MetroMPG.com mailbag: is lowest RPM really best for max MPG with big displacement?
 

Got this question recently from a MetroMPG.com visitor:


This isn't the first time I've read a comment like this about larger displacement motors.

I can say definitively that in all the vehicles I've driven with fuel economy instrumentation, the best cruising efficiency is achieved in top gear at the the lowest RPM. (Level road, constant speed, no other traffic creating aerodynamic effects.)

That said, most of the vehicles I've driven have had small displacement 3 & 4-cyl motors. I've only tried a couple of V6's with instrumentation.

I'm not saying it's not possible that much bigger 6- and 8-cyl motors may behave differently. I just don't have experience with them.

The only way to know for sure is to get instrumentation and try it.

Theoretically, but I recall a discussion that an engine has an ideal "piston speed" of 1350 ft/min (total travel) for the best Economy. I know in my 1.8L, constant load at 1500 RPM is less efficient than the 1800-1900 RPM, which is closer to the 1350 speed mark...

More testing/research may be required :turtle:

RH77

Piston speed? Have to say that's a new one to me.

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Had a followup question from the Jeep guy:

Fortunately, we can answer that one.

Pre-OBD2 vehicles like yours can use a custom electronic gauge called the SuperMID. Check this thread for more information: [CRXMPG FAQ] SuperMID and 1990 Toyota Truck

I remember my dad's Chrysler Intrepid having a flat MPG curve to it too.
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Originally posted April 24th, 2007:

A jeep is not very aero. I would think that penalty alone should kill the FE with speed. Plus with out instrumentation who knows what really going on. I have also heard this before but not from anyone who was using instrumentation.

Interesting. Hopefully some one will chime in who drive a car with excessive power.

Hey, "The Jeep Guy" here, I just wanted to say that, I don't seem to see any difference between 55 and 60mph, I actually see a better miles per tank when I drive 60 rather than 55. My ZJ has a 23 gallon fuel tank, so I'm guessing that it's pretty accurate at fill-up because more gallons are used than smaller vehicles. I've taken my factory roof racks to try and improve the mpg, but I don't feel or see a difference at fill up. But one thing I do notice while driving is it's much easier to accelerate on a half full tank than a completely full one. I've also noticed that I've gotten a slight but noticeable increase in fuel economy after replacing my OEM Air filter to a K&N Filter.

I'm Also thinking back to the whole truck thing where a truck actually gets better highway mpg with the tailgate up than down because of the dome effect created within the tailgate. Theoretically this would create drag, but in a wind tunnel it in fact creates the opposite.

With a large displacement engine, there's also usually a large car. I think that if your going at 60mph, the weight of the vehicle causes the momentum to continue on longer and, since momentum has built up at this higher speed it's easier on the engine cause the weight is pulling the car forward and helping out the engine. I've been in V8 Cadillac's with Instant fuel economy, and when I watch it, it seems that when as the car accelerates to a faster speed, the fuel economy goes up when they put on the gas. I'm guessing that from the large displacement, lets say 4.7L, at idle speed of like 15 mph there is still alot of extra fuel being injected into the engine and fuel is wasted, but with a little bit on the gas you go faster and there isn't much more fuel being injected cause it doesn't take much more RPM to get it from 0-60. I never have to go over 2000 RPM to go anywhere, and My Jeep shifts at 1500 if I accelerate lightly. I can easily get from 0 to sixty hitting the gas barely at all. If I wait long enough The Jeep can Idle at 20mph.

This could be why that Intrepid had a flat mpg curve at speeds over 60, it was a heavier car, and once you get a heavy car going, it goes, and the car doesn't have to do much more to go from 60 or from 70mph. I love coasting in my Jeep, cause it coasts for miles and my rpm's are near idle.

Also, with the SuperMid, Where do I buy one and can I get it in US Customary?

ah, yea, but it did somehow lower my rpm's a bit after moving to a K&N, why is this?

What are some other things that can improve the MPG of my Jeep, Not including removing unnecessary weight.

Driver technique is the biggest saver of all. Take a look here for ideas. Also increasing tire pressure to max listed on the sidewall will help. Sounds like you do a lot of highway miles check out the aero tips also.

