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Discussion Starter #1
So I have been reading a few things on a higher shift speed like S6 will make your smg htadulic system work harder, but will engage the clutch faster with less slip for less wear.. Also how a lower shift speed like S3 or D3 will have more clutch slip, but make the hydraulic system work less.

Is this true? One would think that modulating the clutch for slower engagement like S3/D3 would take more work from the hydraulic system, and slipping the clutch in faster would take less work of the hydraulic system..
 

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I think it has something with the pump having to exert a greater pressure to engage the clutch faster than having to feather the clutch like in D3. In the long run its going to meed replaced anyways.
 
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Short answer, no. No harder on hydraulic system at all in regards to fluid usage, duty cycle of pump/motor, etc. Your clutch and syncros? Different story. The shift gates that the actuators push can crack as well if syncros are worn and engaging rough.
 

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Discussion Starter #4
Short answer, no. No harder on hydraulic system at all in regards to fluid usage, duty cycle of pump/motor, etc. Your clutch and syncros? Different story. The shift gates that the actuators push can crack as well if syncros are worn and engaging rough.
So what your saying is, no matter what shift speed ( 1-6 ) that is used the hydraulic system work or wear doesnt change?

In general im speaking if we was part throttle driving low rpms, not wot full rpms..
 

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Correct. The exact same volume of fluid is used for each piston (clutch slave or shift rod) regardless of how quickly it moves. The primary difference in shift modes is the position if the clutch valve for the clutch and the two PWVs for the shift rods. Making the valves "more open" restricts less flow causing faster actuation.

Again, I'm speaking strictly on the hydraulic side of the house about only accumulator load/release and pump run time. Obviously when you get to the mechanical interface with the shift rods get whacked around, it's a different story.

But no, from the pump/accumulator/valves, no difference at all within the service life of the car. Maybe at 500k miles you might see some wear.

If anything, the failure of BMW to specify a fluid service life is more harmful. Pentosin is good stuff, but still accumulates wear particles and breaks down. I'm thinking of draining mine and flushing it just to clean things up.

I took apart a VANOS solenoid last night (same basic design as the clutch valve) and it had a metal spool sliding in a metal cartridge. There are no soft seals between the two. This means that even in fluid, there can be some wear, so I'd like to remove all that wear product from the SMG system and see if it feels any different.


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Discussion Starter #6 (Edited)
Correct. The exact same volume of fluid is used for each piston (clutch slave or shift rod) regardless of how quickly it moves. The primary difference in shift modes is the position if the clutch valve for the clutch and the two PWVs for the shift rods. Making the valves "more open" restricts less flow causing faster actuation.

Again, I'm speaking strictly on the hydraulic side of the house about only accumulator load/release and pump run time. Obviously when you get to the mechanical interface with the shift rods get whacked around, it's a different story.

But no, from the pump/accumulator/valves, no difference at all within the service life of the car. Maybe at 500k miles you might see some wear.

If anything, the failure of BMW to specify a fluid service life is more harmful. Pentosin is good stuff, but still accumulates wear particles and breaks down. I'm thinking of draining mine and flushing it just to clean things up.

I took apart a VANOS solenoid last night (same basic design as the clutch valve) and it had a metal spool sliding in a metal cartridge. There are no soft seals between the two. This means that even in fluid, there can be some wear, so I'd like to remove all that wear product from the SMG system and see if it feels any different.


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Thank you for this valuable lesson and information.. Everywhere on the forum the default was faster shift speed more hydraulic work and wear..

I think what people assumed is with a faster shift speed like s5/6 the hydraulic pump runs more and longer, and system builds more pressure, as to a lower shift speed, the system builds less pressure..but looks like the system builds the same pressure regardless of shift speed setting 1-6.. Its the clutch valve and two valves for the shift rods that change flow for faster/ slower shifting.. Excellent lesson here sir!

So on a clutch stand point, higher shift speed s5/6 quicker clutch grab, less slip, less clutch wear.. Correct? Basic clutch function here..

Im happy i started this thread, got some new info to dismiss the false info that has been running around here for years
 

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Correct. The exact same volume of fluid is used for each piston (clutch slave or shift rod) regardless of how quickly it moves. The primary difference in shift modes is the position if the clutch valve for the clutch and the two PWVs for the shift rods. Making the valves "more open" restricts less flow causing faster actuation.

Again, I'm speaking strictly on the hydraulic side of the house about only accumulator load/release and pump run time. Obviously when you get to the mechanical interface with the shift rods get whacked around, it's a different story.

But no, from the pump/accumulator/valves, no difference at all within the service life of the car. Maybe at 500k miles you might see some wear.

If anything, the failure of BMW to specify a fluid service life is more harmful. Pentosin is good stuff, but still accumulates wear particles and breaks down. I'm thinking of draining mine and flushing it just to clean things up.

I took apart a VANOS solenoid last night (same basic design as the clutch valve) and it had a metal spool sliding in a metal cartridge. There are no soft seals between the two. This means that even in fluid, there can be some wear, so I'd like to remove all that wear product from the SMG system and see if it feels any different.


