How does Porsche determine redline RPM on various engines?

[supersignman]

Thanks Josh & Townsen. That's the kind of information I was looking for. These flat sixes just feel so well balanced internally compared to a V8 it seems like they should be able to turn more RPM. It makes sense that the margin for error would be smaller though. The best way to describe to sweep up through the RPM band is 'effortless' compared to any other engine I've had experience with. There's no discernible harmonic like your get with V8 engines. Sure, a big V8 can make lots of power and lots of noise when forced to but my 911 seems like it's "comfortable" at any RPM in the range and I don't feel even slightly guilty when winding it out. I'm the first to want to enjoy a car at the limits but I don't want to feel like I'm abusing it and my 3.2 seems "happier" at high RPM than lugging around town at 2500.

[spuggy]

Originally Posted by supersignman

I can certainly see where rod bolts might be the weak link. Do the higher revving earlier engines have better bolts?

For the 3.3/3.2 motors, rod bolts are a weak link. They sometimes just undo for no apparent reason. They're well-known to do exactly that if you overrev, usually on a mech overrev from a bad shift. The factory reverted to the larger bolts with the 964.

The factory bolts permanently deform when installed, so are one-use only. This is standard practice & unremarkable - but factory bolts are close to their limit when installed correctly; if they stretch more, you lose tension on the bolt and spin a big end bearing.

ARP or Raceware rod bolts are made from better metal (and can be re-used, because when installed correctly, they don't exceed their elastic limit). In fact, if you're serious about revving high, look into aftermarket rods like Pauter or Carrillo, which are both lighter and stronger - and come standard with better rod bolts.

Revving an SC or 3.2 beyond redline doesn't get you any more power on its own. The cam profile (which gives good low/mid-range response) is already out of its sweet spot. To actually take advantage of the higher redline, you need to change cams.

A chip remap/exhaust on a 3.2 might get you 15-20HP I'm told. That's about it for "easy". It takes significant work/money for a 3.2 to gain 50-60HP - and those changes might reasonably be expected to result in a motor less flexible than stock that will wear out faster.

This just doesn't make sense; forced induction is cheaper and makes more power. A low-boost turbo conversion would get you 300 at the wheels (or more than a 993RS) with a wide powerband and great flexibility for a fraction the cost, all of the mods being external/bolt-on.

A more detailed/comprehensive conversion would make a lot more power. Would still be cheaper than building a highly tuned 3.2 N/A motor.

[supersignman]

Thanks Supggy... I didn't even know there was the option of a low boost bolt-on turbo until reading about it a couple of days ago on another post. I'm definitely going to look into that also.

[Quicksilver]

The 3.2s rod bolts are "stretch to torque" and are the biggest weak link. But there isn't any "mystery" to them letting go. It is just a delayed reaction. The bolts have yielded at their install torque so if you over rev the motor past the bolt's holding limit anything extra will yield the bolt farther. And every time you do that the clamp load is lowered. With the lowered clamp load the RPM that it takes to open the parting line is reduced. So your safe redline is reduced and you don't know it. And each time you exceed it you are hammering on the bearings and reducing the bolt's clamp load.
End result is you have one good over rev and you are possibly on a trail to a failure quite a ways down the road when you have completely forgotten about the original over rev.

I suspect that the absolute limit on the bolts is right around the ClubSport chip's redline which is 6800 (or 6850).
Porsche didn't leave much on the table in the way of easy power. The one spot is the exhaust. There is about 20 HP available if you get a good set of 1-5/8 primary headers. With that it will pull hard to 6500. (I haven't changed the 6500 redline in my car so I can't tell you about how it pulls past that.)

- Increasing the RPM reduces a engine's life exponentially. The rule of thumb I was taught about 30 years ago was "Every time you increase the RPM by 1000 the engine life is reduced by a factor of 10." (It probably isn't accurate with modern materials and enginer design but it helps keep it in perspective.)

If you want to go past these limits you need to open up you wallet (and motor) and completely go through it. Valve train, rotating assembly, cam, induction, twin plug, Ps & Cs, ...

Or you you could buy a 3.6 and save some money...

[supersignman]

Thanks Quicksilver! Very good explanation. That really answers my question about whether the limiting factor is in the valve train or the bottom end. I think I'll keep gathering information at this point in my quest for the "ultimate daily-driver-torque-monster-instant-throttle-response-streetable-well-mannered" air cooled flat six.

[GH85Carrera]

Yep, the only "easy" power is the exhaust. All of your neighbors will hate your car, and you will hate it on any highway trip with all the racket. But if you are just looking for fun back woods drives dodging deer, exhaust is the way to start. Before you dump a ton of money in the car remember is is all just gone, and the next buyer may hate it all, or love the free upgrades you paid for.

Any of the water cooled 911s will have the power you crave. And the brakes and suspension to handle it all.

In the end, it is YOUR car, do to it want you want.

