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Old 05-01-2008, 10:45 AM   #1
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P&G Questions

Is there a sticky here that:

1. Explains, in detail, how to drive using P&G techniques,

-or-

2. Has links to sites that explain P&G?

Yesterday, after reading a site (IIRC linked by PaleMelanesian) about BSFC, I can understand the fundamentals behind P&G. Question is, it seems like I'm accelerating "violently" (relatively speaking) ... not sure if I'm doing it right...
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Old 05-01-2008, 12:13 PM   #2
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Look in the articles section at CleanMPG. Or look it up on MetroMPG. Both have explanations of how it works. Yes, the higher acceleration is counterintuitive at first, but it works. If you keep the rpm low enough, though, you're not going to be all that violent.
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Old 05-01-2008, 01:03 PM   #3
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Quote:
Originally Posted by PaleMelanesian View Post
Look in the articles section at CleanMPG. Or look it up on MetroMPG. Both have explanations of how it works. Yes, the higher acceleration is counterintuitive at first, but it works. If you keep the rpm low enough, though, you're not going to be all that violent.
I read some stuff on both sites. The CleanMPG info seemed mostly about hybrids (admittedly I didn't dig real hard or deep) and the MetroMPG stuff lacked detail.

I guess my question is this: do I focus on my speedometer or on throttle angle and tach?

The reason I ask is due to the issue of the BHP SFC charts linked on the other site (that I think you provided). If the optimal SFC is at a particular load and RPM, should I try and hit that or simply focus on the car's speed? I've been focusing on RPM and throttle angle, and as such the accelerations seem rather brisk.

I realize one answer will be "buy a ScanGauge II." I will eventually, but in the meantime, can I P&G and improve FE?
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Old 05-01-2008, 01:32 PM   #4
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I highly recommend the scangauge. In the mean time:
Manual trans:
Keep the rpm below about 2,200. Use 1/2 to 3/4 throttle until you get up to your top speed. Let's say that's 55. Then pop into neutral and coast down to 45 or 40. Back into gear (probably top ear, but whatever will give you 2,000ish rpm) and pulse up again. Up-down-up-down. Sometimes I start the pulse in 4th and shift into 5th partway through.

Automatic - you'll have to use less throttle, to avoid a transmission kickdown. That's a killer on fuel economy.
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Old 05-01-2008, 01:57 PM   #5
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Quote:
Originally Posted by PaleMelanesian View Post
Manual trans:
Keep the rpm below about 2,200. Use 1/2 to 3/4 throttle until you get up to your top speed. Let's say that's 55. Then pop into neutral and coast down to 45 or 40. Back into gear (probably top ear, but whatever will give you 2,000ish rpm) and pulse up again. Up-down-up-down. Sometimes I start the pulse in 4th and shift into 5th partway through.

Automatic - you'll have to use less throttle, to avoid a transmission kickdown. That's a killer on fuel economy.
Don't have a manual trans.

OK. I've been trying to emulate what you've described by using "1," "2," and the OD button. Will continue to do so and track MPG.
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Old 05-01-2008, 02:05 PM   #6
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There's no need to use the 1 and 2. You want to be out of those gears as soon as you can, not hold it in low gear. I'd leave in Drive and work at keeping the rpm low. Once you know your car's shift points, let up when you reach that speed, let it shift, then get back on it, only now you're in a higher gear / lower rpm.
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Old 05-01-2008, 02:15 PM   #7
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Originally Posted by PaleMelanesian View Post
There's no need to use the 1 and 2. You want to be out of those gears as soon as you can, not hold it in low gear.
Indeed. What I've been doing is at lower speeds, to use the "1" or "2" to get to an appropriate throttle angle/RPM, followed by a shift to "N." Of course, I don't do this at higher speeds.

Quote:
I'd leave in Drive and work at keeping the rpm low.
I was under the impression, from the BHP SFC graphs, that too low of an RPM is bad (of course is worse with low loads), and that a target RPM was desirable...? No?

Gonna have to get that damned scangauge....
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Old 05-01-2008, 02:35 PM   #8
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I think you're over thinking the low speed part of it. Just let the auto do its thing up to 20-25 mph, with a gentle throttle. Then work on P&G. Also, with an automatic, DWL is a bigger help. Driving with Load - lose speed on the uphills and gain it on the downhills.
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Old 05-01-2008, 02:48 PM   #9
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Quote:
Originally Posted by PaleMelanesian View Post
I think you're over thinking the low speed part of it.
Probably. Heh.

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Also, with an automatic, DWL is a bigger help. Driving with Load - lose speed on the uphills and gain it on the downhills.
Got it.
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Old 05-01-2008, 03:25 PM   #10
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I've only recently started doing P&G, so I'm no expert. But I think I've arrived at a specific way of thinking about it that makes the technique fairly easy for me to understand and implement. I haven't seen anyone else explain it exactly this way, although my technique is 100% derived from reading the material that others have kindly provided.

The basic underlying idea is that a gas engine is most efficient with a large throttle opening. Theoretically, the most efficient setting is when the throttle is completely open (otherwise you get what are called pumping losses, where the engine is doing extra work to overcome the vacuum created by the throttle). But as a practical matter, most (all?) EFI cars open the injectors completely upon full throttle, and this wastes gas (it dumps in more gas than the engine can use). So in real terms, the most efficient throttle setting is perhaps, say, 70% open.

