Rev limiter recommendations???

[obrut]

I have a 964 distributor I'm modifiying to fit to the 2.7 MFI engine in my RS replica. I'm currently using the stock rotor button rev limiter in my 6 plug distributor - but that type of button will not fit the 964 dizzy.

Can anyone recommend a stand alond rev limiter for this application?

[DinFtown]

If you put any kind of multi spark unit on the system you will be able to set a red line with it. Not sure how replica you are going with it, but you will need some up to date unit to control your rev........ or what do you need a rev limiter for, are you hard of hearing???Just let her spin till she stops giving power, and shift.. lol

[Grady Clay]

I was out at the track today and observed many non-Porsche “rev limiter tests.”

My observation is that it is counter productive to run an engine where the rev-limiter has an effect. A rev-limiter should not be a shift-point indicator, it is a safety device. There are electronic shift point indicators. These are totally different functions.

Yes, there is value in having a rev-limiter preventing a less than competent driver from detonating his (Ferrari) engine. There is also value in forcing the driver to “short shift” with a fresh engine.

A competent driver needs a rev-limiter to protect from a mechanical malfunction.

Best,
Grady

[obrut]

the rev limiter is purely for safety in the event of a missed shift, clutch failure etc - i also run a shift light set a few hundred rpm earlier

what do people use in cars built for historic racing in the US?

[obrut]

bump

i've found a lot of rev limiters for sale but none seem to be compatible with CDI boxes - anyone?

[sketchers356]

Rev limiter wont help you too much for a missed shift

No matter how much ignition you pull if your tires are rotating at 90 mph and you are in first things are going to go bad fast.

[obrut]

rant on -

why does everybody keep mentioning missed shifts!!! we all know when rev limiters work and when they don't. however, they are very useful in many circumstances. that's why porsche fitted them in the 1st place.

- rant off.

[Fred999]

Originally posted by obrut
rant on -

why does everybody keep mentioning missed shifts!!!

- rant off.

Didn't you mention in your previous post
"the rev limiter is purely for safety in the event of a missed shift "

I think that is why sketchers356 mentioned it.

Personally, I don't have that much use for one as my main worry is downshifting to the wrong gear and no rev limiter will help a mechanical overrev.

[obrut]

everyone seems to be missing the point that these are very useful things to have.

1. there are ways to miss a shift that does not include down shifting into too low a gear (ie. put into no gear).
2. a throttle over-rev due to clutch slippage is stopped by a rev limiter.
3. a throttle over-rev due to wheel slippage is stopped by a rev limiter.
4. holding a specific gear at redline to avoid an extra shift is made easier by a rev limiter.
etc etc etc

all i was asking for was a recommendation for a stand alone unit that will work with a CDI box - not a list of situations in which they are not effective...

[Jack Olsen]

IMO, you definitely should not drive a 911 aggressively without a rev limiter.

[Early_S_Man]

Ryan,

How about using the USA-spec Rev-limiter that uses the 'TD' signal from the 6-pin CDI unit, shown in the diagram below?

By emperically testing on a distributor machine or on a test bench using a variable ramp or function generator ... the factory-selected resistor[s] could be changed to recalibrate to a higher rev limit. A different method could be utilized to operate as a rev-limiter if a high-speed relay was used to interrupt the CDI 'A' signal to the coil[s] within 10 ms.

[Elombard]

Wow you amaze me Warren. I wonder if a switch could be purchased inexpensively from a wrecker (or new) and wired in easily? I would love to do this!

[obrut]

Warren

Thanks: however, this is why did studied ME not EE at university, all but the simplest of circuit diagrams stump me.

So, if I understand it correctly: the SC's used a CDI based rev limiter with a resistor to set the rpm, so replacing the limitor (perhaps with a variable pot?) allows you to alter the rev limit. Am I on the right track?

[Early_S_Man]

Ryan,

Yes, that is what I am suggesting ...

You could use a precision 10-turn or 20-turn trimpot, or for a really elegant solution, a helipot with a Decadial display/knob. in any case, after modification you could set the rev limiter on a home test bench using appropriate test equipment, in a shop on a distributor machine, or in the car while monitoring rpms with a digital Dwell/Tach.

[obrut]

el bumpo

thanks to a little early xmas present, my twin plug conversion is finally going forward in a period style with a single bosch 12 plug distributor and 2 (3pin) CDI boxes.

does anyone know of a rev limiter compatible with this setup?

[Early_S_Man]

Ryan,

Do you have any friends or co-workers good with electronics ... capable of building a small device on a prototype board with one IC chip, one relay, and a handfull of other components?

If so, have him/her look at the datasheet and application note AN-162 for the LM2917 chip here:

http://www.national.com/pf/LM/LM2917.html

The applications, including a rev-limiter, were discussed in the following thread

$1.49 LM2917 IC does tachometer, MFI Speed Switch, rev limiter, or shift light ...

[obrut]

thanks warren - but i don't think i would be comfortable running a home made device of this type

do you have any idea what system, if any, the early porsche race cars used? or why all the modern systems exclude CDI?

[obrut]

warren

this crane unit looks to be CDI compatible

http://www.cranecams.com/?show=browseParts&lvl=4&prt=92&action=partSpec&partNumber=9000-0005

it looks like it connects to the points output - any thoughts/comments?

[Early_S_Man]

Ryan,

Yes, it looks like it would work just fine!

Here is the installation manual:

http://www.cranecams.com/pdf/90009518.pdf

It probably has three LM2917 chips inside ...

[HawgRyder]

Warren...I like the idea you had for the rev limiter.
I wonder if you could install 2 with power to one switched to provide a "valet" position of say 2500 rpm?
This would protect your investment from spirited parking attendants.
Bob