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Old 03-29-2010, 09:27 AM   #11
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At low rpm it doesn't take much throttle to drop vacuum to near 0 on the intake since even a small throttle opening allows more air to flow into the engine than it can pump so you really are not doing much by pushing the pedal further other than feeding the ECU with throttle position signals that may richen the mixture even further and push yourself into open loop.

In my Old Geo 1 Liter I got great mileage by gently reving the engine with small throttle - no tach so I don't know how high I was reving but it definately liked high rev very light throttle maybe because it was throttle body injection and vaporized the fuel better that way.
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Old 03-29-2010, 09:45 AM   #12
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I knew quite a few people that had 300k+ on subarus and wouldn't own anything else. that was the family 4X4 where I used to live.

I never got one because they were (and still are) kind of expensive. when snow and ice are something that you have to deal with every winter (along with terrain), you have to have something to get you back and forth to work.

even the police force had a subaru outback sport. saw it around a few times and had a few friends that had the grand experience to see it up close (traffic tickets)

BTW, how did this old thread get brought up???

spot, it seems that all old cars (unless they are saturns) are junk. what's up with that?
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Old 03-29-2010, 10:00 AM   #13
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Quote:
Originally Posted by JanGeo View Post
At low rpm it doesn't take much throttle to drop vacuum to near 0
That is a good thing. Making all that vacuum wastes energy.

Quote:
on the intake since even a small throttle opening allows more air to flow into the engine than it can pump so you really are not doing much by pushing the pedal further other than feeding the ECU with throttle position signals that may richen the mixture even further and push yourself into open loop.
The ECU won't enrich the mixture just because of increased throttle position, unless you're talking about WOT.
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Old 03-31-2010, 05:01 AM   #14
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That is a good thing. Making all that vacuum wastes energy.
Very confused by this.

Any engine is an air pump. It was designed to make vacuum.

I thought it was better to see more vacuum if you're maintaining speed. That would mean you're using less pedal.

Anyway, however tight money is, you can't just neglect the car because you're broke but, at the same time say you hope the engine doesn't blow up... because you're broke. You've got to make a choice here. Spend some loot on repairs now, or be S.O.L. when your neglect pushes the engine over it's limits and it fails and the repair bills come in at 20x what you could've spent to save it. What is $200-$300 in sensors/tune up if you get another 200k miles out of the car vs. $2,500 in expenses for a new engine swap in 6 months.

Vehicle maintenance is mandatory to ensure longevity. Stop making excuses. You pay $ now, of you pay $$$$$$$$$$$$$$ later.
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Old 03-31-2010, 06:54 AM   #15
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Yes, vacuum means you're using less pedal, but that's not the most efficient way to run. Vacuum in the intake doesn't make the car go forward, it's just energy wasted on making a vacuum.

The engine is an air pump, and a vacuum on the supply side (intake) is a waste. You want atmospheric pressure there and then you want it pumped out the other end (exhaust) easily. You want pressure is in the center of the system where the work gets done (cylinders).

One of the reasons that diesels are more efficient is that they don't waste energy pulling a vacuum by throttling (strangling) the air supply. The new direct injection gas engines try to emulate this. To a lesser extent, normal indirect injection gas engines with drive-by-wire throttle can (and some do) do it too, the same way I do it.

I always floor the gas pedal and shift low to take advantage of not wasting energy making vacuum.

The reasons that using less pedal and seeing more vacuum can be more efficient are:
- Most cars will go rich if you floor it (my car is an exception); but you could use 75% of the pedal to avoid that.
- In an automatic, the pedal also controls the transmission, which will shift higher if you lay on it.
- If you're not controlling your acceleration and speed by shifting low, then using more pedal means you're going to go faster, which uses more energy (which you've just produced by flooring it up to 3000RPM instead of 1500).
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Old 03-31-2010, 07:34 AM   #16
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What about torque curves? Powerbands?

Why not shift where the engine is making optimal forward momentum at it's designed rate, at the torque peak?

I think we should clear the air on this subject really. How do your techniques related to highway driving? I assume you use light throttle pressure the maintain a speed, like most people, or do you not? Do you get on the gas and then coast (in gear) until you've slowed down to a point you must throttle up again?

I get what you're saying, it's clicking. I drive like that to an extent in my automatic... sometimes. When I'm feeling frisky I think it may be more beneficial to "gun-it" up to 35mph where my transmission shifts into OD then ease on off the pedal and maintain speed.

