Unpredictable RPM During Shifts

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mapman85
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Unpredictable RPM During Shifts

Post by mapman85 »

Apologies if this has been covered elsewhere.

After 6 months driving a MT, I am happy with my progress. I've largely managed to smooth out overly-jerky 1>2 shifts, which were the bane of my existence for a long time. However, I still get random subtle jerks during my other shifts, which is frustrating me because the problem is this:

Although I "feel" like I am driving the same all the time, sometimes the revs drop like a rock (like down to idle speed as soon as I depress the clutch) while I am shifting, thus producing the jerk as they are matched back up. I'll occasionally get rev hang, but the drop seems much more frequent. In other words, it seems very difficult to produce consistent rev-matched shifts.

This issue has cropped up under different shifting styles, different terrain, different speeds, etc... thus why I called it "unpredictable." If anything it might be more common in gentle, around-town driving (would make sense since I'm shifting at lower revs) -- which would also usually be a colder engine -- but not sure if I can narrow it down to that singular cause.

Am I still just too green for my muscles to have committed the smooth shift movements to memory (I do have plenty of normal shifts as well)? Is my car being kooky (seriously, some days it's an absolute joy, and others it feels like it wants to be a pain in the rear)? Is there something I can focus on to improve my shifting? I'd rather not have to stare at the tach on each shift to see what the revs are doing.
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Shadow
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Re: Unpredictable RPM During Shifts

Post by Shadow »

First, you have to understand that a rev-matched shift is usually used to describe a downshift, not an upshift. That's because by design, upshifts normally fall very close to the RPM that they should be as you release the clutch after shifting into the next gear. In other words, if your RPM is at 3,000 RPM in 3rd gear, it will be a bit lower than that as you shift into 4th gear. So naturally, the RPM will drop as you clutch in/shift/clutch out. And if your timing is correct, the RPM will fall right into the correct place as you're re-engaging the clutch.

Now a downshift is an entirely different story. The RPM needs to be HIGHER than it was for a smooth transition into a lower gear. For example, if you're at 3,000 RPM in 3rd gear and want to drop down to 2nd gear, you'll have to blip the throttle to match the revs to get that same smooth transition.

If you have to rev the engine during upshifts, my guess is that you're taking too long to shift. Generally, the RPM shouldn't drop down to idle speed unless you're taking a LOOOONG time to shift. Trying speeding up the process and see if that helps.
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Re: Unpredictable RPM During Shifts

Post by Warner »

Maybe I'm pointing out the obvious, but have you been using your air conditioning recently? This will make a huge difference in how your car feels and how quickly your RPMs drop. The less powerful your engine is, the more you'll notice that.

Also, like you said, your RPMs will definitely drop more quickly while your engine is cold.

If your engine is at a constant temperature and your AC is off, your RPMs should drop at a fairly consistent rate, assuming nothing else is loading the engine. The only other thing I can think of is power steering, but that would only really make a difference if you're quickly turning your steering while at the same time you're pressing in your clutch, which seems unlikely (and is definitely not ideal).
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Re: Unpredictable RPM During Shifts

Post by theholycow »

Warner wrote:your RPMs should drop at a fairly consistent rate, assuming nothing else is loading the engine. The only other thing I can think of is power steering, but that would only really make a difference if you're quickly turning your steering while at the same time you're pressing in your clutch, which seems unlikely (and is definitely not ideal).
Another intermittent load difference could be the alternator. The ECM turns the alternator field on or off (or somewhere in between) depending on battery state of charge and current electrical load.

It could just be kooky drive-by-wire programming. In 45,000 miles with the VW I think there were some inconsistencies whose pattern I never found (if indeed there was a pattern at all).
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Re: Unpredictable RPM During Shifts

Post by bk7794 »

theholycow wrote:
Warner wrote:your RPMs should drop at a fairly consistent rate, assuming nothing else is loading the engine. The only other thing I can think of is power steering, but that would only really make a difference if you're quickly turning your steering while at the same time you're pressing in your clutch, which seems unlikely (and is definitely not ideal).
Another intermittent load difference could be the alternator. The ECM turns the alternator field on or off (or somewhere in between) depending on battery state of charge and current electrical load.

It could just be kooky drive-by-wire programming. In 45,000 miles with the VW I think there were some inconsistencies whose pattern I never found (if indeed there was a pattern at all).
I second the possibility of an electrical load on the engine. I know in my honda, with my lights on and the fan on three or four the rpms drop like a stone..I actually feel the same way sometimes as you, the car is a joy to drive sometimes and other times I hate it. I am not sure how to compensate for that though.
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mapman85
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Re: Unpredictable RPM During Shifts

Post by mapman85 »

Shadow wrote:First, you have to understand that a rev-matched shift is usually used to describe a downshift, not an upshift. That's because by design, upshifts normally fall very close to the RPM that they should be as you release the clutch after shifting into the next gear. In other words, if your RPM is at 3,000 RPM in 3rd gear, it will be a bit lower than that as you shift into 4th gear. So naturally, the RPM will drop as you clutch in/shift/clutch out. And if your timing is correct, the RPM will fall right into the correct place as you're re-engaging the clutch.