[Tippy]

What is really wonderful about the 3.2 valvesprings are you can run some aggressive cam profiles that get you to 7K (or somewhere abouts) without needing to change them.

If you put in ARP rodbolts, I wouldn't be scared to turn up a 3.2 to 7K all day long.

The rods are extremely short and wide girth wise compared to V8's that regularly get turned up to 7-8K (I created a thread about V8 rods to ours long ago).

I think why we keep these motors to such a low RPM is simply the expense of blowing out the cases with a rod failure.

Very, very costly, so we are genuinely very conservative on RPM when in fact, the low end should EASILY take it.

[Trakrat]

valve springs are the first limiting factor in the redline.
rod bolts would be the second limiting factor
finally... air/fuel mix. EFI has programmed limit and would need to be modified.

[Tippy]

Originally Posted by Trakrat

valve springs are the first limiting factor in the redline.

I don't want to speak for Steve Weiner, but he has a lot more faith in stock valvesprings than most on here.

People overload the valvetrain with unnecessary spring rates causing premature cam failure for nothing.

[pampadori]

Originally Posted by supersignman

I can certainly see where rod bolts might be the weak link. Do the higher revving earlier engines have better bolts?

I think they have less mass in the pistons and slower piston speeds so the rods/bolts aren't under as much stress at the same RPM as say a stock 3.2.

[kent olsen]

Again I say when you are looking for power and rpm don't forget to look at the torque curve. For instance if you have an early 911S you have to turn it up to near 7500 rpm to get that power. A 911T with 40 less power with stay with the S for the first 100 yards because of it's peak torque (around 4000) is much lower the the S (around 5000).

[spuggy]

Originally Posted by kent olsen

Again I say when you are looking for power and rpm don't forget to look at the torque curve. For instance if you have an early 911S you have to turn it up to near 7500 rpm to get that power. A 911T with 40 less power with stay with the S for the first 100 yards because of it's peak torque (around 4000) is much lower the the S (around 5000).

My 930 with CIS and factory wastegate made 80% of peak torque @ 3000 RPM (where boost was all-in @ .8 bar). Peak torque around 3500 - and enough of it that transmission longevity is a consideration. My hybrid K27 made 6 PSI boost at 2400 RPM - that, with SC cams, meant it scooted pretty smartly from low RPM.

Power band done by 6K (cams), but a wall of torque is very useful/flexible - and very accessible in most driving conditions.

[Walt Fricke]

I think it helps to consider two "redlines." The mechanical redline is the RPM above which things might break. Valve float, for instance, possibly leading to valve to piston contact. In the 3.2, the rod bolts are not as strong as they were on the earlier engines - Porsche reduced the diameter by 1mm to deal with the 74.4mm crank. The fix for this is to use aftermarket super rod bolts when rebuilding. Short of a missed shift overrev, a stock 3.2 is apt to run out of steam well before it reaches this point.

If you really have valve float, you should feel it. I once had most of my outer valve springs break, and where I had formerly reved to 8,000 rpm, now the motor wouldn't go above 7,600. So I shifted there and finished the race and didn't seem to lose anything, but it was very obvious that the valves were floating. I guess I had enough clearance, as there was no damage to valves or pistons.

The other "redline" is the upshift point - the RPM for each upshift, beyond which you are getting less acceleration than you could if you had shifted earlier (but not too early, if maximum acceleration is what you want).

As a practical matter, this one doesn't matter for street driving - other than the occasional pass on the highway, how often are you at wide open throttle - and upshifting. So it is kind of academic, but for the track you want to maximize acceleration with what you have.

Where to upshift for max accell is determined by your gears, and your torque curve. If you multiply torque at a given RPM by the gear ratio, you have what amounts to thrust at the rear wheels. The ring and pinion and the tire radius are technically part of this, but for the purposes of deciding where the optimum upshift point is, they can be ignored, because they are constant (absolutely for the R&P, basically so for the tire rolling radius), and you aren't calculating a lunar orbit.

You can make a chart of this using graph paper and your torque curve from a dyno. Here it doesn't matter if the dyno was flywheel or rear wheel, the curve is the same for these purposes. I suppose you could use a factory curve if you have a fully stock motor and exhaust.

Porsche calculated this stuff, especially for its race cars, and Pano had an article on it, saved in one of the Upfixin's.

Here is one made using a spread sheet:




This US 1982 3.0 stock motored SC, with a good exhaust with stingers, is best shifted from 2 to 3 at about 6,250, and 3 to 4 and 4 to 5 at 6,000 rpm.
Porsche says redline is 6,700 rpm.

The speeds shown are irrelevant to these shift points (assuming you use your tach), because they use an arbitrary tire size. They will be at the same RPM no matter what tire is used.

