Swap diff gearing for lower ratio & convert to rwd?

[nick325xit 5spd]

Originally Posted by Happy

Even though mine is just as loud as GT class racers, I have zero drone on the inside. And with the widows up, and the sound system playing I cannot even hear it at all on the inside.

The ratio allows me to make very strong passes on the highway without the need to shift to 4th gear. I can go from 60 mph to 90 mph very quickly. The manual E53’s were designed this way. It isn’t hurting anything. I love it!



E53 RiPPeR
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ReVELaTiON 22:21

IIRC, folks have reported worse gas mileage with taller gear swaps. The easier cruise was presumably offset by the greater challenge of getting the rig going.

[Bdc101]

Originally Posted by nick325xit 5spd

IIRC, folks have reported worse gas mileage with taller gear swaps. The easier cruise was presumably offset by the greater challenge of getting the rig going.

I read through a bunch of posts, particularly from Happy, claiming that taller gears would cause the car to use more gas to get up to speed -- I don't think there is any merit to that argument in the slightest.



It all depends on how the car is driven of course (the manual transmission means you can pick the gear you want to drive in at any time, with or without a logical reason) but there is nothing about taller ratios that would cause the engine to use more gas to get the car up to the same speed. It might not use less around town, but it certainly wouldn't use more. There is no merit to that argument at all.

[Happy]

Originally Posted by Bdc101

I read through a bunch of posts, particularly from Happy, claiming that taller gears would cause the car to use more gas to get up to speed -- I don't think there is any merit to that argument in the slightest.

It all depends on how the car is driven of course (the manual transmission means you can pick the gear you want to drive in at any time, with or without a logical reason) but there is nothing about taller ratios that would cause the engine to use more gas to get the car up to the same speed. It might not use less around town, but it certainly wouldn't use more. There is no merit to that argument at all.

Well let’s dig a bit deeper here.

Modifying the stock ratio to a taller ratio will result in a decrease in acceleration. If you decrease acceleration by applying a taller ratio, then that lost acceleration will need to be gained by some other means.

The only place on a stock E53 to gain more acceleration would be, the throttle. To compensate for the lost acceleration more fuel will need to be applied to make up for the loss.

Now this will create other issues as well. One being that, applying more air and fuel will naturally create more heat. This will create even more of a handicap because, not only have you decreased acceleration on an already underpowered 230 hp M54 trying to move 2 tons, you are adding additional heat. Heat is not your friend. Who wants a heat soaked 230 hp power plant trying to push 2 ton around. LoL..

The whole thing is a bit more complicated than just, “I want to lower my 5th gear rpm’s”. BMW engineered the manual 5 speed E53’s gearing this way for a reason, and that reason is due to its weight to horsepower ratio.

As far as my merit is concerned, well that’s a discussion for another thread. LoL.. But, I think I’m doing pretty well with a combined mpg of 20. I’m 25-26 on the highway, and thats burning 80% ethanol / 20% petrol. Who said alcohol isn’t efficient? LoL..



E53 RiPPeR
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ReVELaTiON 22:21

[nick325xit 5spd]

Originally Posted by Bdc101

I read through a bunch of posts, particularly from Happy, claiming that taller gears would cause the car to use more gas to get up to speed -- I don't think there is any merit to that argument in the slightest.

It all depends on how the car is driven of course (the manual transmission means you can pick the gear you want to drive in at any time, with or without a logical reason) but there is nothing about taller ratios that would cause the engine to use more gas to get the car up to the same speed. It might not use less around town, but it certainly wouldn't use more. There is no merit to that argument at all.

I'm basing this off of reports from a couple folks that did it.

And too tall gearing can absolutely increase fuel usage, especially in a somewhat underpowered vehicle. Especially in a manual that doesn't get the benefit of a sloppy torque converter like the automatics have. Sure, you can slip the clutch, but there's only so much of that that you can do.

[Bdc101]

Originally Posted by nick325xit 5spd

I'm basing this off of reports from a couple folks that did it.

