If ruuning an aftermarket computer...how can you tell if the displayed RPM (on laptop) is the actual RPM??? since spark and everything else is controlled by the computer???

ie computer may be saying engine is rotating at 3000 RPM....but sounds more like 6000 RPM..

If you have access to a mulitmeter with an inductive pickup - you put this pickup on a spark plug wire, tell it how many cylinders it has and it displays your rpm. (There about $40-$80 to buy)

that will tell you how many times the spark plug is firing....NOT how many RPM the engine is doing.

ie if engine is doing 3000 RPM...but computer thinks engine is doing 6000 RPM...the computer, which controls spark..will make the spark plug fire 6000 times per minute (for a waste spark system) and so your inductive pickup multimeter will tell you the engine is doing 6000 RPM !!!

The only way I can think of is on the road or dyno....knowing gear ratios etc..pick a speed...calculate
engine RPM..then check this against computer RPM??

oops

"oops"

Not really...

How accurate is the speedo??? or dyno speed readout???

If the road speed is wrong by 10% then I will be under/over revving the engine by 10%. So at max revs, the engine may be turning at 10% MORE what it should be, won't make for a happy engine!!!

(although if I use the Datto speedo it will most likely be 10% less RPM)

If the ECU controlling the spark and doesnt know what RPM its doing... you need a new ECU.

The only thing which may mess things up is the Cam/Crank angle sensor, or a sensor off the dizzy.

This will not make it read 1/2 the RPM.

I'd check the ECU is not giving readouts for a V8.

I guess an inductive pickup at idle might tell you if the computer is spaz too....if the mulitmeter reckons the car is idling at 5000rpm or so at least you will be able to detemrine that the computer is wrong.

You might be able to measure the pulses out of the CAS if you have a meter that has a scope on it. Don't know if you could measure that with a normal meter.

You could do it with as normal meter that measures frequency.

You need to figure out how many pulses per rev too.