915 RPM's higher in 4th gear than 5th?

[matt930s]

Hi-

Edit: in title, meant to state RPM's higher in 5th than 4th...

I've owned a lot of different 901 and 915 cars, but this new to me 78 SC CIS project has got me confused. While driving and running through the gears 1-4 work normally, but when I shift into 5th the revs are a couple of hundred RPM's higher than 4th... When I drop back to 4th from 5th, revs drop. I have a receipt for a complete rebuild by the PO.... Any ideas? Tried a search but I get everything but this issue.

Thanks in advance...

Matt

[hcoles]

subscribed to hear what the root cause is, first I've heard of this issue.

[Matt Monson]

Based on the records of the rebuild what ratios do you expect to be in the box?

[Arne2]

Linkage mal-adjusted and/or bad coupler bushings? Perhaps you're not really getting 5th, but dropping back into 3rd instead?

[Tremelune]

I, too, am boggled. Underdrive.

[Walt Fricke]

1) 4th in these cars is a 1:1 ratio. One turn of the engine = one turn of the pinion gear. With the likely 8/31 R&P, your rear tires should rotate 8/31sts of 360 degrees.

2) Try this: jack car up on one side. Rotate motor 360 degrees. Figure out how many degrees the tire rotated. If it is twice what you calculated in 1), you have the standard 4th. If you have a digital level it is easy to bungee it to a rim and measure angles. Or you could use a degree wheel, or any number of other methods. And you can compare what you get with what happens in 5th and 3d doing the same thing. I do just this when checking race cars to see if they have the required gears in stock classes where you can't just swap gears around. Takes some technique to deal with slop in the system, but highly accurate - I can tell a one tooth difference in a gear set.

3) It is very unlikely that anyone installed a 4th higher than 1:1, or a 5th lower than 1:1. You'd need custom gears for that, and it would be simply bizarre to do so.

4) Arne may have it figured out. Pull the boot up from your shift console so you can see what is going on there. Is what you think is 5th as far to the right as reverse? Is the reverse lockout pawl where it should be when you are in 5th? For that matter, do this simple test. At 60 mph, record what your RPMs are in 3d. Then in 4th. And again in 5th. If 5th = 3d, there you are - fix the linkage. Just search shift coupler here for a world of information on all that can go wrong and what can be done pretty easily and inexpensively to fix it.

5) I once had "higher than expected RPM in 5th" syndrome. Shift into 5th, RPMs went up. But only for a while - if I backed off, things would settle down. Felt a bit like driving a 1950s automatic transmission car. This turned out to be a clutch disk which had reached the end of the road. It was slipping when I hit full power after the upshift. Did it in 5th because that puts the most strain on the clutch because that is where you get the least torque multiplication from the gears helping push the car forward. New clutch disk and all was well. Probably not what you describe, but thought I'd mention it.

[matt930s]

I will have a look agiain tomorrow; seems like the "shifting back into third" could somehow be the culprit (but I swear i was pushing that dog leg all the way over, but now I'm second guessing)...

Thanks for all the thorough assistance.

Matt

[al lkosmal]

An easy way to check would be to check RPM's/MPH in 3rd and compare it to RPMs/MPH when you think that you are in 5th....if they match............you have shifted back into 3rd.

regards,
Al

PS: most likely a badly adjusted coupler.

[tctnd]

4th and 5th may have been transposed when it was rebuilt.

regards,
Phil

[matt930s]

I hope it's the linkage... I will check in the AM....

Matt

[matt930s]

I feel sorry for those up north!!!

Matt

[Walt Fricke]

You can swap 3d and 4th on a 915, but nothing will swap with 5th other than another flavor of 5th.

[Porsche84]

My guess is that a wrong gear was used in either 4th or 5th. Walt is correct, you can not swap 4th and 5th in a 915. Hopefully, it is fifth gear as that is real easy to change. Changing 4th is a big project.