And too tall gearing can absolutely increase fuel usage, especially in a somewhat underpowered vehicle. Especially in a manual that doesn't get the benefit of a sloppy torque converter like the automatics have. Sure, you can slip the clutch, but there's only so much of that that you can do.

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.

Modifying the stock ratio to a taller ratio will result in a decrease in acceleration. If you decrease acceleration by applying a taller ratio, then that lost acceleration will need to be gained by some other means...


...The only place on a stock E53 to gain more acceleration would be, the throttle. To compensate for the lost acceleration more fuel will need to be applied to make up for the loss.

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.



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.)



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.

[Happy]

Originally Posted by Bdc101

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.

So let’s apply this to the diff swap. 3950 rpm’s in 5th gear with the 4.10 diff vs 3525 rpm’s in 5th gear with the 3.64 diff.

At full throttle, both diffs will have max fuel consumption. However, at a cruising throttle position, fuel consumption will be minuscule. The throttle position is still key to the fuel consumption. You’ll need more throttle with taller ratios. Taller ratios deduct acceleration. That’s is why the engine speed decreases with the 3.64 diff. That speed will need to be up elsewhere.

Originally Posted by Bdc101

Partly closing the throttle restricts the flow of air (since the throttle is just a restrictor plate).

Not only does it reduce air flow, it reduces fuel flow.

Originally Posted by Bdc101

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.

When the throttle is in a cruising position on a blown rig, boost is discarded through the valve. But depending on the type of valve setup, this parasitic loss can be harvested at a lower pressure, and redelivered. This adds more air without more throttle, resulting in better volumetric air flow rate efficiency thus raising the mpg by 2 , maybe 3 . Detonation will not be an issue because the pressure is not excessive. Again N/A, FI fuel consumption is determined by throttle position. Detonation is indeed a different subject.

Originally Posted by Bdc101

The end result is that you pay in gasoline for all the acceleration that you ask for with your right foot.

LoL.. Ain’t that the truth! Well in my case, ethanol.



E53 RiPPeR
XOuTPoST jUNkiE
ReVELaTiON 22:21

[nick325xit 5spd]

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.

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.

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.

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.

[Bdc101]

Originally Posted by nick325xit 5spd

The point is that in a manual X5, you CAN'T slip the clutch much. Whereas the automatic X5s slip to, what, 3K?

Then what are you talking about? What does slipping the clutch do except waste energy (through friction) getting the engine and the drivetrain up to the same speed? I don't follow.

Originally Posted by nick325xit 5spd

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.

Yes, I'm aware that higher throttle positions are nominally more efficient. However, there is a work over time trade off.

So by this math, you would need to accelerate in the absolute lowest gear possible (max RPM without exceeding redline), anytime you are accelerating, in order to get peak efficiency -- am I following? Where does the trade-off end if you can just get more efficient by accelerating faster and in a lower gear? Why don't we just accelerate to redline all the time in order to minimize our fuel consumption?

Gearing advantage is just torque multiplication -- it is not power multiplication. Power is conserved no matter what gear you are in. Power (which is energy per unit time) is roughly directly proportional to the rate of fuel flow to the engine. So if you are running at a lower RPM, your power is lower (because power = torque times RPM) hence it takes longer to accelerate. This is confirmed as true from a torque perspective as well: even if the engine brake torque on the dyno chart was the same at both RPMs for the two gears, the multiplication from gearing is lower when you shift to a higher gear, so the acceleration is lower. So your fuel flow rate is lower, but your acceleration is lower, and when you reach your chosen speed you will have consumed the same amount of fuel -- either a high fuel flow rate for less time (faster acceleration), or lower fuel flow rate for more time (slower acceleration). The end result in terms of fuel used will be roughly the same. The "loss of gearing advantage" doesn't have anything to do with power, it only has to do with torque, which affects the rate of acceleration but not the amount of energy that you have to expend to reach a given speed.

Originally Posted by nick325xit 5spd

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.

But we're talking about acceleration, not cruising... plus, I was only talking about superchargers because I was expecting Happy would bring it up. I'm confused about the rest of your statement but I don't think it matters to the point of our discussion at all, so I'll opt out of replying to it.

