Good fuel efficiency strategies

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I just recently installed my vacuum gauge and am hoping to improve my mpg's. Mainly this gauge allows you to read the throttle, which is the pressure at the manifold. The pressure is proportional to the air density, and with a fixed air/fuel ratio, this is proportional to the cubic millimeters of fuel injected into the engine, per revolution of the crankshaft.

Driving at 2/3'rds WOT (wide open throttle)--corresponding to 10 inches of mercury-- is most efficient for the typical gas engine, but staying between 85% WOT to 40% is nearly as efficient for a good range of rpm's.

Below is a bsfc chart for a typical 4 cylinder gas engine:

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(taken from this thread)

It surprised me how efficient these engines run at higher rpm's. i assume this chart is for a oversquare short stoke engine; long strokes would be shifted to the left and better for lower rpms. my subie is slightly oversquare, and so i guess i won't hesitate to accelerate at 3500 anymore for fuel concerns.

So, I'm going to assume this for my car:

--Most optimal operation near 2/3 WOT is from 1500 to 3600; I'll try to use this range when power is required (getting up to speed, hills).

--Higher throttles ~80% WOT, will work better at higher rpm's; over 2500.

--At lower RPM's (less than 1750) apply even less throttle, under 1/2 WOT but over 40% WOT.

--Idle seems to be a little under 1/3 WOT (10 inches of mercury) and efficiency near this pressure is 50% that of the peak efficiency.
 
In this day it's dangerous to universally apply BSFC maps to different motors. Stuff like variable valve timing and even EGR can widen the "good zone" or make hot spots of poor MPG.

I put together a hunk o junk Saturn SL1 with sport gearing from a SL2. Highway RPMs at 60 were 2850 instead of 2100. You'd think this was a disaster for MPG but I got 42 driving 60 MPH getting passed by everyone.

I know, too many variables, anecdotal evidence, etc, but my point-- don't be afraid to rev, a fear you recently got over.
 
For better MPG and to eliminate speeding tickets, I try to limit my speeds on the road/hiwy and use the CC often even around town.

The speeding tickets alone in my neck of the woods are costing people ~ $400.00 these days, points on your licence and possibly higher insurance premiums.

Slowing down and using CC has reduced all of these thing for us in our household and maybe even reduced to cost or frequency of brake repairs it seems. Maybe tires too!
 
Originally Posted By: Char Baby
For better MPG and to eliminate speeding tickets, I try to limit my speeds on the road/hiwy and use the CC often even around town.

The speeding tickets alone in my neck of the woods are costing people ~ $400.00 these days, points on your licence and possibly higher insurance premiums.

Slowing down and using CC has reduced all of these thing for us in our household and maybe even reduced to cost or frequency of brake repairs it seems. Maybe tires too!
Cost efficiency is where you find it. Smart!
 
The cam sets the sweet spot for fuel efficiency at a given rpm. Variable valve timing & fuel maps can be different for the same vehicle depending upon the ECU program. If this engine was a straight cammed engine then you would be 100% correct. There are just too many variables to apply this concept across the board.

Most and I do emphasize “most” vehicles designed for USA operation achieve the greatest fuel economy at 45 MPH. Using synthetic oils, premium fuels on a high compression engine, tires inflated to max pressures, and a clean air filter will reflect a positive difference at the pump.
Using proper maintenance and good driving techniques have yielded 38 MPG on my Esteem, 36 MPG on my Sentra, and 30 MPG on my Maxima. All this attention to maintenance & driving habits yield vehicles lasting for 350K miles and still run like new. Now that is cost effective!
 
Here's one: Traffic. I commute 120 miles a day on Southern California freeways, and I've noticed the effect a large mass of vehicles moving at 70 or 80 MPH has on the air my car is pushing against. With my trusty UltraGauge and it's instantaneous MPG readout (calibrated, of course), I've come up with a few data points. On a flat, open highway with no other cars in front of me, cruising at 80 gets me right about 27MPG. On the 91 freeway with 5 lanes packed with cars moving fast, the center lanes seem to have more of a moving airstream than the outside ones. A steady-state cruise in one of those lanes at 80 gives me 33-35MPG. Obviously, drafting another car or big truck boosts things waaaay up there, but you can only do that so long before the driver in front of you gets angry
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I'm talking about normal (for CA, anyway) freeway following distance. If I make a conscious effort to tuck into a string of cars and minimize the time I spend in "open" lanes, I can achieve an extra 2-3 MPG per tank, consistently. I imagine it's partly turbulence and partly the cars "fanning" the air along in one direction, but it works.
 
