MFI "speed switch" control from CDI+

As you know, the MFI pump has a solenoid which operates a fuel cut off. The principle is that, on a trailing throttle at higher revs, the unit stops pumping fuel into the engine. Anyone who has followed an MFI car on which it's not working (fairly common fault) will recognise the bangs, pops and occasional flames. Quite cool, but not especially good for the engine.

One component of the system is the "speed switch", which detects when the revs are above approx 1500 rpm. Above that level, when the throttle closes (detected by a microswitch), the MFI solenoid should energise and the fuel shut off. On the way back down the revs, below 1300 rpm the solenoid needs to de-energise to resume fuelling and prevent the engine from stalling.

The CDI+ unit from Classic Retrofit offers a (programmable) replacement for the speed switch.

There's a wire coming out of the CDI+ box:

Porsche CDI speed switch control

Which needs to be connected to the microswitch + solenoid via a 40A relay, thus:


40A relays are readily available. Mine is a little grey Hella one which should really live in a Skoda:


I know what you're thinking, that looks crap and why did you choose that ugly relay? Answer: because it fits into an old speed switch case. Stealthy:


Output programmable via the CDI+ software:


I've tested the microswitch and solenoid, all fine, so I reckon we should be in good shape for working fuel cut-off.

It's all plug and play, no butchering of the loom required, so could easily revert. Thanks to Jonny (DDK-er and owner of Classic Retrofit who make the CDI+) for the help and exemplary technical support.

And that, in a nutshell, is the kind of mod I'm doing. Criteria:

  • Easily reversible
  • Makes the car work better (higher performance or more reliable), or look cooler
  • Does not change the character of the car. A 2.2S is fine for me. In this case the way it feels (apart from the soft limiter) should be exactly the same as an MFI car on which the CDI, coil, distributor and speed switch are all working perfectly.
  • Aesthetically hard to detect, or deliberately different in a way that I think looks better.