Anyone heard of a rev limiting rotor arm for >7300rpm ?

[amallagh]

Anyone heard of a rev limiting rotor arm for >7300rpm ?

[Lightweight_911]

Yes - fitted to 911S & 2.7RS models - part no 911.602.929.00

Sorry Andrew - just re-read your question & see you're asking for something with a greater rev limit - not familiar with a suitable rotor arm

[majordad]

There is some method to remove the spring in the rotor arm that allows it to rev un controlled, not as mad as it seems as you can easily watch the revs going up and coming down you could over rev it with ot without the limiter.

[amallagh]

Yes - fitted to 911S & 2.7RS models - part no 911.602.929.00

Sorry Andrew - just re-read your question & see you're asking for something with a greater rev limit - not familiar with a suitable rotor arm

I suppose I could shave a bit of weight off the end of the free end of the shuttle mechanism ?

[fetuhoe]

Just fit a rotor without the cut out.

Rev limiting rotors are not much use as they don't work when you either miss a downshift or shift into low gear down hill or other situations where inertia is dominant.

[Gary71]

Just don't add a blob of Araldite after qualifying or one of the other teams will report you (and get you disqualified) as the only car that isn't briefly hitting the limiter between two second gear bends in a single make championship...

Not that that happened of course. Irony is we would have won the race anyway, it just felt better not sitting on the limiter .

[Steve Warson]

Just don't add a blob of Araldite after qualifying or one of the other teams will report you (and get you disqualified) as the only car that isn't briefly hitting the limiter between two second gear bends in a single make championship...

Not that that happened of course. Irony is we would have won the race anyway, it just felt better not sitting on the limiter .

[gridgway]

Just fit a rotor without the cut out.

Rev limiting rotors are not much use as they don't work when you either miss a downshift or shift into low gear down hill or other situations where inertia is dominant.

There's a big difference to limiting the revs on acceleration to a mechanical over-rev. Just because you can do (and can't stop) the latter, doesn't mean you want a free for all on the former.

[Jonny Hart]

Our CDI+ boxes have two rev limiters. The 'soft' limiter backs off the advance, and the 'hard' limiter cuts the ignition. The soft feels like the engine power just been turned down (like you took you foot off the throttle a bit). The hard cyclically cuts the ignition but still fires the odd cyclinder to prevent fuel build up and backfiring. The two combined are a much nicer experience than being thrown through the windscreen with a mechanical rotor arm limiter.

Say you want to protect above 7500. We typically set the soft limiter to 7250 and hard to 7500. You can 'push through' the soft one but it slows the engine enough for you to feel it. Both are configurable naturally.

[fetuhoe]

There's a big difference to limiting the revs on acceleration to a mechanical over-rev. Just because you can do (and can't stop) the latter, doesn't mean you want a free for all on the former.

I have found that in too many years of driving Rally cars the only issues with over-revving have been on downshifts.

If you consider the rate at which an engine will 'spin up' when connected to the mass of the car, even in first gear, it is a relatively slow process in a typical road based car and should be relatively easy to avoid. On occasion it may even be preferable to lightly 'buzz' the engine to keep control in awkward situations.

Mechanical rev limiters, which were the thrust of the question, are notoriously unreliable and often problematic which is why I would never fit one to a completion car. The 'cut' is also quite brutal and at very high revs on a competition engine can become damaging.

'Soft Cut' systems are much more sensible but again need to be set at least a few hundred revs beyond the ideal gear shift point to allow a little scope for the occasional adventure.

Fitting Electronic Ignition to cars that are being used in FIA Historic Competitions is more problematic and Appendix K states the following:

Cars of Period F homologated with electronic ignition systems
and cars of Periods G1 and G2 may use a non-period electronic
ignition system provided this system is triggered by contact
breaker(s), utilises an ignition coil with a minimum resistance of
3 ohms, the spark is distributed by a rotor arm and the timing
of the spark is controlled entirely by mechanical means. Multispark
systems and systems where the timing of the spark is altered
electronically are not permitted

A Porsche 911 complying with Period F was never fitted with Electronic Ignition so if a modern system was used it would not comply with these regs.

It is even an issue for G1 and G2 but I imagine that some competitors will look the other way and run the risk of having a query placed on their HTP.

[Jonny Hart]

Cars of Period F homologated with electronic ignition systems
and cars of Periods G1 and G2 may use a non-period electronic
ignition system provided this system is triggered by contact
breaker(s), utilises an ignition coil with a minimum resistance of
3 ohms, the spark is distributed by a rotor arm and the timing
of the spark is controlled entirely by mechanical means. Multispark
systems and systems where the timing of the spark is altered
electronically are not permitted

Very interesting. I read this as an acknowledgement that multi-spark gives an unfair advantage e.g. more power. That is exactly what we have found with the CDI+ system on the dyno and in the customer feedback we have received. Yet there are still plenty of naysayers?

Regarding the mapping, our boxes can be used as a development tool to tune the advance curve. Touch of a button to change the map, instead of remove, strip, recurve, rebuild, retest. Once the best curve is found, the curve is recreated conventionally using weights/springs and a standard CDI box is used.

[fetuhoe]

The only issue I would have with changing an ignition is trying to comply with FIA Regs.

Advance Curves are much more interesting and I would agree that programmable systems are helpful particularly in modified road cars.

There was an interesting paper presented by Lucas some years ago when they developing the OPUS System used on the FVA and eventually the DFV and BDA engines.

I have looked for a copy of the paper which I read too many years ago but have failed to find.

The basic argument was that as these engines were rarely used below 5000rpm they didn't need an advance curve and were designed to run a 'fixed' advance.

They were used for a number of years on Mk2 Escorts and seemed to perform well.

I would be keen to know what the impact of changing an advance curve would be on a 2.0 litre FIA spec engine fitted with 906 or similar cam when used between 5000 and 8500rpm.