'm concerned about oil pooling at the back of the 930 case.
2) Lower the engine an inch or so to help improve the geometry, but risk damage to an aluminum oil pan,
3) Dry sump (probably my last option)
4) Combination of 1 & 2
5) Find some really huge tires for the rear so I can lower the car, and re-align the half-shafts.

So what kind of oil pan clearance is had on the 930 inverted boxes out there, and what size tires and chassis ground clearance is being run?
Terry,

Or.....

6) Build a custom pan that is shallower, maybe with wings to regain lost oil capacity.
7) Fit a skid plate under the existing sump.

Actually, I'm with Andrew on this one. Dry sump is a no brainer in a McLaren! Do it now, coz if you don't do it now, you'll do it later! No oil surge, lower CoG are the major reasons, plus the side benefit of reducing loads on the distributor gear.(Edit. Sorry Terry, slight brain fade. Automatically slipped into GT40/Windsor mode. I have absolutely no idea about the oil pump drive on the LS engines!)

However if you must run a wet sump I'm sure you would be OK at much less than 5" under it considering it's so close to the rear wheels. If you are worried fit an alloy skid plate for peace of mind during the odd scrape. Also any reasonable sort of angle you may run on the motor is not going to gain you a lot of extra clearance at the rear. And if you were worried about low oil at the cw&p I'm sure adding a little extra to compensate would have no ill effects, but honestly I don't see that as a problem.

Since you wanted a comparison, and mine is basically a track car on 26" rears. My rear chassis rails sit at 4" above the ground and the bottom of the dry sump is slightly above that. The lowest point on my car is the 930 bellhousing which is two and a half inches above the the ground. My drive shafts are one inch lower at the output shafts than at the wheels.

Hope that gives you something to think about....
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Last edited by Russ Noble; 22nd September 2009 at 09:10 AM.
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Old 22nd September 2009, 12:15 PM   #5 (permalink)
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Re: 930 inverted box ---- halfshaft angle issue

Thanks all for the suggestions.
Russ, I appreciate the data you've provided. It lets me see perhaps some light at the end of this seemingly dark tunnel I'm in. I'm going to look into the dry-sump options and lower the motor some more. Another issue not mentioned is the fact that the pick-up point for the lateral upper link for the rear suspension lies directly over the axles (but about 1.5" above it, giving an upright bump travel of about 3"), which means the only alternative is to lower the transaxle (or move the entire assembly rearward another couple of inches). Putting bigger tires on fhe car and then lowering the chassis will make this issue alone even worse.
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