Cruise efficiency

[malice10985]

I grew some food for thought as I drove home from work tonite.

Every day, I cruise home on the same road at 48mph. I keep my M5 in 4th gear and it stays at 2000rpm. Manifold vacuum is 13-15 inches, depending on the weather.

If I shift into 5th gear, I can lower the rpm by about 400, but I have to open the throttle much more to maintain speed. Thus, the manifold vacuum goes down to about 6-8 inches.

WHich cruise setting is more efficient? low rpm and low man. vac. or moderate rpm and high man. vac? In fligt training, we have been taught that lower manifold vacuum will be substantially less efficient. But most any car driver will tell you that low rpm is the most efficient.

I wonder if they would be equal. Does anyone know the equation for finding out required cfm? I know you need to plug in displacement and rpm.

I think we would also need to look at a fuel map graph to solve this dilema.

Now that I am done thinking out loud, what do you all think?

[malice10985]

ok, I did some research.

The required cfm equation is:

cfm= (CID x rpm) x volumetric efficiency.
----------
(3456)

WIth headers, I have 75.5% V.E. at 2000 rpm.
(estimated with desktop dyno)

I probably have about 70% V.E. at 1500 rpm. Thus,

at 2000rpm I use up 100.9 cfm of air @ full throttle.

at 1500rpm I use up roughly 70 cfm of air @ full throttle.

I am not at full throttle though. standard atmospheric pressure is 29.92 inches. Subtract 15 inches for vacuum and I have 14.92 inches of atmospheric pressure in the manifold. That is 49% of maximum available air intake, or, 49.4cfm of actual air consumption at 2000 rpm.

That same thought process nets me 51 cfm actual air consumption at 1500 rpm. Like I thought, they are about the same.

Does the MAF see that the air intake is the same and put in equal quantities of fuel for each scenario?

[black98V6]

I always just figured that the wider your throttle is open the more fuel the ECM will dump into each cylinder, therefore the more open the throttle the more fuel you are burning. Now sure there are other factors here such as the density and temp of the air which the computer takes into account but it seems that the biggest factor in deciding how much fuel to spray is the throttle position.

I don't know if our cars are that accurate to see 1 or 2 cfm difference and make up for it, I guess one of the better ways to see how efficient this would be is to look at 02 readings and see what they are..

[ September 09, 2003: Message edited by: black98V6 ]</p>

[Jason McCallister]

Wouldn't it be easier to monitor the injector pulse widths to determine fuel efficiency of your various driving conditions?

It may be possible that in 5th gear, you're applying more throttle but the injector pulse widths are still lower than it would be in 4th gear due to the final gear ratio!? [img]graemlins/dunce.gif[/img]

[Dominic]

48mph in 5th? with 3.23s? Mine doesn't like doing that [img]smile.gif[/img]

I usually don't shift into 5th until Im cruising 55 or more, it just runs better in 4th and seems to even get better fuel economy for me.

[UNFNick2k]

If the road is very level then i can get into 5th at 40mph...otherwise i don't shift into it till 45-50mph

[imported_admin]

My non-scientific experience has been that rpm is not as much an indicator of fuel consumption as vacuum/boost is (i.e. throttle position).

The 3.8 likes to hover above 1800 rpm. Lower than that and it's struggling.

I personally do not like to drive for more than a few seconds at anything more than 10 in the vacuum range (i.e. I like to keep it between 10 and 22--what I see at idle). The car just feels like it's pushing itself, and I just know it's guzzling gas by doing so. This is particularly noticeable if you're running a blower with a 2.73 rear. Sure, on a highway trip my gas mileage was 35+ mpg, but in town it sucked because the engine was struggling so hard just to get the car moving.

RPM does not immediately mean excess fuel is being burned. Throttle position does.

Good to see I'm not the only person noticing this! [img]smile.gif[/img]

EDIT: Assuming 3.23 gears, I don't shift into 5th until 55 mph, to answer your original question.

[ September 11, 2003: Message edited by: Stefan ]</p>

[chas]

is this likely to start the age old debate, should i drive in D or 3?

for automatics, personally, if you arent going to go over 45mph for significant time drive around in 3. why? engine will produce more torque at the higher rpms (2200 or so compared to 1800 or so). more torque means the torque convertor should setttle down faster when locking (less slipping = less heat). shifiting in and out of overdrive (4th) creates heat. also, isnt peak torque the point were the engine is most efficient?

