very crude rev limiter on 7747 ecm.....

[diver14_98]

I was thinking of using the shift light pin to activate a solid state relay that would control the power to the coil. Can you guys think of a reason this wouldn't work.

[1project2many]

It would work but might not be smooth or nice. Potential problems include point corrosion causing poor/no-start, wider than desired RPM swings, rpm limit lower than desired, unexpected / unknown variables causing inadvertent light operation.

[scottyd]

Reminds me of when we were kids we'd run an engine up to speed shut off the switch let it spin down and load up fuel and turn the switch on causing a huge back fire POP... this is essentially what you'd be doing.. I never really hurt an engine doing that as a kid but the older much more wiser guys at the time told me that it's not exactly a good idea.... Most soft touch rev limiters hit individual cylinders at intervals to kinda avoid that. So you'd have yourself a hard touch rev limiter. Does the 7747 not already have rev limiting?

[diver14_98]

no it does not. and I know its not going to be pretty when I hit it but its better than blowing up my motor if I pop a u joint try to climb a hill

[diver14_98]

rather than letting it load up I could cut the power to the injectors

[Fast355]

I was thinking of using the shift light pin to activate a solid state relay that would control the power to the coil. Can you guys think of a reason this wouldn't work.

Repin to a 7427, fix the lack of a rev limiter and gain a better PCM in one move.

[JeepsAndGuns]

I think personally I would rather cut power to the injectors. But instead of doing that, I agree with fast355. The 7427 swap is not that hard and you gain so much more tuneability. You also will get a rev limiter, and faster data logging. When I swapped to the 7427 I never looked back.

[RobertISaar]

I think personally I would rather cut power to the injectors.

I'm not sure I would recommend it based on if you cut the injector pulse while an injector is spraying, you're going to get an engine cycle that is going to do some really funny things, could either be not enough fuel to light off or enough fuel to light, but could be a super lean mixture.

cutting the coil power could possibly be worse.... if you cut power while it is in dwell, it's going to spark.... either on the previous cylinder very late into the combustion process or super early into the next one.

if you could monitor if the circuits are active before cutting power, that would solve both of those problems. depending on where the code lays, you might be able to write in a very small patch to quit delivering fuel during engine cycles where engine speed is above x RPM.

but honestly..... i'll never recommend a C3 where a P4/P6/P66 can be used.

[diver14_98]

looks like its time to upgrade ecms

[JeepsAndGuns]

I bet you will not regret it. I dont think I have seen where anyone swapped to a 7427 from a 7747 and wanted to go back.
I will warn you, there is a slightly steeper learning curve as there is probably twice as many parameters. So get a bin file, XDF and just spend some time looking through it in tuner pro and get yourself famillar with it before the actual swap takes place. If you do that, it will help make the swap go easyer.
With the change to the 7427, you will open yourself up to being able to run a E-trans if you want. The ability to have wide band o2 sensor data in the datastream, not to mention hacks/patches to be able to control a electric cooling fan, or PFI.

[delcowizzid]

we used to use tachos with adjustable shift lights for rpm limiting on carby dirt speedway cars just cutting the coil power with a relay activated by the tacho works mint.dont do it off the earth or you can do engine and exhaust damage LOL.if you isolate the coil power only the ignition module should still run the ecu fine without spitting the dummy.if it doesnt cut and return fast enough for your liking a solid state relay might switch faster if you can find one that will handle the amps required

[1project2many]

no it does not. and I know its not going to be pretty when I hit it but its better than blowing up my motor if I pop a u joint try to climb a hill

I have some code written to limit rpm in a 7747 based on speed. I believe it could be rewritten for an RPM variable.

[diver14_98]

im clueless when it comes to moding the code

[scottyd]

I think I'd rather kill fuel than fire, but as pointed out depending on circumstances leaning out could be an issue.

Just a theory, but I wonder if you can't power the coil with two alternative sources, a 12 volt and a much weaker voltage say 4-6 volt. Have a switching relay setup that when your shift light is triggered it kills the 12 volt source but activates the lower volt source. My thinking is that it should still fire to prevent loading, but with much less voltage maybe even enough to keep the engine from raising RPMS. Another theory..lol, I have lots.. is maybe using an flasher relay (turn signals) or similar that quickly switches on/off coil voltage during times when the shift light is active.

[Six_Shooter]

Sounds like it's time to add a 6AL, and have a proven limiter.

[Fast355]

Sounds like it's time to add a 6AL, and have a proven limiter.

Stock RPM/Speed limiters kill fuel so I don't see killing fuel as overly harmful.

[Six_Shooter]

Stock RPM/Speed limiters kill fuel so I don't see killing fuel as overly harmful.

Nothing wrong with killing fuel, provided it's the ECM doing it, so that the entire injection event is killed and not cut off part way through an injection event causing a lean condition.

My suggestion was based more on the "add this" and "add that" posts and ideas that were starting to creep into the discussion. The 6AL is a proven and reliable soft touch rev limiter that prevents fuel loading up, due to the way the cylinders are dropped out.

[lionelhutz]

You can't ensure the fuel is present at the right ratio for one combustion cycle and then cut-off the next when using a TBI, no matter how good the PCM is.

[Fast355]

You can't ensure the fuel is present at the right ratio for one combustion cycle and then cut-off the next when using a TBI, no matter how good the PCM is.

^^^^