Max engine RPM

[pope]

What is the max rpm my engine can go to? Where should the cut off be?

[Formerly Steve Wilkinson]

Assuming you mean a 1970 911E, there's not much point in going any higher than 6,200 rpm, which is the hp peak. (Torque peak is 4,500 rpm.) I don't know for sure, but I would assume that redline is 6,500 rpm.

If by "cutoff" you mean the most you should ever turn the engine, I'd say 6,500. If by "cutoff" you mean some kind of mechanical or electronic rev limiter, I do hope you know there's no such thing on your car: that engine will rev until it swallows a valve, or whatever, if you don't back off the throttle. But I'm confident you already know that.

[pope]

Yes, I meant a rev limiter of some sort.

Basically:
- I've changed the distributor cap and the new one (911E specs) was advertised as rev-limiting to 7100.
- The red line on my tach is at 6900.
- I'm thinking of getting an MSD ignition.

I know that there's no need to go above 6500 but I'm wondering, is my tach red line correct or is the one they installed after (because I don't think it's the original) from another engine. Which set of 'pills' would I be looking at for the MSD rev limiter?

[Formerly Steve Wilkinson]

That's interesting. I don't know how that works--a rev-limiting distributor cap, never heard of one. I can't imagine how a distributor cap alone, which has no moving parts, would limit revs, but what do I know?

I don't know whether you have the correct tach or not. I'm pretty conservative; my modified twin-plug engine has 7,500-rpm redline, but I rarely go over 6,000.

I made MSD ignition with two rev-limiter pills, as you call them: one set of plugs goes off at 7,250, the other at 7,500, for "soft" shutdown. Never used 'em, though.

[silverc4s]

Pope,
I think the rev limiter you speak of is actually in the rotor, inside the cap. It was like that on my old Marelli, and I believe the Bosch rotors in that era had the same sprong load against centrifugal force type of limiter. They do work okay with standard ignition, but are not easy to change if you want a different than stock rev limit.

[silverc4s]

Ignition Rotors
For the early cars there were two types of ignition rotors available, one with a rev limiter, and one without. The rev limiter protects your engine from overzealous drivers. A small floating weight on the rotor grounds out the coil at the RPM limit preventing valve train and other engine damage. In 1978 and later, rev-limiting is controlled by the CD-box.


911-602-925-00-M14 Rev-Limiting Rotor, 7100 RPM, 911 All Except 911S (1965-69), 911E (thru 1973), Each
Brand: Bosch [Photo] [Photo]

$18.50

911-602-929-00-M14 Rev-Limiting Rotor, 7300 RPM, 911S (1967) and (1969-73), Each
Brand: Bosch [Photo] [Photo]

$18.50

911-602-928-00-M14 Rev-Limiting Rotor, 6500 RPM, 911T (1970-73), Each
Brand: Bosch [Photo] [Photo]

$18.50

[copbait73]

Yes, if you have a standard rotor in the distributor it has a rev limiter.

[pope]

Of course I meant rotor. Not cap What was I thinking?


So, does

rotor rev limit = MSD 'pill' limit = tach red line ?

[Jeff Higgins]

MSD chips (for the 6AL, or "adjustable limit" box) only come in 200 rpm increments. Your MSD will come with a 7,000 rpm chip (along with several others). Just use the 7,000 rpm chip; a 2.2 liter "E" motor will happily rev to 7,000 with no ill affect whatsoever.

When trying to make your "best pace", it pays to rev the motor past its horsepower peak. Say you lose 1,600 rpm on a shift. You probably want to go half of that, or 800 rpm, past peak horsepower. So let's go with the 2.2 E's peak hp at 6,200; shift at 7,000, dropping to 5,400 to start the pull in the next gear. You'll pull harder through more of each gear ratio that way. Of course you won't do this in normal day to day driving; just when you feel the need to make some time. That said, these smaller early motors like to rev. Anything under 3,000 rpm I would consider "lugging" it, and will be far more harmfull than spinning it up a bit.

When you get the MSD 6AL, just ditch the rev limiting rotor. They are more trouble than they are worth. A standard VW Beetle rotor (the later tall one) is a perfect fit and has no moving parts to fail, unlike the fancy rev limit rotor.

[pope]

OK Jeff, thanks! I won't purchase any of the pills then.

Do you by any chance know where my red line should be on the tach? I know it is of little importance, I'm just curious.

[laughac]

I had a rev-limiting rotor for awhile. Centrifugal force pushed against a spring and disconnected the circuit at some point around 6500 RPMs. It was the cause of some misfires, however.

Now I use an MSD 6AL ignition amplification system that includes the rev-limiting feature mentioned by Jeff Higgins above. I have a recently rebuilt 2.7 engine and have set the limit at a conservative 6500 RPMs to match the specs recommended by the rotor manufacturer. I usually only hit the limit when the tires break loose in 1st gear, but maybe I should increase this.

[Jeff Higgins]

I'm not sure, pope, but 6,900 sounds about right for a 2.2 E. My 2.4 T tach shows 6,600, and I seem to recall early S tachs are 7,200 or 7,300 red lines.

Sometimes Porsche's rev limits seem a bit arbitrary. Or at least based more upon the motors' powerband than on its mechanical limits. For example, the 2.2 T had a cast, non counterweighted crank. One would think it should not be revved as high as the forged, counterweighted crank of the E and the S. Yet vintage racers lust after the early T cranks and rev the bejeebers out of them. The later 2.4 T had the same forged, counterweighted crank as the E and S, but still held to the lower rev limit of its non-counterweighted 2.2 predecessor. This, with identical valves, valve springs, and rocker arms as the E and S, but less lift and duration in the cams (stressing the valve train less) than the E and S. The rods in the T and E are identical, but the S rods were nitrided. That doesn't seem to do anything for them at the rpm's seen in a non-race motor; I think it was a homologation move. Same with the forged pistons in the S, where the T and E made do with cast pistons. So one would think the mechanical rev limit of all of these would be roughly the same, and it essentially is.

Sales had something to do with it as well. Pretty handy for a salesman to be able to show the increased performance (and cost) models with increasing redlines. Wonderful sales tool...