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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Powertrain and Drivetrain Discussions > N54 Turbo Engine / Drivetrain / Exhaust Modifications - 335i > Stress Reduction During Downshifting



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      12-02-2008, 04:40 AM   #1
Unky
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Stress Reduction During Downshifting

I'm interested in knowing the tolerances of 3-series engineering for the purposes of downshifting safely. This is potentially a broad arena and values can vary greatly depending on the the car's redline, but many people use 3500 rpm as a ceiling for downshifting on declines or to slow the car. The idea being anything higher than this causes wear and premature aging of the transmission.

For those of you that have advanced knowledge of the newer 3-series engines and transmissions, especially the N54, what would be a good rpm benchmark for downshifting? What would be an appropriate rpm ceiling for effectively slowing the car without causing undue wear on the transmission and stressing the synchros?

I'm not trying to start collateral discussions about double-clutching or rev matching. I'm just trying to get a better understanding of the engineering tolerances and how to distribute force reduction between braking and downshifting as it pertains specifically to the newer BMW applications.

Last edited by Unky; 12-02-2008 at 05:08 AM.
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      12-02-2008, 10:09 AM   #2
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Your question has no real answer. There will certainly be more wear on the engine components if you keep the engine at higher RPMs and use the compression to slow the car down but I am going to say that the car will take it just fine.

I think 3500 is too low, I usually blip the thottle to 4500, maybe even 5000 if I want to downshift during slow downs.

In city traffic I often find myself just dropping into the neutral until I need to accelerate and choose the right gear then.
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      12-02-2008, 12:01 PM   #3
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Well the idea of bliping the throttle on downshifts is to RPM match so you are always in the 'power band'.

Coming from NA high reving motors I would always have my revs near the top end to have instant power.

How would this translate to a turbo which makes most of it power in the low rev range? I have never owned a turbo car before.
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      12-02-2008, 12:29 PM   #4
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IMHO I think as long as you rev match properly you'll be fine.

If you're going from 3rd to 2nd and shift into 2nd at 5Krpm, I'd assume this to be the same as accelerating in 2nd all the way to 5Krpm no?

The only play as far as wearing down the tranny goes would most likely come from a bad rev matching job, which would make the synchros compensate for the difference.

But that's just understanding of it. I've only been driving MT for 3 years, while some of the members have probably been driving for 30. lol
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      12-02-2008, 12:41 PM   #5
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Are you referring to a manual transmission or an auto? For a manual transmission, assuming that you are revmatching properly, I wouldn't worry about haivng a "ceiling" for what RPM you can reach when revmatching a downshift. If you are revmatching your downshifts then what's the difference between accelerating to say 4500rpm or downshifting to the same RPM? What are you concerned with, engine load or drivetrain wear?
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      12-02-2008, 12:53 PM   #6
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Quote:
Originally Posted by T i h o r View Post
IMHO I think as long as you rev match properly you'll be fine.

If you're going from 3rd to 2nd and shift into 2nd at 5Krpm, I'd assume this to be the same as accelerating in 2nd all the way to 5Krpm no?

The only play as far as wearing down the tranny goes would most likely come from a bad rev matching job, which would make the synchros compensate for the difference.

But that's just understanding of it. I've only been driving MT for 3 years, while some of the members have probably been driving for 30. lol
Actually not rev-matching is very bad thing thing depending on how hard you are hitting it.

Clutches are usually designed with spring hubs with something like 5-7 springs in the center of a hub (through which the trans input shaft passes through) that are compressed each time the clutch goes from (pushed) "in" to "out" on an upshift.

This only is designed to work for upshifts. There is essentially no spring cushioning the clutch if you try to use the clutch to spin the engine up.

The springs in the clutch hub aren't necessarily there just to lower the stress on the driveline components (trans, axles etc) they also go a long way towards protecting the clutch disc. I've seen clutch hubs that have been completely torn free from the rest of the disc (friction surface) making a nice doughnut out of the clutch.

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      12-02-2008, 01:11 PM   #7
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I down shift all the time and I rev match 99% of the time.
Recently when I was on the track, I was downshifting but not to slow down my car. It was to get into the correct gear to make the turn.
Another bad thing about downshifting without rev matching on the track is that ABS sometimes gets activated during the braking.
This is where the heal/toe becomes handy.

If people are really downshifting just to slow down the car then think of it this way;
I'd rather spend a couple hundred on a nice set of new pads then thousands on a new tranny.
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      12-02-2008, 01:15 PM   #8
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Quote:
Originally Posted by BavarianBullet View Post
Actually not rev-matching is very bad thing thing depending on how hard you are hitting it.

