Strength of a 915 'box.
Discussion
Just wondering how much bhp and torque you could reliably put through a 915 gearbox ie without having to rebuild it every few months. I suppose it depends on whether you have the oil cooler too as well (mine doesn't).
I use my car for spirited road driving and am wondering whether the gearbox would be able to take the adding of a supercharger ie anything up to about 330 BHP (and similar torque?).
Obviously the gearbox would wear a lot quicker but do you think that 330BHP is a little optimistic?
:thumbup:
I use my car for spirited road driving and am wondering whether the gearbox would be able to take the adding of a supercharger ie anything up to about 330 BHP (and similar torque?).
Obviously the gearbox would wear a lot quicker but do you think that 330BHP is a little optimistic?
:thumbup:
993Sc said:
Obviously the gearbox would wear a lot quicker but do you think that 330BHP is a little optimistic?
Not sure it would wear quicker... you just run the risk of stripping cogs if it can't take the torque. The G50 can take up to 640bhp in an Ultima and IRC they used a 4 speed 915 variant in the 930 Turbo (or am I wrong here? Dunno), so it may be OK. If it does break, maybe chuck a G50 in. There are plenty around for circa 1.5k.
Swapping shouldn't be too bad as the 3.2 had 915 boxes and g50 boxes in it during its time. So no major re-engineering, but obviously some things will differ.
915 may not need strengthening, but if it does it is likely to be the synchro that gets it first, not stuff like the casing. New internals will be expensive. Try the 915. If it doesn't like the torque, put a G50 in. It will be a better and cheaper option, at a guess.
915 may not need strengthening, but if it does it is likely to be the synchro that gets it first, not stuff like the casing. New internals will be expensive. Try the 915. If it doesn't like the torque, put a G50 in. It will be a better and cheaper option, at a guess.
Fair enough. Thanks for the positive response by the way - most people just say 'Oh no mate, what are you doing? Tut tut.'
When I put the 993 engine in everyone said that the gearbox was likely to fall out but in 3 years driving the 'box is still miles better than my bro's std 3.2 915.
Cheers
When I put the 993 engine in everyone said that the gearbox was likely to fall out but in 3 years driving the 'box is still miles better than my bro's std 3.2 915.
Cheers
I know of another SC with a 993 engine, owned by ringers list guru Ben Lovejoy.
Remember the six speed G50 from the 993 will mate easily, so maybe keep an eye out. I was offered one a year ago for a grand...
Colin at 9M will also be a general expert for these types of questions. He knows far more than me and will be just as positive
Remember the six speed G50 from the 993 will mate easily, so maybe keep an eye out. I was offered one a year ago for a grand...
Colin at 9M will also be a general expert for these types of questions. He knows far more than me and will be just as positive
It's accepted fact that the 915 was at its limit in the 3.2 Carrera with 231 bhp. In fact Porsche fitted a gearbox oil cooler on that model to help preserve it.
So I guess you must have treated your car with kid gloves if the 'box has survived 270 odd bhp - no fiercesome standing starts perhaps?
So I guess you must have treated your car with kid gloves if the 'box has survived 270 odd bhp - no fiercesome standing starts perhaps?
Gearboxes are limited by torque, not power.
The design load of the 915 gearbox was around 285Nm or just over 200lbft, and since Porsche use a 100% safety margin it is generally accepted that a standard 915 will take 500Nm before gears start to break, depending on the way that it is used. The 915 was a very popular conversion among the old 930 fraternity and would happily live with modified engines of up to 360bhp & 475Nm. With the right modified components (bearing support insert & cover) they can be pushed to 400lbft or 550Nm.
The 930 4 speed gearbox is not a variant of the 915, it is much stronger. The bottom line with this one is that you wil not break it with less than 1000Nm, they are incredibly strong.
The G50 as mentioned above will take 700-800Nm for short periods, the G50-50 turbo versions can cope with more.
So going back to the supercharger conversion, the 993 engine mit blower makes around 450Nm, so your box will cope with it provided that you are careful in 1st & 2nd, since the lower ratios are the weakest gears.
The design load of the 915 gearbox was around 285Nm or just over 200lbft, and since Porsche use a 100% safety margin it is generally accepted that a standard 915 will take 500Nm before gears start to break, depending on the way that it is used. The 915 was a very popular conversion among the old 930 fraternity and would happily live with modified engines of up to 360bhp & 475Nm. With the right modified components (bearing support insert & cover) they can be pushed to 400lbft or 550Nm.
The 930 4 speed gearbox is not a variant of the 915, it is much stronger. The bottom line with this one is that you wil not break it with less than 1000Nm, they are incredibly strong.
The G50 as mentioned above will take 700-800Nm for short periods, the G50-50 turbo versions can cope with more.
So going back to the supercharger conversion, the 993 engine mit blower makes around 450Nm, so your box will cope with it provided that you are careful in 1st & 2nd, since the lower ratios are the weakest gears.
rubystone said:
So I guess you must have treated your car with kid gloves if the 'box has survived 270 odd bhp - no fiercesome standing starts perhaps?
I've done a few enthusiastic standing starts but most of my foot to the floor stuff is in 3rd gear so I guess damage to the 'box will be a lot less.
Interesting to read what Colin had to say about the 100% safety margin though.
Re the engineering of a 915 to G50 swap, I've always understood that because the G50 box is longer you have to change the torsion bar tube in the chassis to the later type which has a curve in the middle to accomodate the box. That's a serious job, or does this only apply to the Turbo ?
out of interest i was pushing 270 bhp out of a 901 strengthened box at prescott hillclimb the other idea, at least 10 starts of min 5000rpm on grippy rubber.
Also been running a 915 box for 7 years with 250bhp,cant count how many 5,000rpm standing starts we have done hillclimbing and sprinting over this time. The boxes have held up very well so far!
Just need to watch that torque with a super charger!
Also been running a 915 box for 7 years with 250bhp,cant count how many 5,000rpm standing starts we have done hillclimbing and sprinting over this time. The boxes have held up very well so far!
Just need to watch that torque with a super charger!
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