S2 SHUTTER DOWNSHIFTING FROM 3RD TO 2ND
10-08-2015, 04:11 PM
#46
All interesting comments to keep in mind. Yes, rev matching should be done, but it also shouldn't be necessary. I do a lot of track coaching, and it's amazing how many folks can't heel and toe. They let out the clutch slowly on a downshift. My S2 would not let me do that the way it is without shaking apart. The answer, buy a new Cayman GTS. I drove one not too long ago and it automatically "blips" the engine on a downshift. Even more surprising, if you know how to heel-toe, the car will not automatically do it, it lets you have the honor. Technology is wonderful.
10-10-2015, 02:45 AM
#47
Rennlist Member
You are correct that you should always rev match any manual downshifting. But the fact remains that this is some sort of major flaw as you can downshift about any other car on the road without rev matching and it won't shutter like this! I have a supercharged Miata that you can downshift all day long and it's as smooth as can be!
That said, I'll toss out a theory for individual consumption...
I suspect this is not an issues with the 2.5L M44/02 ['83-'85] or M44/07 ['86-'87]. Both motors run a 9.5 CR but only develop ~140 lb-ft. Probably not an issue with the 2.5L M44/09 ['88], either. Even though it runs a 10.2 CR, it only develops ~150 lb-ft. The 2.7L M44/11 runs a 10.9 CR, but still only develops ~160 lb-ft. Now, the 3.0L M44/41 runs the same 10.9 CR, but develops a very healthy 208 lb-ft.
My theory...1) the motor's high CR, and 2) an ability to develop some healthy torque [and a relative amount of healthy negative torque under engine braking], a 3) short 3.875 FDR [exacerbating RPM differences between gear changes], 4) and a spring-centered clutch disc without a vibration damper, create a perfect storm of sorts.
The resultant shock to the driveline downshifting from 3rd [~3800rpm] to 2nd [~5500rpm] at 50 is just asking too much. Without rev-matching, as soon as you disengage the clutch, engine RPMs fall. By the time you shift to 2nd and re-engage the clutch, RPMs are down to 2000, or less. At this point, you're basically asking the driveline to spin the motor up ~3500rpm while on the over-run under engine braking. That's a tall order...
I bet your Miata has half the displacement of your 944...with only half the torque.
10-10-2015, 08:56 AM
#48
Rennlist Member
Happy to hear we all agree on rev-matching...and that the issue goes away under that scenario.
That said, I'll toss out a theory for individual consumption...
I suspect this is not an issues with the 2.5L M44/02 ['83-'85] or M44/07 ['86-'87]. Both motors run a 9.5 CR but only develop ~140 lb-ft. Probably not an issue with the 2.5L M44/09 ['88], either. Even though it runs a 10.2 CR, it only develops ~150 lb-ft. The 2.7L M44/11 runs a 10.9 CR, but still only develops ~160 lb-ft. Now, the 3.0L M44/41 runs the same 10.9 CR, but develops a very healthy 208 lb-ft.
My theory...1) the motor's high CR, and 2) an ability to develop some healthy torque [and a relative amount of healthy negative torque under engine braking], a 3) short 3.875 FDR [exacerbating RPM differences between gear changes], 4) and a spring-centered clutch disc without a vibration damper, create a perfect storm of sorts.
The resultant shock to the driveline downshifting from 3rd [~3800rpm] to 2nd [~5500rpm] at 50 is just asking too much. Without rev-matching, as soon as you disengage the clutch, engine RPMs fall. By the time you shift to 2nd and re-engage the clutch, RPMs are down to 2000, or less. At this point, you're basically asking the driveline to spin the motor up ~3500rpm while on the over-run under engine braking. That's a tall order...
I bet your Miata has half the displacement of your 944...with only half the torque.
That said, I'll toss out a theory for individual consumption...
I suspect this is not an issues with the 2.5L M44/02 ['83-'85] or M44/07 ['86-'87]. Both motors run a 9.5 CR but only develop ~140 lb-ft. Probably not an issue with the 2.5L M44/09 ['88], either. Even though it runs a 10.2 CR, it only develops ~150 lb-ft. The 2.7L M44/11 runs a 10.9 CR, but still only develops ~160 lb-ft. Now, the 3.0L M44/41 runs the same 10.9 CR, but develops a very healthy 208 lb-ft.
My theory...1) the motor's high CR, and 2) an ability to develop some healthy torque [and a relative amount of healthy negative torque under engine braking], a 3) short 3.875 FDR [exacerbating RPM differences between gear changes], 4) and a spring-centered clutch disc without a vibration damper, create a perfect storm of sorts.
The resultant shock to the driveline downshifting from 3rd [~3800rpm] to 2nd [~5500rpm] at 50 is just asking too much. Without rev-matching, as soon as you disengage the clutch, engine RPMs fall. By the time you shift to 2nd and re-engage the clutch, RPMs are down to 2000, or less. At this point, you're basically asking the driveline to spin the motor up ~3500rpm while on the over-run under engine braking. That's a tall order...
I bet your Miata has half the displacement of your 944...with only half the torque.
Anyone think it might be worn synchos?
10-11-2015, 01:45 AM
#49
Rennlist Member
The term "minus" or "negative" typically refers to direction of motion, whether linear or rotational, opposite that of the stated normal direction of motion within a particular reference frame. In this case, it simply refers to the twisting force placed on the clutch/driveline in the non-normal direction.
Two questions:
1. Does anyone with a 3.0L M44/41 and rubber-centered disc experience this issue?
2. Does anyone with a 2.5L M44/40 and spring-centered disc experience this issue? While the S motor runs the same 10.9 CR, it develops ~30 lb-ft less than the S2 at the crank.
10-19-2015, 02:26 AM
#51
Rennlist Member
Is there anyone here with a 3.0L M44/41 running a spring-centered disc that is not experiencing this issue?
11-02-2015, 03:03 PM
#53
Rennlist Member
My new-to-me '89 S2 had a spring-centered clutch installed 5k miles ago (before I bought it) and I haven't felt any strangeness in the 3-2 downshift. Though I do tend to rev-match 80% of my downshifts out of habit. I'll try it without rev-matching next time I drive the car.
06-29-2017, 03:21 PM
#54
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Well, it's been about 1 1/2 years since the last reply. Does anyone have any update on this shutter problem? I think mel's theory on this may well be correct. Perhaps the shock this vehicle puts to the driveline when downshifting from 3rd to 2nd combined with a spring centered clutch is just too much over time. I know RJ80 said his doesn't do this but maybe it takes some time. Mine did not do it originally either but over a few thousand miles with a new spring centered clutch developed this shutter. Hey 944hal! Do you have any news?
01-31-2018, 03:17 PM
#56
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I was told that this shuttering was caused by worn torque tube bearings allowing the drive shaft to vibrate while downshifting. Also, this problem was amplified by the fact that S2 torque tubes have a split which over time allows the bearing retainers to move. I purchased a late turbo torque tube and had it rebuilt with a new drive shaft and new bearings. Installation coming soon.
cds
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