Rev limit discussion

[ggtttt3]

I'm aircooled noob starting to educate himself, and I was curious where each successive limit is once you start going up in RPMs on these engine, say with a 964 3.6 as a starting point.

As interested in failure modes than power\performance issues, though I think the latter is better explored here already and is much less problematic of course...

[GH85Carrera]

If your engine is totally stock, don't raise the redline. Porsche engine engineers know their stuff. If you want a long lived engine, leave it alone. The torque is dropping off before redline anyway, just upshift and stay in the power band.

[stownsen914]

Conventional wisdom for later aircooled cars is that if you want to rev above 7000 RPM, you want at least better rod bolts, if not upgraded rods. For 8000+ race applications, you need better rods along with reduced reciprocating weight including valvetrain.

[spuggy]

Originally Posted by GH85Carrera

The torque is dropping off before redline anyway, just upshift and stay in the power band.

Where the torque and HP cross over on the dyno chart isn't a bad place to shift up @ WOT. Especially if the shift point leaves you above boost threshold, or otherwise in a good place in the torque band. Very little point holding RPM past peak HP. On stock motors without "hot" cams, both usually happen well before redline.

That said my motor is built for mid-range throttle response/torque. Although with Pauter rods, ARP bolts, stiffer valve springs etc

So why do I still take it to 7000 from time-to-time, even though I know perfectly well that with SC cams, HP falls off a cliff after 5500 RPM?

1) To dodge a 2->3 shift, followed 50 ft later by a 3-2> shift. Those who've driven PIR will know immediately which pair of corners I'm referring to... Yes, taller 2nd is the correct answer
2) Because the noise it makes @ 7000 RPM with the wastegate open (and especially in a tunnel), makes all the hairs on my arms stand up, and I can't stop grinning. It's. Freakin'. Awesome...

Word says that 964 cams make their power over 4000 RPM (~1000 RPM higher than SC) and the fat lady is singing by 6100 RPM.

Personally, I think SC cams are perfect for >90% of the driving I do. Boost control means I don't exactly feel short-changed at the top, either.

[PeteKz]

The forces and rates of wear on reciprocating and rotating engine parts do not just increase linearly with RPM, but roughly as a square. I don't like to rev mine above 6000, because I want to drive it for many years and a couple hundred thousand miles.

Your engine, your choice.

[David Inc.]

Originally Posted by spuggy

Where the torque and HP cross over on the dyno chart isn't a bad place to shift up @ WOT. Especially if the shift point leaves you above boost threshold, or otherwise in a good place in the torque band. Very little point holding RPM past peak HP. On stock motors without "hot" cams, both usually happen well before redline.

WEEOOOWEEEOOOWEEEOOO

Physics police!

Vehicle acceleration is directly correlated to horsepower, not torque, so maximum acceleration is obtained by shifting to maximize time near peak horsepower and not torque.

acceleration = power / (mass * velocity)

It's not until the 3-4 shift that you'd want to shift at less than redline for peak acceleration, at least in a stock SC.

[Flojo]

Drive it like you stole it.

When I was "young" to the 911-world I limited myself to 5000-5500 rpms.
and crucially made sure the engine was warm and oil level was perfect.

today years old I have my rpm gauge tilted to 6500 at 12-o-clock and often notice that the needle optically jumps to 7k.
if it's the case i already have gained speeds way above 200kmh.
good enough.

I know that regular rockers do break at 10k rpm.
dont ask.

[GH85Carrera]

I rebuilt my engine with all ARB rod bolts, and case bolts, and 964 cams. It went from 200 HP at the crank (factory rating) to 200 HP at the rear wheels on a dyno. Still safe HP. I did have Steve Wong raise up my red line a couple of hundred RPM, and I still shift at 6,400 max.

With a 915, no doubt sticking with 2nd and not up shifting and down shifting a half a second later is likely easier. It is not like a modern PDK and shifts in miliseconds.

With a stock 3.2 I would not want to run past the stock rev limiter. The rod bolts are a know weak spot.

