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

[304065]

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: MFI Fuel Shut off solenoid questions

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?

[Grady Clay]

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

[304065]

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:

Our RPM activated switch uses internal DIP switches to select the rpm setting instead of expensive pills or modules which can get lost or vibrate loose. Our switch also has an adjusting knob inside the unit that lets you fine-tune to any rpm from 2,000 to 9,800 rpm. A green LED light lets you know when the desired rpm is reached--no need to turn on a connected device to see if the switch is working. We also include a red LED power indicator, an internal fuse, and extensive input protection circuitry to keep the switch from blowing up if you connect it to something that draws too much current.

[Lorenfb]

"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!

[Grady Clay]

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