I am looking for a fuel flow meter for my boat. I run a pair of Ilmor 710s.
Has anyone used one that actually works and gives reasonably accurate reading?

I have done some testing with the Lowrance LMF-200 gauge and their EP-60R flowmeter. The thing you have to determine is given that the 710 is fuel injected, it likely has a fuel return coming from the fuel regulator on the rail. Does this return go back to the tank or to the fuel filter like Merc does?

If it goes to the tank, you're stuck, as you can't measure flow in and out and subtract the two (that I'm aware of). If it returns to the filter housing, then you just put your meter before the filter and it will read total fuel volume delivered to the enging.