[ggtttt3]

What breaks around 8k+? Common upgrade seems to be lighter 8mm valves (w inconel exhaust?), springs and titanium retainers. Is there a point where valve float or some valvetrain failure become an issue again with such an upgrade? Titanium valves don't seem very popular with these engines so I'm assuming they don't really become necessary?

Some good info\experience in here already! Keep it coming

[Quicksilver]

Valve train rev limits aren't a Porsche specific issue. A valve, retainer, and spring is a really generic system. You have a mass of the pieces, and a size limitation for the spring package.
Another variable is that higher RPM will require a different cam and those will be designed with more "ramp" that can deal with controlling the valve acceleration for higher RPMs.

The big Porsche limitation will be that rebuilds are going to be more expensive than other engines and that most people expect a Porsche engine to have a very long life that goes far beyond the lifespan of other high performance motors.

[stownsen914]

Many builders seem to recommend upgraded valvetrain if you're planning to rev much above 7000. And start to speak about titanium if you're revving above 8000. It's not so much that valves float or things fly apart at 8100, but you lose cushion for an overrev event.

[kent olsen]

Porsche engines are pretty reliable. I've had mine (in my 72T) for over 20 years. First the stock 2.4 then a 2.7 and now a 3.0L. I've autoxed and tracked the car for 240,000 miles. I built this last 3.0L to be bullet proof. Run the daylights out of it and then drive home and adjust the valves and change the oil once a year. To get it to the "bullet proof" level my friend (Mike Bruns) and I built it to turn 8500rpm. The cam I selected has max torque at 4800 and max HP at 6900. As stated above the issue of a missed shift is probably the only concern. And I had that happen. On the track shifting from 4th to 5th my aftermarket linkage slipped and it went into 3rd instead of 5th. Well I saw the tack go to 8000 before I got off the power and there was no damage. Still runs great.

[ggtttt3]

Kent what was done to it to get it to run 8500rpm?

[DaytonaCoupe66]

The redline on my 1982 stock 930 with an original well functioning 42 year old engine is 6800 rpm. Even before 6K the kick is tapering off. P engineers likely had a good reason for putting that red mark where they did. I'm sure there's a margin for error, but are the rewards close to or over the red line worth the risk of a grenade-d engine? Russian roulette with old metallurgy. Driving aggressively has it's place, but there's no upside to tempting an exploded engine, unless the stakes for that win at the track are huge, or you hate your car.

No judgement, just an opinion.

[kent olsen]

ggtttt3
Long time ago but as I remember, ARP rod bolts, an added bearing on the crank, and I think special rocker arms, oh and a special oil pump.

[spuggy]

Originally Posted by DaytonaCoupe66

The redline on my 1982 stock 930 with an original well functioning 42 year old engine is 6800 rpm. Even before 6K the kick is tapering off. P engineers likely had a good reason for putting that red mark where they did.

Original 930 cams are really soft, minimal overlap/low lift/duration; no point holding RPM past the power band. Even SC cams - which are a lot "hotter" - are completely done by 5500 RPM and power is falling off by 6K; it's very obvious on the dyno chart. Without other changes, SC cams don't make much more power (10%) at the top - but, although they can't do anything about the appalling turbo lag with the stock boat anchor, they do really improve mid-range response - especially with a better turbo/intercooler and headers.

The 3.2/3.3 share the same rod bolts. Neither the SC nor the 964 used those undersized rod bolts. And neither let go. Whereas the 3.2/3.3 ones do - so ARP or Raceware bolts (which are NOT stretch to fit and discard after, but are instead infinitely reusable as long as elastic limit is not exceeded) are a must if you're going to ignore the rev limit. They may even save you $$$$ if you miss a down shift - although it's better not to rely on that...

[targa72e]

"The 3.2/3.3 share the same rod bolts. Neither the SC nor the 964 used those undersized rod bolts"

Only partly right. The SC uses the 10mm bolts that were used on earlier engines as well.

