How good is the stock rotor with the rev limiter in it? Is it reliable, does do what it is supposed when it is supposed to consistently?
I am currently running a non-rev limited rotor, but found myself wishing for some sort of limiter at the last autocross as the valves were floating!

Get the rev-limiting rotor. It's only like $20 which is dirt cheap considering it may save your engine. It is reliable and consistent and works well. It works by grounding out the voltage from the coil, which isn't the best method, but your other alternative is to spend 2 or 3 hundred for a new ignition system which has the ability to do rev limiting. I had a non rev limiting rotor when I first started autocrossing 6 years ago. I remember looking down once at my tach and seeing 6500rpm. I immediately bought the rev limiting rotor and have used it ever since.

Demick

There are positives and negatives about the rev limiter rotors. Rev limiting rotors work by cutting the spark, but the fuel is still being pumped into the engine so when spark comes back at a lower RPM there can be large bangs from the build up. Another point is that most over revs occur on down shifts where the wrong gear is selected in the heat of the moment so they won't help at all in that case. Many racers and hard core auto-x drivers don't use them instead using a shift light to help with up shifts and getting lots of practice to help with the down shifts. But if you are starting out and one will make you feel more comfortable then it is a good idea to use one. Good luck.

John and Demick, thanks for your replies. My concern with limiting the revs comes from the faster portions of an AX course. I had a section last week where I probably needed third gear, but for all of about 200 feet. My thought is that I probably wasted more time going up to third and back down to second than I would have if I just kept it pegged at red line, and with so much else going on, I don't want to have to limit the revs myself, rather let some device do it.
Interesting point about the fuel continuing to flow, John. So, the limiting rotor just shuts down the spark completely untill centrifigual force slows down enough for it to contact the cap again - how does some of the other ignitions do it? Also, is there any problem with that rev limiting rotor and a Pertronix?

Jdogg

Your reasoning is exactly how I 'use' my rev limiter. In autocross, I often (nearly every event) find myself at the top end of 2nd gear, but in a place where it doesn't make sense to go to 3rd and then immediately back to 2nd. And like you say, there is too much going on (and too much adrenaline) to try and limit your rpm's yourself. In these cases, I just ride the rev limiter until it's time to hit the brakes. Adding a shift light will not help. I have no problem knowing when to shift, I am simply choosing not to shift.

I've never had a problem with backfires or 'bangs' from the fuel build-up. Any rev limiting system that you will add to your ignition system will limit the revs by cutting the spark. To do it any other way would require an entire engine management system (one that can control timing and/or fuel flow).

A rev limiting system is well worth the money - even if it costs a few hundred bucks. But the $20 solution works just fine if that meets your needs. It will work fine with the Pertronix system assuming you already have the pertronix. If you are planning to buy a Pertronix and a rev limiting rotor, then I would suggest looking into an ignition system that will provide the rev limiting electronically (MSD, Crane Cams). I cannot recommend a system, but others on this list can.

Demick

Demick-
I do have the pertronix already, so the rev limiting rotor it is.

No problems running them with a Pertronix. I find it helps to have one at the AX kinda like a warning buzzer, if it is on alot I need to shift more. If you are at those high rev's you aren't in the powerband. There is little hp/torque in a 4cyl engine at 6500rpms. The powerband tops out at 5000 rpms so thats where you want to hang out so shifting to 3rd will help on those high speed sections by keeping in hp/torque band (3500-5000). Learn to be smooth shifting and that will help going thru those sections. As posted in the other thread, at first try to learn the course and where to shift up and down smoothly, then go faster.
Geoff

Geoff is quite correct about shifting and being in the "power band". I have noticed in San Diego that the really fast cars seem to "short shift" or shift so they are always in the begining of the power band and not winding past it. The exceptions are the very high reving 6's that have less torque than the fours at lower RPMs so it makes sense to rev them much higher. As an example on my race car the torque peaks at 3200 RPM and stays pretty flat until 5100 RPM so I try to get RPMs in that range all the time in the vintage races I do. I find many times that if I come out of a corner at 4000 RPM following a 2L six cylinder 911 I have to feather the throttle or I run up their back bumper really fast. As I said, try one to see if you like it, good luck.

I like the REV limit rotors.. but its hard to beat the Smooth Touch REV control and Multi spark that the MSD's offer. SummitRacing sells them for 179.00. You can pick and choose the rpm limit with the little chips. They cut out every other cylinder to rev limit instead of letting fuel build up.

Something that wasnt brought up: When the factory rotor cuts off and you lose all power... wouldnt this upset the car ?? Like lifting throttle. I know the MSD is smooth enough that this doesnt happen.


Oh.. I think the MSD is worth the 179.00. It helps with throttle response also...

B

It does upset the car somewhat when you hit the limiter, but if you're really close enough to the edge then any reduction in power will transfer weight off the rear wheels and spin you...

My car was running very rich, and it banged and popped like an Oakland drive-by when I hit the limiter. It didn't use to do that when the mixture was more reasonable.

--DD

With the rev limiting rotor, it doesn't cut the engine completely and doesn't upset the car (since it is not making any power at the red line, there isn't much to upset). It just feels like the engine is mis-firing. It is probably firing randomly on about half the cylinders. I truthfully doubt that it feels much different than the MSD setup.

Demick

I think that depends on 2 things, how fast the spring
reacts and what gear you are in. I don't have any AX
experience with the rev-limit rotor, but in the electronic
rev-limit box I made it seems to operate so fast that
there is no feeling of losing power. This is pretty much
the only electronic control situation I can think of where
you want almost no hysteresis.
Mark S.
'70 914-6

Is there a rev limiting rotor or other system for the 3.2?

Douglas

Douglas:

Avoid floating the valves at all costs. If I have it right, you have a 3.2L? The 3.2L is particularly sensetive to problem$$$$ if over reved. Rod bolts are a weak point.

Is it a EFI engine? I thought the DME had a built in rev limiter.....but I've been wrong before.

Generally, when vavles "float" the springs can overheat to the point where they can loose their temper....and that's hot. As a result, the springs are less effective and will float at a lower rpm.

Piston/valve contact is disaster, again particularly with CIS shaped pistons (not sure about EFI) as the valves don't hit the pistons square. Bending the valve is bad enuff, breaking off the valve head is gawd awful.

As to rev limiters. I run a 7300 rpm rotor and a 7100 or 7300 rpm chip in the MDS. The MSD is a bit "softer" when it hits.....you get this ....tunge,tunge,tunge noise and it goes kinda flat
as the motor is no longer pulling. Feather the throttle or up shift. I've spent some time there.

Selecting shift points is all part of leaning an AX course. Short shifting at an opportune time is a gud plan. 200 feet in third gear will get ya moving pretty damn fast, 300 feet via a short shift will get ya hauling ass...and you better do it, cause the competition will.

The rev limiting rotors are a good line of defense to stop from over revving your engine BUT it does have its downsides. Massive unloading then loading is not good for the rod bearings as it causes babbiting (sp?) of the bearing surface. Babbitt appears like an alligator skin type of wear.

If you’re using the rotor to protect yourself from blown shifts, etc. then it is a good deal. BUT if you are going to be using the rev limiting rotor as a type of shift light please give me a call because I want your engine building contract.

The soft touch type is better for the engine if you plan to hit the limiter all the time. The best way is to use a shift light and to try not to hit the limiter at all.

BTW my experience is with the Type 1 beetle engines, street and drag racing. I’ve built hundreds of engines over the years.