I always drive economically, especially when no one is looking. I can't really drop any weight, I could dump the spare, but you never know with the Jeep (the side of the road seems to be more interesting than the road itself!) I've got the tires about 1.5psi overinflated and removed the roof racks. I was wondering about making a home-made intake from pvc and the cold air intake filter with some heat deflectors to try and make it breath a little better. How much difference in mpg is a clean Jeep vs a Muddy one?

This varies widely among vehicle manufacturers, but you may want to actually warming-up the incoming air.

I'm not sure about Jeeps or Chrysler brands, but Saturns and some Hondas respond to this.

Same for cold starts -- intake air temp is generally one of the feedback parameters for the FI system. Generally, warmer = more efficient.

As far as mud, it depends on how thick the coating is :thumbup:

RH77

I'm pretty sure that in this automobile, that the car is more peppy while cold air is being put into the engine. The engine may be warm, but it wants that cooler air to be forced in and used. Also as with some of my Jeepin' friends have tested some things and have come to the conclusion that a cold air intake on a Jeep I6 definitely increases fuel economy by far. Also, owning a Jeep I'm sort of a nubcake with some of these terms. What's a Grille Block exactly and how does it work? Also, With the Kardboard Kammback, how well would that work with my Jeep? it slightly tapers but not by much. Also, if you had a really big long one at a slighter angle would that be a lot more effective than a smaller more angled one?

Driving efficiently when nobody's looking? C'mon! :P

Cold air is good for power. The idea behind a warm air intake is to intentionally DEpower the engine, which requires a wider throttle opening to do the same amount of work. A wider throttle opening means reduced pumping losses. One of several reasons diesel engines are more efficient is because they have no throttle plate.

A grille block is just that: closing up some of the front of the vehicle ahead of the radiator. A significant amount of the total aero drag of a vehicle is airflow through the radiator. If you don't NEED the airflow, closing off that opening will help. Don't melt your engine though.

I drive Economically all the time, I drive a lot more economically when there's no one around to pass me when oncoming traffic is too close. I've had to slow down a few times because people try to pass with not enough time. I'm not gonna drive 47 all the time.. Especially when lots of cars are behind me and it's a busy day.

I'm still thinking about that Kammback. What would be a sufficient material that's weather resistant, easy to mod, and lightweight? I also don't wanna ruin the paint job either. I wonder if I could use the Torx screws on the roof of the Jeep that used to attach the roof rack to attach the Kammback. It wouldn't require any reworking of the body to do, and it would stay on there easily. I'm wondering about visibility? Would this be legal?

I can't answer the legality question - depends on your local/state situation.

Material choice for a partial Kammback is a tough one. I'm going to try making a fiberglass one myself. It'll be light, and perhaps easier to get the desired shape (by making a foam mockup).

Visibility may be an issue. You'll note that Mighty Mira used clear plastic on a metal frame partly for that reason (I'm assuming):

http://img225.imageshack.us/img225/8...x480bp0.th.jpg

My kardboard kammback mockup blocked the upper 1/3 of the vertical height of the rear window, and I still had a clear view of traffic on a level road behind me in the rear view mirror.

http://metrompg.com/posts/photos/aer...ail-rear-s.jpg

I was thinking about a thicker plastic, but clear as tight surran wrap. I could do thin sheet metal for framing and a layer of the clear plastic right over it. And I could rainX the thing, that way the water sheets right off of it during the rain.

That could work.

Question for you: what type of driving do you do most? (urban/highway/freeway?)

Definitely not Urban, I live in the middle of nowhere, Most of my driving is highway and some "city" with the occasional off-road adventure after it rains. But mostly the miles are highway. Freeway, not likely, maybe every 2 weeks. and that's even controversial. But yea, mostly highway.

Cold air is good for power cause air expands as it gets hotter. When it is cold the volume/density changes and you get more air/oxygen in the same amount of space.

(K&N filter discussion moved to: Debate continues: K&N filter vs. MPG)

well when driving my brick(67 GMC truck) I have to say I don't notice much difference in mileage between third gear and OD( yes I have a period correct borg warner 3 speed with a 30% oD). The difference I do notice is in the vacumn the engine runs in and out of OD. In 3rd gear I am usually pulling 15-16 inches of vacumn at highway speed(about 2400 rpm , in OD abnout 10 inches of vacumn( about 16-1700 rpm). I am guessing my 3.5 to 1 rear gear isn't low enough to effectively use the OD.

How flat are your roads? OD works better as the roads get flatter in my experience...