Does a small V6 give better MPG than a 4 Cyl?

Elwood Balboa said:

then please explain.

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Right.

Think of the engine simply as a powerplant. It puts out torque through the gearbox and into the axle/axles and to the wheels. Depending on the engine, the revolutions per minute vary from about 800-900 rpm (tickover) to around 4,000 on a diesel, and around 6,500 on a petrol. This varies greatly from engine to engine, but by and large they are around those figures. The size of the engine (2 litre, 3 litre, etc) is simply a measure of the total volume of the cylinders in the engine. So a 3.0 litre V6 engine has 6 cylinders, each of which are 500cc (cubic centimetres) in volume.

Now, the job of the gearbox is to take these raw 'rpms' and transfer them to the axles, which in turn transfer them to the wheels. Using different sized gears, the gearbox can take in for example 1,000 rpm from the engine, and output 500 rpm to the wheels, or take in 2,000 rpm and output 4,000 rpm to the wheels. These are just examples and again they vary greatly from vehicle to vehicle and gearbox to gearbox.

So, the size of the engine has no direct relationship to your rpm/speed ratio - there are many, many factors which contribute to this including wheel and tyre size, differential ratios, transfer box ratios and the gearbox as I mentioned.

Hope that clears it up... not the best explanation but that's the basics of it.

paul_c said:

Cause & effect. Larger engines tend to be matched to higher gear ratios (typically, a higher final drive ratio).

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They tend to be with 'normal' cars, but it is not a direct relationship as the OP seems to think - think about big rig trucks, with 12 litre engines and low ratio gears, or tractors with massive engines and low gears.

Also what about small capacity high output engines? (Turbo or supercharged)

Elwood Balboa said:

I get that, but more cylinders=less revolutions when coupled with the same gearbox, is that right? I can't even remember what I thought just now.

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Number of cylinders has nothing to do with the rotational speed of the engine. If the gearbox is the same, that is. They'll sound different, and there will be more ignition events per revolution, but the rotational speed would be the same.

Langers said:

They tend to be with 'normal' cars, but it is not a direct relationship as the OP seems to think - think about big rig trucks, with 12 litre engines and low ratio gears, or tractors with massive engines and low gears.

Also what about small capacity high output engines? (Turbo or supercharged)

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You're right, I was thinking more in terms of what you'd find within a model range of a few cars. For example, Mk1 Golf GTI 1.6 has a 3.89 final drive ratio, the 1.8 has a 3.67 final drive ratio (the gearbox ratios are all the same).

Trucks, big cars, little cars, high revving ones, performance ones etc all have slightly different requirements so will have gearing to suit its job.

paul_c said:

Number of cylinders has nothing to do with the rotational speed of the engine. If the gearbox is the same, that is. They'll sound different, and there will be more ignition events per revolution, but the rotational speed would be the same.

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OK, I think I'm satisfied, I think I'm coming down with flu, my brain can't make connections I'll try and sum it up-- 'An engine is nothing without a gearbox'

Cheers everyone, who needs school.