Supercharger boost = higher economy?
 

I drive a 2004 MINI Cooper S which I've begun aeromodding, and I've noticed something that seems counterintuitive: particularly when going uphill, my KiwiMPG instantaneous MPG readout gets higher as the RPMs climb, to about 4,000, at which point it begins to drop off again. I've hypothesized one of two things to be true:

a) My KiwiMPG doesn't know what the heck it's talking about, because I can't possibly be getting 37mpg under load uphill in 2nd gear at 3,500rpm.

or

b) As the supercharger kicks in, the air forced into the engine actually leans out the air/fuel mixture when I'm not mashing the throttle.

If a) is true, then I'll just have to live with it until I can convince myself to buy a ScanGauge. If b) is true, then it logically follows that if I were to induce higher boost at lower RPMs, if I'm able to tame the impulse to bury the needle, my economy should improve pretty dramatically.

Of course, it's entirely possible neither is true, and it's something I never would have thought of that's happening. I may just be looking for a reason to buy a 19% smaller supercharger pulley, thereby making the car faster, in my ever-self-defeating internal conflict of speed vs. economy.

Simple.... your engine is operating under optimum load and in the proper rpm range for your motor which produces the most torque efficiently. Brake Specific Fuel Consumption. Google it or search here for it.

supercharger is belt driven, turbo is spun by exhaust pressure.

As manifold pressure increases, there is a linear relationship to the amount of fuel needed, regardless of effiency of the engine, a lower load will always require less fuel a set RPM. unless your getting more then 37mpg in second at low loads it couldn't be getting better in boost.

right , something doesn't sound right there . Supercharges also "generally" don't get better fuel economy , turbos have better chance doing that (thats generally by using smaller sized turbo engine combo ).

What is your throttle position whilst this is happening, are you mashing it or letting it pick up speed relatively slowly ?

Well well well. This is the first time in any fuel economy forum that I've seen an admission that engines have zones of efficiency. The mantra I've heard so far has been "low RPM in a high gear" and not much else.

I live at the top of a hill, and so I always have a long climb just before getting home; natually this plays havoc with my carefully collected trip MPGs. I've tried going really slow, using low RPMs, and the like, without seeing much difference. On the other hand if I keep my speed high enough to hold 3rd gear and 2500-3000 RPMs, the car seems happier, the MPG loss is a tad less, and as a bonus I get home a little sooner.

Low rpm in high gear applies to cruising.

The idea is to accelerate to cruising speed by staying inside the lowest BSFC island while going through the gears. The load and RPM will vary by engine. Once you reach cruising speed (whether it be 40mph or 60mph), lowest rpm in highest gear while holding a steady vacuum is great. Pulse and Glide + EOC is the best, but some people may not like doing that.

There's always a point where going lower in rpm doesn't help you anymore - going below that you'll make it even worse.
At some point the engine revs so slow it can't efficiently produce the required power anymore, and trying to squeeze that power out of it regardless leads to very high fuel consumption (and ultimately engine damage).

It's also the reason why shortshifting and driving at low rpm is so beneficial to large engines - they have more (enough) power low down simply because of their size.
It won't work quite as well on smaller, "downsized" engines.

You've got a "downsized" 1.5L petrol engine, so when it needs to work very hard, you'll also need higher rpm.

Not here as far as I know, we have debated hill climbing a few times - I started a thread on it nearly a year ago, and there are loads of others.

The issue the OP is saying is 2 fold, he says its better at 4k rpm than lower and also with supercharger boosting .

I don't think there many superchargers that can give decent boost down low (lugging), they generally need decent amount of throttle and rpm .I am not familiar with newer mini coopers maybe they have secondary set of injectors and scangauge isn't reading it ?