RPM varies, cold start, clutch in and out

[00NissanNinja]

I've been looking on here for awhile and I haven't seen anyone else talk about this. When I start my car cold in the morning the RPMs vary in neutral depending on if the clutch is in or out. RPM goes up slightly with the clutch in and goes down slightly with it out. Its maybe around a 50-100 rev difference. When warm I don't notice anything, seems to be mainly during the fast idle period. Just wondering in case my clutch is out of adjustment or if its just something the car is programmed to do for whatever reason.

[z3ro]

Originally Posted by 00NissanNinja

I've been looking on here for awhile and I haven't seen anyone else talk about this. When I start my car cold in the morning the RPMs vary in neutral depending on if the clutch is in or out. RPM goes up slightly with the clutch in and goes down slightly with it out. Its maybe around a 50-100 rev difference. When warm I don't notice anything, seems to be mainly during the fast idle period. Just wondering in case my clutch is out of adjustment or if its just something the car is programmed to do for whatever reason.

mine does this to but i think its just because with the clutch out there might be slightly more load on the engine.

Never seen any adverse effects so i'm not concerned but i'd still like to see a more technical response.

[00NissanNinja]

Cool thanks. I haven't seen any issues either. I too wonder the technical reason for it.

[humfrz]

Originally Posted by 00NissanNinja

Cool thanks. I haven't seen any issues either. I too wonder the technical reason for it.

I think z3ro nailed it up above .....

humfrz

[Porsche]

Originally Posted by 00NissanNinja

When I start my car cold in the morning the RPMs vary in neutral depending on if the clutch is in or out. ... RPM goes up slightly with the clutch in and goes down slightly with it out.

Even when the gearbox is in neutral, if the clutch is engaged (foot off the pedal), the engine is spinning the gearbox input shaft at engine rpms, which in turn has ALL OF THE GEARS spinning at their respective rpms. In neutral, of course, the output shaft has not been engaged, so the drive wheels are not being driven.

This is strictly speculation, but my first guess is that when the clutch is engaged, with all the gears being spun in the cold, and relatively thick, bath of transmission oil, there's real resistance there to be overcome when the trans is cold.

De-clutch and that drag on the engine from the transmission is now removed, and so the rpms edge up a little.

When the transmission oil heats up, it creates less drag, so you don't notice it much, then.

*******

Do I get a prize?

[z3ro]

Originally Posted by Porsche

Even when the gearbox is in neutral, if the clutch is engaged (foot off the pedal), the engine is spinning the gearbox input shaft at engine rpms, which in turn has ALL OF THE GEARS spinning at their respective rpms. In neutral, of course, the output shaft has not been engaged, so the drive wheels are not being driven.

This is strictly speculation, but my first guess is that when the clutch is engaged, with all the gears being spun in the cold, and relatively thick, bath of transmission oil, there's real resistance there to be overcome when the trans is cold.

De-clutch and that drag on the engine from the transmission is now removed, and so the rpms edge up a little.

When the transmission oil heats up, it creates less drag, so you don't notice it much, then.

*******

Do I get a prize?

Swift kick in the nuts work because thats about all I have right now :0)

Sent from my SCH-I545 using Tapatalk

[humfrz]

Originally Posted by Porsche

Even when the gearbox is in neutral, if the clutch is engaged ...........

Do I get a prize?

Well, no, ....... ol @z3ro made that discovery up in post #2.

However, you do get recognition for your excellent explanation .....


humfrz

[00NissanNinja]

Originally Posted by Porsche

This is strictly speculation, but my first guess is that when the clutch is engaged, with all the gears being spun in the cold, and relatively thick, bath of transmission oil, there's real resistance there to be overcome when the trans is cold.
Do I get a prize?

Nice and logical just the way I like it. I did not think even think to consider the resistance of the oil. Makes a lot of sense considering how viscous gear oil is.

Alright, I'll come by your place and do some donuts how bout that as a prize haha.

[Porsche]

Originally Posted by 00NissanNinja

Nice and logical just the way I like it. I did not think even think to consider the resistance of the oil. Makes a lot of sense considering how viscous gear oil is.

Alright, I'll come by your place and do some donuts how bout that as a prize haha.

Cool!