Auto trans questions.

[Bellow]

A couple of questions about auto trans boxes as used.

First question, I watched EE's video >>

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re Five things not do and on the subject of coasting in neutral it says you shouldn't - but doesn't at any point say it is damaging to the transmission. Can you go further and stop the engine and coast (to a halt if desired) without damaging the transmission? (Not something I intend making a habit of, but there are reasons I might occasionally want/need to).

Second question, revving in neutral. ZF's HP4 could suffer severe damage from this - other boxes also? Again, not something I'll be making a habit of but I probably have the need for holding rpm higher than idle (circa 3000rpm) with no load (hence neutral) for the purpose of LPG calibration. Can this cause harm? By what I've heard of the ZF HP4 box, it would be inadvisable but is the HP4 uniquely sensitive to this and others not at all, or is it a sliding scale with all auto boxes affected to some degree? My box (not ZF) is also a 4 speed (with TC lock up).
TIA.

 

[whitenemesis]

Coasting in neutral may stop the atf pump circulating the fluid which may damage the box, akin to towing?
The car systems should prevent revs from going too high whilst in neutral

 

[Gazwould]

What my handbook says .

 

[Bellow]

Coasting in neutral may stop the atf pump circulating the fluid which may damage the box, akin to towing?

That's kind of what I suspected - a bad idea then. No matter, less important than.....

The car systems should prevent revs from going too high whilst in neutral

...... the ability to rev in neutral (just realised - and edited OP to show - 3000 rpm is required) and on this car it will rev in neutral.

 

[Dryce]

A couple of questions about auto trans boxes as used.

First question, I watched EE's video

I don't think the source is necessarily that good.

ISTR watching a video from this lot a while back on AWD / 4x4 and it was ..... to be polite ..... confused.

I'm not sure they actually understand what they try to explain.

As regards automatic transmissions - then there are lots of variations these days. Most people have little idea what they have. Manufacturers don't seem to really want to explain the nuances.

 

[Bellow]

I don't think the source is necessarily that good.

ISTR watching a video from this lot a while back on AWD / 4x4 and it was ..... to be polite ..... confused.

I'm not sure they actually understand what they try to explain.

EE is usually pretty good (I prefer his pacy delivery over the usual sludge punctuated with fireworks) but I've seen the odd inaccuracy.

As regards automatic transmissions - then there are lots of variations these days. Most people have little idea what they have. Manufacturers don't seem to really want to explain the nuances.

I'm wondering if the issue with the ZF HP4 is unique or all auto boxes can or do suffer from the same. I need to pop a seal inside the box like I need another orifice. If it really is a problem I'd rather raise the drive wheels from the ground and do the necessary in 'drive'.
Thinking about it, a shed load of revs in neutral - isn't that an MOT emissions test for diesels?

 

[Bellow]

I don't think the source is necessarily that good.

ISTR watching a video from this lot a while back on AWD / 4x4 and it was ..... to be polite ..... confused.

I'm not sure they actually understand what they try to explain.

EE is usually pretty good (I prefer his pacy delivery over the usual sludge punctuated with fireworks) but I've seen the odd inaccuracy.

As regards automatic transmissions - then there are lots of variations these days. Most people have little idea what they have. Manufacturers don't seem to really want to explain the nuances.

I'm wondering if the issue with the ZF HP4 is unique or all auto boxes can or do suffer from the same. I need to pop a seal inside the box like I need another orifice. If it really is a problem I'd rather raise the drive wheels from the ground and do the necessary in 'drive'.
Thinking about it, a shed load of revs in neutral - isn't that an MOT emissions test for diesels?

 

[Dryce]

I'm wondering if the issue with the ZF HP4 is unique or all auto boxes can or do suffer from the same.

I would guess that if there are variations between ZF transmissions it will be down to (a) whether it has a clutch setup and (b) how that is configured to operate.

Back in the day I used to have a bike with a multiplate clutch and it was 'sticky' until well warmed up.

 

[Bellow]

I would guess that if there are variations between ZF transmissions it will be down to (a) whether it has a clutch setup and (b) how that is configured to operate.

.

It's an oil pressure thing from the primary pump.
In neutral the box doesn't need or want the oil, the revs push up the pressure. Result - displaced seal (IIRC).

