Additional thoughts:

 

I think it is probably dependant on your setup. I would go for oil pressure on a standard car but perhaps A/F ratio on a tuned/FI car.

 

Many of those can be monitored using a smartphone app, no need for gauges.

I've managed without any of those for the last 9 years, including many track days, so you're probably wasting your money.

 

Exactly, it all depends what you're running. For example AFR gauge on my 100% stock car wouldn't make much sense.

I would definitely go with a oil temp gauge, letting you know exactly when you can hit VTEC.

I'm not sure how useful an oil pressure gauge would be, the engine damage would be already done before the gauge shows you're too low on pressure (and hence oil).

 

yeah there are loads of different ways of monitoring, just interested which people think is the most important.

 

Those won't work with an aftermarket ecu though will they? That's what I understood unless you retain the obd functionality?

 

I'd say oil pressure and temp.

I once had an oil filter that cracked on the b16 in my crx and as soon as I saw the oil pressure fluctuate a bit I pulled over on the hard shoulder, after installing a new filter and oil in it ran happily for another 5 years in my posession and is still going strong now in someone elses car.

 

depends--

AFR for any boosted application

Boost for a turbo car, (shows potential engine destroying over boost conditions)

Water temp for a track car. (probably not otherwise required on a standard car in the uk)

Oil temp for a non oil cooler track car.

Fuel pressure initial setup and fault finding (?)- not so sure on other uses for that one

Intake temp is handy to see how your intercooler setup is doing and how much power you maybe losing.