Better Mileage With Tranny Switch Set To Pwr

[daveGSLooker]

On my 01 LS430 I usually drive with the transmission shift switch set to PWR. Two weeks ago I decided I'd set the switch to the normal position to see if my MPG's improved. I was shocked to see that for a whole tank's worth of driving I went from averaging 18.4mpg down to 16.5 mpg. I always fill up with 93 octane.

I have a short 9 mile commute with lots of red lights, and often I drive somewhat aggressive on the throttle to prevent people that love to cut in.

I guess it makes sense that my mileage would go down since in the normal mode the tranny would have more of a delay on the downshift which isn't as efficient.

What have you guys experienced? Is this just a symptom of my driving style?

[jcrome04]

Well generally "PWR" should give you worse MPG since it holds the RPMs longer before shifting.

But since you're consistently in "stop and go" traffic, maybe your MPGs went down in NORM mode since the tranny is shifting more...

That would be my guess... :D

[wwest]

On my 01 LS430 I usually drive with the transmission shift switch set to PWR. Two weeks ago I decided I'd set the switch to the normal position to see if my MPG's improved. I was shocked to see that for a whole tank's worth of driving I went from averaging 18.4mpg down to 16.5 mpg. I always fill up with 93 octane.

I have a short 9 mile commute with lots of red lights, and often I drive somewhat aggressive on the throttle to prevent people that love to cut in.

I guess it makes sense that my mileage would go down since in the normal mode the tranny would have more of a delay on the downshift which isn't as efficient.

What have you guys experienced? Is this just a symptom of my driving style?

"..I drive somewhat aggressive..."

Driving in "pwr" mode made you a match with the car.

Drive more sedately, consistently sedately, and the normal mode will exceed 18.4MPG.

[eatingupblacktop]

Jcrome's exactly right. When a tranny is shifting it's similar to idling waiting for the lights to change. You get 0 mpg, although shifting is a lot quicker of course. So the less shifting your tranny does the more mileage you get. One more reason for locking out the o/d when driving in the city. Another reason you're getting better mileage might be that you're spending more time in the engine's optimum revving range. Every engine has a range where it gets optimum fuel efficiency just as peak horsepower and torque have their power bands.

[wwest]

Jcrome's exactly right. When a tranny is shifting it's similar to idling waiting for the lights to change. You get 0 mpg, although shifting is a lot quicker of course. So the less shifting your tranny does the more mileage you get. One more reason for locking out the o/d when driving in the city. Another reason you're getting better mileage might be that you're spending more time in the engine's optimum revving range. Every engine has a range where it gets optimum fuel efficiency just as peak horsepower and torque have their power bands.

I disagree...

The 2001 LS430 has DBW, E-throttle. With DBW the engine ECU can be programmed to react entirely differently (slower rate of torque rise..??) to gas pedal depressions in normal vs "pwr".

Pedal to the metal, WOT, may not be ACTUALLY WOT in normal mode.

Think about buying a car with a smallish I4 engine primarily for FE and then driving it aggressively. If you wish to drive aggressively then a higher HP V6 will be a better choice and will yeild better gas mileage.

Think of your V8 in the "normal" mode as having been detuned, derated down to the equivalent of a V6, in order to improve FE. Now when you drive in "PWR" mode you're simply switching to the V8 which is clearly more appropreate, FE wise, for your personal driving style.

[eatingupblacktop]

Jcrome's exactly right. When a tranny is shifting it's similar to idling waiting for the lights to change. You get 0 mpg, although shifting is a lot quicker of course. So the less shifting your tranny does the more mileage you get. One more reason for locking out the o/d when driving in the city. Another reason you're getting better mileage might be that you're spending more time in the engine's optimum revving range. Every engine has a range where it gets optimum fuel efficiency just as peak horsepower and torque have their power bands.

I disagree...

The 2001 LS430 has DBW, E-throttle. With DBW the engine ECU can be programmed to react entirely differently (slower rate of torque rise..??) to gas pedal depressions in normal vs "pwr".

Pedal to the metal, WOT, may not be ACTUALLY WOT in normal mode.

Think about buying a car with a smallish I4 engine primarily for FE and then driving it aggressively. If you wish to drive aggressively then a higher HP V6 will be a better choice and will yeild better gas mileage.

Think of your V8 in the "normal" mode as having been detuned, derated down to the equivalent of a V6, in order to improve FE. Now when you drive in "PWR" mode you're simply switching to the V8 which is clearly more appropreate, FE wise, for your personal driving style.

