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Originally Posted by baxster
I didn't think about that. So as the temps go up, so should the revs.
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Well, not really... if the pumps aren't nominal, the temps will go up, period. Once nominal, the temps will essentially stabilize. The pumps are certainly operating at a good flow rate by 2500 RPM. Too high, and the pumps can cavitate and actually reduce flow.
Your temp gauge isn't telling you what temp the engine critical parts are at, it's telling you what your coolant is at. On a Boxster, this is typically around 180 degrees.
But the temps at the crank and big end journals can exceed 600 degrees. That's OK, they're designed for this. But it is the oil and the oil flow which keeps it at this temp, and there's no gauge for this, except maybe extrapolating the Tach.
So far as Range (MPG) is concerned, there are many variables, such as weight (don't keep the Trunk/Frunk loaded with unneeded items), tire pressure, tire rolling resistance, make/model of tire, road surface, elevation changes (hills), time at idle (stop & go), weather, top up/down, windows up/down, even the prevailing winds (headwind = lower MPG, tailwind may increase MPG). Then there is that variable right foot.
But also, every engine has a
sweet spot, and if you can determine it, you can take advantage of it.
For instance, I bought my E-30 (325is) in NC and drove it 1400 miles home. Borrowed a friend's V1 for the trip. So confident, I had the cruise set to 85 MPH most of the trip (3100 RPM). MPG was 32.4 - reported electronically and verified manually.
One week later, with a fresh oil change, I took a trip 400 miles. No major differences in road, elevation, weather, etc. No V1, so I reduced the speed to 76 MPH on cruise. I ran at about 400 RPM lower because of this speed, and got only 26.2 MPG. This engine just likes to run at 3100 RPM - probably reaches it's best volumetric efficiency in this range.
This is not going to be a Boxster thing, it's unique to each individual engine. They will all have similar MPG overall, but every engine is different for lots of reasons - miles, type and mileage of oil, quality of fuel, regional fuel differences, etc. And each will have it's own 'preferred' RPM to yield maximum Range.
It's not easy to determine, you need some long trips and a bit of luck to spot it (or lots of record keeping). But if you can find it, and can operate the car in this range consistently, you can maximize your Range.
Cheers!