It is easy to see that if you shift early, you lose a bit because you now are at a lower thrust than if you had reached the optimum point. Similarly, if you shift late (stretch it), you lose a bit because you spent some time at a lower thrust before the shift. If you have on track data of sufficient granularity, you can see these effects - if the acceleration number is different between the lower and higher gear at upshift, you missed the optimum.

Note this has nothing to do with peak horsepower directly. The shift point is apt to be above that RPM, but that isn't useful. It is way above max torque.

I'd be careful about thinking the engine is still pulling well, etc., above the optimum shift point. My 8,000 rpm race motor felt like it could keep pulling above that figure, which was where I shifted. Then I put it on the dyno, and 7,600 turned out to be where to upshift (other than stretching it on a track approaching a braking zone).

[supersignman]

Very interesting Walt. Thanks for the info!

[kent olsen]

My point exactly Walt. If you just want to twist your engine high and hear it scream 500 rpm over the redline is probably safe, if not done to regular. Above that is dangerous territory.

If you want max acceleration, racing, autoxing or a stop light drag race, then as Walt's post shows you need to consider torque as well as horsepower.

40 years ago I raced corvettes with my old friend in SCCA. We found out that bigger is not always better, carbs and exhaust. We found that if we could tune and gear for the best pull out of each turn we could get jump out of the turn and hold them off at the end of each straight.

[WP0ZZZ]

Walt's explanation is really nice and clear.

It could be made just a bit clearer by talking about the power curve instead of the torque curve. The cool thing about thinking in terms of power is that the expression linking power to car acceleration does not require the gear ratios and tire diameter. Then, solving the maximum acceleration problem is simple: at any given car speed, one needs to choose the gear that results in the engine delivering the most power.

As a consequence, it is almost always the case that it is necessary to drive beyond the peak power point in order to have maximum acceleration. The peak torque point of the engine is irrelevant to the choice of gear for max acceleration.

[kent olsen]

http://forums.pelicanparts.com/uploads10/rolling+road+run+May+19_20071179576551.jpg

Maybe we should use the term performance rather than power. Power equates to HP and torque is the pulling power to move the vehicle weight. You can see where this engine makes around 250 hp but it doesn't do that till near 6000 rpm. Look back at the torque curve and see when you have only 150-200 hp available the torque has an almost steady pull until the horsepower catches up.

Also after the horsepower peaks it drops off rapidly. My experience running on a dyno with 3 different cars showed the operator ran the engine wide open until the horsepower peaked, then let it run another 2-300 rpm to confirm the power was indeed going down and then they took their foot off the throttle.

[WP0ZZZ]

Originally Posted by kent olsen

http://forums.pelicanparts.com/uploads10/rolling+road+run+May+19_20071179576551.jpg

Maybe we should use the term performance rather than power. Power equates to HP and torque is the pulling power to move the vehicle weight. You can see where this engine makes around 250 hp but it doesn't do that till near 6000 rpm. Look back at the torque curve and see when you have only 150-200 hp available the torque has an almost steady pull until the horsepower catches up.

Also after the horsepower peaks it drops off rapidly. My experience running on a dyno with 3 different cars showed the operator ran the engine wide open until the horsepower peaked, then let it run another 2-300 rpm to confirm the power was indeed going down and then they took their foot off the throttle.

In my message above I've used the word "power" very strictly. It could be measured in hp or, as Porsche likes to do it, in kW.

[Bill Verburg]

Originally Posted by WP0ZZZ

Walt's explanation is really nice and clear.

It could be made just a bit clearer by talking about the power curve instead of the torque curve. The cool thing about thinking in terms of power is that the expression linking power to car acceleration does not require the gear ratios and tire diameter. Then, solving the maximum acceleration problem is simple: at any given car speed, one needs to choose the gear that results in the engine delivering the most power.

As a consequence, it is almost always the case that it is necessary to drive beyond the peak power point in order to have maximum acceleration. The peak torque point of the engine is irrelevant to the choice of gear for max acceleration.

I have to disagree, HP is derived from torque and rpm, and does indicate the ability to move the car. But what actually moves the car is torque and what you monitor(besides the seat of your pants) is rpm.

To fully understand the performance potential you do need to know torque, + rpm + gearing(including tires) You can gloss over some w/ hp but gearing et al is still critical to full understanding.

You also need to include vehicle all up weight and aero to really see what's going on

here are some examples
say i want to know the performance potential and when to shift a '73 2.7RS. It is spec'd at a nominal 210hp @6300, what does that tell you? Suppose you want to compare it to say a 993RS speced 300hp @6500

if you know the torque curve, gearing weight & aero you can predict the acceleration in each gear, you can also see that the 2.7 will be as fast or faster up to ~60mph where the edge shifts to the 993
2.7RS


993RS


Even easier to the 2.7 will out accelerate the 993 in 1 & 2 but lose from there up.



Looking closely at the 2.7 curve in 4 & 5 w/ & w/o aero you can see where to shift for best acceleration

[RWebb]

I wonder what Dr. Max Thrust would think of this discussion