[harkes]

Why does the M4 consume more gas on the combined cycle? I dont think there is any WOT action included in the calculation? At the same AFR the N55 and the S55 will burn the same amount of fuel so is the S55 running richer?

[the_phew]

I don't have a definitive answer, but some ideas:
-Gearing? I know the 6MT in the 4-cyl F3x has a taller 6th than the 6-cyl
-Aero? The F82 is probably set up for more downforce, which usually means more drag
-Two turbos vs. one=more losses
-Throttle losses: the S55 requires a more closed throttle to achieve a given power output=more throttle losses. Otto cycle engines are most efficient (thermodynamic efficiency, not fuel efficiency) at WOT. Partial throttle is not usually very efficient for Otto engines; that's Diesel's domain
-Cooling: the M cars have a lot more heat exchanger area, which means more aero drag and more parasitic power to drive the water pump
-M cars have huge-ass brake rotors, wheels, and tires, which take more power to accelerate than non-M cars with less rotating mass
-M cars come with sticky rubber, EPA #s for non-M cars are for the most common setup, which are low rolling resistance tires

It all adds up.

[harkes]

All valid ideas.......adding up to a 14% loss in fuel economy. Seems like a lot. Anyhow I cam check some JB4 logs to find out about the AFR

[jinnrice]

Normal driving in the m3 you get 19mpg average
Normal driving in 335 sport mode 19mpg.
Pretty much what the other gentleman said about turbos and gear ratio.

[JOHNBMWM5]

My LCI M3 gave 28 MPG UK gallons, general driving with some blasting, I think the MPG will be about the same TBH, I would not want a 435i IF MPG was the reason to buy.

[RoundelM3]

M3/M4 engine is in much higher state of tune that allows more fuel and air into the engine (more boost, likely larger injectors, less intake/exhaust restriction, etc.). Regardless of throttle state, it's going to consume more than one in a more moderate state of tune. Just because they're the same displacement it doesn't mean both engines are going to get the same mileage under all conditions. Besides, those "calculated" combined driving averages are just that - no real relation between that and what a given driver under his/her local conditions will get.

Having the same air/fuel ratio has nothing to do with fuel consumption - it's the amount of air/fuel mixture that goes into each cylinder. Under higher boost/higher valve lift and timing, a larger amount of fuel and air are pushed into each cylinder before combustion. Also, the mixture may be slightly more rich on the M3/M4 to avoid lean-burn detonation at higher throttle opening. The M3/M4 engine is tuned to produce more power at a higher RPM range, as were the normally-aspirated M engines prior to the current forced-induction variant. The N55 power peak is roughly 6K RPM, where the M3/M4 engine is still in its powerband. Higher RPMs will always consume more fuel, and the M3/M4's gearing is numerically higher, particularly in top gear, resulting in higher RPM at a given speed.

I normally get 28 MPG in comfort mode in combined city/highway driving with my 8AT 335i equipped with MPPK and MPE. Manual trans would probably get less due to not having two overdrive gears. I can also get 34 MPG on the highway with A/C on and 36 with it turned off. If I drive in Sport mode, which cuts out 8th gear unless manually shifted and shifts up at higher RPM, the mileage is lowered by a few MPG.

[floydarogers]

Originally Posted by the_phew

-Throttle losses: the S55 requires a more closed throttle to achieve a given power output=more throttle losses. Otto cycle engines are most efficient (thermodynamic efficiency, not fuel efficiency) at WOT. Partial throttle is not usually very efficient for Otto engines; that's Diesel's domain

All of BMW's engines in the 3/4, including the M3/4, are Valvetronic engines, and don't have throttles, hence don't have pumping losses.

[the_phew]

Originally Posted by floydarogers

All of BMW's engines in the 3/4, including the M3/4, are Valvetronic engines, and don't have throttles, hence don't have pumping losses.

It's true that they don't have throttle bodies/butterfly valves, but the intake valves accomplish the same function. Less valve lift=more pressure drop=more losses.

