Throttle spring...pedal vs RPM? - Page 2 - Fuelly Forums

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Old 08-01-2006, 01:41 AM   #11
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Is this the same car that you have the Condensator on?
No...this is a V6 auto with the FA2000. Actually this doesn't change things too much...the detent just works to remind where the typical throttle position is.

On this car it's at ~2100 RPM and 55 MPH or so. This car has a tach...hit the detent and it will shift pretty much a 2100 RPM.
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Old 08-01-2006, 02:57 AM   #12
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With a 4 cyl car...I'd say generally that 2500 RPM is usually needed to get down the road in a reasonable fashion.
I guess it depends on the car. With the Del Sol I could get down the road just fine at 1200 rpm in 5th. I could even climb moderate hills like that. With the VX, I find myself cruising at 1000 RPM due to the additional available torque and taller gearing.
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Old 08-01-2006, 05:04 AM   #13
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I've tried experimenting with FE vs. gear ratio and in my case the best thing to do is stay in the tallest gear possible as long as I don't hit 100% load and open loop fuel magement. I have to climb a steep hill to get to my apartment, which is where I tried down-shifting and watching the scangauge. At 30mph in 4th gear the ScanGauge reports 18-20mpg. Same speed in third gear results in easier climbing but a reported 13-15mpg.
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Old 08-01-2006, 06:28 AM   #14
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yeah..but that is uphill...
of course at load higher rpm is going to use more fuel
at cruise...depending on the car...i think higher rpm with lower tps % (higher vacuum) could decrease fuel consumption
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Old 08-01-2006, 07:34 AM   #15
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Originally Posted by molecule
yeah..but that is uphill...
of course at load higher rpm is going to use more fuel
at cruise...depending on the car...i think higher rpm with lower tps % (higher vacuum) could decrease fuel consumption
All of my testing shows that low RPM wins hands down. But then again, I have only tested two Hondas. Other cars may differ.
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Old 08-01-2006, 10:18 AM   #16
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Originally Posted by molecule
yeah..but that is uphill...
of course at load higher rpm is going to use more fuel
at cruise...depending on the car...i think higher rpm with lower tps % (higher vacuum) could decrease fuel consumption
Ehh...uphill or steady cruise, the engine needs to make power. The only difference is how much. To look at it another way, the aero drag at 65 or 70mph could just as easily be replaced by the gravitational pull of going up a hill of some grade at a speed where aerodynamics don't play as much of a factor.

The point was not that going up hill makes my gas mileage lousy. The point was that climbing the same hill at the same rate takes less fuel if the engine is turnig more slowly with more open throttle. I think that there are a couple of rasons for this.

First, pumping losses are produced when the engine tried to draw air against a closed or nearly closed throttle. The high manifold vacuum under such a condition acts against the piston during the intake stroke, which takes energy to overcome. All things being equal, one advantage to running at a wider throttle position is the reduction of those losses.

Another thing to consider is mechanical drivetrain losses vs. RPM. Overcoming internal engine friction is not trivial, and does require more power as engine speed increases. Producing the same power with fewer revolutions should result in less wasted energy.

The confounding factor here, which varies from engine to engine, is thermodynamic efficiency vs. RPM. All engines have trouble making power below a certain point, which is a function of displacement, number of cylinders, cam timing, etc. If you ask too much of the engine below that point, you may end up feeding it fuel and getting very little in return. That might, in some cases, overcome the reduced pumping and frictional losses of operating at low RPM/more open throttle. I think you just need to experiment to find out where that point is. Maintain one speed in the highest gear possible, use your instrumentation to check fuel consumption, and repeat in a lower gear to see if it increases or decreases. Most likely, you will find that higher RPMs are more efficient in some circumstances, less efficient in others. I simply haven't run across a situation where higher RPM benefits my particular vehicle.
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Old 08-01-2006, 05:11 PM   #17
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There are 2 things most IC engines have at cruise that are "wasted"...lots of vacuum and lots of heat. Both can be used to vaporize fuel?
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Old 08-01-2006, 05:17 PM   #18
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I guess it depends on the car. With the Del Sol I could get down the road just fine at 1200 rpm in 5th. I could even climb moderate hills like that. With the VX, I find myself cruising at 1000 RPM due to the additional available torque and taller gearing.
The Tercel is only rated at 62 HP...but I'm probably getting 70 or so out of it.

It runs about 2500 at 55 MPH. Seems shifting at 2500 is about right to be able to get down the road..maybe 2K on a slow day.

Those Hondas are tweaked a little more? My brother has an S2000...about the 2nd year made....it will wind up pretty well for sure. I think it redlines at 8K...torque all the way.
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