Hill climb: 80% Load or "slow speed bleed"

[cosmick]

Originally Posted by ShadeTreeMech

What I have found is if you can stay in overdrive, do so. This will of course entail a speed bleed, otherwise you will downshift and it's all lost.

I disagree, because my own testing has repeatedly proven, in a variety of applications, that 200 RPM too much is better than 50 RPM not enough.
Now, if one is willing to add axle ratio to enable staying in OD, then that matches my findings. But most of my testing has been in hilly to mountainous country.
For example, I had an '06 GMC Sierra RCLB 2WD 4.3L, a/t, 3.23:1, did a regular best hwy avg of 20 MPG. It wouldn't quite hold 3rd when towing on flat land. I should have gone 3.73:1, but even after swapping in the 4.10:1 rear axle and getting its computer recalibrated, my MPG improved to 22 nearly all the time. Further, my acceleration and towing both improved dramatically, and it did end the downshifting out of OD for grades when empty. And I can give other examples.

[cosmick]

Originally Posted by ShinyMew

the maximum load the automatic transmission will allow before it shifts down.

Unless you have a 'yota, which are the least friendly to aftermarket tuning, this can be adjusted, typically on a chassis dyno using electronics like EFILive or HPTuners. But if the change causes damage, it's your liability and expense.
Best way to circumvent the risk while still getting the results is to try different final drive gear ratios. Then it's just correcting the speedometer / odometer, and for automatics, the shift points. Not a big deal, no impact on the annual emissions test.

[spacemanspif]

This is interesting, thanks to ShinyMew for bringing this topic back up from the dead. I'm going to have to select "load" on my SGII and watch what my car is doing on the small hills I have on my commute. One day I hope to get my vacuum gauge installed because I like it's "instant" feedback compared to the SG's slight delay.

Just to clarify, you guys are saying that ALL hills are best climbed at 80% load? Is there a cut off where it's better to keep low load vs increasing load to 80%? For instance, let's say I set the cruise control and it's able to maintain the set speed up a hill and only gets up to 40% load; is it better to increase load, or just hang out at 40?

Thanks,

[Focus-Ak]

Hi Spaceman,

See "Brake specific fuel consumption explained": http://ecomodder.com/forum/showthrea...ned-30791.html

Other searches in this forum will also be helpful. The best search terms are "specific fuel consumption" or BSFC.

One thread has a series of BSFC curves for various cars (but unfortunately not our Duratec Foci). Generally, the best rough guess is -- I think -- 65% to 80% load. There can be hidden sweet spots in the curves, and RPM is also a factor. For most of us the 1,000 to 2,500 range will be the most important.

[redpoint5]

Originally Posted by spacemanspif

Just to clarify, you guys are saying that ALL hills are best climbed at 80% load?

Yes, that is correct.

Is there a cut off where it's better to keep low load vs increasing load to 80%?

No, unless the vehicle is already in top gear and bringing the engine to 80% load would mean exceeding the speed you wish to travel.

For instance, let's say I set the cruise control and it's able to maintain the set speed up a hill and only gets up to 40% load; is it better to increase load, or just hang out at 40?

It would be best to shift to a taller gear. If you are already in top gear and only at 40% load, then the hill isn't very steep, or your car is incredibly powerful.

[cosmick]

Originally Posted by spacemanspif

This is interesting, thanks to ShinyMew for bringing this topic back up from the dead. I'm going to have to select "load" on my SGII and watch what my car is doing on the small hills I have on my commute. One day I hope to get my vacuum gauge installed because I like it's "instant" feedback compared to the SG's slight delay.

Just to clarify, you guys are saying that ALL hills are best climbed at 80% load? Is there a cut off where it's better to keep low load vs increasing load to 80%? For instance, let's say I set the cruise control and it's able to maintain the set speed up a hill and only gets up to 40% load; is it better to increase load, or just hang out at 40?

Thanks,

Don't watch load, watch either A:F, or closed loop vs open loop.

[jcp123]

So far, the results with extra load aren't good, but I'll plug away at it a bit more. UG says a 10% drop in FE...

[redpoint5]

Originally Posted by jcp123

So far, the results with extra load aren't good, but I'll plug away at it a bit more. UG says a 10% drop in FE...

The other thing to consider is the back side of the hill. If you have to use your brakes coming back down the other side of the hill, then it's more efficient to let the speed bleed off as you approach the top. Let gravity build the speed up again on the back side.

[jcp123]

On the back side of hills here, I generally eiter coast or use light throttle depending on the grade or what's coming next (another hill?), few are long or steep enough to where too much speed is a concern.

[Focus-Ak]

Originally Posted by jcp123

On the back side of hills here, I generally either coast or use light throttle depending on the grade or what's coming next ...

If throttle is needed, I make a pulse (even if only a very short one) somewhere in the range of 65-80% throttle, for reasons several have given above. However, particularly on a known route, I try to crest the hill going to idle with enough speed that I can idle-coast to the next up-grade or stop sign. I think either of these strategies is easier on fuel than using light throttle.