Throttle losses are an unavoidable downside of Otto cycle IC engines, at least ones that have to meet emissions requirements.

[floydarogers]

Originally Posted by the_phew

It's true that they don't have throttle bodies/butterfly valves, but the intake valves accomplish the same function. Less valve lift=more pressure drop=more losses.

Throttle losses are an unavoidable downside of Otto cycle IC engines, at least ones that have to meet emissions requirements.

Throttle losses result in a vacuum between the throttle and the intake manifold. There is none with Valvetronic - there are no losses. In fact, the N20/26/N55 have vacuum pumps to provide brake assist and other functions.

You are right in that the air-fuel mixture is modulated by the valve lift rather than a throttle, but you are wrong when you call that throttle losses.

[pikkagtr]

Real world F80 mpg with efficient throttle over a long freeway trip 25mpg
Sports throttle around town with some aggressive WOT runs ~ 19 MPG

[the_phew]

Originally Posted by floydarogers

Throttle losses result in a vacuum between the throttle and the intake manifold. There is none with Valvetronic - there are no losses. In fact, the N20/26/N55 have vacuum pumps to provide brake assist and other functions.

You are right in that the air-fuel mixture is modulated by the valve lift rather than a throttle, but you are wrong when you call that throttle losses.

Valvetronic just moves the throttle losses to the intake valves; if there were no losses, there would be no airflow throttling and the mixture would be too lean at part-load. Valvetronic just kills two birds with one stone. You're arguing semantics, I'm arguing fluid mechanics.

Besides, I'm even semantically correct. From Wikipedia:
"A throttle is the mechanism by which fluid flow is managed by constriction or obstruction"
...which is exactly what is happening with the intake valves in a Valvetronic engine. The intake valves accelerate airflow and result in head losses since the geometry dictates rather poor pressure recovery in the cylinder. Valvetronic is a more elegant system than separate throttle butterfly valves, but ultimately it still incurs induction losses inherent to all modern Otto cycle engines at part load.

Ref: I'm a mechanical engineer specializing in fluid flow.

[tex2670]

I'm going to go out on a limb and say that M4 buyers aren't really concerned about fuel economy comparisons.

[JunkStory]

also the non-M cars have the automatic engine start/stop feature. From my experience, if you disable that feature the M4 has virtually identical MPG as an 335/435 xDrive. All wheel drive has the worst beating on MPG

[harkes]

Originally Posted by tex2670

I'm going to go out on a limb and say that M4 buyers aren't really concerned about fuel economy comparisons.

I think you are right and those people are more than welcome to stay out of this discussion. My intent was not to open up for personal preferences towards fuel economy, but to have a technological discussion

Agreed that fuel economy isn't that sexy but I still find it interesting from a technological perspective. Also, I dont own a money tree so I do appreciate it.

[harkes]

Originally Posted by floydarogers

All of BMW's engines in the 3/4, including the M3/4, are Valvetronic engines, and don't have throttles, hence don't have pumping losses.

I found this on my car (N55) and I would go as far as to calling it a throttle body Maybe what you mean is that it is fully open even at part throttle driving?

[floydarogers]

Originally Posted by harkes

I found this on my car (N55) and I would go as far as to calling it a throttle body Maybe what you mean is that it is fully open even at part throttle driving?

All the valvetronic cars have an "emergency" throttle that is only used if the valvetronic fails. Actually, my M57 and my wife's N47 diesel engines have a throttle - to prevent run-aways.

[nicknaz]

guys, I have a DCT F80 M3, and wife previously had a sport 8AT F30 335

regardless of the window sticker, the real world MPG is pretty similar (IF I am able to resist using the throttle on the F80...). I am 50/50 highway and city and get like 20mpg in the M3 and got 22mpg in hte 335

both have auto start stop. 335 cruising rpm is generally a few hundred rpm lower. S55 has a lot more power compared to N55 everywhere in the powerband, even in part throttle, so that also uses more fuel.