What breaks in the 915?

[TeeJayHoward]

What are the 915's weak points, and what solutions are available for them? I see that billet side plates are an option, but is that really something that would prevent the 915 grenading if I were to hook it up to a crazy twin-turbo 4L mill? How about an LT5? A 6.7 Power Stroke? At some point, the torque capacity of the transaxle is going to be tested. What exactly is it that snaps?

[Marc Bixen]

Enough power and you can break anything! Usually high power will break an input shaft.

[dannobee]

A 915 will not take the torque from any of those engines. They simply were not made for that kind of abuse.

You'll need a 930 transmission or one of the G50's.

[TeeJayHoward]

Originally Posted by Marc Bixen

Usually high power will break an input shaft.

Awesome, now that I've got an idea of the first thing that breaks... Are aftermarket up-rated replacements available?

[stownsen914]

There are many failure modes for a trans. A few to mention:
- Put enough hp and RPMs through a gearbox and it will get hot. Heat will cause accelerated wear of parts. A cooler will help with this.
- Enough torque will physically deform and break parts. No amount of cooling will help that.
- Shock loads like dropping the clutch from standing start is abusive even if it doesn't necessarily fall into one of the above two categories.

Guard makes gearsets and input shafts that I'm sure are stronger than stock. Maybe side covers too? The one piece bearing retainer is a good upgrade too.

[dannobee]

Oh, you wanted a list?

Differential will break at the ring gear bearing side. Input shaft will break or twist. Pinion and input bearing holes will wear and spread out. Crown wheel and pinion gears will break. Teeth on the synchro hubs will break, jamming it into (usually 2nd) gear.

915's will not reliably take more than about 300lb ft of torque. 350 is pushing it. Two of the three engines you mentioned can have double that amount of torque.

[Geneman]

see Scott winders posts here. he is running a hot rod knightrace race motor with outputs close to those mentioned ... with a relatively garden variety 915... but he is a superb driver! .. my 915s are lasting about 4-5 race events ...dog teeth and synchros for me...

[Matt Monson]

Originally Posted by Geneman

see Scott winders posts here. he is running a hot rod knightrace race motor with outputs close to those mentioned ... with a relatively garden variety 915... but he is a superb driver! .. my 915s are lasting about 4-5 race events ...dog teeth and synchros for me...

Scott is running my mainshaft with an active first and tall 2&3. This does substantially increase the strength and durability of the gears that break most often in a racing application. They use stock synchro teeth, sliders, and shift sleeves, which are wear items that are subject more to individual driving style than anything else.

I don’t think Scott’s engine makes the kind of torque that this guy is talking about making. Really the ring and pinion is going to be the limiting factor in at the limit strength on these boxes. This is doubly true if one puts in a bunch of new gears but reuses the ring and pinion.

Regards,

Matt Monson
Guard Transmission Llc

[winders]

My engine has 430 HP and 340 FT-lbs TQ at the crank.

As Matt says, I am running a 33% taller 1st gear than stock but I use it in races. 2nd gear is 13% taller than stock while 3rd gear is only 3.41% taller than stock. 4th and 5th are shorter than stock.

I have been running this same transmission since 2012 and with this stronger engine since 2021. But this 915 has always been in front of a 3.6 race engine since I have had it. I bought it already setup from another racer and he had it in front of a 3.8L engine.

The ring and pinion still look perfect after all these years. I had to replace 3rd gear once due to wear issues. I did regear the box for 2021 and only 3rd gear stayed the same. But, all the gears I took out looked great. I attribute the stellar life I am getting out the ring and pinion and gears to Swepco 210......

I do send the 915 to Bill Rader each off-season to go through the box to see what wear items need replacing and to make everything else is looking good.

I would never put a 915 in front of strong Turbo or diesel engine. Just too much torque!