More often than not though, I just give moderate gas (15-20 TPS) and let off a little (maybe 5 or 6 TPS) to let the car shift into the next gear a little earlier.
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Old 03-31-2010, 07:56 AM   #17
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I don't have graphs of torque curves, powerband, etc for my car. Besides, those aren't what matter for FE; a BSFC graph would matter though, if you want to do it that way. Dyno charts don't have enough dimensions; they only tell you what happens at WOT. That's fine if you're only driving at WOT but we're talking about partial throttle now too.

Anyway, I do it by experimenting and seeing what works for me.

For highway driving, if I really want to work hard to save gas I'll P&G. WOT, then neutral, then back into gear and WOT again. It's a lot of work, though. Usually I don't want to put so much effort into it so I mostly set my cruise control and forget about it. For decent descents (hah!), I will cancel the cruise and coast in neutral (engine on or off per my mood).

I tried "get on the gas and then coast (in gear)", a technique which I named "Pulse & DFCO". It was less efficient than steady cruising...while being a whole lot of trouble. It was really a terrible way to drive and I couldn't tolerate it even if I got 100MPG doing it. Maybe it wouldn't be so awful in a different car with taller gears or an automatic with an unlocked torque converter.

My driving is 40% highway. The rest is light stop-and-go driving through towns and rural areas. In that kind of context, you really get to use the WOT+shortshifting combination as well as P&G and EOC. In dense, hyper city driving you can't EOC and you can't P&G as much but the rest applies well.
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Old 03-31-2010, 05:02 PM   #18
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What about torque curves? Powerbands?

Why not shift where the engine is making optimal forward momentum at it's designed rate, at the torque peak?

I think we should clear the air on this subject really. How do your techniques related to highway driving? I assume you use light throttle pressure the maintain a speed, like most people, or do you not? Do you get on the gas and then coast (in gear) until you've slowed down to a point you must throttle up again?

I get what you're saying, it's clicking. I drive like that to an extent in my automatic... sometimes. When I'm feeling frisky I think it may be more beneficial to "gun-it" up to 35mph where my transmission shifts into OD then ease on off the pedal and maintain speed.

More often than not though, I just give moderate gas (15-20 TPS) and let off a little (maybe 5 or 6 TPS) to let the car shift into the next gear a little earlier.
gunning a tad right before you're about to shift a manual, gives your car a little momentum boost that helps you get it into the next gear more quickly hence saving a little of gas instead of floundering the wading pool of your clutch and gears at low rpms
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Old 04-01-2010, 12:31 PM   #19
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did anyone ever figure out which one is more efficient?

Full Throttle at low RPM, vs High RPM and low throttle
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Old 04-01-2010, 10:41 PM   #20
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did anyone ever figure out which one is more efficient?

Full Throttle at low RPM, vs High RPM and low throttle
I think it depends mainly on the vehicle. Where the engine goes into open loop and starts reading the pre-set fuel tables for heavy load. Once it does go into open loop usually the fuel tables will be richer then stoich. The reason is to keep the engine from killing it self due to low air velocity and all the heat that's created from a high load low rpm condition.

In the beginning I was confused on why people were claiming better FE by using a higher load, low rpm shifting technique. Because anytime you push the gas pedal down the engine will produce more HP and with more Hp means more fuel that needs to burned.

So I crunched some BSFC numbers with different load values into my spreadsheet to do some comparisons. The first thing I found out was that even by running a much better BSFC number as seen in the BSFC maps of certain engines, the data would show more fuel was being used for the mere fact of a higher HP number from high load.

Then it hit me while doing some RTP testing on my Honda. The one thing I forgot all about was time.

As a example:

A engine running a BSFC of 273 g/kwh will use 0.0277 lbs/sec. and make 10HP with a load of -15.7 inch/hg of vacuum

A engine running a BSFC of 213 g/kwh will use 0.0549 lbs/sec. and make 20HP with a load of -2.04 inch/hg of vacuum

So from these two examples it looks like the one with a the lighter load (-15.7inch/hg) will consume less fuel at 0.0277 lbs/sec even with a poor BSFC of 273 g/kwh.

But what about the amount of time the first engine takes to get to the say 50mph or the next shift point? It could take as much as four times the amount of time verses the second engine. The second engine is running 10 more Hp so it will accelerate faster to 50mph or the next shift point then the first engine.

So now we have 0.0277 lbs/second x 4 = .1108 lbs/4 seconds verses 0.0549 lbs/1 second to reach the same 50mph or next shift point.

BSFC map from a Saturn
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