red85berlinetta

hey guys i have a pretty good question for ya, i was reading te other day somewhere about engine efficieny in the fact that at lower rpms sometime it will use more gas because its less efficient. now heres my question, when im cruising around 40 mph in OD im at about 13-1400 rpm and when in drive im at about 1900 rpm...which is right where i am on the highway when i can get close to 20 mpg. so the main question is with the cam i have and intake shifting the power band at cruises around 35 - 45 mph can i actually get better gas milage leaving it in drive because of the motor actually being more "efficient" at this point? thanks i would try it out and see the difference but dont want to find out its way worse after i waste a tank of gas... thanks alot guys

DuronClocker

iTrader: (1)

I've actually been wondering the exact same thing. I'm not sure yet, I've been driving around in drive for a little bit the last day or so. Haven't checked mileage yet. I'll probably do that tomorrow after I burn some more gas.

I'm not too sure though..at 40mph, in 3rd gear I was at 1850rpms and an injector pulse width of 4.xx I think. Yet I was at 1250rpms or so in 4th gear and had an injector pulse of 2.35ish. I'll do some more experimenting tomorrow, but this sounds like I'm better off in 4th gear..

Though in 3rd gear, there should be less strain on the motor (less load) and it should be a bit more efficient, though the higher RPMs may off-set that..

Gumby

http://www.viragotech.com/fixit/Fuel...encyPower.html

VILeninDM

I am no expert at this, but I think you misinterpreted what you've read. Engine is more efficient when it is under more load, not necessarily more RPM.

So if you think about gears, 4th gear gives you less torque than 3rd gear, and therefore, engine in 4th engine has to work harder (i.e. more throttle) than in 3rd. When you shift into 3rd, you would probably notice that no your foot has to press less on the throttle to hold the same speed.

If you think about it another way, considering you want to maintain a constant speed, in 3rd you have more power that you are not using than in 4th. Meaning that if you press the pedal in 3rd you will accelerate harder than in 4th. Some of that power you are not using will get wasted by the engine, so 3rd wastes more.

Gumby

I have come to the same conclusion. Gas last longer when I leave it in 4th {I have a T5} rather then hit 5th and drive around at almost idle.

I do all around town driving 40-50

I run a vacuum load gauge. Hooked into my egr circut so it only show when the pedal is used. I get the most vacuum/load
-13 -14 when I am in 4th doing 2000-2300

Other wise id have to be flying to get anything close in 5th.

Gumby

A vacuum/load gauge is them using them old vacuum gas mileage gauges. Most newer cars show your MPG using the same or similar method but a tad more accurate with computer input.

AJ_92RS

I would say it's one of those "it depends on....." answers.

My mom and dad were moving down to KY, taking a load of antiques with them in and behind their '98 Tahoe at least once a week.

They would make it Paducha on one tank and that was it.

After 2 months of doing this I had made one trip with my dad to help him carry big stuff. A few hours after we were cruising along I asked him "Do you have it in OD?" He did. It just seemed like the engine was having a hell of a time.

I suggested to him to leave it in Drive and see how that does.

The next two trips he left it in drive and made it to their house on one tank. No stop in Paducha needed.

ME Leigh

An engine is most effecient when running at maximum torque output. Thats not to say that your engine will use more gas in OD at lower RPM's. There is really know way to know what is most efficient. But the best way to get more efficiency is to have is much torque as possible at your cruise RPM. This is what makes an LS1 so good. It still makes tons of power at lower rpms where it is cruising in 2OD. It all depends upon where your cruising and how much power your making. But an engine turning less rpms will use less power also because its turning slower and using less air and gas because it not using as much per given distance traveled.

I'm not sure it there is an actually theory or equation to calculate this so...empircal data is what really determines it.

red85berlinetta

so then that means i should defintly drive in od seems how my fan is a big drag and i guess to what ive read that since its lower rpm it ther by is taking in less air meaning less fuel is need?

ME Leigh

Not necessarily the car requires a certain amount of power whatever it is at cruising. But cruising at a lower rpm (od) will require more torque from the engine to make the same power required. More torque means more air and fuel are required. Really cruising at a lower rpm requires less power because there is less friction.

Now if an engine is really bad at making lowend torque and power (high revving engines) then it will suck gas up the *** because the engine is really inefficient in the cruise range. That is why the 400 hp Corvette gets way better gasmileage than say a 250hp 4-cylinder, even though then engine is smaller.

I'm not purvey to a simple formula to calculate this, it will require extensive engine, automobile, and driving cycle data. You might be able to come up with a abominable estimate using a road load equation and some dyno data. I have done these computer models before, modeling several prototype hybrids that i have worked on. The modeling software alone is hundreds of thousands of dollars.

ME Leigh

Oh yeah it is generally assumed that cruising in OD above 55mph will save gas. You will really need to get some empirical or test data for your car and driving cycle, to be sure.

Tom 400 CFI

I read the article from the link above and agree with it. It is well written.

I think you should be able to read that article and figure out how it applies to YOUR car/combo.

As a "high miler", and **** retentive milage recorder etc., I will add these points;
*More RPM increases friction, hurting economy. The lower the RPM the lower the friction.
*Though the engine has less friction as speeds below the torque peak, it is possible that it may consume more fuel, due to volumetric inefficiencies and poorer combustion
*Finding the best compromise is a trial and error approach.
*Anything above the torque peak will definitely be worse fuel economy; more friction & less efficient VE

In my two Trans Ams with 4 engine combos, three trans combo's, and three rear gear combos over the years, I have noticed one utterly consistent thing: They all get the BEST fuel economy (in mpg) between 75-80 mph. I could make a graph that would look the typical "bell curve" from all the records I've kept showing fuel economy in OD from 55mph in 5 mph increments, up to 85 mph, and with ALL the combos I have had, 75-80 yielded me the best results consistently; 24 mpg. 75-80 may not correspond with the torque peak exactly, but it is the point where engine efficiency and aerodynamic drag cross.

