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Optimal RPM...
Topic Started: May 2 2013, 10:38 AM (1,550 Views)
Coche Blanco
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This graph leads me to believe that 3000ish rpms is the sweet spot for the G10, unfortunately this is for the non XFi.

Have we had a BSFC discussion on this forum? I searched and saw some stuff....but nothing BSFC specific.
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Turbo Dan-O
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Obsessive Car Detailer

Joel told me this about a week ago when we were in FL. He told me NOT to swap to 13" tires because it would hurt my mileage. My car(G10,3.79,12" tires) runs right at 3000 RPM at 65 MPH. I always try to keep it around 60 MPH on the interstate but on the way home from FL I was ready to get home and did not care about mileage so I hammered it all the way doing 65-70(3000+RPM) and without trying I got the best mileage to date(I make this trip a lot) on the "up hill" trip back home to TN. Maybe you guys are on to something... :hmm

BTW my factory XFi cam is advanced 4*
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Coche Blanco
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I think you should run some 155/80-13 for a trip down to the beach and do a test Dan-O...
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jonathan180iq
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Not Really All That Smart

I've been doing a lot of interstate runs going back and forth from PAPs and obviously down to GOTG. I've been contemplating this a lot and trying to gauge it by feel, which I know isn't science, but it's not horrible either.

I've been "feeling" an obvious sweet spot where I don't have to pressure the engine to do more work and I also have pop whenever I need it, and it's around 3200rpm. Less than that, and I feel an obvious loss of "pep" and the car has to work harder to work up to speed. More than that and it feels like it starts to hollow out, meaning I am going faster but it doesn't feel as rich and fluid and in-tune. The sweet spot is 3200-3400 rpm. I can go faster obviously, in engine speed, but it just doesn't feel like it gives me as much or likes it as much.

I've been doing this ever since I got home from GOTG and I should be buying gas tomorrow to see if I am right at all. I have established a consistent baseline for my car of 43mpg so we'll see if there is anything significant over that.

I need an MPG calculating device...
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91xfi1.0


Hi, I have the xfi, and I am interested in what the optimal rpm would be for the xfi. Hey, if I can drive faster and get better gas mileage!

Dan-o: how close is your Geo to my stock xfi? Do you think 65 mph would be optimal for my car?
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dayle1960
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Fastest Hampster EVER

I can hit 3000 rpm's at 60 mph while in fourth gear, but when I shift into fifth gear the rmp's drop down to about 2500. At 60 mph I get a steady 48-49 mpg.

Now you are saying to stay in fourth gear at 60 mph OR bump my speed up to 65mph in order to get my fifth gear to hit 3000 rpm.......correct?

Seems counterintuitive to run a higher rpm to gain mpg....That has me scratching my head.

CB, throw some more discussion into this thread, please.
Edited by dayle1960, May 2 2013, 08:25 PM.
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redpepe


only a short history since i just added a tach a few days ago, and presuming it's accurate, i realize i'm at 2500-2600rpm around 65mph. 70+mph seemed about 2800. i still have original 12" wheels. my best mileage has been under 60mph. if i remember right, got about 56mpg on a full tank long haul at 45-55 in alaska ... essentially idling in high gears for hours.
Edited by redpepe, May 2 2013, 08:42 PM.
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Woodie
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redpepe
May 2 2013, 08:40 PM
only a short history since i just added a tach a few days ago, and presuming it's accurate, i realize i'm at 2500-2600rpm around 65mph. 70+mph seemed about 2800. i still have original 12" wheels.
I don't think you have the right tach, those numbers are very low. My 98, with 3.79 gears and 13" wheels, spun much faster than that.
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Dattman
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You guys are forgetting about wind resistance and friction, the chart shows where THAT engine is most efficent, it's up to you to gear the car so your cruising speed is in the sweet spot.

Don't forget friction and pumping losses increase with rpm, this is why you want to change your cam and advance your cam timing, bring the sweet spot of efficiency down in the rev range so you can have less engine friction/pumping losses and change your gearing to suit the new lower sweet spot.
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High MX
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Is the data from a controlled experiment or data logged over time in various conditions?
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Cobb
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Watching my sg2 I found the best mpg was 4th gear at 40 mph with my scion xa.

If you advance the valve and ignition I imagine that rpm drops. :popcorn
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Turbo Dan-O
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Coche Blanco
May 2 2013, 03:08 PM
I think you should run some 155/80-13 for a trip down to the beach and do a test Dan-O...


91xfi1.0
May 2 2013, 03:44 PM
Dan-o: how close is your Geo to my stock xfi? Do you think 65 mph would be optimal for my car?



Jake, I still want to try out the 13s with the tires you mentioned and see what happens. :hmm There is a LOT to think about in this thread. I like it! :thumb

91xfi, both of our cars should be very close but it's hard for me to say where your car would run the best at. So many variables...
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arollinstone
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On a Mission from God

Dattman
May 3 2013, 06:59 AM
You guys are forgetting about wind resistance and friction, the chart shows where THAT engine is most efficent, it's up to you to gear the car so your cruising speed is in the sweet spot.

Don't forget friction and pumping losses increase with rpm, this is why you want to change your cam and advance your cam timing, bring the sweet spot of efficiency down in the rev range so you can have less engine friction/pumping losses and change your gearing to suit the new lower sweet spot.
:thumb
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dayle1960
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Fastest Hampster EVER

I'm now wondering if the graph test was run on a dyno or was it real life testing on roads. I'm doing a quick test going 3000 rpm's to see if the mpg's bump up a bit. Will be to see the results.
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redpepe


Woodie
May 3 2013, 05:38 AM
redpepe
May 2 2013, 08:40 PM
only a short history since i just added a tach a few days ago, and presuming it's accurate, i realize i'm at 2500-2600rpm around 65mph. 70+mph seemed about 2800. i still have original 12" wheels.
I don't think you have the right tach, those numbers are very low. My 98, with 3.79 gears and 13" wheels, spun much faster than that.
---------------------
it's certainly possible you're right woodie. i will keep comparing. it's the hf 3cyl special. it actually seemed a bit high below 2000 rpm compared to my trusty old sun tach. it's small too and my eyes aren't what they used to be. it seems steady and responsive overall.

as mentioned above and adding to the original considerations though, there's significant wind resistance and inertia to overcome at higher speeds ... so lower gear with lower speeds could indeed produce higher mpg. the metro has a great aerodynamic coefficient but there are still significant differences and limits at higher speeds. it used to be claimed that drag over 55 was the significant problem generally.
Edited by redpepe, May 4 2013, 09:17 AM.
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