There is no engine I know of that has a flat output. You can see these power and torque curves of any engine on a dyno. That's basically the reasoning behind the cvt...to keep the engine in the optimum part of the curve. As far as hard acceleration, I agree, loading an engine beyond it's designed capabilities will yield less efficiency than comparable acceleration with an engine designed for that load. DaveGS said he drives somewhat aggressively, which to my mind, given the longer hold on each gear and longer duration in the power band could explain his better mileage.

[wwest]

Jcrome's exactly right. When a tranny is shifting it's similar to idling waiting for the lights to change. You get 0 mpg, although shifting is a lot quicker of course. So the less shifting your tranny does the more mileage you get. One more reason for locking out the o/d when driving in the city. Another reason you're getting better mileage might be that you're spending more time in the engine's optimum revving range. Every engine has a range where it gets optimum fuel efficiency just as peak horsepower and torque have their power bands.

I disagree...

The 2001 LS430 has DBW, E-throttle. With DBW the engine ECU can be programmed to react entirely differently (slower rate of torque rise..??) to gas pedal depressions in normal vs "pwr".

Pedal to the metal, WOT, may not be ACTUALLY WOT in normal mode.

Think about buying a car with a smallish I4 engine primarily for FE and then driving it aggressively. If you wish to drive aggressively then a higher HP V6 will be a better choice and will yeild better gas mileage.

Think of your V8 in the "normal" mode as having been detuned, derated down to the equivalent of a V6, in order to improve FE. Now when you drive in "PWR" mode you're simply switching to the V8 which is clearly more appropreate, FE wise, for your personal driving style.

There is no engine I know of that has a flat output. You can see these power and torque curves of any engine on a dyno. That's basically the reasoning behind the cvt...to keep the engine in the optimum part of the curve. As far as hard acceleration, I agree, loading an engine beyond it's designed capabilities will yield less efficiency than comparable acceleration with an engine designed for that load. DaveGS said he drives somewhat aggressively, which to my mind, given the longer hold on each gear and longer duration in the power band could explain his better mileage.

One final point.....

The Power/Normal switch exists, so it must have/serve some purpose. Can that purpose be anything less, anything other than, derating the engine in "normal" mode in order to achieve improved FE...??

And yes, the shift pattern might very well be, should be, altered also.

And driving aggressively with a derated engine will not yeild good FE.

As a for instance it is my opinion that the FE of the Acura RDX's I4 with twin turboes could likely be improved substantually if the turbo wastegates were left open, no boost, unless the driver depresses the gas pedal unusually rapidly and/or beyond the 2/3's point.

In other words an automatic switch to "power" mode.

[jcrome04]

I don't think the pwr/norm switch does anything other than alter the shift patterns of the transmission.

[new2mud]

Every engine has a range where it gets optimum fuel efficiency just as peak horsepower and torque have their power bands.

An engine operates most efficiently (meaning power output per gas consumption, not fuel efficiency) at its torque peak. However, for maximizing real-world fuel economy, it is a function of drag, frictional losses, engine RPM, and gearing. This is why you will likely experience the greatest fuel efficiency at the lowest RPM in the highest gear regardless of what car you're driving.

[new2mud]

As a for instance it is my opinion that the FE of the Acura RDX's I4 with twin turboes could likely be improved substantually if the turbo wastegates were left open, no boost, unless the driver depresses the gas pedal unusually rapidly and/or beyond the 2/3's point.

In other words an automatic switch to "power" mode.

Leaving wastegates open may produce a little more efficiency due to reduced exhaust backpressure, but would result in turbo lag at an unacceptable level for an Acura-branded vehicle (or most modern turbo vehicles, for that matter).

[wwest]

As a for instance it is my opinion that the FE of the Acura RDX's I4 with twin turboes could likely be improved substantually if the turbo wastegates were left open, no boost, unless the driver depresses the gas pedal unusually rapidly and/or beyond the 2/3's point.

In other words an automatic switch to "power" mode.

Leaving wastegates open may produce a little more efficiency due to reduced exhaust backpressure, but would result in turbo lag at an unacceptable level for an Acura-branded vehicle (or most modern turbo vehicles, for that matter).

I wasn't (even) thinking of reduced exhaust backpressure, just less fuel used, a LOT less, with no boost, as long as the driver is satisfied with smooth and slow acceleration.

For some of us, given the FE return on the "investment", a little additional delay in the onset of boost would be perfectly acceptable.

An ideal solution would be to use DFI and have the compression ratio at ~12:1 and then use i-TEC to implement an Atkinson cycle via modifying the intake valve timing. Keep the intake valves open longer as the boost rises making the FULL boost effective compression ratio 8.8:1 to accomodate the turbo boost. DFI, direct

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