The argument here is -- will getting longer gears give you better or worse fuel economy -- everybody knows it will increase your fuel economy when cruising in top gear -- but you are arguing that it will decrease your fuel economy when not in top gear, and I am arguing that it will make no difference.

[nick325xit 5spd]

Originally Posted by Bdc101

Then what are you talking about? What does slipping the clutch do except waste energy (through friction) getting the engine and the drivetrain up to the same speed? I don't follow.

Faster acceleration. Have you ever driven an automatic E53 with the 3.0? The stall limiter is set around 3K. Any time you need acceleration and your natural revs are below that, it'll spin up to 3K. It's kind of like dumping the clutch, but every time and every gear.

So by this math, you would need to accelerate in the absolute lowest gear possible (max RPM without exceeding redline), anytime you are accelerating, in order to get peak efficiency -- am I following? Where does the trade-off end if you can just get more efficient by accelerating faster and in a lower gear? Why don't we just accelerate to redline all the time in order to minimize our fuel consumption?

No, see below.

Gearing advantage is just torque multiplication -- it is not power multiplication. Power is conserved no matter what gear you are in. Power (which is energy per unit time) is roughly directly proportional to the rate of fuel flow to the engine. So if you are running at a lower RPM, your power is lower (because power = torque times RPM) hence it takes longer to accelerate. This is confirmed as true from a torque perspective as well: even if the engine brake torque on the dyno chart was the same at both RPMs for the two gears, the multiplication from gearing is lower when you shift to a higher gear, so the acceleration is lower. So your fuel flow rate is lower, but your acceleration is lower, and when you reach your chosen speed you will have consumed the same amount of fuel -- either a high fuel flow rate for less time (faster acceleration), or lower fuel flow rate for more time (slower acceleration). The end result in terms of fuel used will be roughly the same. The "loss of gearing advantage" doesn't have anything to do with power, it only has to do with torque, which affects the rate of acceleration but not the amount of energy that you have to expend to reach a given speed.

What are you talking about? Power is torque over time and nothing else. Horsepower is torque over time and nothing else.

You are oversimplifying the many factors that go into fuel consumption. First of all, engines are not uniformly fuel efficient across their RPM ranges. Generally speaking, the point of highest efficiency is the torque peak. Different gearing will put you in different RPM ranges and cause you to experience different levels of fuel consumption.

Secondly, higher torque demand means more fuel at a given RPM. If the acceleration is sufficiently reduced by gearing, this can absolutely result in higher fuel consumption during acceleration.

Thirdly, the manual X5 has somewhat weird transmission ratios. The result of a taller final drive is to spread the gear ratios further apart. Thus, you're more likely to fall out of optimal ranges and are likely to run different revs while driving. You will, in particular, need to rev each gear higher in order to not fall out of the driving RPM range when you upshift.

But we're talking about acceleration, not cruising... plus, I was only talking about superchargers because I was expecting Happy would bring it up. I'm confused about the rest of your statement but I don't think it matters to the point of our discussion at all, so I'll opt out of replying to it.

You brought up superchargers and forced induction and focused on a side effect, rather than the point. (In fact, with FI, you might choose taller gearing to give time to build boost.)

The argument here is -- will getting longer gears give you better or worse fuel economy -- everybody knows it will increase your fuel economy when cruising in top gear -- but you are arguing that it will decrease your fuel economy when not in top gear, and I am arguing that it will make no difference.

I'm arguing that it depends. And that people who actually swapped 4.10s for 3.64s reported worse fuel economy.

Different gearing results in different driving. The 6MT gets you a better highway cruise without any tradeoff. Taller final drive results in compromises everywhere but highway cruise.

[Effduration]

Originally Posted by Happy

Well let’s dig a bit deeper here.
............

The whole thing is a bit more complicated than just, “I want to lower my 5th gear rpm’s”. BMW engineered the manual 5 speed E53’s gearing this way for a reason, and that reason is due to its weight to horsepower ratio.
........

How does towing work into this? I presume the selection of gearing BMW would have included the 6,000 lb towing capacity?

I am not saying you can ignore towing, but If you don't tow much, would a 3.64 make even more sense ?