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Home Made Rev-limiter For Carby Car


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Have had this theory for a while now....


Most rev-limiters work by cutting power to the coil temporarly.

A convention change over relay can be used to 2 main ways: To power something when triggered, or to cut power to something when triggered. This is as simple as wiring the item to be powered to pin 87a rather than 87.

So......by doing this you can easily cut the power from your coil when triggered.


Next bit is the trigger!!!

Many tacho's are avaiable with shift lights, and these can come on at basically any desired RPM.

By patching into the wires for the shift light, you can get a your relay trigger at any RPM!

This is easy with any conventional 12v bulb, but if using an LED shift light the lower voltage may not be enough to trigger the relay.


So......wire the shift light to the trigger of the realy - pin 85, ground 86, wire the negative side if the coil though pins 30 and 87a and you just made your self a rev limiter that will work at any RPM set for the shift light!! :y:



Casey (flat_out) generously lent me a autometre shift light tacho last week, to see if my theory would work. Took about 15 minutes to wire the whole thing up (already had tacho wiring run), only issue was the shift light in these is pulled to ground rather than powered, so that was just a matter of powering pin 85 from the positive side of the coil and running 86 back to the shift light for grounding.


The result:


....an effective rev-limiter for next to nothing.

That video was off by phone, as the camera was flat and isn't the best quality. I'll get another one up soon.



Anybody who isn't competent with wiring probably shouldn't do try this themselves, ask a mate who is. But for anybody who knows what I'm on about with pin numbers etc why not give it a crack and have a rev-limiter for next too nothing!!! :jamie:

For these guys also, here's a hint at another idea for a laungh:

exhasut tip + 1x extra spark plug + 1x extra coil + pin 87 = :wink: 8/

Edited by Taz_Rx
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Just remember to check the relay first, most 87 and 87a are the same for "on" as they are ment for driving lights, one pin for each light, you have to check that they are seperate.


Pulling the power from a coil can be dangerous. when power is connected the coil charges, when power is removed the coil lets out the spark (eg when the points open it fires not when they close, the points complete the circuit to ground when closed, thus charging the coil). you can get pre ignition causing a missfire and detonation on a cylinder and we all know what happens then.


Would be better to pull the negitive side of the coil with the relay to ground the spark will stop (charging the coil) and when the revs come down it will fire in the correct cylinder as the points will take over pulling to ground untill the next ignition fire comes.

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Yeah mate, I'm breaking the circuit on the negative side of the coil. Learn all out that once when we were trying to put a flame thrower on jade's mr2. We were running the positive side of the coil through a 2 position switch, and wondered why it would just die when switched. Swapped it over to the neg side and it was all good.

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I think i should have explained it different, don't brake the circuit, braking the positive or the negitive will result in the same thing, missfire and detonation, but you can use your setup to pull the negitive side of the coil to ground. will have the same effect as rev limiting but save the missfires.

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wasn't really think factory rev limiters, but after markets ones like an MSD. I am talking about carby cars here anyway as if it was fuel injected its going to have a rev limiter anyway! 8/


So what your saying Nick, is if I've got the negative running through 30 and 87a, then ground 87 rather than having nothing on it?

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30 would be on the coils negitive, 87a (normaly closed) not connected to anything and 87 connected direct to ground, what will happen is the car will run as normal, when the shiftlight comes on the turns on the relay and that will connect 30 and 87 together , making the negitive side of the coil go direct to ground. That will stop the coil from firing (will be in charge mode for the next spark and this is what the points do when they are closed) untill the revs drop, when the shift light goes out the relay will let go of 30 and 87 and letting the points take over from there ready to fire the next spark and then it will fire at the correct time.


so 30 to negitive on coil

87 to ground

No cutting of wires just splice 30 in to the negitive coil wire

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