wpgshark

I'm confused, how does the clutch being in or out have an impact on engine oil pressure?

Oil pump is positive displacement and driven off of the timing belt, pressure should be determined by RPM, looseness of the engine, and relief.

Where does the clutch come in?

hwyengr

It's not that, it's that the pressure plate loads the crank against the thrust bearing before oil pressure comes up.

Mrmerlin

To add more info to HWYENGR post .
when the engine is stopped then there isnt any oil pressure.
if the clutch is pushed to the floor then the release arm will pull the crank shaft towards the rear of the car as it releases the clutch disc.

The thrust bearing is now holding the load of the crank and the clutch.

With no oil pressure the parts will have increased wear as they are turned towards a running condition as the starter is engaged.

SO its best to start the engine with your foot off the clutch,
no loads are placed on the thrust bearing,
and oil pressure will build ,
then the clutch pedal can be pressed.

davek9

Sorry I don't buy in to that at all, if the eng spins more freely the oil pressure will build.
How does applying the clutch stop the oil pressure, really? ??

The Deputy

Yes, and take it from a guy that had his legs pinned between a work bench and a F-150's bumper when I asked someone, with an IQ less than ten, to bump the key. It is not a good practice to get into...in my opinion...not pushing in the clutch for whatever reason.

Brian.

hwyengr

It's not that it stops the oil pressure, it's that you're putting the full axial design load on an bearing that's unlubricated until the pressure comes up to normal.

To me, it's just wear that can be avoided. Probably not lethal to the engine, but there have been 5-spd TBF cases, and that's pretty much where all the load and wear on the bearing comes from.

Also, your starter isn't as powerful as your brakes, if you're concerned about the car jumping in gear.

Mrmerlin

Maybe I am not explaining this in the proper way.

ideally to reduce wear in the engine you want oil to be flowing under pressure,
the thrust bearing is a part that is fed oil,it is the number 3 main bearing in the block.

This oil under pressure provides a liquid film to absorb force and prevent metal to metal contact.

THUS with the engine stopped this oil film under pressure will not be happening.

NOTE pressing the clutch to the floor without the engine running and no oil pressure being generated from a running engine,
will increase the chances of metal to metal contact at the thrust bearing.

This will translate to increased wear of the crank thrust surface and the thrust bearing surface.

NOTE pressing the clutch once or twice it may be hardly noticeable,

But figure a car thats been started for many years with the clutch being pushed before the engine is started and no oil pressure,

The wear of these parts will be more than the engine that was started without the clutch being depressed.
prior to engine start.

So this is how a 5 speed thrust bearing can fail, it will take a long time to manifest,
but some owners have had their cars for 10, 20 or 30 years

NOTE this condition will get even worse with an engine with oil thats isnt changed as frequently

wpgshark

Although I agree that there is a possibility of reduced load on the thrust bearing if the clutch is in during starting. If there is sufficient good quality proper viscosity oil in the car, the residual oil film should be more than enough to handle the extra load for the first partial revolution of the engine, much like every other bearing.

I would tend to believe that TBF on 5 speeds occurred because of some manner of oil issue, which would be tougher on the Thrust bearing as every shift would cause wear, not just the starting.

I recall reading a very good post from one of the listers down under with an alternate explanation of TBF, a lot of which was based on oil viscosity, I was impressed with the reasoning at the time and continue to believe it has merit.

Given the arguments made it is unlikely that releasing the clutch at stop lights will have any effect at all.

Given the 40 years of being told to push the clutch in, and the fact that all of our modern standards require it, I'm unlikely to stop pushing it in on the 928.

Thanks all for the clarification.

Wisconsin Joe

I understood you just fine the first time.

There's pressure on the Thrust bearing when the clutch pedal is pushed down.

Without the engine running and the oil pressure up, there's no lubrication on the thrust bearing (when you start it with the clutch pedal down).

Unfortunately, my 928 & 944 are the only two standard trans cars that I've ever owned that don't have a starter interlock switch. Even my Suzuki Katana needs to have the clutch lever in for the starter to turn.