Clutches are usually designed with spring hubs with something like 5-7 springs in the center of a hub (through which the trans input shaft passes through) that are compressed each time the clutch goes from (pushed) "in" to "out" on an upshift.

This only is designed to work for upshifts. There is essentially no spring cushioning the clutch if you try to use the clutch to spin the engine up.

The springs in the clutch hub aren't necessarily there just to lower the stress on the driveline components (trans, axles etc) they also go a long way towards protecting the clutch disc. I've seen clutch hubs that have been completely torn free from the rest of the disc (friction surface) making a nice doughnut out of the clutch.

BB
For the record, the OEM clutch is not sprung hubbed.
Most aftermarket replacments are.
Here is a picture of my old clutch compared to the new one that I recently got.

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      12-02-2008, 01:25 PM   #9
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Quote:
Originally Posted by Mr. 5 View Post
For the record, the OEM clutch is not sprung hubbed.
Most aftermarket replacments are.
Here is a picture of my old clutch compared to the new one that I recently got.

Just curious: How much would something like that cost (parts & labour)?

Also, what kind of a difference would you expect from the new one versus OEM? Would it make the clutch feel heavier/lighter/ the same, or is it just to allow the tranny to take more hp?
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      12-02-2008, 01:28 PM   #10
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Quote:
Originally Posted by T i h o r View Post
Just curious: How much would something like that cost (parts & labour)?

Also, what kind of a difference would you expect from the new one versus OEM? Would it make the clutch feel heavier/lighter/ the same, or is it just to allow the tranny to take more hp?
My clutch wasn't functioning properly and slipping occasionaly so I received a new OEM clutch under the maintenance agreement.
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      12-02-2008, 01:42 PM   #11
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Quote:
Originally Posted by Unky View Post
I'm interested in knowing the tolerances of 3-series engineering for the purposes of downshifting safely. This is potentially a broad arena and values can vary greatly depending on the the car's redline, but many people use 3500 rpm as a ceiling for downshifting on declines or to slow the car. The idea being anything higher than this causes wear and premature aging of the transmission.
as long as you're matching revs when you downshift, it really doesn't matter where you are in the rev-band when you do it, generally speaking - i'll get to the exception to the rule shortly. the premature wear associated with downshifting is caused by NOT matching revs. in other words, if the drivetrain rpm does not match the engine rpm when downshifting, there will be slippage between the flywheel and the clutch disk during engagement. this slippage may last only briefly while the flywheel spins up and the clutch spins down to a "middle-of-the-road" rpm. but if do it all the time, and all these brief moments of slippage (additional clutch wear) start to add up. if you match your engine's revs to the driveline's revs however (or come close - to within a few hundred rpm), the difference in engine rpm and driveline rpm will be minimal (or nonexistent), resulting in a minimal (or nonexistent) amount of slippage/unnecessary wear.

the exception to the general rule (the general rule that you can downshift from pretty much anywhere in the revband so long as you're rev-matching) is quite common sense and is best explained by example: if your redline is 7000rpm for instance, and you're cruising @ 6500rpm in 3rd gear, DON'T downshift to 2nd gear or else your revs will quickly jump beyond the 7000rpm redline. there's probably a scientific term already in existence for this concept, but i'll call it the Critical RPM. Critical RPM = the rpm at which one needs to be in order to get the revs to jump exactly to redline by downshifting to an adjacent gear (5th -> 4th, 4th -> 3rd, 3rd -> 2nd, etc.). so for instance, if i take it to redline in 2nd gear, shift to 3rd, and notice that my revs are now at 5500rpm, i can then confidently say that, by downshifting from 5500rpm in 3rd gear that i'll end up at redline in 2nd gear. if i downshift from any rpm less than 5500rpm in 3rd gear, i'll end up safely below redline in 2nd. likewise, if i downshift from any rpm greater than 5500rpm in 3rd gear, i'll end up ABOVE redline in 2nd gear (which can cause serious damage depending on how high above redline one accidentally takes it).

so in a nutshell, matching revs while downshifting will result in FAR less wear on the drivetrain than downshifting without matching revs. just be careful not to downshift above your Critical RPM (which can vary from gear to gear due to each gear in the box having a different ratio).
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      12-02-2008, 01:47 PM   #12
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Quote:
Originally Posted by Mr. 5 View Post
I down shift all the time and I rev match 99% of the time.
+1



