Hills - Which gear?

[serialk11r]

Originally Posted by YeahPete

I approach hills alot different than you guys. What I do is "coast down the hill" really slow and "POWER" over the next hill. I just floor it up the hills about 80% throttle and when I get to about 10mph over the speed limit I pop into nuetral and coast down and as far out until im going about 5 mph under the speed limit. Slower if no ones behind me.

Yea but see the problem is you're going faster at the top! Higher speed = more drag = bad. Aside from the inherently increased load from the hill, it's the same principle as on flat ground, higher speed = less mpg. 60% load is not much worse for efficiency than 80% load, but going 10mph faster is a lot worse for efficiency! Of course if your car has a larger engine then maybe it could be better to just go a little faster to increase load on the engine on the hill.

PaleMelanesian, I'm surprised your engine works so well at low rpm! It's like freaking magic to me... Maybe it's just that my engine is in bad shape :/ Rolling in 4th gear at 800rpm is 55mpg, rolling in 5th is only 40mpg. If I'm under 1200rpm, accelerating in 5th is almost impossible (lots of groans and vibration, tach needle barely moves up until it's over 1200). I noticed that the groans get better after the engine warms up though, kinda weird.

[PaleMelanesian]

Originally Posted by serialk11r

PaleMelanesian, I'm surprised your engine works so well at low rpm! It's like freaking magic to me... Maybe it's just that my engine is in bad shape :/ Rolling in 4th gear at 800rpm is 55mpg, rolling in 5th is only 40mpg. If I'm under 1200rpm, accelerating in 5th is almost impossible (lots of groans and vibration, tach needle barely moves up until it's over 1200). I noticed that the groans get better after the engine warms up though, kinda weird.

Yeah, funny how these "gutless", "no torque" and "no power until 6000 rpm" Honda engines actually work in the real world.

[wmjinman]

Originally Posted by YeahPete

I approach hills alot different than you guys. What I do is "coast down the hill" really slow and "POWER" over the next hill. I just floor it up the hills about 80% throttle and when I get to about 10mph over the speed limit I pop into nuetral and coast down and as far out until im going about 5 mph under the speed limit. Slower if no ones behind me.

Yeah, I don't think that would work on Donner Summit (I-80 over the Sierras west of Reno). LOL!!

[wmjinman]

Well, had another trip over the Sierras the other day. It was raining & the carboard got all soggy & started falling apart, so I had to pull it off. Worst part was the engine overheating real bad (252 degrees at one point). When I got to Sacramento, I checked the radiator fluid level & ended up adding over a gallon and a half. No more overheating problems after that. DUH!!!



I did take one picture before it all went to pot, though....

[Gasoline Fumes]

Originally Posted by PaleMelanesian

I have a long hill I tackle every day. It's about almost 1/2 mile long, ~3% grade. It's shallow enough to gain speed in 5th gear. I note the change in trip mpg from bottom to top each day. It's at the same point in my commute, so that's a consistent mark. The best approach, that loses the least mpg, is the old "slowest speed in top gear". If I start the hill at 25 mph in 5th gear (1000 rpm), hold 80-85% LOD, I accelerate to the 45 speed limit at the top, with the least mpg lost. Other gears, higher starting speed, lower LOD, all give worse results.

But climbing the whole hill at 25 MPH would be even better, right? I usually try to lose speed up hills, not accelerate. With no instrumentation, I'm just guessing. I've been thinking about this post every time I climb a hill.

[serialk11r]

Originally Posted by Gasoline Fumes

But climbing the whole hill at 25 MPH would be even better, right? I usually try to lose speed up hills, not accelerate. With no instrumentation, I'm just guessing. I've been thinking about this post every time I climb a hill.

Problem with that is it's not really efficient use of the engine to go 25mph. If you're already pulse and gliding, using the hill as the pulse is better, since you're going to have speed fluctuation anyways.

[JRMichler]

Originally Posted by JRMichler

I ran some quick tests to find which gear was best to climb a local hill. This testing does not come close to meeting the standards in http://ecomodder.com/forum/showthrea...ery-11445.html, but I thought the results might be of interest.

I regularly climb a certain hill. The engine is always fully warmed up, and I normally drive it at 55 MPH in 4th gear. For the test, I also tried 3rd and 5th gears while eyeball averaging the Scangauge MAP and instantaneous MPG displays. The results:

Gear RPM MAP MPG OAT
3rd 3380 10.0 11.7 43 deg F
4th 2470 11.7 13.5 37
4th 2470 12.0 13.2 35
5th 1800 13.9 15.0 27 Unable to maintain speed.

I got another data point. This time I went up the hill in 4th gear, setting the throttle so as to bleed off the same amount speed as in the 5th gear test. Results:

Gear RPM MAP MPG OAT
4th 2470 10.7 15.1 32 deg F

I estimate the best efficiency RPM at about 2800. Fourth gear puts the engine closer to its best efficiency point than 5th gear, but at an inefficient MAP. It appears that those two effects cancel each other out at this power setting and speed.

[serialk11r]

Originally Posted by JRMichler

I got another data point. This time I went up the hill in 4th gear, setting the throttle so as to bleed off the same amount speed as in the 5th gear test. Results:

Gear RPM MAP MPG OAT
4th 2470 10.7 15.1 32 deg F

I estimate the best efficiency RPM at about 2800. Fourth gear puts the engine closer to its best efficiency point than 5th gear, but at an inefficient MAP. It appears that those two effects cancel each other out at this power setting and speed.

That's pretty interesting! Not what I would've expected.

Though, from the typical BSFC chart we can see that at lower rpms and maximum load the BSFC is quite high, presumably because of high cooling losses. I think it might be interesting to try going up that hill a bit faster if your engine has more torque at a higher speed, the maximum load BSFC might improve quite a bit and you might see better results despite going faster. For example I can't climb some "mountain roads" in 5th at 1400rpm but if I downshift, accelerate, and then floor the pedal at 1600-1700rpm then the engine purrs along nicely and doesn't lose speed. I imagine the mpgs are better that way.

[razor02097]

Originally Posted by serialk11r

Problem with that is it's not really efficient use of the engine to go 25mph. If you're already pulse and gliding, using the hill as the pulse is better, since you're going to have speed fluctuation anyways.

I can vouch for that. There is a massive hill I have to climb and there is a stop sign at the bottom. When I accelerate I am in 3rd by the time I get to the top. With each shift the vacuum dips low then starts to climb again. Once I reach the speed limit of 25mph the vacuum gets to about 9". If I creep along at lower speeds the vacuum is lower or the RPMs are too high.

[JRMichler]

Originally Posted by serialk11r

I think it might be interesting to try going up that hill a bit faster if your engine has more torque at a higher speed, the maximum load BSFC might improve quite a bit and you might see better results despite going faster.

I'd like to get a running start, but the speed limit changes from 25 to 55 at the bottom of the hill. I can just get to 55 MPH in 4th gear near the bottom. I would need to use 3rd gear to hit the hill any faster.

As it is, I approach the bottom of the hill in 5th at about 12 lbs MAP at 30 to 35 MPH, shift down to 4th, hold about 12 lbs MAP until 55 MPH, then maintain 55 MPH in 4th until near the top, then shift to 5th. There is a transition from a low grade to a steeper grade about the point where I reach 55 MPH.