Who drives at 2/3 - 80% throttle?
Only for short bursts, maybe.

And then you have to figure that if you accelerated harder than that, you would not be at or near full throttle for such a long time - you would be accelerating for a shorter duration.
This may overcome any theoretical advantage to accelerating with partial throttle settings.
 
Originally Posted By: mechtech2
Who drives at 2/3 - 80% throttle?
Only for short bursts, maybe.

I do, almost all the time.
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When I want to save fuel, I do it by shifting early and staying in high gears. I seem to get better fuel economy that way than I do by shifting later and using less throttle.
 
My understanding is, that as a general rule. It is best to accelerate moderately hard up to cruising speed and then let off to cruise. That should yield higher MPG than using a light throttle up to speed.
 
SFCs are great if you have a known load and need to optimize alignment of a plant to minimize fuel consumption. For a highly variable situation like an automobile, I have to wonder how useful they are besides trying to estimate efficiency.

you arent going to load an engine higher to get fewer lb of fuel per hp-hr. Higher loading is still going to correspond in most cases to higher consumption, even if the engine is using the fuel more efficiently.

My best advice is to appreciate physics and drive fr both acceleration and braking as if you have an egg between your shoe and the pedal.
 
I have motorcycles and drive a RAV. I don't seem as concerned about fuel like I did when I had full size mod 4wd trucks. But I don't tromp around the hills like I used to (camping, hunting, fishing, gold prospecting, metal detecting etc.)
And I use a lot less gas now for my work drive.
Going from thirsty mongers to fuel sipping economy was like a extra pay day when I made the change.
 
You'll never save enough gas to pay for the guage. All you need to know is that the smoother and slower you go in top gear,the better your mpg. Don't lug the engine.
 
The best thing you can do to save fuel is look far ahead. Anticipate what you are going to do ahead and then respond early.

I.E. if you see the light ahead turn red, no need to stay at 2/3rds to 80% throttle. Let off the throttle and your car will probably shut off the fuel injectors as it coasts to the next light. (Within reason of course, if you are a mile away, maybe that's not a good strategy...)

Drive SMOOTHLY, anticipate. Moderately aggressive acceleration, but try to preserve momentum. No need to add more energy to the system by staying on the gas if traffic ahead is slowing or stopping.

Also, leave a good gap between you and the car ahead. That way, you are not slamming on the brakes and can simply slow if they turn, etc.

If you want to save energy, then you have to preserve the energy you've already used by avoiding when possible giving up that energy by braking.
 
Hey thanks everyone for the tidbits of advice.

Whether its practical or significant in savings or not, I think there's some fun in knowing when the engine is operating closer to the peak of its capable efficiency.

My previous mental model of reading engine load by ear proved totally wrong and unreliable, so I'm glad I ended up installing this thing.

I didn't spend much on this at all; $25 for a cheap combined vacuum/boost gauge (see below) at pepboys. I did end up spending combined about half a day installing this thing (which involved mainly removing an almost impossible to remove screw plug with a nonstandard 7mm square bit, which didn't quite fit on a 1/4 in square drive--so after mangling it with pliers I filed the 7mm bit down to a 1/4 in bit and finally removed it).

If I had the choice of a scangauge for reading the vacuum I would've done it that way, but since my car is and OBD 1 type that wasn't an option. I wonder why manufacturers don't put this in there, esp for some of the newer cars where you access the on board computer for everything from batt voltage to instantaneous mpg.

I still have to find a better permanent location for the gauge; probably on top of the dash, so I don't take my eyes off the road for a reading.

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Not a bad deal. Wish these automakers would put a numerical temp guage in these new cars.
 
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