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Old 02-22-2016, 07:38 PM   #1
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3000 rpm

I read somewhere a while back that the M96 engine has a natural vibration at 3000 RPM and you should avoid maintaining that RPM for an extended period.

I have a 99 with a 5 speed, and it just happens that 73 mph is 3000 RPM.

Can anyone confirm that maintaining 3K RPM is a bad thing?

Thanks
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Old 02-22-2016, 07:46 PM   #2
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I read that too at some point. It does seem to have a harmonic at 3000, and that rpm does come at inconvenient points in each gear. As to whether it's harmful, I've flown planes that have placards and yellow arcs that were stay out zones so it causes me to avoid sustained operation. But it's not in the manual that I've found and other than interewebs stuff I've not read much
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Old 02-22-2016, 07:57 PM   #3
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I have never heard of the 986 having a natural vibration at 3000RPM. Excessive vibration at 3000RPM is normally an indication of a failing motor mount.
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Old 02-22-2016, 08:11 PM   #4
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I think what you're hearing is advice to keep the engine above 3000rpm to avoid IMS bearing problems. The IMS bearing is rumored to fail earlier if the engine is driven at low rpm (<3000rpm) on a continual basis (supposedly especially bad is lugging the engine at low rpm).

I have no idea if this bit of Boxster-lore has any real validity but if you do a search on the IMS bearing I am sure that you can read all about it.
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Old 02-22-2016, 08:34 PM   #5
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And I've read that tips have a lower rate of failure and they tend to run at lower revs than 3000 automatically !
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Old 02-22-2016, 08:48 PM   #6
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I do notice a vibration that happens only in 6th gear and only when my foot is entirely off the gas at 3k rpm. If I have my foot at all on the gas no vibration but if I let off there is a vibration until the revs drop.

I'm assuming it's just my motor mount getting old though.
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Old 02-23-2016, 03:01 AM   #7
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And I've read that tips have a lower rate of failure and they tend to run at lower revs than 3000 automatically !
That's because there were less cars built with tip. Don't believe everything you read.
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Old 02-23-2016, 09:50 AM   #8
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And I've read that tips have a lower rate of failure and they tend to run at lower revs than 3000 automatically !
Tips never lug. They downshift.
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Old 02-23-2016, 10:47 AM   #9
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I seem to notice a slight vibration/flat spot at about 3200 RPM, if I'm just cruising. '97 manual.
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Old 02-23-2016, 11:05 AM   #10
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IIRC, the variocam kicks in at 3200rpm, which is why there is s flat spot and why it's advised not to maintain that rpm.
I'm not sure if it applies to all 986 models...
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Old 02-23-2016, 11:51 AM   #11
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Let me share my 2 cents.
I have been following this forum (and others) for the past 6 years.
The first time I read about the vibration, was in one of Jake Raby posts about the potential reasons for the IMSB failure.
If to be honest, no one really convinced me with the true reason for the failure. Although there are a liot of theories out there, there is no logic explanation (to me) why some fail and some don't.
The only theory that made a sense to me, is the one presented by Jake, talking about the vibration at the 3000 RPM range.
If I remember correctly, he mentioned that the flywheel is acting as a harmonic balancer, and there is a natural vibration due to the tension on the IMS to crank chain.
He also implied that low mileage cars are more prone to failure under the assumption that they are not driven hard enough. Also explain some documented cases of multiple failures with the same driver, to suggest the way you drive your car contributes to the IMSB failure.
Up to here, these are words of my memory, so I might not be 100% accurate (and I apologize for that) with Mr. Raby words.
To speak from my personal experience, I can tell you that there is a significant vibration in the 2700-3200 RPM range. It is mostly noticeable on deceleration. (Meaning when the engine RPM drops from 3200 to 2700) and RPM fluctuation.
On my ex 99 base, the flywheel was completely shot at 160K miles.
I could clearly feel the vibration at the above mentioned RPM range, and it was gone, as soon as I installed a new flywheel, just before e I sold the car.
Since then (and if to be honest also before) I try to keep the RPM on the 3000-4000.
It is also more fun, although less fuel efficient.
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Old 02-23-2016, 05:01 PM   #12
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Replace the Dual Mass Flywheel.
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Old 02-24-2016, 08:58 AM   #13
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Replace the Dual Mass Flywheel.
With What?
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Old 02-24-2016, 05:37 PM   #14
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With What?
A new Dual Mass Flywheel.

