Using MSD "RPM Activated Switch" instead of MFI speed switch
 

928or911? posted something in the classifieds that jogged my memory of the MSD "RPM Activated Switch."

We know from Warren's reverse-engineering of the MFI speed switch that:

IF (RPM > 1800) AND (Microswitch ON) THEN (Solenoid ON)
which moves the rack and cuts the fuel flow

AND

IF (RPM < 1300) AND (Microswitch ON) THEN (Solenoid OFF)
which releases the rack and starts fuel again

As Warren posted in this thread: http://forums.pelicanparts.com/showthread.php?s=&threadid=160057&highlight=mfi+sp eed+switch

The original circuit uses a "Schmitt Trigger" with different RPM thresholds.

Why couldn't you use an MSD RPM activated switch with an 2000 RPM limit? The fuel would start flowing again below 2000 RPM, which might diminish efficiency somewhat, but it's cheaper than the original part at $46.

Anybody tried this?

John,

Is the RPM adjustable and is the RPM hysteresis adjustable? Sounds like a good idea and has the potential to adjust for better performance. $49 is a bargain.

Best,
Grady

Grady,

Excellent questions. The RPM is adjustable, the hysterisis is NOT.

The MSD is more expensive and uses "pills" to set the RPM. Here's one by Summit Racing that looks better:

"Why couldn't you use an MSD RPM activated switch with an 2000 RPM limit? The fuel would start flowing again below 2000 RPM, which might diminish efficiency somewhat, but it's cheaper than the original part at $46."

You can find rebuilt units for $75 - $85 from Pelican and others. Just ask.
Why go through all the headaches, when you can get an OEM solution?
It's hardly even worth a thread discussion!

John,

It likes like several of these are designed to operate multiple RPM lights on the dash or steering wheel with no hysteresis.

Perhaps some reverse engineering or an e-mail to one of the manufacturers could produce an inexpensive switch with both RPM settings adjustable to provide adjustable hysterisis. Good ideas.

Best,
Grady