What Happens When 7:31 Pinion Breaks?

[jamesjedi]

Hi,

What kind of damage can be expected when a 7:31 pinion breaks?

What can be expected for repair/replacement?

Thanks in advance.

[Flieger]

At the worst, Kablammo. As in going down the front straight, 100mph, hard on the power and BANG! the gears seize, wheels lock, the tranny case explodes, crankshaft breaks, drive shafts shear and flail around, possibly causing the rear wheels to come off, while you go spinning into the wall backwards and on fire.

At the best, it looses a piece tooth but the piece finds its way to the drain plug magnet and stays there so the car keeps running until the next piece breaks off, just with more noise, less smoothness, and accelerated wear.

Do you feel lucky?

[jamesjedi]

Thanks for the reply.

If the ring and pinion is changed to 8:31, is it the only part that changes? Or do all gears have to be changed etc.?

[Flieger]

I believe you can just change the ring gear and pinion shaft, but that means taking the whole 'box apart and all the gears off the pinion shaft. So you are only saving the cost of the new parts, no labor.

I'm still using a 7:31 by the way.

[jamesjedi]

Thanks for the replies.

With the 7:31 removed and replaced with an 8:31, I realize that the gearing will change, but will it still have have "shorter" gears than a 915 that originally had a 8:31 ring and pinion?

[Steve@Rennsport]

Originally Posted by jamesjedi

Thanks for the replies.

With the 7:31 removed and replaced with an 8:31, I realize that the gearing will change, but will it still have have "shorter" gears than a 915 that originally had a 8:31 ring and pinion?

While the final drive changes from 4.43:1 to 3.87:1, the gear splits are unaffected.

Since everything is shifted higher, top speed is (potentially) extended at the expense of acceleration. You will need to do the math, based on your rear tire diameter, to know how this affects your car.

You can also install an 8:35 R&P for much improved durability over the 7:31, while keeping the overall ratios and speeds the same.

[jamesjedi]

Steve...Thanks. Surprised to see you here on a Sunday.

Thanks again.

[Flieger]

If you install a taller final drive ratio the rpm drops between gears will be larger.

[jamesjedi]

I just checked the price on the 8:35. R&P. They are $1800+. They look very nice, as they should.

[Steve@Rennsport]

Originally Posted by Flieger

If you install a taller final drive ratio the rpm drops between gears will be larger.

Run the numbers for yourself using some transmission software and you'll see that changing the final drive has zero effect on the RPM splits at each shift.

[Steve@Rennsport]

Originally Posted by jamesjedi

Steve...Thanks. Surprised to see you here on a Sunday.

Thanks again.

My pleasure, anytime.

Working today,...too far behind to take any time off.

[Flieger]

Originally Posted by Steve@Rennsport

Run the numbers for yourself using some transmission software and you'll see that changing the final drive has zero effect on the RPM splits at each shift.

I don't have any transmission software but my hand calcs are saying that for discussion purposes a doubling of the r+p ratio halves the engine's rpm drop all else equal.

The geartrain value is the ratio of the gears, final drive, and tire rolling diameter multiplied together.

[Jon B]

Originally Posted by jamesjedi

If the ring and pinion is changed to 8:31, is it the only part that changes?

You cannot use an 8:31 ring & pinion with a 7:31 differential, there is a 2.0mm offset difference to the ring gear flange. Depending on what year your transmission is, you might also have no speedometer drive.

[Bill Verburg]

Originally Posted by Flieger

I don't have any transmission software but my hand calcs are saying that for discussion purposes a doubling of the r+p ratio halves the engine's rpm drop all else equal.

The geartrain value is the ratio of the gears, final drive, and tire rolling diameter multiplied together.

Listen to Steve, changing cwp or tire rolling radius has zero effect on the drops between gears

[mamut]

It just changes the time for speed in each gear, it will either extend or shorten the speed in each gear, Steve is absolutely correct.

[jamesjedi]

As always, thanks.

[Fishcop]

Originally Posted by Werkstatt

You cannot use an 8:31 ring & pinion with a 7:31 differential, there is a 2.0mm offset difference to the ring gear flange. Depending on what year your transmission is, you might also have no speedometer drive.

Jon, it can be done - a 2mm spacer plate is available, and if you had a desirable LSD you'd probably be trying to salvage its use due to the rarer drive hubs. The hens teeth rare 3.0 Carrera gearbox (and some other 77 change over year boxes) had a 8:31 with a mechanical speedo drive. I've also had later speedos modified by NH Speedos to look identical to an earlier cable drive speedo when using a later box in an earlier car.

Yes you do end up with a "Frankenbox"

[Flieger]

Originally Posted by Bill Verburg

Listen to Steve, changing cwp or tire rolling radius has zero effect on the drops between gears

Maybe I am getting confused with how the thrust vs. speed curves are flattened out by a taller final drive ratio, meaning that there is a larger thrust drop between gears at a given speed.

[Jon B]

Originally Posted by Fishcop

Jon, it can be done - a 2mm spacer plate is available...

John, a 2mm spacer is for the opposite combination, a 7:31 ring & pinion with an 8:31 differential...

[Bill Verburg]

Originally Posted by Flieger

Maybe I am getting confused with how the thrust vs. speed curves are flattened out by a taller final drive ratio, meaning that there is a larger thrust drop between gears at a given speed.

RPM drops are the subject
stock 915/67


915/67 w/ 7:31 but otherwise the same as above


Obviously the peak on thrust vs speed curves is lower and to the right(at a higher speed) as you go up through the gears, changing cwp or tire rolling radius is like a mini gear shift as can be seen here in a comparison of 275/35x15 vs 275/35x18 tires


any time a higher gear is selected either in a build or by a shift the speed at which max toque or thrust is achieved will move to the right the reverse will be true if a lower gear is selected that can be seen here where the cwp and tire are fixed but 2 different transmissions w/ different internal ratios are compared