Lotus Elan

Cutout rotors

PostPost by: benymazz » Tue Nov 12, 2019 8:13 pm


Had an interesting thought today. For those of you that go to car meets where modern cars are present, I'm sure you've heard of a "2-step" rev limiter. Typically used when launching, it partially cuts the ignition rather than the fuel (and possibly also significantly retards the timing) for two purposes: quicker "recovery" after the launch begins, and also to spool up the turbo by effectively dumping raw gas into the exhaust which ignites and gets the turbo spooled. The latter results in a deafening popping and more often than not flames out of the exhaust.

Our old "analog" cutout rotors do the same thing albeit with the intent of preventing destruction rather than facilitating it :roll: So my question is, when the rotor cuts out is there any popping from the unburnt gas igniting in the exhaust? Or does it go into a sort of harmonic motion where the rotor cuts out and then the revs drop, the rotors comes back in and the rpms go back up, and so on.

Since I haven't read anything about cutout rotors having this effect I would suspect that it's relatively tame and there's no popping or flame shooting :lol: . This is also somewhat supported by what I've read which states that the rotor basically exists in two states - cut out or cut in - and that the transition period between the two is extremely brief.

Asking because my engine is set up with a solid non-speed limiting rotor so I can't test this.

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PostPost by: rgh0 » Wed Nov 13, 2019 11:11 am

Done a bit of playing with this. I found the original cut out rotors gave a very hard cut and the engine died until the car slowed and the rotor started working again. (A large dead band in process control terms if your into the details of that)

In my road cars I just rely on paying attention to the revs and have removed the cut out rotor. In my race Elan I have a soft rev limiter that progressively cuts more and more cylinders ignition cycles to keep the rev limit I have set at 8300 rpm and this works well to stop me over revving in the heat of the moment while not suddenly upsetting the cars balance with power loss at a critical moment

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