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Andy Zarse
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« on: December 01, 2003, 05:19:12 pm » |
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What was unusual, in racing terms, about the gearbox of the 956?
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I wouldn't sit there if I were you, it's still a bit wet.
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Gilles
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« Reply #1 on: December 01, 2003, 05:22:32 pm » |
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What was unusual, in racing terms, about the gearbox of the 956?
No reverse speed ? Only 4 or 7 gears ? |
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Le Mans is life, anything before and after is just waiting...
... it's not the taking part but the winning that counts !
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Andy Zarse
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« Reply #2 on: December 01, 2003, 05:31:13 pm » |
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Nope, neither of those.
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I wouldn't sit there if I were you, it's still a bit wet.
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Fax
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« Reply #3 on: December 01, 2003, 06:29:24 pm » |
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Come on Andy,
Thats fricking easy, its a syncromesh box, not straight cut like most racing boxes.
John
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Andy Zarse
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« Reply #4 on: December 01, 2003, 06:38:50 pm » |
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Come on Andy,
Thats fricking easy, its a syncromesh box, not straight cut like most racing boxes.
John
Half a point (I think)! You're right about the syncro bit, but I have no reason to believe the gears were not straight cut. The two are not mutually exclusive, so far as I know. For a bonus point, why did they opt for syncro? |
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I wouldn't sit there if I were you, it's still a bit wet.
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Fax
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« Reply #5 on: December 01, 2003, 06:52:43 pm » |
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Hmm..
Got me on the reason why, can't think of it off the top of my head.
I do know that the gearboxes were always the achilles heal of the 936's until they installed a 917/30 Can-Am box in the 936/81 for the 1981 race. Does it have anything to do with the fact that the 956/962 engine was derived from the stillborn Indy car project?
I don't think they were straight cut gears. With straight cut gears you can crash the box (shift without de-clutching) and I think in the
956 if the clutch went you were f@#cked.
John
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Grand_Fromage
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« Reply #6 on: December 01, 2003, 07:54:53 pm » |
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AFAIK it's a load easier to make a crash change on a
synchro helical cut gear than a straight-cut. The main
problem with heli gears is the end thrust at high
torque. Thats one of the reasons for opting for
straight.
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Matt Harper
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« Reply #7 on: December 01, 2003, 08:02:10 pm » |
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Synchro and straight cut (rock-crusher) gears are very different from each other.
Synchromesh gears are quieter and easier to manipulate than straight cut gears, but they're not as strong.
Aside from synchro, I can't think what was peculiar about the 956 transmission. The early ones had a very unorthodox means of attaching the cylinder heads to the barrels however - I presume everyone is familiar with that engineering oddity???
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If it\'s good and fast, it won\'t be cheap. If it\'s fast and cheap, it won\'t be good. If it\'s good and cheap, it won\'t be fast.
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Fax
Guest
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« Reply #8 on: December 01, 2003, 11:08:05 pm » |
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Matt,
No I wasn't familiar with that. I remember that the engine itself was a bit of an oddity. It was originally designed for Porsche's planned assualt on the Indy 500 but when USAC changed the turbo boost regs at the last minute, Porsche bailed out. They officially said that the engine would not run properly at the new boost limits but I've got it from several people who would know that the new boost pressure was no problem, just that Porsche felt they were being jerked around by USAC. The engines they built ended up sitting on shelves at Stuttgart until someone got the idea to plug them into the back of a pair of old 936's and mate them to some old Can-Am gearboxes lying around to cope with the additional BHP and torque. The Ickx-Bell car ran like a Swiss watch to win in '81 and when the new Group C regs were announced, it was a natural powerplant for the new formula.
John
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Matt Harper
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« Reply #9 on: December 01, 2003, 11:31:55 pm » |
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hey John,
Cylinder heads were water cooled, even though the rest of the engine was conventionally air cooled. This caused all manner of problems in making gas and pressurised water seals between the heads and the cyl blocks. With typical german inginuity, they simply welded the heads to the blocks. Not great if you need to service the heads, but effective in curing the leaks.
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If it\'s good and fast, it won\'t be cheap. If it\'s fast and cheap, it won\'t be good. If it\'s good and cheap, it won\'t be fast.
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Andy Zarse
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« Reply #10 on: December 02, 2003, 06:26:41 pm » |
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Matt, I thought there was some element of water cooling on the block too? I seem to remember a story whereby the drive belt snapped once and the fan obviously stopped working. The car raced on for a further two hours with no ill effects. Eventually I thought they did away with the fan altogether on the 962.
Fax, I still cannot see why you can't have a syncro straight cut box. The helical cut on most gears is simply to reduce noise; straight cut refers just to the angle of the cut on the drive teeth. But surely syncro rings can still be fitted? Are you sure you're not confusing this with drop gears, (otherwise known as a dog box?) which require no clutch to be used?
Someone please help. Nordic, you must know.
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I wouldn't sit there if I were you, it's still a bit wet.
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Matt Harper
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« Reply #11 on: December 02, 2003, 08:04:28 pm » |
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Nope - the 956 motor (935/82) had liquid cooled heads - Gear driven DOHC and air cooled cylinder barrels - fan assisted. At Le Mans in '82/'83 it knocked out 620bhp @ 8200 and 2.2 bar - not too shabby for a 2.6 litre flat 6.
I am not an engineer and can only quote others in this regard, but it is my understanding that helically cut gears lack the tensile strength of 'opposing mesh', i.e. non-synchronised straight cut gears. This makes them largely unsuited to racing applications, whereas a rock-crusher is better suited to handle excessive torque and transmission shunt - and the abuse dished out by the driver, over a sustained period, high temps etc.etc.
Why they couldn't just go with a DAF Variomatic, I'm not sure.
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If it\'s good and fast, it won\'t be cheap. If it\'s fast and cheap, it won\'t be good. If it\'s good and cheap, it won\'t be fast.
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Nordic
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« Reply #12 on: December 03, 2003, 09:54:53 am » |
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Someone please help. Nordic, you must know. Sorry I Have'nt a clue on this one. But on the subject of gears, the 956 or was it the 62 did run with the PDK box for a while, before it found its way into the Quattro rally car. |
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Some people will tell you that slow is good - and it may be, on some days - but I am here to tell you that fast is better.
H S Thompson 1937 - 2005
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Steve Pyro
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« Reply #13 on: December 03, 2003, 10:35:32 am » |
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From my searching, I've found that the 956 had an all new fully synchromesh five-speed gearbox plus reverse gear with a single plate clutch.
The later 962 had a sequential gearbox with a multi plate clutch.
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Steve  East Anglian cobras 
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Sherm
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« Reply #14 on: June 30, 2006, 06:17:11 pm » |
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Did it employ that PDK system - I think it stands for Porsche Dopple Kubblung, or something like that - where it pre-selects the next gear?
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