Soft Rev Limit & Fuel Cut Off?

FULMNTE · Post by **FULMNTE** » Tue Apr 05, 2011 9:33 pm

Hi,

I am in the process of installing a MJ & EDIS-8 in my Big Block Chevy.

I have a MSD RPM switch left over which I could use to cut off the electric fuel pump at the rev limit.

Anyone see any disadvantages using the regular MJ Soft Rev limit together (100 RPM before rev limit) with a fuel pump cutoff at the rev limit? Anyone done something smilar?

Or could I even use the inverted output feature of the MJ to deactivate a fuel pump relay at a certain RPM?

Post by **brentp** » Tue Apr 05, 2011 9:52 pm

Assuming you're running a carb'd setup, disabling the fuel pump might be a bad option as the cut would not work instantaneously as fuel remains in the carbureator bowl for some time- likely long enough to power the engine through an over-rev condition.

FULMNTE · Post by **FULMNTE** » Tue Apr 05, 2011 9:56 pm

brentp wrote:Assuming you're running a carb'd setup, disabling the fuel pump might be a bad option as the cut would not work instantaneously as fuel remains in the carbureator bowl for some time- likely long enough to power the engine through an over-rev condition.

Thanks Brent,
I should have mentioned that this is for a Throttle Body Injection system, no carb...

Post by **brentp** » Tue Apr 05, 2011 10:49 pm

Ah, got it.

Still, you'd have residual fuel pressure in the lines, which might power the engine into a danger zone.

is the fuel injection system capable of doing a fuel cut at the injector?

You could still do an ignition cut. The plus side is you might get some pretty exhaust flames

FULMNTE · Post by **FULMNTE** » Tue Apr 05, 2011 11:40 pm

brentp wrote:Ah, got it.

Still, you'd have residual fuel pressure in the lines, which might power the engine into a danger zone.

is the fuel injection system capable of doing a fuel cut at the injector?

You could still do an ignition cut. The plus side is you might get some pretty exhaust flames

Thanks!

In that context, short of upgrading to the hard rev limiter, what would happen if one were to use the user output of the MJ to ground the + VR sensor signal to the EDIS at upper rev limit?

Post by **brentp** » Wed Apr 06, 2011 2:52 am

Interfering with the VR sensor signal is an interesting idea and might work to act as a rev limit. It might cause plug mis-firing when interrupted at any random time (e.g. interrupted when the coils were dwelling). Plus there might be a delay in re-acquiring the signal.

However, the user outputs of the Megajolt have a +12V pullup when in the "off" state, so it would interfere with the VR sensor signal regardless of the state.

Spockie-Tech · Post by **Spockie-Tech** » Wed Apr 06, 2011 3:04 am

Use a normally closed relay to turn off the +12v power to the injectors ?

They tend to be wired with a common positive, and the injector signals pull the negative side down, so just break into common the +12v to the injectors and turn that off when the hard-rev-limit is reached ?

FULMNTE · Post by **FULMNTE** » Wed Apr 06, 2011 2:35 pm

Thanks for the replies....

Just bouncing around some ideas.

I think I will try to shut off the injectors at rev limit, seems to be the best way to go...

bobclevenger · Post by **bobclevenger** » Tue Apr 12, 2011 9:26 am

Just thinking out loud here, but why not connect a relay to the coil output wire of the MJLJ (along with the coils, of course) and use this relay to run your fuel pump? You'd have fuel being pumped whenever the coils are powered up, but not when they are shut off (either normally or due to over-rev condition).

FULMNTE · Post by **FULMNTE** » Tue Apr 12, 2011 6:05 pm

bobclevenger wrote:Just thinking out loud here, but why not connect a relay to the coil output wire of the MJLJ (along with the coils, of course) and use this relay to run your fuel pump? You'd have fuel being pumped whenever the coils are powered up, but not when they are shut off (either normally or due to over-rev condition).

I did not know that there is a coil output on the MJLJ without the hard rev limiter installed... Is there?

FULMNTE · Post by **FULMNTE** » Tue Apr 12, 2011 6:06 pm

bobclevenger wrote:Just thinking out loud here, but why not connect a relay to the coil output wire of the MJLJ (along with the coils, of course) and use this relay to run your fuel pump? You'd have fuel being pumped whenever the coils are powered up, but not when they are shut off (either normally or due to over-rev condition).

I did not know that there is a coil output on the MJLJ without the hard rev limiter installed... Is there?

bobclevenger · Post by **bobclevenger** » Tue Apr 12, 2011 7:13 pm

FULMNTE wrote:
bobclevenger wrote:Just thinking out loud here, but why not connect a relay to the coil output wire of the MJLJ (along with the coils, of course) and use this relay to run your fuel pump? You'd have fuel being pumped whenever the coils are powered up, but not when they are shut off (either normally or due to over-rev condition).
I did not know that there is a coil output on the MJLJ without the hard rev limiter installed... Is there?

My error. I was thinking of the hard rev limiter.