merchauser

helping a friend deal with his 996 IMS bearing, I have read some interesting articles about best way to avoid IMS issues. in addition to frequent oil changes, most experts agree that the IMS does better with owners who drop back a gear and keep revs above 3500. it is said that the cars that suffer the most are owned by lady boy, girlie man drivers that don't ever crank the car up. higher spinning IMS bearings seem to last longer.

with that in mind, is it "better" to keep revs higher with our 928's? as often as possible, respecting modern traffic patterns, I love to push to 6k, and keep revs above 3500. a lot easier to take advantage of power band. but, with my wife in the car, crowded roads, and stop lights, I find that I frequently go to the next gear, and revs are often just below 2k.

are there advantages to keeping our cars higher up in the RPM range?

hacker-pschorr

It's a common misconception that low RPM's are better for motors, that high spinning to "red line" is somehow dangerous. When in fact the opposite can be true.

The greatest risk of engine damaging detonation is at peak torque. Constantly forcing an engine to pull around in this area isn't considered a good thing.

Every time I drive any car, I'll hit red line when it's reasonable to do so and the engine is fully warmed up. I never short shift, even in my high mileage 2003 Yukon XL. Multiple cars in my history still running strong past 200,000 miles tells me I'm not exactly hurting anything.

It's very much an American thing to lug engines around at low RPM's thinking all that torque is where you should be driving. Comes from the muscle car era where cars had gobs of torque and a 2-speed power-glide transmission.
Meanwhile those of us who grew up with hot hatches from Europe, designed and geared to always be in the upper RPM rage, tend to drive every car that way. It's not uncommon for European cars to be at 4,000+rpm at highway speeds in top gear.
The difficulty with this is the owners of US 16V cars which are torque engines down low and fall on their face upstairs. Even still, I won't lug one of those around below 3k, it's just more tempting.

I always chuckle when I ride in 5-speed 928's or any performance car with a 6k+ redline and people are shifting at 4,500 rpm or so during a WOT acceleration run or bogged down at 2k when trying to speed up. Makes me wonder why someone would bother buying a manual if you're not going to use all of the gears.

I have no idea how this translates to the IMS issue though. Maybe those motors are prone to detonation when being lugged at low RPM's on a cold engine and that's damaging the bearing. It's highly possible.

Remember, Todd's Twin Turbo is spinning to 8,500rpm multiple times every time he drives it (it's his daily driver) with STOCK rod bearings. His pistons and rods are lighter than stock, but the rotational speed has yet to be an issue.
The best way to foul up the plugs on a Ferrari V8 is keeping the RPM's low.

I've taught my wife to drive this way, she keeps her TurboS Beetle at or above 3k. Now yea, if she drops just under 3k it's not panic zone...and she knows that. More time spent with a car the more you get to know it's nuisances.

Easy way to get to know the power-band is get going about 70-80mph and see what the RPM's are in top gear. When cruising through town at say, 45mph, that's your target engine speed.
This gets complicated with modern cars with multiple overdrive ratios and direct injection.
Ignore the above, doesn't make as much sense as I thought it would when I first typed it. Not deleted to avoid confusion with posts that follow.


Lugging an engine around is a great way to save fuel, which is obvious to anyone who has a car with a shift light. If that's the primary focus when driving somewhere, maybe leave the 928 at home.

G8RB8

I like to let my cars spool up. I've got over 125K on my Boxster with no issues and I drive it hard every day. I keep the 928 at a rolling boil around town and in traffic. It's an auto so I have to manage it myself.

hacker-pschorr

I'm itching to install the brand new LSD equipped 89 automatic transmission I have into my 87 which will help keep the RPM's more in the fun zone.

These are not Cadillac's

merchauser

got it! but, wondering if there are "advantages" in keeping up the RPMs? if the IMS prefers to spin fast, are there components in our cars that prefer revs as well?

hacker-pschorr

I don't think the IMS necessarily likes to spin at a higher speed to last longer, it's the force subjected to the drive-train when lugging an engine that I suspect is considered bad for the IMS bearing.