[malice10985]

<blockquote>quote:</font><hr>Originally posted by Stefan:
My non-scientific experience has been that rpm is not as much an indicator of fuel consumption as vacuum/boost is (i.e. throttle position).

The 3.8 likes to hover above 1800 rpm. Lower than that and it's struggling.

I personally do not like to drive for more than a few seconds at anything more than 10 in the vacuum range (i.e. I like to keep it between 10 and 22--what I see at idle). The car just feels like it's pushing itself, and I just know it's guzzling gas by doing so. This is particularly noticeable if you're running a blower with a 2.73 rear. Sure, on a highway trip my gas mileage was 35+ mpg, but in town it sucked because the engine was struggling so hard just to get the car moving.

RPM does not immediately mean excess fuel is being burned. Throttle position does.

Good to see I'm not the only person noticing this! [img]smile.gif[/img]

EDIT: Assuming 3.23 gears, I don't shift into 5th until 55 mph, to answer your original question.

[ September 11, 2003: Message edited by: Stefan ]<hr></blockquote>

I agree with you regarding the manifold pressure cruise settings. I also like the sotp feeling of cruising at slighty higher rpm and vac.

I want numbers to back up our assumtions though.

I always thought that the "ideal" fuel ratio, 14.7:1; was a ratio of volume. WRONG! I found out today that it is a comparison of mass. How I am curious, how much does air weigh? It's sea level pressure is 14.7psi, but how much does a parcel of air actually weigh?

I will research this weekend, and update.

[Camarorulz]

I think I heard somewhere that the converter does NOT lock up when you have it in D, only OD. Makes sense, the car won't shift into 4th in D.

[Ex-5.0]

Generally, lower RPM with larger throttle opening means less pumping losses and less friction so MPG should be higher in 5th gear.

It might not be that simple, however. In addition to the effect of RPM and manifold vacuum on the fuel, the spark advance/knock charictaristics will be different. Probably, the motor can tolerate a lot more spark advance at 2k RPM & 13" vacuum than at 1500 RPM and 8" vacuum. This alone would probably make it more efficient in 4th gear than 5th gear at 48mph, assuming that the PCM allows the spark to be advanced to be near the optimum advance for each condition. That may not be a good assumption because sometimes spark is deliberately retarded at low RPM to improve emissions.

Also, regarding peak efficiency being at peak torque RPM, this is only true at full throttle. In this case, the question is about peak efficiency at part throttle (actually a barely open throttle). Peak efficiency under this condition will be at a much lower RPM than peak torque.

My guess is that the MPG would be about the same in either gear at 48mph. You could experiment and run through a couple of tanks of gas using 4th gear every day and then a couple more tanks using 5th gear to see if you can quantify any difference in the MPG.

[Dominic]

<blockquote>quote:</font><hr>Originally posted by Ex-5.0:
Generally, lower RPM with larger throttle opening means less pumping losses and less friction so MPG should be higher in 5th gear.

It might not be that simple, however. In addition to the effect of RPM and manifold vacuum on the fuel, the spark advance/knock charictaristics will be different. Probably, the motor can tolerate a lot more spark advance at 2k RPM & 13" vacuum than at 1500 RPM and 8" vacuum. This alone would probably make it more efficient in 4th gear than 5th gear at 48mph, assuming that the PCM allows the spark to be advanced to be near the optimum advance for each condition. That may not be a good assumption because sometimes spark is deliberately retarded at low RPM to improve emissions.

Also, regarding peak efficiency being at peak torque RPM, this is only true at full throttle. In this case, the question is about peak efficiency at part throttle (actually a barely open throttle). Peak efficiency under this condition will be at a much lower RPM than peak torque.

My guess is that the MPG would be about the same in either gear at 48mph. You could experiment and run through a couple of tanks of gas using 4th gear every day and then a couple more tanks using 5th gear to see if you can quantify any difference in the MPG.<hr></blockquote>


And I have done this experiment and my mpg is far higher in 4th than in 5th when cruising around town below 55mph for extended durations (down 45mph roads with traffic say doung 45-50mph). max rpm is around 2100 or so and my fuel economy is great. Shifting into 5th (barely) cuts into my fuel horridly.