Fuel econemy question

I had a discussion with a pal about comparing vehicle fuel economy. Would a 6 Cylinder at 65 MPH showing 2200 RPM use less fuel than a 4 cylinder showing 2600 RPM at 65 MPH?

Reply to
Jesse
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Jesse wrote in news:ib95361v126e4f9msaaovpehaaa7o2i9eh@

4ax.com:

I think the question is impossible to answer as a generic thing; there are too many variables.

What are the characteristics of each engine and each car? How much power is being created by each engine, and where in their rev-range? How heavy is each car? How much wind-resistance has each car? How much throttle-opening is required to maintain speed in each car?

Reply to
Tegger

It's not the number of cylinders, it is total displacement. A big four, say a 3 liter four, is likely to get poorer gas milage than a 2.5 liter six, all else being equal.

That all else being equal is a big gotcha, though. One of the most important parameters is final drive ratio (used to be rear axle ratio in days of rear drive cars).

After the first gas embargo in late seventies, Ford offered a high milage version of the Mustang, the Mustang MPG. Only difference was final drive ratio.

While cylinder sizes are narrowing, in olden days you could find very big cylinders on some cars, very small ones on others.

Reply to
Don Stauffer

all other factors being equal, the 4 will be more efficient because of the lower component count and therefore lower friction losses. BUT, it's not that simple in real life, and all factors are not equal - a well tuned 6 can easily be more efficient than a poorly tuned 4. so, there's not "definitive" answer to your question unless you want to compare specific vehicles/engines.

Reply to
jim beam

All other factors being equal, a 6 might show a few % less effeciency than a 4 due to friction. (more surface area of piston/ring and more rockers/cam followers) Does this sound reasonable Dan S.? Ben

Reply to
ben91932

The simple part of the calculation is the six cylinder will fire its sparkplugs (and therefore burn gas) 13,200 times in a minute and the 4 cylinder will spark 10,400 times. So, IF they are both injecting the same amount of gas into each cylinder, then the 4 cylinder should use less gas.

The missing piece here is that the amount of gas being injected is not specified for either engine, so you can't solve the problem analytically, even if you have them running in a controlled environment (e.g. Windtunnel) without more information.

Reply to
E. Meyer

There are way to many variables to properly answer this questions.

Do the engines have the same displacement? What sort of valve train? Compression ratio? Why are you running the four faster? Bearing sizes, ring type/tension, intake tract efficiency, exhaust design, etc. all effect the results.

Maybe you should ask if all things are equal, would a x size 6 cylinder consume more fuel than the same size four cylinder at a given speed. Even that is uncertain, although I would expect the four cylinder to do slightly better (if in truth all things were equal, including gearing).

Ed

Reply to
C. E. White

"C. E. White" wrote in news:i0vvoa$298$ snipped-for-privacy@news.eternal-september.org:

And not in a ratio 6/4 either begging questions of the power and ratios on the vehicles.

However the bottom line in fuel economy is the feet on the other side of the firewall.

Reply to
chuckcar

I knew a guy who replaced the engine in his '65 Ford Econoline 6 with a small V8, and his economy went UP! The smaller engine was busting it's ass to haul the bulk around, where the 8 hardly worked up a sweat.

Reply to
Hachiroku $B%O%A%m%/(B

Perhaps I should clarift my situation. The discussion followed an attempt to select a car for my daughter, she prefers good milage. I drove both cars on the same road, at the same speed and observed at 65 MPH the 4 cyl at 2700 rpm and the 6 cyl at 2200 rpm. The question was which car should get the best fuel economy at these conditions. Neither myfriend or I know.

Reply to
Jesse

`

` The federal fuel economy numbers will probably be more accurate way to compare the two cars than asking strangers to guess based on the number of cylinders and RPM.

-jim

Reply to
jim

Nope. Given the information you've provided, nobody can know.

--scott

Reply to
Scott Dorsey

Jesse wrote in news: snipped-for-privacy@4ax.com:

And neither does anybody else.

As I and several others have already pointed out, there are way too many variables present to be able to give a black-and-white answer to your extremely general question. Plus you've provided zero information about the cars you've driven.

So, what are the particular cars in question?

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has pretty accurate figures for all US-market vehicles. You may wish to check there.

Reply to
Tegger

snipped-for-privacy@panix.com (Scott Dorsey) wrote in news:i12p32$134$1 @panix2.panix.com:

A bicycle will return superlative gas-mileage, I think.

Reply to
Tegger

If this 6 cyl has 3 litre, then 3 x 4/6 x 26/22 = 2363 cc, so if this

4 cyl has >2363cc, both have same fuel ratio @ these rpm, no super / turbo charging, then theoretically this 4 cyl will use more.
Reply to
TE Cheah

I revise my calcn to 3 x 22/26 = 2538 cc.

Reply to
TE Cheah

Thanks everyone, I'll give this up. I hoped that there was some correlation of mpg to rpm.

Reply to
Jesse

Why would there be?

Every time the engine goes around, it sucks in some amount of fuel proportional to the displacement and the throttle. Every time the engine goes around, it moves the car forward some amount proportional to the transmission and differential gearing.

So... if you knew the displacement and you knew the throttle percentage and you knew the transmission and differential ratios, you could figure the mpg at any point in time. However, the throttle percentage is something that changes a lot as you drive.

And one big hint: the lighter a car is, the less throttle you need to make it accelerate. The lighter car will almost invariably have the best mileage, in the long run.

--scott

Reply to
Scott Dorsey

There is not enough data to make a definitive selection. BUT, I have seen some of these four cylinders that run higher RPM's, as you mentioned, get terrible mileage. Part of that equation is transmission and gearing, BUT if you have an anemic little four you may find you are in the accelerator too much, trying to make a hill or trying to pass something, and it can be an unpleasant event, both from driving satisfaction and from economy.

My father-on-law has a 2.3 litre Jimmy pickup that has no power, is not at all pleasant to drive, and has never gotten 20 mpg in its life

I drove a 4 cylinder Honda before I bought Toyota, and it was a gutless little machine (to me).

A well set up 6 cylinder can get very good mileage, and can be a heck of a lot more driveable on the highway. For example, my old Buick Regal (89) had the 2.8 litre V6, and always got 35 mpg on the highways. Other than that, it was a pleasure to drive, but one of the most fragile, undependable turdcars I have ever seen.

You can look at the estimated mileage figures for each car, but those may be off a good bit.

Our 268 hp Avalon will average 32 mpg on the highwar. And it has great power for when you really need power.

Look at the mileage, certainly, but look a little further befure you buy her a car.

Reply to
hls

No, there isnt. For your daughter, you want something very dependable, with reasonably good mileage, that handles well and can keep her relatively safe.

I suggest you forget the mpg to rpm ratios.

Reply to
hls

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