Title: Porsche 956
Post by: Andy Zarse on December 01, 2003, 05:19:12 pm
What was unusual, in racing terms, about the gearbox of the 956?
Title: Re:Porsche 956
Post by: Gilles on December 01, 2003, 05:22:32 pm
What was unusual, in racing terms, about the gearbox of the 956?
No reverse speed ? Only 4 or 7 gears ?
Title: Re:Porsche 956
Post by: Andy Zarse on December 01, 2003, 05:31:13 pm
Nope, neither of those.
Title: Re:Porsche 956
Post by: Fax on December 01, 2003, 06:29:24 pm
Come on Andy,
Thats fricking easy, its a syncromesh box, not straight cut like most racing boxes.
John
Thats fricking easy, its a syncromesh box, not straight cut like most racing boxes.
John
Title: Re:Porsche 956
Post by: Andy Zarse on December 01, 2003, 06:38:50 pm
Come on Andy,
Thats fricking easy, its a syncromesh box, not straight cut like most racing boxes.
John
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?
Title: Re:Porsche 956
Post by: Fax on December 01, 2003, 06:52:43 pm
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
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
Title: Re:Porsche 956
Post by: Grand_Fromage on December 01, 2003, 07:54:53 pm
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.
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.
Title: Re:Porsche 956
Post by: Matt Harper on December 01, 2003, 08:02:10 pm
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???
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???
Title: Re:Porsche 956
Post by: Fax on December 01, 2003, 11:08:05 pm
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
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
Title: Re:Porsche 956
Post by: Matt Harper on December 01, 2003, 11:31:55 pm
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.
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.
Title: Re:Porsche 956
Post by: Andy Zarse on December 02, 2003, 06:26:41 pm
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.
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.
Title: Re:Porsche 956
Post by: Matt Harper on December 02, 2003, 08:04:28 pm
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.
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.
Title: Re:Porsche 956
Post by: Nordic on December 03, 2003, 09:54:53 am
Someone please help. Nordic, you must know.
Quote
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.
Title: Re:Porsche 956
Post by: Steve Pyro on December 03, 2003, 10:35:32 am
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.
The later 962 had a sequential gearbox with a multi plate clutch.
Title: Re: Porsche 956
Post by: Sherm on June 30, 2006, 06:17:11 pm
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?
Title: Re: Porsche 956
Post by: LangTall on June 30, 2006, 10:21:47 pm
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?
That would be Porsche Doppel Kupplung then. ;) Has never been used in the 24 du Mans, was only used in so called 'Flugplatzrennen'. Porsche is adapting the Aud/VW DSG gearbox, and will call that the DPK again. Will be available on the 911/998Link on this in German (http://www.pm-magazin.de/de/vermischtes/vm_id267.htm)
Title: Re: Porsche 956
Post by: Lorry on June 30, 2006, 11:07:57 pm
Going back to the synchro 956, I thought the reason was that you could change gear faster, as it helps the dogs engage, and puts less stress on them making it more reliable. Plus they were planning to sell them to privateers, who would like it if they were used to less exotic machinery.
Title: Re: Porsche 956
Post by: Rhino on July 01, 2006, 10:54:14 am
They did use it at other races, i seem to remember Derek Bell having to use it in i think the Malaysian race when they had torrential rain. On the fasted part it started changing down on it's own! Only the rev limiter saved the pistons from going into low orbit.
