I don't get it? I only got 3 more MPG??? - Fuelly Forums

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Old 05-28-2008, 09:35 PM   #1
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I don't get it? I only got 3 more MPG???

1991 Miata with a supercharger. Yeah, I know it isn't the perfect car to experiment with but I did it anyway. The car is normally driven VERY hard and I keep it in boost as much as possible. Over the past two weeks I kept it out of boost, shifted around 2500 rpm (redline is 7500 in a Miata), and drove around 45-50 MPH to work instead of my normal 65 MPH. After 213 miles (not a great experiment but again I was just playing) I check my MPG and saw it had risen from 20 to 23.9. I figured that I would have been close to 30 or so. Any idea why my mileage didn't jump more than that? The car runs pretty rich but I figured staying off the supercharger would have at least given me +5MPG.
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Old 05-28-2008, 09:47 PM   #2
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Your programming probably has less aggressive timing to err on the side of safety. less timing means less mpg and power down low. I would sell the super and bolt on a turbo. As soon as a computer sees boost it starts dumping fuel. You will only spool and get into boost dependent of load, not rpm's. There are very easy manifold kits out there. Some other mazdas came turbo from factory. My other car has a turbocharged engine and i get 215mi on a tank if I baby it or drive pretty hard. I've noticed that cars with power adders generally dont lose mileage as they speed up as fast as a regular car. I think its the timing advance finally catches up with slightly better airflow and it finds a sweet spot. I get the same mileage in the dodge going 65 or 80 when it should be dropping like a rock. After 80 it sure does but I dont drive that fast anymore
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Old 05-29-2008, 04:42 AM   #3
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Shift at lower RPM, use higher gears, stay out of boost.

The Miata is not a high torque engine, try driving like you have a rev limiter set at 3000 RPM.

regards
gary
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Old 05-29-2008, 06:48 AM   #4
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If I shifted at 2500 rpm, I'd lose at least 5 mpg. Every car is different but I bet you can shift a LOT lower than 2500. I rarely shift higher than 1500, often around 1250, and occasionally close to 1100 in my VW Rabbit.

The supercharger could make that strategy fail, though; getnpsi mentioned that you'll boost at high loads independent of RPM and then the a/f ratio will go rich.
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Old 05-29-2008, 07:51 AM   #5
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its a shame mazda chose not to put the VICS on the Miata intake manifold. That killed a sizeable amount of under 3000rpms torque, while gaining hp up top. The SC drag also makes shifting at lower rpms marginally more difficult (probably not noticeable though).

Shifting at 2500 is probably a good start, but I know I could keep my 4wd protege under 2000rpms and still drive decently. You've got 400lbs on me, and the same or more power available.

On the flipside, wth are you doing trying to get high FE out of a gocart with a SC? I personally would flog that, because that's what it was made for.

oh, and you should get way better than low 20s. the 4wd in town got 25 when I was nice to it, 20 when flogged. I'd look at some other things that might be holding your FE down.
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Old 05-29-2008, 11:20 AM   #6
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I still disagree that the best mpg's out of a miata are at 45 mph. A brick isuzu rodeo with nothing going for it at all gets mpg increases up to 51mph.
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Old 05-29-2008, 11:55 AM   #7
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Quote:
Originally Posted by getnpsi View Post
I still disagree that the best mpg's out of a miata are at 45 mph. A brick isuzu rodeo with nothing going for it at all gets mpg increases up to 51mph.
That'a very different vehicle, and being more bricky isn't necessarily going to lower its most efficient speed. Gearing is probably the most important part. I suspect that the best MPG in my 2008 VW Rabbit, with a .32 drag coefficient and decently efficient setup all around (except for the gears), is about 35mph. That's something like 1500rpm in high gear. My full size pickup, OTOH, probably gets its best FE at 60mph, where it can stay in OD with low RPMs.
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