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Old 11-13-2014, 12:28 PM   #1 (permalink)
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engine load vs hills on more powerful cars

I was wondering what peoples theories are on going uphill. In a 4cyl avg passenger car like a corolla I used to hold about 70% engine load at roughly 1700 rpm to get up this hill. I would bleed a bit of speed but apparently this was the best way.

On the Subaru which has more torque and slightly shorter gearing , at top gear its churning 1850 rpm with a locked torque converter on the hill. However with only 55-60% engine load the speed stays constant...like cruise control. I can induce vacuum by reducing throttle to bleed speed and not to fight gravity but I'm scared for pumping efficiencies at 50% load.

So how does one balance pumping losses when dealing with the terrain? What's the best way to deal with hills with a bigger displacement engine? Bsfc sweet spot 'window' is narrow and at moderate engine speeds on these cars (between 2400 and 3200 rpm in closed loop).

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Old 11-13-2014, 12:45 PM   #2 (permalink)
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Pulse & Glide.
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Old 11-13-2014, 01:38 PM   #3 (permalink)
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FWIW 2400-3200 rpm = 1244-1659 ft/mn piston speed; well above the 1000-1200 ft/mn "guideline".
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Old 11-13-2014, 04:49 PM   #4 (permalink)
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Your question reminds me of my first theory on hill climbing.

As Pale pointed out, pulse and gliding will get you the best fuel economy even though your speed will quickly fall off. Your engine has the torque available, even in top gear, to gain speed up the hill after a short glide.

I don't pulse and glide up hills because I have a manual gearbox, and don't want to spend my effort and clutch pad on frequent P&G. For this reason, depending on the hill, I'll often gain speed up the hill to crest it quickly and then glide down the backside.

If the hill is so steep that on the backside I will go over the speed limit, I'll coast up the last portion of the hill, allowing my speed to fall off, and then gain the speed back on the downhill.

I never brake going downhill unless there is a stop at the bottom. On the rare hill that is extremely steep, I'll DFCO to control speed.
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Old 11-13-2014, 05:38 PM   #5 (permalink)
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On the topic of clutch wear, my 96 Civic still had its fully-functional original clutch when I retired it at 200,000 miles. I'm not too worried about that one.
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Old 11-13-2014, 09:18 PM   #6 (permalink)
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Clutch wear is a non-issue. The aerocivic's original clutch still had 1/4 of its lining remaining when I replaced the transmission at 510,000 miles after the 5th gear's bearings failed.
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Old 11-13-2014, 09:56 PM   #7 (permalink)
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I wondered this myself and when I moved 3 years ago I drove a diesel straight truck and a cab over style moving truck to name a few. Each one upshifted and sounded lugged at the same spot my honda did. I had a late model gmc van with a 6 speed automatic I believe. It up shifted multiple times on the same spot.
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Old 11-14-2014, 05:00 PM   #8 (permalink)
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Uh, I'd say just don't worry about it, why not press the gas on the Corolla harder so you maintain speed (unless you're trying to slow down)? 75% load or 80% load is probably almost just as efficient if not more than 70%. On the Subie, why would you bleed speed if you don't have to? You can always coast down the hill. If you want to bleed speed just push in the clutch like other people said. Oh I guess you have an auto, go to neutral then? Or just don't slow down until you're over the hill.

The inefficient nature of the typical gasoline engine means that you have to really drastically change the load to see a large efficiency difference. Above 60% load there's not that much gain to be had. Avoiding sub 50% load where you can is the best way to save fuel because that's where the engine starts to get thirstier and thirstier per unit energy produced.
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Old 11-14-2014, 08:06 PM   #9 (permalink)
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In my Mustang and my F-250 I just set the cruise control at the speed limit, unless the speed limit is over 65MPH.... Then cruise control is set at 65 MPH. I'm OK with the results. 40+ "% over EPA combined" is better than the average "nut behind the steering wheel" can claim.
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Old 11-14-2014, 08:26 PM   #10 (permalink)
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If you are looking for best FE, just look at MPG numbers and stick to a relatively high number, I wouldn't worry about engine load and RPM, those don't mean that much in Final drive. If your car can keep its speed consistent up a hill while keeping the same FE, then that's actually really good. I'd only be worried if you gained speed up hill, that would mean you are doing something wrong!

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