Real Fiero Tech

Shaun and I were discussing building high rpm engines for fieros.

Is there a limit to the amount of rpm you can put through a getrag? I heard it becomes hard to shift at arounf 7500-8k rpm

Thoughts?

Am I smoking, or did I read somewhere that it's rated to like 7K maximum input shaft speed?

I dunno how much you could spin it and still get it to work acceptably. Or for how long.

I first heard this and thought "bullshit".

1970's 4spds can be spun to 9000 rpm and still shift with no issues. BUT im no expert at this. I could see this being an issue with automatics not being able to shift at high rpms. but not manual trannies.

with good synchros, the speed of the shafts should not matter. The only difference in 1k rpm and 10k rpm is the centripital force applied to the synchros. This may have a gyroscopic effect on the lateral movement of the synchro, but I highly doubt it will effect it enough that your strong arm cant shift it.

This assuming your tranny is in good condition, no chipped gears, no pitted bearings. end serious post here

when I was building my lumina tranny to hold 15k rpm so I could do high speed runs on the salt flats, I had all the gears and shafts ballanced on a small crank ballancer, then shot peened and cryotreated. cost me like 7 grand but the car did 700mph. Thats a car that impressed me.

p8ntman442 wrote:with good synchros, the speed of the shafts should not matter. The only difference in 1k rpm and 10k rpm is the centripital force applied to the synchros. This may have a gyroscopic effect on the lateral movement of the synchro, but I highly doubt it will effect it enough that your strong arm cant shift it.

This assuming your tranny is in good condition, no chipped gears, no pitted bearings. end serious post here

when I was building my lumina tranny to hold 15k rpm so I could do high speed runs on the salt flats, I had all the gears and shafts ballanced on a small crank ballancer, then shot peened and cryotreated. cost me like 7 grand but the car did 700mph. Thats a car that impressed me.

lol! classic.

Thanks for the answers.

I would like to build my N* with a bunch of high performance goodies from chrfab, but it would be pretty useless if I'm limited by the amount of rpm I can put into a getrag.... With their forged pistons and rods, springs and retainers the motor should go to 8000rpm and beyond. So yeah, if the trani is going to hold me back, there isn't much point in building a high HP NA N*.

p8ntman442 wrote:with good synchros, the speed of the shafts should not matter. The only difference in 1k rpm and 10k rpm is the centripital force applied to the synchros. This may have a gyroscopic effect on the lateral movement of the synchro, but I highly doubt it will effect it enough that your strong arm cant shift it.

This assuming your tranny is in good condition, no chipped gears, no pitted bearings. end serious post here

when I was building my lumina tranny to hold 15k rpm so I could do high speed runs on the salt flats, I had all the gears and shafts ballanced on a small crank ballancer, then shot peened and cryotreated. cost me like 7 grand but the car did 700mph. Thats a car that impressed me.

*snicker*

I have felt and driven a 5spd V6 car that reved to 8000 rpm. I have personally felt what it felt like to try and shift the bitch even at 7300 rpm from 1st to 2nd gear. You could not shift that thing quick at all. Every time you had to slowly shift it.

This is of course Curtis Walkers turbo z24. I've driven it a few times. He's had 3 transmissions in that car all of them were always a terrible shift at high RPM. We just blew out the trans again on it back when I was down visiting him in Dallas back in October. I don't know what everyone else here has done for that amount of RPM but, You WILL have to make changes to the synchro'/Dogs/hubs to allow it to shift properly that high. It was never designed to shift that high and does not like to do so.
I know on my sunbird at the 6500+ shifts it was starting to get a bit finicky.

On my muncie 4 speed I would shift at 7100rpm or so with the 3.4dohc, it didn't seem to shift any differently.

Here is a question, what is different in a transmission that is designed for high rpm shifting?

Most honda 4cyl engines have a redline somewhere around 7k rpm, it should be very difficult for them to shift at 9k when modded then correct?

The synchros have to match speeds as gears are engaged. This means rapidly changing the speeds of spinning shafts. There is energy involved and because of the time constraints, the synchros have a certain amount of power involved.

The 1-2 shift in a 282 drops the engine back to 59% of the shift RPM. Double the RPM and the amount of energy that the synchro has to remove from the input shaft (via friction) quadruples. However, since the RPM and thus surface speed as the synchro friction interface doubles, the horsepower of the synchro for a given shift fork load doubles... so you get double the power and 4 times the energy to deal with. Shift times with constant force on the shifter will increase linearly with RPM. This is a natural consequence of having a synchro.

The 282 and 4 speeds use single cone synchros. Modern transmissions for high RPM apps use double or even triple cone synchros. This has the same effect as going to a double or triple disk clutch. You get 2 or 3 times the torque from a given shifter load and shift times are cut by the same proportion.

I've also read somewhere that the max rated input shaft speed for the 282 was 7K.

The newer Isuzu 5 speed has triple synchros for 2nd gear.