Problem Shifting at High RPM's and Hot
 

I had my 93 RX7 (around 400 hp at crank, single turbo) to a track day. On the front stretch, when the car got hot (after about 7 or 8 laps) it would not shift into 4th gear at 6500 rpm. No grinding, no noises, it just would not go into gear. On the back stretch I tried to shift into 4th at 6000 rpm and no issues. When the car cooled down it would go into 4th at 6500 rpm with no issues. Any ideas on what could be causing this?

Thanks.

Wade

When's the last time the fluid was changed and what are you using? Perhaps try a synthetic gear oil if you're not already.

Bleed the clutch as well. Where does your clutch fully release? If it is at the floor adjust your clutch master to release slightly higher. That should solve your problem.

Thanks. I have bled the clutch. I am not sure where the clutch is fully released (I am away from my garage this month). How do you adjust the clutch master to release slightly higher?

Wade

The car is relatively new to me. I do not know what is in the trans or when it was changed last, so I just picked up some synthetic to change the trans and diff fluid. In hindsight, I should have changed both before going to the track. :(

There is an actuator rod with a clevis attached to the clutch pedal. Loosen the jam nut and unthread the rod from the clevis 1/2 a turn. Reset the jam nut and drive the car. You may need more or less adjustment than that, but the fact that is only does it when hot at high RPM means if that if clutch release is the problem, it just needs a little adjustment.

I wonder if the trans is "growing" from heat and throwing off the distance from the output shaft to the input shaft, which the slider has to connect in order to get 4th.

Very odd that the issue is only in 4th gear and not any others. That points away from a clutch issue and towards a transmission issue. Normally difficulties in engaging 4th gear are related to engine/trans concentricity (or a lack of it) but that isn't something that will change with temperature.

So absent any new data, my theory is the trans is getting really hot, either from internal loading or radiated exhaust heat (most likely both) causing the trans to get longer. Even if all of the metal expanded at the same rate, the aluminum case will expand at 3x the rate of the steel shafts so the effect of heat would be the gap between the input and output shafts would get longer. If this clearance were marginal, then adding more clearance would stuff up the ability for the synchro cone to work if it starts wobbling around so the slider can't push on it concentrically when you try to shift.

I'd definitely change the fluid as a start. If that doesn't help, I'd look into shielding the right side of the trans from exhaust heat. You don't have an open wastegate dump, do you?

FWIW, don't discount the effects of exhaust heat. I fought a fuel pumpproblem for a while until I determined that it was due to heat from the exhaust system causing a vapor lock issue (external pump).

I would say if you have just got the car and tracking it and don't know what is in the diff/trans/engine as far as oil, time to change it all with some synthetic and see if that fixes that issues. For all we know you might be low on oil in the trans! That would be my first logical step.

Thanks. I did check all the fluids before the track day. I will be changing all the fluids next week as it will soon be time for the next track day.

Had that issue in a transaxle from exhaust and (inboard) brake heat, only resolved when the car was parked in the pits for an hour and then it would select again. Can't say I've heard of a late Mazda box doing the same after those few laps! I'd imagine something internal is amiss.