Now it gets much more confusing in around town driving because the driving itself is so inconsistant, but here are a few things to think about:
*If you have a EFI car, and you're coasting down a hill that is steep enough that the engine alone won't hold it back, or if you're decell'ing to a stop, the best fuel economy fuel economy w/your foot off the gas and RPM's above 1200, due to enleanment mode (no fuel injected into the engine)
*If going down a hill such that some throttle is required so as not to slow down, (like nuetral throttle) you will achieve better fuel economy in nuetral at idle.
*If carb'ed, down hill coasting in nuetral will always give the best results.
*Upshift as soon as possible and use little throttle to accelerate.
*Use the highest gear practical -even climbing hills.
That's about it that I can think of. This comes from a guy that consistently got 24 mpg out a 3.45 geared, 5 speed Trans Am w/a 400 CID engine, and currently gets up to 22 mpg out a 5 speed, Chevy 1500 4x4, xcab.

Streetiron85

I spoke with a guy named Dave Emmanuel once. At the time he was a writer for PHR and back at that time, writers didn't seem to have a problem with talking to readers they've never met before. Anyhow I had some questions about a project car that was featured in the mag named Project Econo Performer, and one of the points that he stressed was that best economy was achieved below 2000 rpm. So if you want fuel efficiency, you have to build a combo that focuses on developing power below the torque peak.
Project Econo Performer was a Monte Carlo with a 350 that got 26+mpg hwy and ran in the low 14s.
It had a QJet, some 4-2-1 headers, reworked factory heads, a small Crower cam, 3.08 gears... you probably get the picture.

I've searched for the article, with no luck.
Hopefully something here is helpful to you though.

ShiftyCapone

iTrader: (7)

Only viscous friction will increase as RPM increases. That would be friction due to oil and air resistance. All dry friction stays the same no matter what RPM (ring to cylinder wall contact and bearing friction). As Mr Leigh said an engine is most efficient when it is opperating at its peak torque. It makes the most power per fuel consumption. However, you do not need peak power when cruising. In fact, a typical car on a flat surface only needs 20 to 60hp to maintain it at 60mph with no head wind. The reasons engine make more are for acceleration (we all know this). Running an engine at its peak efficiency would also increase wear and tear. The more rotations you have per given distance the faster the engine wears out. The car and engine are desinged as one big compromise. Low engine speed to increase longevity, all opperating at a low engine load to keep fuel consumption down. They will sacrifice fuel consumption for longevity. I have created some pretty good fuel consumption calculation models that are really extensive and highly accurate. You can basically punch in a bunch of numbers and it will theoretically tell you your fuel consumption at every rpm point for ever gear. I had to create it and use it as a working model for various IC engines problems.

ME Leigh

That is completly wrong, there is more piston to cylinder friction because there are more rpms. Which means that the piston travels further distance in the bore per unit time, and distance traveled by the car. The same for the bearings, even though they are riding on a film of oil.

Apeiron

I think that what you mean is that more energy is expended overcoming that friction. The amount of actual friction is the same.

ME Leigh

Ok, yes, the coefficient of friction does not change nor does the friction force, but the work due to friction sure as hell does.

Getting all technical and **** on me!

Tom 400 CFI

If I gave you some numbers from my old car, could you crunch them and tell me what fuel economy I SHOULD have gotten? I'm curious about this.

-Tom

ME Leigh

Yes there actually is a pretty cheap program that calculates gas mileage. I believe its called gas mileage calculator. It could be a Performance Trends product, i'm not going to look now because i'm busy.

ShiftyCapone

iTrader: (7)

This is what I meant and should have said.

Correct. This is what I really should have elaborated on. I was clarifying the basics for those that were getting confused up top.

IROC_385Z

I think an ez way to look at this would be this:

An engine puts out X amount of power at a givin RPM, using a certain amount of fuel.

At a higher RPM the engine can make the same or MORE power using the same amount of gas....but since you will be using more RPMs it will use more total gas....just that it makes the same or better power comparativly.

Does that make sense or is it too wordy?

Look at it in terms of how many times the motor spins. If the motor spins 1500 times a minute and uses 10 gallons (I know it wont use that much but...) in that minute it would have used 10/1500 gallons each individual revolution in the minute.

Now if you spin the motor twice as fast, 3000 times a min (RPM) and used 15 gallons of gas, then the motor used 15/3000 per spin...

10/1500 = .007 gallons per revolution
15/3000 = .005 gallons per revolution

So technically the engine uses less gas...per single RPM...but in that same 1 minute time it will eat 5 gallons more RPM uses more gas!

I hope I didn't bore or confuse anybody....thats my .02

rx7speed

just curious where and how much did it cost you to get something to mesure pulse width?
I have been looking for something like that to use on my car


another piece of info I read regarding fuel efficiency (though I by no way claim it to be true though it does seem to hold up with my car) is when accelerating from a stop to not granny it but give it maybe 70% of throttle or so.
what this does it granted use more fuel at that instant but gets you up to cruising speed and throttle usage (ie more efficient rage) a lot quicker which in turn will give better overall gas mileage

nsimmons

Have to agree with this. I read an article in c&d a few years ago and they concluded stabbing the throttle and short shifting provided the best in town mileage.

Of course i drive full throttle to redline from every stop light...

Gummie

That makes sense considering work = force * distance so you can actually do more work (burn more gas) by applying a smaller force over a greater distance. How much exactly would varry from car to car.