andy-gts

2.2 in my gts and turns 2300 at 80 mph

James Bailey

The myth that you have to not lug Porsche engines came about BECAUSE the 4 cylinder overhead cam 356 Carrera engines uses roller bearings on the crank which did not tolerate lugging well at all but there were few of those actually made.....Basic fact on most engines is if you spin it 6,000 rpm vs 3,000 rpm many, wear items like piston rings and cylinder bores , valve guides etc. are going to wear out twice as fast. Using high RPM often just makes more noise and burns more fuel but if that floats your boat go for it !!!

dr bob

FIFY!

Makes one wonder how diesel engines survive more than a few minutes between overhauls.

hacker-pschorr

Exactly, it's a case of which is better / worse. The fact is, modern engines (and yes the 928 is part of that club) simply do not "wear out" piston rings / bores, bearings etc.... like engines did in the past. So OK, maybe running them up to redline will cause them to only last for 200,000 - 300,000 miles instead of 400,000

Some people act like this conversation is saying you should run around town at 6,000rpm all day long. If you want to accelerate at the fastest rate, which I do regularly when driving any car, ruining it up to redline **then shifting** is not going to hurt anything - as long as everything is all warmed up.

But lugging them around at idle speed in 5th is a horrible idea, yet some of you apparently think this is A-OK.


This has nothing to do with any 356 engine and is not Porsche specific. Gotta be kidding me with this comparison.

Why do you even own a 928 if you think they are so fragile? A 32V 928 only makes more noise between 3,000 & 6,000 rpm? Are you serious? So to obtain maximum acceleration you shift at 3,000rpm?

Please explain how every single car I've ever owned was regularly run up to redline and lasted to well over 200,000 miles and sold still running like a top. This includes Saab's, VW's, Honda's, Audi's..... Every single one of these engines would buck and ping if lugged at a low RPM. So again, as I stated before, with a torque ready V8 you can "get away with" it a bit more, but that still doesn't make it good for the motor or ideal, especially when trying to accelerate.
My wife's previous 2004 VW Bug sits at over 4,000rpm at highway speeds by design!! These motors easily exceed 250,000 miles, the bodies / chassis are usually too far gone to go much further.

Driving my 87 automatic through my favorite cloverleaf, I'm traveling about 40mph ~3,000rpm in 2nd gear when I need to accelerate to about 80mph in order to safely blend into traffic. Per you, I should just shift into high gear and roll into it, complete and utter BS. Keep in 2nd, run it up to redline and catch 3rd.....away I go. That's why we own these cars. If I want to save gas I'll buy a Tesla.

Even the most anemic 928's, the 80-82 US models the HP doesn't start to fall off until 5,200RPM...but per you, they only make more noise above 3k.....sure thing.

The idea that engines need to be kept low and slow in order to protect them is "old school" American Iron nonsense, not 356 folklore.

I'm fully aware of the fragility of classic American V8's in their stock form when revved too much. This kind of conversation reminds me of a dyno day years ago with the Corvette club. Friend of mine in attendance, who is a dedicated Corvette mechanic and restoration expert, was trying to talk all the big block owners out of dynoing their cars. First off, he was afraid of them coming apart and second, he knew they would be very disappointed in the numbers.

Fact is, the 928 / 944 engines are so over designed, 6,000 rpm isn't even breaking a sweat. Run the right oil, get everything nice and warmed up and have fun!!!


Even my new Husqvarna Yamaha powered riding lawn mower says to keep it at max RPM while under load for best engine longevity.

I knew someone would bring this up.

Anyone who doesn't understand the fundamental differences between a high pressure turbo diesel (or diesel in general) and a 928 motor.....well, I'll just leave it at that.

For starters where was the engine designed to run? Ferrari V8's are the best example. PLEASE go buy a 355 and lug it around at 3,000 rpm, never getting into the upper rev ranges and let me know how that goes over the long term.


I think someone needs to tell George Suennen he's just making noise and needlessly wearing out his engine running at such dangerous RPM levels.

dr bob

I think the division here is whether it's "OK" to putz around at low RPM's and low load, and the answer is of course Yes. If you need more performance, drop it down a gear or three so you get the Horsepower needed to meet your performance expectation. As you do, I drop to third or second early on those ramps, so I can blend relatively effortlessly in to faster traffic. Roll the ramp at whatever G load you feel is comfortable, then launch into traffic. Other times the traffic I'm merging with might be crawling along at 20.

It turns out that the Rogerbox in my car has the amazing ability to select gear ratios based on load demand and speed. I dink with the TV cable adjustment to find a balance between our 4000ft altitude and my feather-foot driving style. It works amazingly well at selecting a good gear for the duty. You poor sods who are stuck with clutch-and-row cars have to do so much more thinking and work just to get to work or the pool hall. I just point it and go.

docmirror

This is an old question. First I'd like to dispel a theory that floats around the M96 engine. No one, but no one KNOWS that high RPM helps the M96 avoid catastrophic failure. What is needed(we think) is for the car to be driven regularly, and not let them sit unmoved for months at a time. There HAS been some investigation and some vague probability study with poor R squared value that shows a link between IMS failure and low mileage cars that sit for long periods. Not trying to pick a fight with anyone but I studied this from the writings and some long articles written by folks who have taken apart a bunch of engines, and made a real study(not like my ersatz thoughts) of the failure modes. The general rule of thumb is drive it often, drive it for > 30 minutes, and don't baby it(which does not really translate to high RPM).

