Hills - Which gear?

[wmjinman]

Originally Posted by redpoint5

The answer to my question that seemed most reasonable is this;

OK, question # 2, then: How do I find my car's BSFC?

And I guess my test showed that 60 mph must have moved the engine's RPM out of the BSFC range? - and I guess 35 mph moved it out of that range on the other end?

One thing I did find interesting was that 40 mph, which I believe (from some basic testing) is my best MPG on "nearly level" ground is also "in the grouping" of best MPG speed on the hill (that paarticular hill, at least)

And from what redpoint is saying (and quoting), maybe the reason it was just as good at 45, 50, and *maybe* 55 is because the BSFC (what is that, anyway - Best Specific Fuel Comsumption?) increases with speed at a rate that just offsets aerodynamic drag???? - but on level ground, with fighting gravity not being in the equation, the BSFC and wind resistance balance out at 40?

[redpoint5]

Originally Posted by wmjinman

OK, question # 2, then: How do I find my car's BSFC?

Good question, and I am not as knowledgeable as others on this subject. There is a thread on here that lists the BSFC of many engines, and similar engines will have roughly the same BSFC. The abbreviation is Brake Specific Fuel Consumption, and I believe it is a map that shows how much power is produced per unit of fuel throughout a range of RPM and throttle positions.

And I guess my test showed that 60 mph must have moved the engine's RPM out of the BSFC range? - and I guess 35 mph moved it out of that range on the other end?

More likely is that exceeding 60mph reduces efficiency due to aerodynamic drag.

You may be correct that dropping below 35mph put you in an inefficient engine operating speed. I would also believe moving this slowly up a hill is inefficient due to how much time is spent getting to the top.

One thing I did find interesting was that 40 mph, which I believe (from some basic testing) is my best MPG on "nearly level" ground is also "in the grouping" of best MPG speed on the hill (that paarticular hill, at least)

I also find this speed to be the most efficient on flat ground in top gear. It doesn't surprise me that it would also be an efficient speed when going up a hill because it's still fairly quick, but aero drag is relatively low.

And from what redpoint is saying (and quoting), maybe the reason it was just as good at 45, 50, and *maybe* 55 is because the BSFC (what is that, anyway - Best Specific Fuel Comsumption?) increases with speed at a rate that just offsets aerodynamic drag???? - but on level ground, with fighting gravity not being in the equation, the BSFC and wind resistance balance out at 40?

This sounds reasonable to me. Larger throttle openings are more efficient due to reduced pumping losses. Climbing hills requires more throttle opening, so it is relatively more efficient than the reduced throttle opening required to travel a flat road.

[wmjinman]

I found the BSFC thread on here and read the whole thing. Verrrrrry interesting. Fortunately, there was a chart for the GM 4.3 liter Vortec V-6, which is what my Jimmy has. Looks like I'm best at between 2000 and 2800 rpm, with 2800 being the best, but VERY close through that whole range. Guess that supports my wide range of "good" speeds going up that hill.

I also noted "Old Mechanic" said a simple vacuum gauge will tell you a lot - keep it between 1 and 2 inches of vacuum and between 1500 and 2500 RPM (I guess that would be 2000 and 2800 in my case), and you'll get your best BSFC. I'll have to re-read the ScanGauge instructions & see if I can get manifold vacuum on it. If so, I'll have to start looking at that. If not, maybe I'll just have to go out & buy a vacuum gauge!!

This is exciting - learning new stuff!! I'd love to hit 35 MPG in that thing. I thought I might hit 33 today, but only got to a best of 32.9 on the "current trip"

[user removed]

I would guess that the scan gauge reads MAP which is the opposite of vacuum. It's called Manifold Absolute Pressure. If your ambient pressure is 30 inches then 0 on a vacuum gauge would be maximum maifold pressure. Most here will tell you 80% load is ideal which would work out to around 24 inches of manifold vacuum. What you want to avoid is enough throttle to bring on WOT (wide open throttle) enrichment which lowers you air fuel ratio. Most BSFC maps show full throttle to be less efficient than 80% load or a few inches of vacuum reading. Basically you want to avoid the last few points of throttle just before it is floored to stay out of WOT enrichment. If you don't have precision instrumentation then just avoid the last 10-15% of throttle position.

regards
Mech

[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.

[redneck]

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.

I also find this to be true.

It is more easily seen with sensetive instrumentation. (MPGuino in my case)

>

[wmjinman]

Thanks a million, guys. I'll go out & see if I can apply that and get positive results. This may help my averages a lot, because there are a LOT of hills/grades around here & I never really knew how to tackle them before.

But what about a gradual enough grade to keep it at best mileage speed with cruise control? I guess as long as I don't go above about 80% throttle and/or lose too much vacuum, eh? And if it's "too low" for maximum efficiency vacuum/throttle, it just means it's gradual enough to just treat it as "level" and continue at max mpg speed?

(just a bit confused by PaleMelanesian's report of getting better mpg by slowing at the bottom of the hill & actually acceleration up it. ???? All I can think is that's getting into the P&G techniques - keep throttle/engine at max efficiency even if it means accelerate, then when speed gets too high, due to aero, coast back down & start all over??)

[PaleMelanesian]

Exactly. I pulse and glide everywhere, all the time. In this case, the hill just makes for a very long pulse, with a long glide down the other side.

[serialk11r]

redpoint, it does not take energy to hold position on a hill :bonk: basic physics.

The problem with crawling up a hill slowly is only a matter of engine efficiency.

PaleMelanesian's method of trying to accelerate up a hill is what I try to do, although I don't start that slowly (when there's traffic around I try to reduce speed fluctuations), and I try to pick up less speed going up because my 5th gear is extremely short and prone to overspeeding and thus wasting fuel on aero drag. Above 60% load the efficiency gains from increasing load start to diminish, so I try to hold at ~60% load (I don't use a gauge anymore but my butt dyno is somewhat well calibrated at this point), which allows me to pick up speed just slightly up a lot of hills. My personal guideline is to not pick up speed too quickly because it bothers other drivers, and my load is adjusted for each hill.

[redpoint5]

Originally Posted by serialk11r

redpoint, it does not take energy to hold position on a hill :bonk: basic physics.

The problem with crawling up a hill slowly is only a matter of engine efficiency.

Your basic physics are incorrect. Without a force counteracting gravity, a vehicle will roll downhill. That force is provided by the engine and can be tested by stopping on a steep hill in an automatic transmission car. It takes some amount of energy to prevent the car from rolling downhill when the brakes are released. It takes yet more energy to produce forward movement.