There's a bypass valve in the oil pump limiting the pressure. Peak pressure should develop
pretty low in the RPM range, so the pressure idea would be the same as trying to figure out
the RPM based on alternator output.

If this is an electronic ignition, why don't you tap into the crank/cam position sensor (or what
ever you vehicle uses). These sensors produce a set number of pulses per rotation. You could
use the PIC to calculate the RPM based on the time between pulses averaged out over a few
pulses to get the RPM. That's how the engine control module does it anyway... A high input
impedence OpAmp (6-10Mohm) set up as a voltage follower would allow you to tap into the
signal without loading the actual CPS circuit appreciably. another benefit is this can all be
done inside the car so you can hide the wires and not worry about anything happening to
them.


On 15 Jul 2003 at 21:21, Picdude wrote:

> On Tuesday 15 July 2003 20:05, Alex Harford scribbled:
> > > I don't have any links, but I do have a suggestion.  Someone suggested
> > > a few weeks ago that monitoring the cigarette lighter voltage on a car
> > > can be used to extrapolate the speed of the engine.  I don't know what
> > > the relationship is; my suggestion, stick an o-scope on it and find out!
> >
> > Whoever proposed the idea probably figured that the voltage output would
> > increase with increasing RPM.  In my experience, there are too many
> > other factors that affect voltage (power windows, cooling fan,
> > headlights) for that to be accurate.
>
>
> BTW, a more accurate hokey idea for measuring RPM would be to measure
> oil-pressure and have some proper temp compensation (actually measuring oil
> temperature).  I am willing to bet it would be FAR more accurate than
> measuring voltage, but I stress, it's still a hokey way to do things.
>
> Cheers,
> -Neil.
>
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