Next, I was going to point out the physical forces involved in RPM, but Dr Bob did an eloquent job on that. A few more thoughts. At the same speed, the difference in RPM of 2000 would translate to significant piston friction distance. Frex - a 3.5 inch stroke, running 2000RPM faster would equate to an additional 1166.667 feet of friction distance(7 in x 2000). There are other frictional wear issues with higher RPM.

Now, on to engine design. The biggest engine in the world is in those huge container ships that cross the ocean. They redline about 100RPM. The flat 16 piston engine in an F1 car will redline around 19,000RPM. Different strokes for different folks. A good rule of thumb for spark ICE is that max torque occurs at .787 x redline. There are a bazillion design elements that will move this around, but it's a good starting point. There's absolutely nothing wrong with running the modern engine right at that max torque location, with a wide open throttle for hours on end. IF - it is designed right, and has the right cooling. Airplane piston engines do it all the time.

The 928. Like many performance cars, the 928 is a compromise of engine design and use. Our engine will go for half million miles with modest care. It likely lives it's life at about 18% of rated power, for hours, and hours, and hours, and not a care in the world(provided the flex plate is checked regularly). Running it faster is just going to increase friction wear, at the possible reduction in detonation. If an early engine, with limited knock detection and limited spark control is run on poor gas, with a lot of carbon build up, it might detonate. That will cause massive wear, very quick. Later engines, with better knock detection and spark control, run on good quality fuel of the proper octane will almost never suffer from detonation, even in the worst case conditions(lugging, and full throttle accel with warm engine) But - my 2 cents says that any marginal reduction in potential detonation by running higher RPM will be well offset by greater friction wear, and bearing stress as Dr Bob has alluded.

YMMV, don't try this at home, objects in mirror, contents have settled, and may cause **** leakage. <edited to correct the torque value, I had it wrong>

Petza914

There were 3 versions of IMS bearings - a small single row and a small double row, both used in the 986 and 996 and then a large single row in the 997 that started mid-year 2005.

Centrifugal force tends to self-center the IMS bearing at higher RPMs vs the load of the shaft on the bearing at low RPMs/idle. The bearing is sealed, which is shouldn't be, and is actually an alternator bearing application placed inside a motor. It's also the wrong type of bearing designed for the inner shaft to be stationary and the outer bearing to rotate, but in this application the outer race of the bearing is stationary and the inner race the one that rotates. The problem with the larger, later bearing is that at high RPMs, the surface speed of the bearing can exceed its design parameters. This is why for the larger 997 bearing there are fewer street driven failures and more track failures.

andy-gts

petraz, that sounds like one shoould avoid those cars PERIOD !!!!! no good fix

Petza914

It's funny actually. There's a permanent fix (IMS Solution product from LN Engineering and Flat 6 Innovations) for the early cars - 996 and early 987s, but because those cars are the ones with the issue, they are avoided for the later 997.1 cars where you have to split the case to make a permanent fix.

The best 997 package to own is a 2005 997 C2S launch car. Sport sport seats, PCCB brakes, and a replaceable IMS bearing.

Adk46

As I think of it, without necessarily finding the answer: we commonly accept that a rebuild is necessary between roughly 100,000 and 200,000 miles. Pick two other numbers if you prefer.

Where in this range would you like to be? What can you do to influence it? Are there degradation processes you can easily avoid while still having fun?

Even after discounting stupid (not changing oil, using the wrong oil, flooring a cold engine to impress the cars and coffee crowd, etc.), engines still wear out. Apparently - I've never worn one out myself. As Dr. Bob says, load matters and there's a nasty squared term in the equations. It's possible that even Porsche engineers do not know the precise answer - long-term durability testing is too expensive to explore all the factors. One thought is that if you operate your engine differently than was used for durability testing, you are on your own (e.g., do you park outside in Minnesota?). I like "Drive it like Porsche tested it" rather than "like you stole it".

But again, load matters - the engineers probably do accelerated testing as well, which must mostly involve imposing high loads. They must have graphs of degradation versus RPM, which they provide to the warranty department - in theory, those are the guys who place the red line on the tach.

