F40 6 speed Transmission ( UPDATED !!! )

[mr_g]

400HP, 440Nm and a traction controll (VEMS launch rpm vs roadspeed table). Stock F28. Don't have quaife...

[mender]

Just as a follow up, the F40 has been in the Grand Prix for about 2 weeks.... but something has gone terribly wrong. The owner says he did a 2nd-3rd-4th pull shifting at redline (about 6200rpm) each gear. When he shifted into 4th, he heard a bang and then crunching. After that, 2nd gear was GONE (as if it was in neutral) and you could not get into 5th and 6th. Since I now work at the performance shop he goes to, I got to take it apart today. Heres the pictures of the destruction!!!

Can anyone figure out how 2nd gear got destroyed while shifting into 4th ? (He didn't mishift so far as we know)

Second gear failed while shifting into fourth. The only way that can happen is if second gear was engaged, as power only flows through one set of gears at a time (other than the transfer gears at the end of the mainshaft and the countershaft).

My take: this failure was because both second and fourth gears were engaged at the same time. The driver was shifting very quickly and was pulling hard on the shifter to do so. As he pulled through the center of the gate, he pulled the lever towards himself and accidentally shifted back into second. I checked my F40 and it takes very little misalignment of the shifter cables to allow that to happen. The shifter mechanism on the transmission has a ramp that directs the shifter mechanism into second rather than allowing it to stop between the 1-2 and 3-4 gates.

So how did he get two gears? The mechanism itself has a lock-out preventing two gears from being selected so it shouldn't happen and can't happen normally. This has been puzzling me for a while but I think I have the answer. By shifting very quickly, the 3-4 synchro is traveling quite quickly and overshoots to engage fourth as second gear is inadvertently also selected because of the misaligned shifter cable, allowing both fourth and second to engage. When this happens, the cluster with the smallest (and weakest) gear breaks, which is second gear.

It's pretty rare to break gears in the upper range (3-4-5), it's almost always first or second because of the shock loading when launching or shifting. Being the smallest gears, they are the weakest and have the most torque multiplication through them.

I have a vested interest in finding the root cause as I will be using an F40 transmission in an upcoming project that hopefully will top the 500 WHP mark. I was initially quite concerned about the two failures but I'll be going ahead now and will make sure the shifter mechanism works perfectly before performing any full-on runs through the gears.

[benzino]

good first post

interesting to hear more about your project mate

[mender]

It'll be a mid-engined kit car with a 3800 SC converted to turbos and F40, target is 500 WHP and 1900 lbs. After all these years, I deserve my own toy - well, I've had a few over the years but I want a serious toy!

I'm gathering parts but likely won't get to it for a while, my real job is building project cars for others and the customer comes first.

[benzino]

that sounds like a pretty serious toy... hahaha

[MMamdouh]

very nice post mender

MMamdouh

[exist3nce]

Yes very informative post, welcome to the forum.

As the original poster of that F40 being destroyed, I can answer any questions you might have. I was in the car when the second one exploded, and believe me he wasn't shifting aggressively, it was a slow smooth shift because he knew he had already broken one! lol I even have the incident on video if you are interested I can post...

Sounds like a good project, is it a Fiero?

[mender]

I was hoping you'd comment.

So it appears my take was a bit premature based on your reply. Part of it still stands though, as the usual way to break second gear is to be in it; odd when the claim is that it was in fourth but that's what makes diagnosis interesting. Let's start with the first transmission failure.

Right after the first one failed, did it feel like a gear was still engaged, or did the engine flare freely? Did the car lurch forward or back as the clutch was released? What gears were "engageable" after the failure? When did he discover second was gone but not fourth? Upon disassembly, was there anything else unusual?

My project is a Manta Mirage kit-car, gonna see how cheaply I can build it. I got rid of most of the original frame and suspension (VW Beetle front, Corvair rear) and will be using Porsche 928 front and rear suspension and brake parts instead, engine is 3800 SC switched over to twin ball bearing turbos and the F40 trans. So far I have less than $1000 into the body and parts but obviously have lots left to go.

[TurboCruze]

i looks to me judging by the picture's of the case opened up the catastrophic failure was caused by the open diff meaning the retaining pin/bolt for the pinion gear "PIN" broke from overloading from the wheel spin it's happened on many diffs and seam's to be the weakest link next to the pinion gears splitting in half. i have done both.
The only remedy is get a ARP retaining bolt/pin and send the whole diff out to be cryo treated and even if you get a limited slip do the same. NO More braking tranny's