Thanks guys. I've seen that pic in other posts here. Its interesting but based on my gauge at least, it seems only partially relevent. Whether the mech gauge reads 30 or 50 psi the stock needle hovers right around that first tick mark... When the mech gauge goes down to 20, the stock drops down to the 18psi mark (or lower). I'd be curious to know what might cause the variability in these stock gauges... Sender? gauge function? electricity?
There are three electro-mechanical issues with the variability:
1) the gauge itself uses a small heater coil and a bi-metallic strip to move the needle. It's nowhere near accurate and it's not even linear (as it heats up the resistance changes).
2) the gauge runs on the 5V regulator behind the dash which is another bi-metallic device and therefore somewhat inaccurate. I've read that this was done since they moved to 12V from 6V electrics way back when. Rather than re-engineer the gauge design they just used a voltage limiter (that's what I read anyway). If that's right then it should really be 6V but I think maybe they specced 5V to be on the safe side = lazy gauges. I've tried a 6V regulator and the gauges do read better.
3) the sender mounted on the engine block is an old fashioned rheostat (a bit like the one in the fuel tank). A slider rubs up and down rows of resistance wire. Again it's not too accurate and after a while the contact with some of the wires can be lost. If you open them up there is an adjuster screw which allows you to move the slider's position relative to the resistance wire. It's an easy way to improve your oil pressure
Overall it's a system installed more for entertainment than being anything very useful.