10mm Connecting Rod Bolt Set - Pro Wave ARP 2000 Complete - 1972-1973 Porsche 911 2.4L - 1974-1983 Porsche 911 2.7L / 3.0L - 1976-1977 Porsche 930 3.0L TURBO - ARP -

The 964 uses the same bolt and rod as 3.2/3.3 engines. 964 rod.



ARP Pro Wave 2000 Engine Connecting Rod Bolt Set - 9mm - (1984-1998) Porsche 911 3.2L 964 / 993 3.6L - (1978-1998) 911 Turbo 3.3L 3.6L Turbo - ARP

The 993 uses the same bolt but the rods are different. 993 rod.



The smaller 9mm bolts are not as strong as the earlier 10mm. Upgrading to higher strength bolts for these engines using 9mm bolts is highly recommended.

john

[spuggy]

Originally Posted by targa72e

Only partly right. The SC uses the 10mm bolts that were used on earlier engines as well.

...

The 964 uses the same bolt and rod as 3.2/3.3 engines. 964 rod.

I sit corrected- thanks!

But you've got me all curious now...

When I dropped a rod bolt & spun #6 (mechanical over-rev/operator error, too much movement in tired trans mounts taking double expected power and a 915 with a sloppy linkage, dog ate my homework, etc), wrench told me that the week after, a long-time (15-20 year) 3.2 owner brought his car in with a dropped rod bolt as well. And "it's always the same bearing(s)".

Said he was pretty sure the owner was a fairly conservative driver and very probably hadn't over-rev'd it. And that "sometimes they just do that".

So if the problem with the smaller bolts is that they stretch on an over-rev (or just sometimes when they feel like it for no apparent reason), come loose (sometimes 3 weeks later) - then why wouldn't 964/993 have the same/similar issue, if they use the same bolts - which presumably can get over-stressed?

[targa72e]

I think there are two different problems. One is strength of rod bolts. On over rev the rod bolts can be stretched which causes spun bearing.
The other problem with spun bearings on 3.2 seems to have more to do with loss of oil pressure at high RPM. Normally the oil is only supplied to the front and rear of the crank. This is one of the reasons to have the center main bearing drilled. This modification adds an additional supply point in the middle of the crankshaft. Normally the oil has to overcome centrifugal forces to get out to the center rods so pressure goes down as RPM's go up.
The picture below if from a old Upfixing that shows oil supply for 911,908 and 917. 917 had oil supplied to center of crankshaft. You can see in diagram at top (911) that the inner rod bearings do not get as much oil at high RPM with the stock oiling.



This seems to be less of a problem on the 964 and later engines. But these also have a much larger oil pump than the 3.2. Apparently, the oiling path and size was changed for the GT3 cranks so that the center oiling is not required.

john

[Walt Fricke]

To figure the optimum upshift RPM point for maximum acceleration, you need the torque values at relevant RPMs, and to know the gearing. Though you can include R&P and tire rolling radius if you want, for easier conversion between RPM and speed in a gear.

The upshift point is where the applied torque (engine torque times gear boost, and calling it thrust may help understand things) of the old gear is the same as the applied torque in the new gear. On my '
82 US SC on the track I upshift from 2d to 3d at about 6,250 RPM, and to 4 and 5 at 6,000. Of course, I may run it up to 7,000 if that saves an upshift. I don't do standing starts, though it looks like 6,500 would be where optimum would be for 1st to 2d.

On the street I'd never push it like this. Why would I?

This is how Porsche described how to figure upshifts, as reproduced 25 years or so ago in an Upfixen. Explained how to do it with graph paper. Spread sheets make it much easier. Still have to do some eyeballing with this sheet to see what the RPM at the crossover points is, but good enough.

Reference to where these points are with relationship to torque or HP peaks refer to purely serendipitous relations. Yes, on any sort of normal torque curve it will be above the torque peak. Mine was at 4,000.

Here is how my SC (rev limiter adjusted to 7,000) came out. Data points are in 500 RPM increments, with the highest RPM being 7,000.