=====

Sealed bearings become unsealed eventually. Best to put them only where they can be easily replaced, and where failure does no serious harm. This is the root evil of the IMS bearing (and 928 water pump bearings). They are within splash lubrication range, but only if you rev up the engine - that's the explanation I like about why gently driven 996/986 and 997.1/987.1 engines are more likely to fail. Only someone with the statistics can really know, however.

dr bob

If ever there was a case for different strokes for different folks, it's this one. Do I drive my sports-touring car in max available performance mode even when I don't expect to need it? I don't. Erik's reference to Ferrari engines is interesting. An engine designed for max performance at max RPM's necessarily has large ports, and that means that running at small fractions of available flows (load and RPM's) encourages deposits on critical pieces. I happen to have spent a bit of time at Ferrari NA in the early 1980's, trying to get those engines to pass some very strict California emissions standards. Choking the exhaust flow, lowering effective compression, managing timing at less than ideal, over-fueling for charge cooling were the available weapons without resorting to internal engine design changes. All those things made driving more work and less fun, at least within the RPM and load ranges required for passing the testing. Remember, the manufacturer had to guarantee that every car will pass, and for 50k miles at that point in regulatory time. Driving at low loads and low RPM's meant serious carbon buildup, and that caused NOx and detonation damage. It's no wonder that owners quickly learned to drive them like they stole them, as the engines would self-destruct if they weren't driven that way. Fast-forward to modern engine management and variable cam timing available today: The engines run and easily survive when driven in a more moderate style. You don't get all the sweet mechanical sounds when you do it of course. The only gearbox options these days are auto-shifting, so the engine management system can protect from detonation-driven bearing and piston hammering that might occur at high loads and low RPM's. Consider how we are artificially generating mechanical noises and piping them through the audio systems in some cars. All to deliver a classic driving experience when the technologies all say it's not just unnecessary but really undesirable in almost all typical US driving conditions.

Drive your cars however you see fit. Meanwhile, I replaced headgaskets on my gently-managed non-928 engine at north of 100k, and found bores that still showed full factory crosshatching and no top deposit or wear ridges. Valve gear that looked brand new. Yes, you can easily affect the life of the engine with proper break-in, maintenance, along with religious warm-up and load/heat management in service. I expect the same from my 928.

ptuomov

It's all relative in the sense what's considered low rpm and what's considered high rpm.

Looking at the S4 engine torque, I think it was designed to be driven at all rpms from idle to 6200 rpm. The engine is overbuilt to handle hugely higher cylinder pressures and torque than what it makes in its stock form. If the oil flow is not interrupted and cooling system works, you could probably leave the S4 engine running at peak torque rpm for a week without any problems. The rod bearings, rods, wrist pins, pistons, etc. are all too strong and heavy.

The big problem in the 928 bottom end is the crankcase gas flows that screw up with oiling at high rpms, I'd say that's the #1 constraint on the engine. The piston assembly and the rods are also very heavy. The very heavy reciprocating weight deforms the rod big end at very high rpms, causing the rod bearings to grip from the sides, especially with cast rods. I'd guess that's something that would become relevant at rpms over 7000. The 1.5mm top ring is also something that can start fluttering near 7000 rpm, which will cause blowby and worse oiling problems. The S4 intake and exhaust manifolds don't like making power much above 6000 rpm anyway, so shooting for the peak power at low 6k's and setting the redline near 7k's is what the S4 engine wants to do naturally anyway.

ptuomov

IIf you look at the 928 S4 pistons and rods, the dimensions are right there in the gray area between gasoline engine and diesel engine. (I'm using the Mahle piston book, Table 2.1, page 26 as the reference.) The 24mm pin is massive, as is the 42.6mm compression height. That rotating assembly can take a huge amount of torque and cylinder pressure. The thermal inertia of the piston assembly is massive, too. I can't see a situation in which one can "lug" the stock 928 S4 engine to destruction, if cooling and oiling are all good.

I think George Suennen doesn't run his engine much past 6000 rpm? If your argument is that the S4 engine was mean to be run at around 6000 rpm, I'd agree.

Trying the turn the stock S4 rotating assembly and stock oiling system over 7000 rpm is IMO asking for trouble, however.

dr bob

With more RPM's come more horsepower and therefore more ability to do "work", assuming the other factors (like net torque) stay constant. Every once in a while I end up with my foot into the kickdown switch, asking more from the engine than it was delivering in a higher gear. It will spin to redline if asked, more quickly accelerating the car in the process after the downshift. It's needed very infrequently, usually only when I've misjudged a traffic situation and another driver. 99.28% of the time though, the engine is just loafing along at a fraction of total available horsepower, at least in typical US street driving service.

Directly to the OP's question: Except for "saving" the fraction of a second that the auto gearbox takes to shift down to advantage higher available net horsepower, there's no real benefit to the engine in spinning it faster "just because". In my experience anyway.

docmirror

This statement troubled me. I would like to offer an amended version. 'Inertial forces at higher RPM tend to counteract the pulse forces(valve train) at low/idle RPM, which may result in a jarring or NRRO force the bearing is not designed to handle.' OK?

Centrifugal forces never, ever 'self-center' a rotating assembly, unless there is a strange, multi-order, phugoid harmonic that not even FEA could reproduce. Provided we postulate a Newtonian physical world. Navier and Stokes might look at the IMS and decide that there is such a moment harmonic, but I really doubt it is all that complex. Just a crap bearing in the application where sitting for long periods with acidic oils(yes, the 'fully synthetic oils are acid Ph) etch the bearing race/*****.