So what I do is I try to adopt the idea that my throttle is a binary switch, with only two acceptable settings: off (completely closed), and 70% open. Let's just call it OFF and ON. That's the key concept, for me. Then I adjust my driving to take this into account. I strive to avoid any throttle setting other than those two.

So let's say I'm driving along a typical suburban road, at about 40 mph. Let's say the speed limit is 45 mph. The traffic is light-to-moderate. I'm in top gear, with the throttle 70% open (ON), and the road is flat. This means I'm accelerating. So I let my speed get to, say, 50 mph. This brings me pretty close to the car ahead of me. So then I drop into neutral (I could also turn the motor off, but the important thing is to at least drop in neutral). And now of course the throttle is OFF (if I didn't turn off the motor, some gas is being used to keep the motor idling, but to keep things simple let's still call this 'OFF'). Now I'm coasting. I let it coast down to, say, 35 mph. Then I shift back into top gear and I turn the throttle back ON, that is, I move the throttle to the other acceptable setting (70% open.) Now I'm accelerating again, from 35 mph. When I reach, say, 50 mph, I drop into neutral again, and turn the throttle OFF. Rinse and repeat.

If conditions are right, and if I'm skillful, I can avoid annoying the driver behind me and the driver ahead of me. At the moment I hit 50 mph and am about to start coasting, I am as close the car in front of me as I am willing to safely go. At the other end of the cycle, when I am done coasting and am about to start accelerating again, I have dropped back and gotten close to the car behind me. But not too close. If traffic isn't very heavy, and there's a reasonable space between those two cars, I'm simply using that space fully, without getting too close to either of them.

Another important idea is keeping revs as low as possible, which means upshifting early (about 1200-1600 rpm, on my car). But when I'm doing a nice long P&G routine, doing (let's say) 35-50 mph over a long distance, the only gears I'm using are 5 and neutral. And I'm not paying attention to the tach. I'm mostly focusing on the two acceptable throttle positions (OFF and ON), and I'm paying attention to the speedo, and my position relative to my neighbors (in front of me and behind me).

"do I focus on my speedometer or on throttle angle and tach?"

As I explained, I'm paying a lot of attention to throttle angle, and also to the speedo. But I'm paying no attention to the tach. This assumes I'm going fast enough (say, over 25 mph) where the only two gears I need are neutral and 5.

Of course if I approach a grade, things change. I might have to use 4, or 3. But I still want to follow the rule about ideal throttle opening (either closed or 70% open), regardless of what gear I'm in. If I'm at 70% throttle in 3 or 4 and getting too close to the car in front of me, that means I should have shifted to 5, or I should be coasting ('gliding'). In fact, I can apply these simple rules to pretty much any driving situation. If throttle ON is giving me too much speed, that means I should be shifting up. And if I'm already in top gear, and I have too much speed, that means I should be coasting.

Conversely, anytime the throttle is OFF (i.e., I'm coasting), and I find I don't have enough speed (to avoid annoying the driver behind me, let's say), that means it's time to turn the throttle ON. And in the highest possible gear.

There's one more idea that's helpful. It has to do with the fact that coasting ('gliding') really comes in three flavors, as follows:

A) coasting in neutral; the engine is idling
B) coasting in neutral; the engine is off ('engine-off coasting,' or EOC)
C) coasting in gear; engine braking is slowing the car down

The interesting thing is that C uses less gas than A. Why? Because when I'm coasting in gear (i.e., using engine braking), then the injectors are completely off. If I'm coasting in neutral (with the engine on, throttle closed), the injectors aren't completely off; they have to provide gas to keep the engine idling. So sometimes it's better to use C instead of A. But this would be the case only when my intention is not to maximize coasting distance.

This concept becomes relevant in the following situation (or in similar situations). I'm cresting a hill. There's a long, moderate downgrade ahead, and then a stop sign. I know I will have to stop, at the bottom of the hill. I know I don't need throttle ON to get down the hill. Gravity and momentum is more than enough to get me there. B would be ideal, but let's imagine I prefer to avoid that, for whatever reason. I still have two choices: I could use A, and then use my brakes at the bottom. Or I could use C, and avoid A, and then perhaps not need to use my brakes. This latter technique saves gas. Why? Because C is more efficient than A.

I could also just have the engine off while I glide down the hill (B). That's fine too. The key point is to realize that in this particular situation I can save a little gas by using C instead of A (and C should consume no more gas than B).

Note this related paradox: an engine at 3000 RPM can be using less gas than the same engine idling. How is that possible? When the engine at 3000 RPM has a closed throttle, and is being driven by the car's momentum (i.e., it's in engine-braking mode).

"I was under the impression, from the BHP SFC graphs, that too low of an RPM is bad"

No, I think this is a bad misunderstanding on your part. Lower RPM is always better, assuming your engine isn't pinging. And assuming your engine is generating enough power to do what's needed (e.g., push your loaded car up a grade at a speed sufficient to avoid annoying the driver behind you). If you can't get up the grade, downshift and get higher RPM. But always seek low RPM, whenever possible.

There are many folks here who know a lot more than me, so I hope they chime in with any corrections. Those folks include Pale. I've learned a lot from his posts. I think what I've said is congruent with what he's saying in this thread.
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