Why do T, E and S engines have different redlines?

[BK911]

You gentlemen were most impressive answering my last question. Ready for another?

On the T, E and S motors, what defines the redline? The bottom end is essentially the same except for the non-counter weighted crank in the early T's. The top end uses the same valve train. Besides the obvious differences such as the cams and compression ratio, the only difference I can think of is the installation height of the springs. Can you install T springs to S spec and increase redline? Or does the cam lift dictate the installation height, which in turn sets the redline? Or am I way off?

Thanks!
BK

[scca_ita]

Because of the different cam profiles. T - mild cam, E - more aggressive than T, and S - yet more aggressive than E. In which case the power and torque peaks are higher in the RPM band.

[CamB]

To put the question another way, is it safe to rev the bollox off your T or E right up to the S redline (regardless of whether the engine is continuing to make power).

[turbo dave]

Doesn't the T have cast pistons, which would be heavier & weaker than the forged pistons of the S (and E?)? Plus, I think(?) that the T only shared connecting rods & crankshaft with the S & E on certain later years, maybe 2.4 only.

[jluetjen]

I would not consider it "safe" to rev a T repeated past it's redline, and the same would apply to an E or an S. Porsche was/is very good about doing all of the detail work to make their engines into reliable power units. The E and the S have a number of features which allow them to rev higher. These higher rev's allowed Porsche to use cam's which deveped their peak HP at higher engine speeds which resulted in more HP. To be honest, compared to American engines, 911 motors are overbuilt with "excess" safety margins for things like over rev'ing. But the ability to finish races has always been a Porsche hallmark. You know the old addage, in order to finish first, first you must finish .

The differences that I'm aware of excluding the camshafts which I've mentioned above are listed below. Note that these varied from year to year.

1) Counterweighted crank shafts on the faster spinning models to ensure that loads on the crank shaft bearings stay reasonable at higher rev's. An added benefit is that it also helps to keep the motor vibrations down which means longer life for just about everything attached to the motor. The longer stroke (2.4) T's got the same counterweighted crankshafts as the E's and S's, but then the 70.4mm stroke cranks had a tendency to loosen flywheel bolts at higher rev's due to the higher forces involved with the longer stroke. So I'm sure they considered it insurance to apply the counterweighted design to the T.

2) Different heads on the S. In order to spin as fast as it does, S heads have larger ports to allow air to flow in and out of the cylinders at a rate consistant with the engine speed. In some cases the valves of the S heads were larger too. Carb'd S's also have larger chokes in the carbs for the same reason. The same applies to the throttle bodies on MFI'd S's.

3) The earliest T's used only single valve springs rather then duel valve springs. Since T's did need to rev as high, they didn't need the benefits of duel springs to keep valve motions in check. Along with being a cost saving measure, single springs' reduced pressure on the cam reducing the amount of internal friction in the engine which can actually increase the HP a little bit in the rev range with T's run. E's and S's have duel valve springs for at least 3 reasons: A) The two springs can provide increased pressure which helps to keep the follower on the cam at higher rev's, B) The two springs have different frequencies which tend to cancel each other out which also helps to keep the followers on the cam and C) Many duel spring designs have an interference fit between the two springs - the friction which results tends to dampen spring movements which also helps to prevent valve float at higher revs.

4) S connecting rods often had a surface treatment (nitrating) which improves the fatigue properties of the metal. This is important since at an S's redline of 7500 RPM, the piston speeds are about 3300 FPM, while at a T's red line the pistons speeds are about 2900 RPM. Keep in mind that the forces involved in accelerating something increase exponentially with the speed.

5) Forged pistons were used in the S, which are stronger then the cast pieces in the T and E. Forged pistons are denser then cast and consequently have a higher tensile strength. So they can withstand higher pressure and heat loads then a comparable cast piston.

Will a T engine break something the first time that you rev it to 7500 RPM? I doubt it. The second or the third? Most likely not. But are you feeling lucky????

Ultimately you don't gain anything by rev'ing a T past its redline since the HP drops off pretty quickly above the redline. This means that the car will accelerate faster at lower rev's in the next higher gear then you will by overreving a T. So in the end -- why bother?

[724doorE]

Nice list John

I belive that the early 2.0, 2.2 T cranks were cast steel instead of forged as the E and S cranks were. Also belive the S cranks themselves were nitrided to help reduce fatige crack initiation and crack growth thereby giving a longer service life at higher fatigue loading (RPMS).

They are all very tough engines though!

[724doorE]

Oh and maybe most importantly, if they (P) listed the redline spec for all three as the same RPM, even if the power were different, some S owners might not feel as "special" for their extra dollars spent!

maybe, maybe not!

[BK911]

Gentlemen!

Thanks for all the info. I had a feeling John would reply to this one!

I still don't understand why they had different spring installation heights. Why not have them all at the "best" height for added insurance?

BK

[jluetjen]

I had a feeling John would reply to this one!

Oh -- am I getting that predictable?

I checked my spec book (for '69's - 71's) and found the following data which I think that you are referring to:

Spring Height:
T: 36 I / 36 E
E: 35.5 I / 35.0 E
S: 35.5 I / 34.5 E
My guess: The E's are a little lower then the T because doing this increases the the pressure required to open the valve. The increased pressure may be needed to support the higher rev's.

The S's exhaust is lower (higher pressure still) still because it is a silicon filled valve. I'm not sure why this would make a difference - possible because it expands more???

Am I close?

[BK911]

Sorry John, I don't know if you are close or not. This wasn't a quiz. I don't know the answer! Sounds reasonable, but what do I know!

Initially I thought the lower spring height would provide more tension allowing higher RPM's without float. So if you installed the E springs to S spec you should increase the redline of an otherwise stock E.

This was purely for educational reasons. I don't plan on gambling with any of my engines!

BK

[TimT]

Another reason besides the non-counterweight crank, cast pistons etc. "S" rods were given a specials stress relieving treatment that "T" rods didnt receive.

im sure you could safely rev higher, but you wouldnt see anymore power. The "T" cams along with the smaller ports kinda run out of breath after the redline. You can spin the engine higher but it wll be past the HP and Torque peaks.

I have pauter rods, a "S" crank, ported heads and a webcam regrind in my "T"

I love the sound at 7500RPM