The negative effects are most noticeable on smaller engines like a 4-banger VW. Take one of those, put it in 5th gear and slow down to engine idle speed and floor it. You'll feel some uncomfortable bucking and vibrations as the engine works it's way to a proper area in the power band (usually above 3k). You may not feel this as much in a 911 or 928, but the effects are still there.

There are other issues related to slow air speed and what not that can cause valve-train buildup and fouling spark plugs. This is where the "Italian tune-up" comes into play.

This is where the stall speed of a torque converter comes into play. When you mash the throttle in an automatic it doesn't take off at idle. The torque converter is designed to spool the engine up first then apply the power. This is why a common upgrade to make automatics faster is a higher stall torque converter.

linderpat

Hmm, not watching IMSA much lately Hacker?

WALTSTAR

Maybe I have a problem, but using your benchmark Hacker, my 928 is at 2000 rpm at 80 MPH in top gear.

hacker-pschorr

OK, 4th gear for you

Not a scientific formula, just something I ponder when driving a new (to me) car. I'm just so used to high revving engines that are geared to be at 3k-4k down the highway in top gear and the early 928 5-speeds with 2.75 R&P.
In my opinion those damn 2.20's should never have been installed in a 928, especially the automatics.

I'll think of a way to re-word it that post.

Honestly though, once you get to know a car you can usually start to tell where it's "happy" and keep it in that range. It always bugs me when I'm driving a new car and it shifts into 8th gear and I'm going 70mph at engine idle speed.
Direct injection and multi-clutch / faster shifting transmission are a game changer in this whole discussion. Roll on the gas in these new cars and they instantly drop 3-4 gears. We had a new Mercedes last year in Estes Park, CO with a 7 or 8 speed transmission. It was unreal how much that SOB was shifting. It was super smooth though, just something to get used to.

My 81 5 speed, per the chart in the owners manual, is at exactly 3,000rpm at 80mph with the 2.75 R&P

Per this chart:
https://www.928gt.com/t-928trans.aspx
Your car has a 2.36 R&P.
I don't think I've ever driven one of those.

Should be noted, thoughts keep coming to me, some experts will say such discussion with high torque V8 motors like ours this is not as big of a deal as say smaller displacement / higher revving engines like the 911 unit in question with the IMS issue.
Even the US 16V engines pull smoothly from lower RPM's under load.

If we pulled Twin Turbo Todd into this discussion he would fill a few pages explaining how rod to stroke ratio plays a big part in this too. The 928 engine is an "over square" design with very much favors higher RPM's. This means the bore size is larger than the stroke length.

I guess my overall point is we should not be afraid to rev these engines to redline and use higher RPM's....once oil / coolant are up to temp.

G8RB8

Yes. Lower pressure in the cylinder decreases wear on conn rod bearings, wrist pins, lifters, cams, and helps avoid detonation and overheating.

hacker-pschorr

Good summary, would have saved me a lot of typing had I started with that

edwardsgw

The M96 engine’s IMS bearing sees its highest loading at idle. This is why higher RPMs are better for those engines.

dr bob

Whew!

Friction on bearings and similar, like bores and skirts, increases with the square of the increase in surface speed. RPM's in this case.

Detonation damage and bearing pounding is a bigger risk in higher gears mostly because the crank won't move out of the way as easily as in lower gears. Pounding is different than friction.

If you aren't demanding a lot from the engine, no reason to spin it up.


There are lots of theories on how to drive your 986/996/earlier 997 to reduce the chance of IMS bearing damage. Like any other rotating friction item, load goes up with the square of the increase in RPM. Meanwhile, lower engine RPM may reduce oil flow to a flooded bearing. My minimal knowledge of the IMS bearing is that it's a sealed multi-row assembly, and there's no oil flow to it/through it unless you remove a bearing seal. So no benefit to spinning the engine faster. Not trying to start a war on IMS failure modes or remedies, just pointing out some basic fizzicz from which you can extrapolate your own hypothesieze.

buccicone

Wow my GT & GTS are 3000 rpm at 80 mph.

merchauser

same with my 90 GT because of 2.73 ratio.