Now a downshift is an entirely different story. The RPM needs to be HIGHER than it was for a smooth transition into a lower gear. For example, if you're at 3,000 RPM in 3rd gear and want to drop down to 2nd gear, you'll have to blip the throttle to match the revs to get that same smooth transition.

If you have to rev the engine during upshifts, my guess is that you're taking too long to shift. Generally, the RPM shouldn't drop down to idle speed unless you're taking a LOOOONG time to shift. Trying speeding up the process and see if that helps.
Yes, sorry for the poor wording. Basically I was trying to describe that as soon as I clutch in to upshift, the engine will drop like a rock to around 1000 RPM, thus missing that "correct" spot, no matter how fast I shift (and it seems to happen inconsistently despite similar driving conditions/techniques).

Everyone else, thanks for the insight. It's not solely the A/C... it's happened plenty of times with no climate control on. I will start paying attention to the other electrical load. And maybe the cold engine is a large part of it. A lot of my around town driving is usually 10 minutes or less of travel time.
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Re: Unpredictable RPM During Shifts

Post by Shadow »

mapman85 wrote: Yes, sorry for the poor wording. Basically I was trying to describe that as soon as I clutch in to upshift, the engine will drop like a rock to around 1000 RPM, thus missing that "correct" spot, no matter how fast I shift (and it seems to happen inconsistently despite similar driving conditions/techniques).
Hmm....I've never experienced that on any car that I've owned or driven. The RPM should not fall as rapidly as you're describing. Do you know if your car has its original flywheel?
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Re: Unpredictable RPM During Shifts

Post by mapman85 »

Shadow wrote: Hmm....I've never experienced that on any car that I've owned or driven. The RPM should not fall as rapidly as you're describing. Do you know if your car has its original flywheel?
I bought it with 25,000 miles, so I would think so. Of course, who knows what driving habits the original owners had.

I should also clarify not to take the 1000 RPM as an exact value every time. The point being, on these jerky shifts, the RPMs fall way faster than on the normal/smooth shifts.
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Re: Unpredictable RPM During Shifts

Post by bk7794 »

Wait you said it happens when Cold? I know that if I let my foot off the gas just right I will get that, and the Cow said that he used to get the same thing your describing when the engine is cold. I think that is what he said..he can correct me if I am wrong.

I get massive rev hang sometimes when its cold.
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Re: Unpredictable RPM During Shifts

Post by theholycow »

bk7794 wrote:Wait you said it happens when Cold? I know that if I let my foot off the gas just right I will get that, and the Cow said that he used to get the same thing your describing when the engine is cold. I think that is what he said..he can correct me if I am wrong.
That's pretty much it, except I framed it differently...when it was cold I loved how the RPM dropped fast with no rev hang, my shifts were always perfect when it did that.
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Re: Unpredictable RPM During Shifts

Post by Shadow »

mapman85 wrote:
Shadow wrote: Hmm....I've never experienced that on any car that I've owned or driven. The RPM should not fall as rapidly as you're describing. Do you know if your car has its original flywheel?
I bought it with 25,000 miles, so I would think so. Of course, who knows what driving habits the original owners had.

I should also clarify not to take the 1000 RPM as an exact value every time. The point being, on these jerky shifts, the RPMs fall way faster than on the normal/smooth shifts.
Have you tried shifting at a higher rpm to buy yourself more time before the revs drop all the way down?
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Re: Unpredictable RPM During Shifts

Post by mapman85 »

bk7794 wrote:Wait you said it happens when Cold? I know that if I let my foot off the gas just right I will get that, and the Cow said that he used to get the same thing your describing when the engine is cold. I think that is what he said..he can correct me if I am wrong.

I get massive rev hang sometimes when its cold.
A large chunk of my driving consists of ~10 minute trips, so I theorized that could very well be one of the causes. I haven't made a point yet to see how often it happens once the engine is nice and warm.
Shadow wrote:Have you tried shifting at a higher rpm to buy yourself more time before the revs drop all the way down?
Yes, and also tried to concentrate on altering my accelerator/clutch transition a little as well. My guess is that it reduces the likelihood, but I'm pretty sure it has happened at higher RPM, and the sequence just moves up the tach. The other thing is that it's so inconsistent (at least to me anyway), that I don't know it will happen until after it's already happened.

I'll have to try to start making a mental logbook of when it occurs so I can provide better statistics.

Also, regarding the previous electrical load comments: I have noticed on random occasion (tonight being one) that my headlights will flicker slightly when I shift. Normal or is there a connection here?
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Re: Unpredictable RPM During Shifts

Post by Shadow »

You should capture the issue on video and post it here. Maybe you can get the revs dropping normally...or hanging...or dropping like a rock. I've never seen such a variation on the way revs fall on any particular engine, other than a slight variation due to a load like the A/C compressor. And even then, it's just a slight variation...nothing that would cause me any problems shifting. I'm curious to see what's going on with your car.
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Re: Unpredictable RPM During Shifts

Post by bk7794 »

Yes please do, I am getting the same issue and I hope it could help me out aswell.
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Re: Unpredictable RPM During Shifts

Post by Squint »

I second what shadow said. I've had a few oddities when the engine was cold, but it was only a small difference. Nothing that really I would call "dropping like a rock". AC/electrical pull can certainly do it too, but it shouldn't make a massive difference.
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