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I'm with you 100% on this. This is why I can't understand this 'lifetime fluid' mentality. On my 530 with the steptronic, I got this all the time when I asked about changing the fluid. Granted the SMG is different alltogether, but the last time I asked my SA about the M5 he gave me the same lifetime fluid story. My car has only 14k on the clock but I'm gonna insist on changing the SMG fluid. And thanks for your input.


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I've yet to really data log this properly but i find it hard to believe that the clutch is actually used to synchronize the RPM in any mode.

In fact there's way more clutch wear in higher modes since the engine doesn't have time to rev down to the next gear rpm before the clutch is engaged. In S3 I've noticed the clutch engages back in just in time for the revs to drop down (this is purely by feel).

The only time the clutch is used to match revs is when you downshift off throttle. The ecu goes into engine braking mode and doesn't apply any power to match revs so that deceleration continues during the shift so I tend to try to help it shift a little by blipping the throttle a tiny bit.
 

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but the last time I asked my SA about the M5 he gave me the same lifetime fluid story. My car has only 14k on the clock but I'm gonna insist on changing the SMG fluid.
Show them your Service and Warranty booklet, it describes the service interval for M5/M6 SMG gearbox oil/differential fluid.
 

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I would have to agree based on the fundamental principle that a smooth shift requires RPMs to be matched *before* the clutch starts to engage. Anything else would transmit the necessary differential forces to the driven wheels in either resulting in a delta of longitudinal forces.

Seeing as how the SMG module has direct control of the idle actuators via the LLS CAN, the SMG can directly control the engine RPM for zero speed take- offs and shift rev matching.

But, I think Lumi was talking specifically about the hydraulic components in his question. As for those, they don't really know the difference. The four shift solenoid valves all do their job for the shift rods, it's the 2 PWVs that tell them how quickly; one for the outgoing gear and one for the incoming gear.


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Show them your Service and Warranty booklet, it describes the service interval for M5/M6 SMG gearbox oil/differential fluid.

That's for the gear oil and diff fluid, but remains silent on the Pentosin SMG fluid. In fact, there is no procedure in TIS to even drain it, so I doubt most dealers would know how. If you pull the plug on the bottom of the hydraulic block, that doesn't do it either unless the pump is running.

I just took a pump block apart last night to try and figure out how to drain it and there's a little check valve inside that may have something to do with that.

I'll post a DIY if I ever figure it out.


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I would have to agree based on the fundamental principle that a smooth shift requires RPMs to be matched *before* the clutch starts to engage. Anything else would transmit the necessary differential forces to the driven wheels in either resulting in a delta of longitudinal forces.

Seeing as how the SMG module has direct control of the idle actuators via the LLS CAN, the SMG can directly control the engine RPM for zero speed take- offs and shift rev matching.

But, I think Lumi was talking specifically about the hydraulic components in his question. As for those, they don't really know the difference. The four shift solenoid valves all do their job for the shift rods, it's the 2 PWVs that tell them how quickly; one for the outgoing gear and one for the incoming gear.


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Only possible extra wear I could think of would be from pure force on the solenoids and the control valves opening further to increase pressure. Both seem microscopic compared to the much higher wear on the synchros during a fast shift. S5 to me feels like if you were to bang into gears in a regular manual, not very healthy for the gearbox. I wonder if they made the synchros way beefier in this gearbox to account for the much higher speeds.
 

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Only possible extra wear I could think of would be from pure force on the solenoids and the control valves opening further to increase pressure. Both seem microscopic compared to the much higher wear on the synchros during a fast shift. S5 to me feels like if you were to bang into gears in a regular manual, not very healthy for the gearbox. I wonder if they made the synchros way beefier in this gearbox to account for the much higher speeds.

I doubt it. Find pics of an SMG taken apart and the reused 6 speed shift forks become apparent. Troy has said before the syncros are the same as well. Getrag keeping cost down...


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I doubt it. Find pics of an SMG taken apart and the reused 6 speed shift forks become apparent. Troy has said before the syncros are the same as well. Getrag keeping cost down...


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wow, really, my e39 m5 gearbox would've hated me if i shifted this fast in it, lol. Obviously i'm not sure since I've never killed one but I do feel like the shifting got a little clunkier after shifting really hard for a while.

Guess the smg might follow some shifting curve which it engages the synchro smooth then bangs it in harder possibly.

Would love to see the smg actuation in action on a bench, in high speed.
 

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Would love to see the smg actuation in action on a bench, in high speed.

Working on that...need more parts donated. Ideally, I would build a full open case housing for the input, drive, and lay shafts and transplant the internals. Then actuators an pump assemblies could be bolted on for and tested under observation. A small Arduino controller similar to what I built a few years ago for an F1000 car would handle testing duties with a few analog pressure gauges installed for leak-down tests.

Meanwhile in the real world, my wife thinks I should finish putting our second floor back together...



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