Why do the syncros seem to work BETTER when my 915 is COLD!

c2 rower · 11-22-2003, 01:39 PM

Specifically, the 2nd gear syncro actually works better when the transmission is cold. Is that weird? Could it suggest something about the tranny fluid? Has anyone ever noticed that kind of thing? The car has 65k miles. I'm gonna ask the PO what fluid was used. I suspect either Swepco or Mobil 1.

Jim Smolka · 11-22-2003, 02:06 PM

The colder oil is 'thicker', allowing the sycros to work better (slow down the rotating parts)

BGCarrera32 · 11-22-2003, 06:37 PM

Which is why it also makes sense to use a modified dino oil like Swepco as opposed to Mobil 1 in the 915.

red-beard · red-beard's Garage

What it means is your synch bands are done and they need replacing.

Jim Smolka · 11-23-2003, 02:55 AM

Changing the rings now may save $$$ as compared to replacing the dog teeth and rings.

MAS · MAS's Garage

I've noticed this too, and I'm pretty sure that it has nothing to do with the gear oil. It's because of the cold start assist keeping the revs higher between gear shifts.

My 1-2 upshift is often stiff, but almost always never during the first few minutes of driving. I finally found out I was able to duplicate the effect by keeping my right foot very lightly on the gas during the shifts... in other words not letting the revs drop below about 1200-1400 rpm. Normally between shifts, my revs drop very very fast... usually down to about 500-600 rpm for a split second before returning to 800rpm. I'm assuming that keeping the revs up slightly during shifts keeps the synchros spinning a bit faster.

-MAS

Gunter · Gunter's Garage

Mas: The RPM should not drop that low. Set your idle to about 950 and reset your mixture and timing. Check clutch adjustments (1mm free play and 25mm travel) Cheers.

dvkk · 12:29 PM

Quote:

Originally posted by MAS
I've noticed this too, and I'm pretty sure that it has nothing to do with the gear oil. It's because of the cold start assist keeping the revs higher between gear shifts.

My 1-2 upshift is often stiff, but almost always never during the first few minutes of driving. I finally found out I was able to duplicate the effect by keeping my right foot very lightly on the gas during the shifts... in other words not letting the revs drop below about 1200-1400 rpm. Normally between shifts, my revs drop very very fast... usually down to about 500-600 rpm for a split second before returning to 800rpm. I'm assuming that keeping the revs up slightly during shifts keeps the synchros spinning a bit faster.

-MAS

Your theory has one flaw, the clutch pedal is in, disconnecting the engine from the input shaft. It doesn't matter what rpm you rev it to when the clutch pedal is on the floor, it won't do anything to the input shaft's speed.

The syncro's job is to slow down (or speed up) the input shaft and clutch disc to match the pinion shaft's speed so the gear can be engaged. This requires friction. When your gear oil is cold, it has more friction than when warm. Thus the syncros work better when cold.

JackT · 11-23-2003, 01:47 PM

I think it is because the motor and tranmission mount rubber is hard when cold. As the motor heats up, the rubber heats up and softens and lets the tranmission and motor move just enough to make quick shifting difficult.

dvkk · 11-23-2003, 11:35 PM

How hot do you think the rubber gets?

Again, the clutch is in, so the trans and engine aren't moving at the time your shifter is.

JackT · 11-24-2003, 03:37 AM

Hot enough to have more flex. Drive a 911 with solid mounts. It is a more consistant shift than sport mounts which is more consistant than regular mounts. Once the engine and transmission settles down, (slow shift), its an easy shift with whichever mounts you have.

Gunter · Gunter's Garage

The transm. mounts have a steady cool air flow, so, let's blame it on?? Spare wheel not inflated?? Air filter clogged??. Where is this going???Check shifter assembly components first, then basic clutch adjustments, then we go into utopia.

JackT · 11-24-2003, 09:29 AM

So Doctor, your saying the mounts do not have any effect on the shifter? Next time you put you car on a lift or get under it. Touch the transmission and see if it is hot. That heat then tranfers to the brace and to the mounts.

dvkk · 11-24-2003, 10:28 AM

Why not just grab onto the rubber mounts?
Since your front trans mounts are way out on the ends of the wings, I highly doubt they will get any heat from thermal conduction especially since they have good airflow over them. Many other cars have their rubber mounts in direct contact to their transmissions without adverse effects.
The temp of the gear oil will affect shifting the most.

JackT · 11-24-2003, 10:32 AM

Ok, Doctor, grab on to the mounts. Whatever, but grab something down there so you can see that it gets warm enough to soften the rubber, which then lets both the tranmission and the engine twist, which in turn, makes it harder to shift fast.

BGCarrera32 · 11-24-2003, 10:40 AM

Quote:

When your gear oil is cold, it has more friction than when warm. Thus the syncros work better when cold.

Yes...from what I gather it allows the brake bands inside the synchro to work effectively at matching the synchro speed to the dog teeth (a certain amount of friction is required there)...so perhaps a gear oil that is less runny than Mobil 1...like Swepco or good old Valvoline gear oil...which allows the brake bands to do their job...

The tendency is to put the newest technology of oils in things that were not designed to use them when they were originally conceived 25-30 years ago. Not necessarily gonna solve the problem.

Now if you go to the G50 with Borg-Warner type synchros, that may run just dandy on Mobil 1.

These are only my observations based on all the info I have sifted through on the 915...