Clutch slip? Or something else?

[alifsyz]

My E30 is 24v swapped with a M50B20 and matching gearbox. Recently, I've changed the Final Drive from 4.10 to 3.46 cuz it was getting annoying on the freeway. However, I'm getting the same 'rpm to speed ratio' with the old 4.10 diff. Is it possible my clutch is slipping? I checked the 3.46 diff manually and it took 3 turns + for 1 revolution of the axle. Car was originally a 318i.

[alifsyz]

bump bump

[e30davie]

Sure the old one was 4.1?

You should have seen about an 18% change in rpm. Which is quite significant. 3000rpm would become ~2460rpm.

[straight6pwr]

find a stretch of road, throw it in 5th, and accelerate from like 15mph up to highway speed and watch to make sure the speed and revs match in a linear fashion.

we can also do the math.
assuming the ZF trans with the 1:1 ratio 5th gear
3.46 diff
and you have stock size tires - 23.9"
you should be seeing 72mph at 3500rpm or 60mph at 2900rpm

assuming the getrag with .81:1 ratio 5th gear
3.46 diff
and you have stock size tires - 23.9"
you should be seeing 71.1mph at 2800rpm or 60mph at 2350rpm

[alifsyz]

Sure the old one was 4.1?

You should have seen about an 18% change in rpm. Which is quite significant. 3000rpm would become ~2460rpm.

It should be. Cuz most of e30s here are 318i, so it should be a 4.10 diff.

[alifsyz]

find a stretch of road, throw it in 5th, and accelerate from like 15mph up to highway speed and watch to make sure the speed and revs match in a linear fashion.

we can also do the math.
assuming the ZF trans with the 1:1 ratio 5th gear
3.46 diff
and you have stock size tires - 23.9"
you should be seeing 72mph at 3500rpm or 60mph at 2900rpm

assuming the getrag with .81:1 ratio 5th gear
3.46 diff
and you have stock size tires - 23.9"
you should be seeing 71.1mph at 2800rpm or 60mph at 2350rpm

Pretty sure the trans is a getrag in the 320i. Tire sizes are 205/50/R16, slightly larger than stock. I'll try to do the test you recommended. If it fails, could it be the clutch? Or maybe a failed speed sensor/gauge cluster?

[alifsyz]

bump

[Andre3127]

Accelerate hard under load and bring the revs up to about 5k then let off the throttle. If the RPM drops suddenly after letting off the gas, your clutch is slipping.

This procedure puts the engine through max load at its peak torque output RPM, then the tach will show you when the clutch "grabs" again.

[AndrewBird]

You would know if your clutch was slipping, it's very obvious. The RPMs will go up, but you won't go any faster, similar to spinning the tires.

[alifsyz]

You would know if your clutch was slipping, it's very obvious. The RPMs will go up, but you won't go any faster, similar to spinning the tires.

It doesnt feel like its slipping. Normal driving wise, everything seems in order. I chose the 3,46 diff because it matches with the e36 320i gearbox (which I am using) that has a 3.45 FD. Maybe a problem with the diff? I feel like the speedo is not 'in sync' (for lack of better words) with the RPM as it begins to rise in a high gear.

[e30davie]

take a video and post it up.

[AndrewBird]

The movement of the speedo and tach changes with different gear ratios, so that is not an indicator.

Download this calculator I made and insert all the numbers for your trans, tires, etc.

https://drive.google.com/file/d/1gLYkRw1KB7WP6o4nzSNnVI8L3LmIvq_p/view?usp=sharing

Then figure out what MPH you are going at X rpm and see if it matches. Do it for a few different speeds to be more accurate. Shouldn't be hard to figure out what diff it actually is.

[Wanganstyle]

My E30 is 24v swapped with a M50B20 and matching gearbox. Recently, I've changed the Final Drive from 4.10 to 3.46 cuz it was getting annoying on the freeway. However, I'm getting the same 'rpm to speed ratio' with the old 4.10 diff. Is it possible my clutch is slipping? I checked the 3.46 diff manually and it took 3 turns + for 1 revolution of the axle. Car was originally a 318i.

Check the axel that came out in the same way, then you will know.

To test for Clutch slip just floor it in first, or just try to do doughnuts.

[TobyB]

Check the axel

If it looks like



you know you have a problem.

t

[alifsyz]

Update!

Okay so. I finally have time to look at the car again. I tested it on the Highway to see if there is any sort of abnormalities. One thing I noticed was when I was cruising at 100kmh(rpm was at 3k and fifth gear) and floored it, the speedo will reach 120kmh just before 4k rpm and then sort of floats around there as the rpm goes higher. It doesnt seem as linear as when it is at lower speeds. Could this be an indicator of a bad clutch?