Quote:
Originally Posted by Mr. 5 View Post
If people are really downshifting just to slow down the car then think of it this way;
I'd rather spend a couple hundred on a nice set of new pads then thousands on a new tranny.
+1
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      12-02-2008, 02:05 PM   #13
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Quote:
Originally Posted by 94JZA80 View Post
as long as you're matching revs when you downshift, it really doesn't matter where you are in the rev-band when you do it, generally speaking - i'll get to the exception to the rule shortly. the premature wear associated with downshifting is caused by NOT matching revs. in other words, if the drivetrain rpm does not match the engine rpm when downshifting, there will be slippage between the flywheel and the clutch disk during engagement. this slippage may last only briefly while the flywheel spins up and the clutch spins down to a "middle-of-the-road" rpm. but if do it all the time, and all these brief moments of slippage (additional clutch wear) start to add up. if you match your engine's revs to the driveline's revs however (or come close - to within a few hundred rpm), the difference in engine rpm and driveline rpm will be minimal (or nonexistent), resulting in a minimal (or nonexistent) amount of slippage/unnecessary wear.
This was particularly helpful... thank you.
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      12-02-2008, 02:27 PM   #14
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I try taking deep breathes when downshifting. That seems to help my stress although I have never found the act of downshifting to be very stressful.






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      12-02-2008, 02:53 PM   #15
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Quote:
Originally Posted by 94JZA80 View Post

the exception to the general rule (the general rule that you can downshift from pretty much anywhere in the revband so long as you're rev-matching) is quite common sense and is best explained by example: if your redline is 7000rpm for instance, and you're cruising @ 6500rpm in 3rd gear, DON'T downshift to 2nd gear or else your revs will quickly jump beyond the 7000rpm redline. there's probably a scientific term already in existence for this concept, but i'll call it the Critical RPM.
"Mechanical over-rev" is the term most often used to describe the type event where you have a 6MT and try to do a hard upshift from 3rd to 4th and wind up throwing the trans into second and let the clutch out fast- BOOM! The ECU would log this FWIW. No warranty on that.

I've seen this once when a guy did it street racing. The whole engine didn't grenade but he managed to pull at least one rocker arm stud out of his cylinder heads (mustang V8).
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      12-02-2008, 03:23 PM   #16
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Quote:
Originally Posted by Mr. 5 View Post
I down shift all the time and I rev match 99% of the time.
I do the same. I feel like downshifting and NOT rev-matching would cause even more wear on the transmission from it not engaging as smoothly. I've been doing this for a good 7 years or so now... and never had any issues with my transmissions ever. Infact, its become such a habit that if I try to show someone what it feels like to downshift without rev-matching... I almost always accidently rev-match anyway.
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      12-02-2008, 03:32 PM   #17
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I've seen this once when a guy did it street racing. The whole engine didn't grenade but he managed to pull at least one rocker arm stud out of his cylinder heads (mustang V8).
Listen for it at about 36 seconds into the video. Yes, that was me but got back in the clutch only to set a code for an over-rev to 7800 RPM. The car only had about 1800 miles on it too. Thankfully Honda/Acura can built a stout engine (just not a AT transaxle).

[u2b]<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/EfsB6YVjKzI&hl=en&fs=1"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/EfsB6YVjKzI&hl=en&fs=1" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object>[/u2b]
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      12-02-2008, 03:50 PM   #18
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Quote:
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For the record, the OEM clutch is not sprung hubbed.
Most aftermarket replacments are.
Here is a picture of my old clutch compared to the new one that I recently got.
Good catch mr5.

Yeah, now that I think about it, the issue is the flywheel- BMW 335s use the ultra-expensive dual mass flywheels. They are an odd contraption and I believe the flywheel itself has springs between the masses that take the shock otherwise absorbed by the clutch disc spring hub. Here is an interesting video that explains how it works as a general concept: http://www.youtube.com/watch?v=YnaXB8q3uzQ

UUC also apparently has at least the stock-style (no spring hub) discs available based on this thread and on the their website: http://www.e90post.com/forums/showth...uuc+clutch+335

So the point I made would still apply but not at the clutch disc but rather the flywheel itself.

Hopefully someone will soon release a solid flywheel that can work with existing sprung-hub discs.

BB

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      12-02-2008, 11:28 PM   #19
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Quote:
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"Mechanical over-rev" is the term most often used
AKA: A money shift. Because doing it costs a lot of money! I came damn close once not long after I got my car. Missed fourth and hit second. Luckily I got back on the clutch fast enough.
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