Tech. theory: Engine Power vs. Fuel Consumption - The Unofficial BMW M5 Messageboard (m5board.com)

MT JTN · 9th March 2004, 04:09

With all the talk of the new E60 M5 as of late, I've been finding myself thinking... okay, for a moment, let's put aside the looks and think of the goods - at least 500 horsepower and probably 100 more lb/ft of torque than our current cars make... ten cylinders, 8500 RPM redline... what about fuel economy??

With our cars currently rated at 13/21, I can't imagine that another 100 HP and another 1500 usable RPMs would be able to match current consumption and I got to thinking...

Is engine power always inversely proportionate to fuel consumption?

-- I simply can't imagine getting 10 miles to the gallon in the new car and with prices of crude supposedly sky-rocketing this summer, returning even less fuel economy than I do now is not something I'd enjoy. --

It's generally true that two engines of the same specifications except a difference in displacement, one displacing more than the other, will not return equal fuel economy. More displacement, less fuel economy.

Will there ever be a time when an engine receives a power upgrade but without a sacrifice in fuel economy? (IMO, the 3.0 liter, and even the 2.5 liter I-sixes could use a little bump...just look at the engine output in the competing G35!)

A Dinan S2 certainly uses more fuel than a stock car, right?

Then I thought... well perhaps an engine just needs to improve its efficiency in order to make more power given an equal fuel economy. How would one go about making an engine more fuel efficient? How could our cars be made to be more fuel efficient and keep output the same? Perhaps it's not possible.

Blah blah, so those are my thoughts for today. I wonder what the fuel economy of the new 10-cylinder will be like and I wonder if in general, more power equals less fuel economy.

I hope some of you thought that was interesting enough to respond to.

-Matthew

schnelleM5 · 9th March 2004, 04:23

One need only to look at what BMW did with the brilliant Valvetronic N62 powerplant, which has increased power and torque output as well as improved fuel efficiency compared with the previous M60 motor to realize that sometimes technical innovation and mechanical efficiency can offset the penalty in fuel consumption that is normally associated with boosting output. It is definitely not a linear relationship.

Richard in NC · 9th March 2004, 06:35

Another thing often added to the performance/economy balance is gear ratios. Often, as is the case of the 325/330, the bigger engined car has a taller (lower numerically) diff. ratio. This reduces the power gained but increases the economy. The V10 M5 may have so much more low end torque, that the 7th gear reduces crusing RPMs and increases economy. The extra gear may also allow closer ratios which can keep the engine at more efficient RPMs for moderate acceleration which also increases fuel economy.

Lscman · 9th March 2004, 12:58

I think the E39 leaves lots of room for improvement in the fuel economy area, thru the use of better gearing, fuel management and variable cam technology. More low end torque and better A/F mgmt at part throttle could significantly improve fuel economy posted by the EPA and the steady-state RPM at highway cruise is too high for efficiency.

The advertized ratings of 13MPG city & 21MPG highway are not impressive. A Corvette with the same cg and 20% less weight gets about 30% better economy. BMW did not sharpened their pencil on the 4.4/5.0L engine design, with respect to economy. BMW V8 5 series customers dishing out $55K or more were not exactly looking for economy

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CSBM5 · 9th March 2004, 14:18

I averaged 21.5mpg running a constant 85mph across Kansas a couple of weeks ago which I found respectable. BTW, the Vette may have a similar Cd, but it's frontal area is much smaller I would think = less drag.

greg · 10th March 2004, 08:09

An engine is at its most efficient when it is at full throttle - and at its torque peak, somewhere below redline. The more "excess capacity" you have, the more all that is associated with it becomes a big boat anchor of drag that doesn't need to be there. I don't know what the figure is for an E39, but I remember an article about an RX-7 I once owned said it required only 15hp to maintain 55mph.

Although it strikes some people as completely counter-intuitive, it is a fact then that to get the best mileage, when you accelerate from speed X to speed Y, you should use a lot of your engine's power for a shorter period of time, rather than "drive like there is an egg under your foot." Floor it, short-shift a bit, get up to speed in a hurry, and immediately lift to cruise power, upshifting to the highest possible gear.

SO - more power almost always means lousier mileage - because most of the time you're just dragging it around using significantly less. Lighter weight, lower aero drag, lower rolling resistance tires - can all help. We already have very efficiently managed engines as far as ECU control - I don't see that getting a lot better.