Quote:
Originally Posted by Bdc101
That would be valid if you slipped the clutch a lot (like A LOT) -- which you don't have to do with the 4.1 final drive and still wouldn't with the 3.64 (that is still a pretty short final drive in the grand scheme of things). If you are slipping the clutch enough to affect your fuel economy, you would also be replacing the clutch on a yearly basis.
|
The point is that in a manual X5, you CAN'T slip the clutch much. Whereas the automatic X5s slip to, what, 3K?
Edit: Also, the 4.10 or 3.64 are not that short when you adjust for tire size vs. other BMWs of similar vintage.
Quote:
|
Do you think that when you are at full throttle at 2,000 rpm it uses the same amount of gas as full throttle at 3,000 rpm? It doesn't -- you are accelerating slower because the motor is using gasoline at a slower rate (because it's at lower RPM and is pumping air and fuel at a slower rate). It's not like using higher RPMs magically gives you extra acceleration without spending any gasoline to do it.
|
No. The point is that you are demanding higher torque for a longer period of time from the engine to make up for the loss of gearing advantage.
Quote:
|
If you think of a piston engine as an air pump, which it first and foremost is, then it's easier to visualize that the engine produces more power depending on how much air is pumped through it. Full throttle allows the engine to pump the maximum amount of air at that RPM (which is 3 liters every two revolutions, for the M54). At 1500 RPM that would be 2,250 liters of air per minute. At 3,000 RPM that would be 4,500 liters of air per minute. Partly closing the throttle restricts the flow of air (since the throttle is just a restrictor plate). The amount of power it produces is roughly proportional to the volumetric flow rate of air through the engine. (This is not 100% exactly true in practice due to the volumetric efficiency, or VE, not being quite constant, but it's pretty close.)
|
Yes, I'm aware that higher throttle positions are nominally more efficient. However, there is a work over time trade off. Moreover, with taller gearing, you have to make different choices about what gear you use when - thus the effective gearing in much of city driving might actually be shorter because you need to hold on to, say, third gear instead of cruising around in fourth.
Quote:
Likewise, pumping the air into the engine using a supercharger increases the volume of air that you can get into it per revolution, because it pumps it in at a higher than atmospheric pressure and thus a higher density. When you supercharge your mileage suffers because you have to inject more fuel to keep charge temperatures lower, to protect from detonating, but that is a different thing from what we're talking about by swapping final drives.
The end result is that you pay in gasoline for all the acceleration that you ask for with your right foot.
|
You're glossing over a lot of factors here. Also, it's not the fuel mix that impacts supercharger mileage in light driving - it's the supercharger drag. At low load, an FI car is still targeting similar air fuel mixtures to NA cars. You put more fuel into an FI car because there's more air to use because there's effectively more displacement. It's not to "avoid detonation," per se. You need more fuel because you're pumping more air. Avoiding detonation is side effect.