Big difference in mpg between 3.0si and Z4M

[alexman]

Ok this might be a dumb question (and I know they're different engines), but can anyone shed any light on why the M engine gets 22mpg and 3.0si gets 32mpg even though the displacement (size) of the M engine is only 0.2l more. What is it in the M engine that pushes it to offer this much more performance, or, is the 3.0si a newer engine and so they've managed to make it more efficient?

I chose an M over the 3.0si no question as I was after the "ultimate" but I do wonder about how the mpg seems to drop disproportionately! What is there in the M engine that makes it offer more performance?

 

[mmm-five]

The reason for the big increase in consumption for such a small increase in displacement is purely down to the large amount of extra power the ///M gets from that extra 200cc.

80bhp or so extra is 25% more power, so even in simple terms you've got to agree a 25% increase in fuel consumption is acceptable.

Plus you tend to spank the ///M version more so that brings the economy down, and it's heavier than the 3 litre.

 

[Smokin]

The 3.0Si is a totally new engine made from aluminum and magnesium and MUCH lighter than the M engine. Plus, as mmm-five said, you gotta burn fuel to get horsepower. The Z4 was the last car to keep the older M engine.

 

[Wondermike]

MPG is proportionate to power so the difference doesn't seem too bad to me.

My two cars doing the same run cross country -

Micra - 82PS - 37mpg (thrashing it)
SLK - 272PS - 26mpg (cruising!)

 

[sammyz]

On a run I can get 32 MPG out of the 'M' if I cruise at 75ish. Put the wellie down press the sport button and she drinks the super unleaded like a fish.

 

[IK.]

Is this MPG recorded under the same conditions? I doubt there would be a 10mpg difference between the two!

 

[Wondermike]

Comes from the official figures I believe, although I thought 23 and 33mpg.

 

[IK.]

Hmm, I'd be surprised if there was a 10mpg difference in reality, personally.

 

[Top pup]

It's all to do with the way the engines are tuned. Basically engines are at their most efficient at the rpm they produce peak torque, which in the case of the M is much further away from the kinds of rpm used in normal driving hence the higher fuel consumption.

 

[IK.]

What are the figures based on though? Doing 70mph? Round town driving?

 

[Top pup]

What are the figures based on though? Doing 70mph? Round town driving?

Those figures will be the official "combined" figure quoted in the handbooks. It's a combination of the urban (town) and extra urban (motorway) figures.

 

[IK.]

Oh right, gotcha. Damn, that's odd. I still think in reality the M won't be that different. I'm sure my friend said he got a pretty reasonable 33mpg with motorway and some town driving, and 27mpg average since he got it, including some 'brisk' drives out with my lot over on Pistonheads.

The si can't be that much better in reality?

 

[Andy]

I'm always stunned at the low mpg of mine, despite deliberately sometimes driving it without moving the pedal more than a milimetre.

On any sort of distance the best I've seen is 30.7mpg, no way I could consistently hold 32.

If I was to pull away from the lights, 1st, 2nd, 5th and sit at 1500rpm for about a minute my average (from the computer) would most likely be about 22, tops.

If i sit at 60mph in 6th for about 30mins idling, that's how I've achieved 30.7.

I'm convinced mine is running rich.. :thumbsdown:

 

[Top pup]

I've got a E46 330i for a daily driver and it's a good 5+mpg better than my M in like for like normal driving. I get @29mpg at 80mph on the motorway for the M and @38mpg for the 330i at the same speed.

 

[fridaypassion]

One thing I will say for the ///M is that when your pressing on the MPG will hovver around 18/19mpg which is not really bad for spirited driving. My old VX220 (non turbo) was only a bit better than that and my daily driver Megane 225 used to be low teens at best when you opened the taps up.

 

[Shipkiller]

I'm always stunned at the low mpg of mine, despite deliberately sometimes driving it without moving the pedal more than a milimetre.

On any sort of distance the best I've seen is 30.7mpg, no way I could consistently hold 32.

If I was to pull away from the lights, 1st, 2nd, 5th and sit at 1500rpm for about a minute my average (from the computer) would most likely be about 22, tops.

If i sit at 60mph in 6th for about 30mins idling, that's how I've achieved 30.7.

I'm convinced mine is running rich.. :thumbsdown:

One possabilty for that is that your two upsteam O2 sensors are going out and need replaced. Mine were bad, not throwing a code and my MPG was 22-24 on a 2.5L. After replacing both upstream O2 sensors, my MPG was up to 30..

Just a thought... but that S54 loves fuel...

 

[Top pup]

One thing I will say for the ///M is that when your pressing on the MPG will hovver around 18/19mpg which is not really bad for spirited driving. My old VX220 (non turbo) was only a bit better than that and my daily driver Megane 225 used to be low teens at best when you opened the taps up.

I find the same thing, you can cover ground extremly quickly and still get @20mpg. It's because when you press on you tend to spend more time near the rpm where the engine produces peak torque.

 

[Phoenix]

I get around 22-24mpg with local driving depending upon how hard i drive it.

On the motorway i get between 31-35mpg depending upon how fast im going. Depends upon the motorway though, if theres traffic its nearer 30.

 

[Z4Kevo]

If i sit at 60mph in 6th for about 30mins idling, that's how I've achieved 30.7.

Yep...60 in 6th is the sweet spot.