rpm sensing circuit?

Is there anyway to make something that senses rpms and then switch something on or off? I am trying to build an adjustable rev-limiter. Basically it will switch a relay to turn off the ignition circuit at a certain rpm.

Yep. Two common methods.... A frequency switch (when frequency exceeds...) A frequency to voltage converter (eg LM2907/2917 or NTE995) and set you voltage. There are kits like Jaycar's KC5378 Frequency Switch kit (currently out of stock).... I'm sure others will have equivalents.

Thought a rev-limiter was already built in to most cars.

fuel cut may be a better option, since with ignition cut, the ECU may still signal injectors to continue supplying fuel.This information is provided only as a reference.

All circuits should be verified with a digital multi-meter prior to making any connections.

I did this once and it didn't work very well at all with a simple frequency switch because there is no "lead in".  From memory what happened was the motor had enough power to overshoot the cutoff point by a significant amount, and then died down below the setpoint, then shot past again due to the motor going back to full power.

In the end, what worked, was using a frequency monitoring device (microcontroller) and setting up three "zones".  The "Low zone" allows full engine power, the "Medium zone" (setpoint - X RPM) allowed only a few ignition cycles to fire in a somewhat random fashion (don't cut the same cylinders each time) and the final zone (> then setpoint) was zero ignition.  This works well because going in to the setpoint you are no longer generating full power and when you go from cut back to the "medium zone" again you are not applying full power.  I have some you tube videos out there - search for KPiersonTec 2 Step

Ignition is typically the source to cut because it is much easier to work with.  An ignition event is so short that it can be killed without risking detonation.  At higher RPMs fuel injectors overlap each other and trying to cut one off could cut another off (assuming one power source and not individual injector control) which can lead to a lean condition and a misfire.

Kevin Pierson

And as a heads up, if you are going to cut ignition make sure you either stay off the limiter or get rid of your cats because that unburnt fuel is going to go somewhere!Kevin Pierson

91stt wrote: fuel cut may be a better option, since with ignition cut, the ECU may still signal injectors to continue supplying fuel.

No way - that's almost as bad as oil pressure to control fuel pumps.... Over-rev limiters usually work by either limiting the injection, else cutting ignition pulses (on an evenly spaced basis until finall totaol kill). Cutting fuel - apart from running injectors dry plus any lag with suge tanks, fuel bowls (cabies) etc - risks piston damage... (lean mixture). Do what oil pressure should do - ignition kill.    BTW - if you have an analog tacho, you have a frequency to voltage converter. You might be able to tap the meter signal.

oldspark wrote: 91stt wrote: fuel cut may be a better option, since with ignition cut, the ECU may still signal injectors to continue supplying fuel. No way - that's almost as bad as oil pressure to control fuel pumps.... Over-rev limiters usually work by either limiting the injection, else cutting ignition pulses (on an evenly spaced basis until finall totaol kill). Cutting fuel - apart from running injectors dry plus any lag with suge tanks, fuel bowls (cabies) etc - risks piston damage... (lean mixture). Do what oil pressure should do - ignition kill.    BTW - if you have an analog tacho, you have a frequency to voltage converter. You might be able to tap the meter signal.

My response wasn't meant to imply cutting power to the fuel pump. I apologize if that is how it came out. Controlling the injectors is much less current to deal with than the fuel pump. The injectors also respond much quicker. As KP has pointed out, you will have to account for the fuel when cutting ignition. I may be possible to trick the ECU into thinking that the throttle has snapped shut which in turn pulls fuel. What kind of car is it?This information is provided only as a reference.

All circuits should be verified with a digital multi-meter prior to making any connections.

This is a toyota AE86. Howie it does have a built in fuel cut but as a drift car, letting it hit fuel cut at 8k rpm repeatedly is already at the engines stress point. This will also greatly help when launching. KPierson that is exactly what i need, a step that can be activated by button or the handbrake switch. There is no need to worry about the flaming exhausts haha the car has a full race exhaust with no cats which currently already shoots out flames under high rpms. Such a product already exists call the Bee-R Rev limiter but at the cost of $300, I think the people here at the12volt can teach me a cheaper way. http://www.neweraparts.com/Default.aspx?tabid=54&List=0&Productid=17 Oldspark's suggestion http://www.jaycar.com.au/productView.asp?ID=KC5378 maybe plausible, is it already assembled or do I have to put the components together my self? Any other suggestions?

Again, in my personal experiance (on my TT G35 coupe) a simple frequency switch will not work well at all as an accurate rev limiter.

Are you looking for a rev limiter, a launch controller, or just a button activated rev limiter?

Kevin Pierson