I use LUK brand.
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Old 02-26-2016, 12:31 PM   #15
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Oh - Great....Something else to add to the "s--t to worry about" list!
3000 rpm seems to be my favorite rpm - running around town and on the freeway 75+ in 5th!
I don't drop much below but normal use I tend to be 3000 to 4500.

In the twisty bits I tend to run 4500 to 5500.

Last edited by njbray; 02-26-2016 at 01:46 PM.
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Old 02-26-2016, 10:58 PM   #16
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I had this issue for some years
Mine is a 2,7 my 2000
Purchased in 05. First thing on the fix list was a new motormount.This did not cure the vibrations.
At that time I noticed that changing the way the rear muffler was hanging changed the amplitude of the vibrations.
Anyway, I replaced the dmf but no improvement at all and learned to live with it for some years.
A rear cat delete was done and vibrations moved up to the 3400 rpm range, out of my cruising speed.
Still a little bit annoying so I devided to do a second motor mount job and found that there was a later version available, even softer rubber. This was the cure but instead I got a feeling that the engine is moving around in the back when I am a little bit inspired.
I consider my engine to be well maintained and in good condition with 10k km on the odo, us import...
But the 3k rpm vibration is there, only hidden by a rubber mount but I gues this is the same for all engines, amplitude is just depending on the internal tolerances.
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Old 02-27-2016, 01:39 AM   #17
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Sigh. I ignored a lot of doom advice and scare stories when I bought these cars and I guess I'll have to ignore this too if I plan on getting any sleep. Mine seems to gravitate to 3000 rpm for all things and I assume has done it for all its 162,000 miles. It shifts between 2500-3200 rpms 90% of the time in normal driving, and cruising on the highway at typical speeds (65-75 mph) falls right around the 3000 rpm mark as well.
If that's really it's unhappiest place to be, then I'd have to file it under the "major design flaw" category.
Honestly, I've never noticed any vibration.
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Old 02-27-2016, 04:01 AM   #18
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It's surprising to me that people are having vibration issues with these engines. It's a pretty well known fact that a flat 6 configuration is inherently balanced, or at least should be in theory. I never cruise at 3k, usually 2500-2800 so I haven't really noticed this on my car.

FWIW, I found this excerpt which was written about a 2001 S in a porsche technical document.

"In operation, the engine control unit operates timing pistons in the intake
camshaft chain tensioners. When the engine reaches 1,200 rpm, the control unit
signals the piston to rotate the intake camshafts by 12.5 degrees. When the
engine reaches 5,120 rpm, the pistons return the camshafts to their normal
positions. The control unit will delay VarioCam until 1,480 rpm if the engine oil
temperature exceeds 266 degrees Fahrenheit (130 degrees Celsius)."

So it looks like the engagement window isn't at 3k, at least not for the 3.2. Interestingly enough, however, the next paragraph says:

"The Boxster S shares its twin-resonance air induction system with the 911
Carrera. The system acts as a “resonance supercharger,” allowing the engine to
draw from higher velocity airflow at certain engine speeds. A crossover pipe
connects the individual air collector/resonance chambers for each cylinder bank.
A flap in the pipe remains closed from idle to about 3,100 rpm. When it opens,
each cylinder bank can draw from airflow “excited” by the resonance created by
alternating induction between all six cylinders. In essence, “dual resonance”
creates two induction paths for each cylinder. Below 3,000 rpm, the cylinders
draw air from a “short” path. From 3,000 rpm to about 5,100 rpm – when the
resonance flap opens – the cylinders draw from a long intake path, which boosts
torque. Above 5,100 rpm, the flap again closes to allow the cylinders to draw
intake air from a shorter intake path to boost horsepower at higher engine speeds"

Total shot in the dark, but I woudn't be that surprised if the vibration when hovering around 3k had at least something to do with the intake paths constantly changing between the long and short paths and the fluctuation of torque it would create.

Source:
http://press.porsche.com/archive/products/press_kits/press_kits_2001/PDF/Boxster_S_in_Depth.pdf p.13
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