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Old 02-08-2010, 05:27 PM   #11 (permalink)
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Quote:
Originally Posted by SentraSE-R View Post
1200 rpm is probably too low an engine speed for cruising. Your engine's BSFC graph should point you to where you want to be. Usually, your best efficiency is between about 1500 and 2200 rpm for 2 liter class engines.
True, but potentially misleading. (It needs qualification.)

Your best efficiency might be in that RPM range when accelerating/climbing and you want high load. But it's not correct to say you'll get better fuel economy cruising at that RPM when you could use a higher gear.

(I realize you then went on to describe pulse & glide, but I just wanted to bring this up in case someone doesn't make the connection.)

For someone who doesn't want to pulse and glide, driving with load (DWL) at low engine RPM is a good alternative.

I'd have no problem cruising at 1200 RPM provided the engine will do it without complaining (and by that I mean physically juddering). Bestclimb's post above describes lugging nicely.

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Old 02-08-2010, 05:35 PM   #12 (permalink)
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Quote:
Originally Posted by greasemonkee View Post
The end result - more negative work applied to the reciprocating assembly, more wasted work than driving higher rpm. This is a general rule with 1000's of variables, but there is a window where your loading/rpm relationship yields the lowest bsfc. Finding that window is the hard part.
My experience: I've yet to find a car where (in real world circumstances) the best cruising fuel economy isn't in the highest gear possible without lugging. I don't think the "best" engine RPM/load window is hard to find.

At cruise, the rule of thumb is: upshift, unless it lugs.
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Old 02-09-2010, 03:06 AM   #13 (permalink)
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What about when it shifts into open loop or sub stoichiometeric operation? Seems like an economy loss would be eminent once that occurs.
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Old 02-09-2010, 03:33 AM   #14 (permalink)
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Lugging an Automatic

So, I have modified the automatic to lockup the Torque Converter at the flick of a switch -- if started in 2nd, it has been possible to stall the engine and override the ECU/TCU.

With that said, I have wondered the same about lugging. Typically, I can get the revs down to 1100, which maxes engine load and has been noted to warm-up the vehicle quicker than slipping with the slush (or even lower with 2nd-gear start). The best feature is the throttle lift-shift that generally produces more of a vibration than "lug". To engage full lockup, it requires significant throttle input -- which is a give-and-take. Transmission warmup is faster as well.

FE has improved with the mod, but with Winter, the percentage is hard to estimate. The benefit of essentially having a "heavy manual" is nice.

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Old 02-09-2010, 08:25 AM   #15 (permalink)
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Quote:
Originally Posted by bestclimb View Post
I lope along at 1000 rpm frequently l, 5th gear and mph, the engine is smooth so no worries. If I try to increase my speed and the engine vibrates it means I am applying too much pressure to the accelerator or I need to down shift.
Very good point - at low load, you can go all the way down to idle. When you open up the gas, though, it starts shuddering at that same rpm.

Quote:
Originally Posted by MetroMPG View Post
My experience: I've yet to find a car where (in real world circumstances) the best cruising fuel economy isn't in the highest gear possible without lugging. I don't think the "best" engine RPM/load window is hard to find.

At cruise, the rule of thumb is: upshift, unless it lugs.
I concur.

Quote:
Originally Posted by greasemonkee
What about when it shifts into open loop or sub stoichiometeric operation? Seems like an economy loss would be eminent once that occurs.
Also a good point. I know, from the Scangauge, that my car goes into Open Loop at >95% LOD - pedal all the way to the floor. I monitor that gauge and keep it below that point.
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Old 02-09-2010, 11:05 AM   #16 (permalink)
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Good points by Darin (metrompg) and Andrew (pale). I agree. Best cruise mpg is during lowest cruise speed in your highest gear.

I just had the "opportunity" to lug my engine due to a clutch problem, and retract my original impression. It is, indeed, possible to damage your engine by lugging it too low in rpm at too low a speed. I had mine shuddering due to starting without a clutch in 4th gear. It's possible, and you should avoid it.
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Old 02-09-2010, 01:17 PM   #17 (permalink)
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So the concensus seems to be: it's OK to cruise at low RPM and low load.

If you need more power, you should downshift if:

1) the engine lugs (shudders) when you push the accelerator further, or

2) you cause the engine to go into open loop (typically at very high loads - ie. pedal near the floor), which can be monitored with a ScanGauge
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Old 02-09-2010, 01:27 PM   #18 (permalink)
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Quote:
Originally Posted by MetroMPG View Post
So the concensus seems to be: it's OK to cruise at low RPM and low load.

If you need more power, you should downshift if:

1) the engine lugs (shudders) when you push the accelerator further, or

2) you cause the engine to go into open loop (typically at very high loads - ie. pedal near the floor), which can be monitored with a ScanGauge
Those vibrations can get really bad really quickly if you're loading up the engine like that, so I'd have to agree. Plus, when you're making the engine vibrate wildly, you're wasting fuel... that vibration energy has to come from somewhere, eh?

I can put my truck into 5th gear at 20 MPH and still get up to speed, but it takes forever and a very light right foot. I rather do a 2/4/5, and I've only had it for a week, and driven it just over 100 miles, so beyond "first impressions", I'm not an authority on how to drive my truck, by any means. I'm still waiting to get it fixed up and put the MPGuino in, so I can have some real-time feedback and try a few different things.

I also think that the tenet we endorse which has people accelerating at high load/low RPM needs to be further explained because of this. It should rather state that accelerating at the highest load you can use without creating excess vibrations or shuddering/detonation at the lowest RPM for your speed (highest gear) may be the most efficient. This introduces a new idea, that maybe the absolute highest gear you can use isn't necessarily best for your situation, and you should adjust accordingly, because even though you're at a higher RPM/lower load in a lower range, you may be closer to "optimum" for the shorter time you're accelerating.

I know it seems like common sense, but that's not nearly as "common" as the name would imply, and people do follow suggestions to the letter, as though they were directives.
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Last edited by Christ; 02-09-2010 at 01:33 PM..
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Old 02-09-2010, 03:31 PM   #19 (permalink)
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I think that for acceleration, you're still best going by the best BSFC window, not lowest RPM at which you can accel without bogging.
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Old 02-09-2010, 07:21 PM   #20 (permalink)
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Quote:
Originally Posted by MetroMPG View Post
I think that for acceleration, you're still best going by the best BSFC window, not lowest RPM at which you can accel without bogging.
I guess I kind of labeled the thought incorrectly. I should have said - "That has been tossed around". After posting that, I was looking around for it, and could only find general mention of it, not necessarily anyone saying that it works/doesn't work.

I'm kinda working on trying both ways, just to see if there is an obvious difference. With 4.9L of displacement when you're used to 2.4 or less, it seems almost monumentally important to keep the RPMs as low as possible, and that may not necessarily be the best way to do it.

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Last edited by Christ; 02-09-2010 at 07:29 PM..
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