 

[Bellow]

The car systems should prevent revs from going too high whilst in neutral

Would that also be true of its counterpart with a manual transmission?

 

[whitenemesis]

That I don't know but my guess would be yes, as it's as much to protect the engine as anything else.

 

[Bellow]

That I don't know but my guess would be yes, as it's as much to protect the engine as anything else.

The absolute (ie red line) limiter should do that though.
In auto applications I'm wondering if the lower rpm limit is to protect the auto box or the driver from themself!
The ZF HP4 box has that known problem. But, I think I read somewhere recently (on the 'net, now unfindable) that other auto boxes are susceptible also.
There is also the diesel MOT emissions test scenario - that is conducted at quite high rpm, manual or auto.
Why are the simplest questions the hardest to get answered?!!!

 

[whitenemesis]

4k rev limit

Another poster says
"Engines shouldn’t be revved without any load. The connecting rods work pushing down to the cranckshaft with each cylinder explosion, however in neutral the cranckshaft is free to rotate & it doesnt give enough resistante to the connecting rods meaning these will expand, to the point where you can bend one or the pistons may slam the valves."
Rev Limiter in Neutral is 4.5k but in Drive it goes all the way to 6.2k, why is this??? And how do I fix it

 

[Bellow]

4k rev limit

Another poster says
"Engines shouldn’t be revved without any load. The connecting rods work pushing down to the cranckshaft with each cylinder explosion, however in neutral the cranckshaft is free to rotate & it doesnt give enough resistante to the connecting rods meaning these will expand, to the point where you can bend one or the pistons may slam the valves."
Rev Limiter in Neutral is 4.5k but in Drive it goes all the way to 6.2k, why is this??? And how do I fix it

I'll pursue the linked pages later but the quote applies only to 2-strokes. With 4-strokes every second upstroke is unbalanced by compression/combustion force - and is the one that has open valves.

 

[whitenemesis]

Clearly the manufacturers believe it to be a possible issue, or they wouldn't have introduced the limit?
It would be interesting to know the rev limit, if there is one, that F1 teams use when running in and warming their engines. They subject the engine to bursts of high revs whilst stationary and presumably in neutral?

 

[Bellow]

Clearly the manufacturers believe it to be a possible issue, or they wouldn't have introduced the limit?

My guess is that it is tied to the possibility of someone slamming it in drive with a shed load of revs. Less likely to happen with a manual as two distinct operations required to cause harm (Clutch x 2, gear selection x 1 ). Just a guess.

It would be interesting to know the rev limit, if there is one, that F1 teams use when running in and warming their engines. They subject the engine to bursts of high revs whilst stationary and presumably in neutral?

Different transmissions. This is only an issue with epicyclic autos with their engine driven primary pump generating pressure with no where to go.

 

[wongl]

I wonder if this is related to the lack of load as mentioned in a previous reply? Without a load, the engine can accelerate very rapidly, so much more than under load and the ECU might struggle with the ever reducing window to complete the calculations of the next injection/combustion cycle.

I know that many years ago when the company tried to adapt the TAG2.12 ECU from F1 race car for a two stroke racing engine, it ran out of computation 'window' because the two stroke engine had such rapid acceleration from idle to 18,000 RPM that the performance exceeded the performance of the then state of the art TAG2.12 ECU!

 

[st13phil]

Without a load, the engine can accelerate very rapidly, so much more than under load and the ECU might struggle with the ever reducing window to complete the calculations of the next injection/combustion cycle.

Different application, but motorcycle engines will happily spin up to the revlimiter in neutral, and they’ll typically spin up much more quickly than a car engine with its relatively heavy flywheel (crank, rods, etc.).

 

[wongl]

Different application, but motorcycle engines will happily spin up to the revlimiter in neutral, and they’ll typically spin up much more quickly than a car engine with its relatively heavy flywheel (crank, rods, etc.).

It wasn't for a motorcycle engine - the ECU was being evaluated for an experimental two-stroke racing engine for car racing!

 

[Bellow]

Different application, but motorcycle engines will happily spin up to the revlimiter in neutral, and they’ll typically spin up much more quickly than a car engine with its relatively heavy flywheel (crank, rods, etc.).

Crossed my mind also - then wondered if the computing power is there for the bike but not the car.