Topic: Engine & economy - does size count?

[stodge]

Ok, a bad cliche in the title, but my question is this.

Consider a heavy vehicle, say a Ford Five Hundred (for a car) or an SUV. Would a bigger engine in such a vehicle be more efficient than a smaller, underpowered engine because it requires less "work" (loosely used) to make it move? I'm a complete engine-car-idiot, so I've often wondered about this.

The reason I mentioned the Five Hundred is that it currently has the 3L V6 200hp Duratec, which is reviewed as being underpowered. The economy figures are ok, but with the new Ford 3.5L 250hp (I think) would it be more efficient?

Thanks!

[ovr50]

Weight exacts a price in fuel consumption - regardless of the vehicle. Larger engines will "feel" better to drive in a heavier vehicle in that the engine won't feel like it's struggling to move the weight; but you will pay at the pump. I think that's pretty much a given/universal rule. 

[weebl]

I believe in theory, that if you pair an underpowered engine with an overweight vehicle, it will burn more fuel than the bigger engine in real world use, as you're going to be asking that little engine to give you 100% most of the time, versus asking the big engine for 100% not nearly as often.

That said, there are many other factors that may tip it one way or another, including driving style, and the relative overall efficiency of both engine designs.  Theoretically, the small engine could be very efficient, and the big engine very inefficient, resulting in better fuel economy with the smaller engine.

So it's a general rule that an underpowered engine would burn more fuel in real world driving, but it won't always be the case.

[Arthur Dent]

I believe in theory, that if you pair an underpowered engine with an overweight vehicle, it will burn more fuel than the bigger engine in real world use, as you're going to be asking that little engine to give you 100% most of the time, versus asking the big engine for 100% not nearly as often.

In the late 70s / early 80s GM put little V8s or V6 in some of their full size cars that got worse mileage than a moderately sized V8.

And of course engine displacement isn't the only predictor of fuel economy. Gearing, vehicle weight and aero drag, technology (carb vs FI for example), operating rpm range, etc all play significant roles.

[barrie1]

A good example of good gearing that gets good gas milage would be the new Corvette which is rated at over 30 MPG on the highway. These have a good sized  V8 engine in them and lots of HP as well. Artherdent is correct in that there are many variables in the reason for good gas milage. 

[articsteve]

Usually if the lower powered engine is optioned for the vehicle the manufacturer will adjust the tranny and diff gearing accordingly so in some instances, like high speed interstate driving, the lower powered motor will get less fuel mileage and be an unpleasant drive.

[Trainman]

Good real world example is the 2000 vs 2001 Nissan Pathfinder.  2000 was the last year for the 3.3 l, while 2001 was the first year for the 3.5 l.  The 3.5 gets better fuel economy, with more power and torque.  But it is a very different motor as well.  And as noted above, there are a lot more variables involved than just the size of your, ehm, "motor"   .

[mrthompson]

The GM midsize sedans with the 3.8L V6 get surprisingly good fuel economy on the highway, given the size of the car. 

[David Mac]

My 4L Mazda/Ford Ranger gets the same mileage as the
3L engine.Keep your foot out of it,drive the speed limit,
run the a/c on the hyw no open windows and you can get good
mileage.

[barrie1]

Actually most GM combinations of engines and transmissions have been giving very good milage for many years now. All of their V6's and 4 cylinder engines have been very good on fuel for the last 20 years or so. Most of the V8's have been quite good as well with the gearing in the trannys and rear ends. 

[Panzer]

The GM midsize sedans with the 3.8L V6 get surprisingly good fuel economy on the highway, given the size of the car. 

I have to admit, GM's 3.8L OHV engine is utterly reliable...

[ktm525]

The Land Rover LR3 V6 and V8 have almost identical fuel economy ratings. Off course the diffenece in displacemnt is not that great 4.0L vs 4.4L.

[mrthompson]

Can the same be said about the Dakota's 3.9L V6 and the 4.7L V8? 

[ktm525]

Can the same be said about the Dakota's 3.9L V6 and the 4.7L V8? 

Yes.

I have had three Dakotas. One each with the 3.8L, 5.2L and now the 4.7L. It seems I was getting the best mileage with the 5.2L. The 4.7L is burdened with an extra 500lbs though because it is a Quad Cab full time  AWD.