Converting a GT3 engine to reverse rotation?

[Henry Schmidt]

Originally Posted by blue72s

Why sequential?

Sequential shift linkage tends to be easier to route and fabricate.
Sequential gear boxes also offer speedy shifts with less misshifts.

[Cupcar]

Originally Posted by blue72s

The GT1 'box is actually upside down. Again not suitable for me.

Interesting, I am not surprised.

I'm curious why a 915 is not an option for you.

[Steve@Rennsport]

A 915 will not handle the power/torque of a GT-3 engine for very long, even with pressurized oiling and a cooler.

[Matt Monson]

Interesting. I'd be interested in knowing more about the exact application.

Isn't one of the problems the Cup Cars have had is breaking axles when they lower the car too much because of the angle? If this car used a similar ride height, that 5" higher flange location might now not be such an issue and actually more of a benefit. Though maybe you've already measured this and checked the actual fit of a GT3 gearbox?

[Cupcar]

Also, could it be possible to easily adapt the cable shift linkage of a GT3 gearbox to mid engine location?

[blue72s]

Originally Posted by Matt Monson

Isn't one of the problems the Cup Cars have had is breaking axles when they lower the car too much because of the angle?

Yes but you're talking about RSR not Cup cars. However, the new RSR (2008 onwards) has a new gearbox - transverse type I believe - to lift the output shafts a little higher.

I would like to see the 2008 RSR manual. Anybody got one here?

[johnman001]

This may be a stupid suggestion, but what about a trans out of a Cayman or a Cayman convertible (aka Boxter Spyder)?

[blue72s]

Originally Posted by Matt Monson

If this car used a similar ride height, that 5" higher flange location might now not be such an issue and actually more of a benefit.

Similar ride height for street use? You've got to be kidding. How could the kerbs, ramps and things be overcome?

[Matt Monson]

Originally Posted by blue72s

Similar ride height for street use? You've got to be kidding. How could the kerbs, ramps and things be overcome?

You never said street use. I asked for more details on your exact configuration and application which you have declined to provide. I can't read your mind, so how am I supposed to know it's street use? I asked a question looking for more guidance from you and this is the response? You don't make it very easy to help you with this...

[blue72s]

^ OK, sorry.

[Cupcar]

Doing a little homework, I am not so sure that the aluminum case 915 would not work with a few modifications, particularly in a street/occasional track application in a light car.

Reading Frere's book on the 911, he states that the 915 was designed for an input torque of 181 pound feet, but the original used a magnesium case. He states that Porsche increased this to 275 pound feet with oil cooling and presumably the later aluminum case and bearing mods. Frere also states the the 915 gearbox worked for the '74 Racing Turbo Carrera 2.14 liter which produced 400 pound feet of torque although this was with "some reinforcements" and "not without some problems".

My point is it seems that the 273 pound feet of a 3.6 GT3 engine should not be a huge issue for the 915 gear cluster, particularly if oil cooling, side plate reinforcement, an 8/31 R&P, and steel differential bearing plate with 930 bearing are used.

The weight of the car has to be factored in too, the lower the inertial loading the better.

Wevo has a product line to do all this, as well as a shifter kit to mount the 915 in a mid-engine. If Guard straight cut gears were used, the torque output would be increased much further for a racing only type modification, or for someone who loves gear whine.

[Flieger]

Originally Posted by Cupcar

The weight of the car has to be factored in too, the lower the inertial loading the better.

I thought about that before. The inertial loading will be proportional to the acceleration. Therefore the gear stress will be less on a light car only if the acceleration is constant. If the thrust is constant, then the acceleration of a light car will be greater and the stress on the gearbox will be equal. Thrust is proportional to throttle, so basically you could not use full throttle if you wanted that to be true.

[Cupcar]

Originally Posted by Flieger

I thought about that before. The inertial loading will be proportional to the acceleration. Therefore the gear stress will be less on a light car only if the acceleration is constant. If the thrust is constant, then the acceleration of a light car will be greater and the stress on the gearbox will be equal. Thrust is proportional to throttle, so basically you could not use full throttle if you wanted that to be true.

Makes sense.

I was thinking more along the lines of the initial stress during a clutch popping hard launch in first gear.

[blue72s]

Originally Posted by YTNUKLR

You can't flip the ring because it's hypoid, right?

Right. Negative hypoid does not work.

[blue72s]

Originally Posted by YTNUKLR

Is this for a 914 or something else?

Originally Posted by Matt Monson

I'd be interested in knowing more about the exact application.

Cayman.
Application: Street/occasional track.
Ride height: Similar to street 997 GT3.

[blue72s]

Originally Posted by Cupcar

Doing a little homework, I am not so sure that the aluminum case 915 would not work with a few modifications, particularly in a street/occasional track application in a light car.

Not a light car. About 2,800 lbs if I'm lucky.

[Flieger]

Originally Posted by johnman001

This may be a stupid suggestion, but what about a trans out of a Cayman or a Cayman convertible (aka Boxter Spyder)?

So, Cayman gearboxes are not strong enough? There are 3.8 and Turbo conversion street cars, and there are a few being raced.

I would think it would be easier to make an adapter plate and flywheel than convert the engine to spin the other way.

[Steve@Rennsport]

Originally Posted by Flieger

So, Cayman gearboxes are not strong enough? There are 3.8 and Turbo conversion street cars, and there are a few being raced.

OMG, no,... These have many issues and once you see one all apart, its soon quite obvious what these really are.

The G50 6-speed is FAR stronger, especially a late Cup or RSR version.

[Flieger]

How about one of these?



Or maybe Audi would like to get rid of their old R10 boxes. That can handle 1,000 foot-pounds of torque or thereabouts from the 5.5 liter V10 turbodiesel.






Or there is the Carrera GT gearbox with the cool, low-inertia clutch.

What did 962s use?

[Matt Monson]

Originally Posted by Flieger

What did 962s use?

A 5spd evolution of the 930/935 4spd. Canepa might have one for sale. It's gonna cost more than a GT3 box.

Now that I know the application, I still think the inverted GT3 box is the way to go. It's been done already. It can be made to work.