Libby's are "stiff"

Sir Sam · **Posted:** Tue Aug 04, 2009 8:37 pm

tjkj2002 wrote:

The KJ's 4wd system is not very parasitic as all you think,even with the front diff always "turning" even in 2wd it does not effect MPG's that much(maybe 1mpg at the max).I ran around for one year with no front drivetrain(no front diff,CV's,or front driveshaft) and did not see any MPG improvement at all with less parts turning and less weight to boot(same driving style,"drive it like you stole it").

You might want to keep your fluids fresh since my KJ coasts just fine.............

I change my F/R diffs every 12,000miles,t-case every 6000miles,and the trans fluid every 15,000miles.

X eleventybillion

many XJ and TJ owners have been in the same place wondering if they could get better MPG by having locking hubs, for those that have done it, its not noticeable at all really.

dgeist · **Posted:** Tue Aug 04, 2009 9:40 pm

Sir Sam wrote:

tjkj2002 wrote:

The KJ's 4wd system is not very parasitic as all you think,even with the front diff always "turning" even in 2wd it does not effect MPG's that much(maybe 1mpg at the max).I ran around for one year with no front drivetrain(no front diff,CV's,or front driveshaft) and did not see any MPG improvement at all with less parts turning and less weight to boot(same driving style,"drive it like you stole it").

You might want to keep your fluids fresh since my KJ coasts just fine.............

I change my F/R diffs every 12,000miles,t-case every 6000miles,and the trans fluid every 15,000miles.

X eleventybillion

many XJ and TJ owners have been in the same place wondering if they could get better MPG by having locking hubs, for those that have done it, its not noticeable at all really.

I thought you were "eleventybillion"-ing the point on fluids. I used the heavier spec oil, valvoline full-syn I think, the first time I changed my diff fluid. Most recently, I switched to the lower weight amsoil and saw immediate benefits in coasting and a bit in economy. 'Course, I may have just had crud in there and putting in fresh salad oil would have done just as well.

Dan

username · **Joined:** Wed Jan 28, 2009 12:42 pm **Posts:** 21

nursecosmo wrote:

Just wondering, how on earth the engine tune could possibly improve the coasting ability of the CRD? No tune is giving any more fuel than another @ 0% pedal depression, whether it is the GDE, Inmotion, or OE. The same goes for the boost parameters.

I think this is a valid question. I figured the transmission computer would determine when and how much the vehicle would coast. Does GDE's tune somehow have access to transmission parameters?

95BadBoy · **Posted:** Tue Aug 04, 2009 10:46 pm

This was posted by GDE in answer to coasting. On thier form.

This can be attributed to how the raw accelerator pedal position is filtered. We made changes to this logic so that on a tip-out maneuver, the filtering lasts longer so that, even though you removed your foot from the pedal, it still injects a very small amount of fuel for a slightly longer duration of time. We also made changes to the driveability governors that allow for an improved coasting performance.

nursecosmo · **Posted:** Tue Aug 04, 2009 11:19 pm

Well, a smoother tip out sounds like a good idea, but what are the "driveablity governors"? Edit: when reading their description, it looks like the driveability governers are simply a more gradual increase and decrease from 0%TPS signal to load and back to 0%. That still should not address the parasitic loss of the drive train while coasting. When I want to coast a long way without using fuel I simply take it out of gear. I can usually get about twice the coasting distance that way.

username · **Joined:** Wed Jan 28, 2009 12:42 pm **Posts:** 21

tjkj2002 wrote:

I ran around for one year with no front drivetrain(no front diff,CV's,or front driveshaft) and did not see any MPG improvement at all with less parts turning and less weight to boot(same driving style,"drive it like you stole it").

I think "driving it like you stole it" is probably why you didn't see much difference :wink:

95BadBoy wrote:

This idea has even been tested on TJ's many times. 1 MPG gain at most - many got 0 differance

Tj's are about as aerodynamic as a mailbox so drag contributes alot more to the overall inefficiency. 1mpg difference is washed out by any change in driving speed or wind direction for example.

I commute 250miles one-way twice/week. I think I will test out the mpg w/o the axels averaged over a few trips to compare with my current average. I'll post back on this thread for anyone interested in a quantitative analysis.

danoid · **Posted:** Wed Aug 05, 2009 12:03 pm

There's also a persistent rumor that 4 Full Time gets better fuel economy than 2WD. Here's the logic...

The front axle has a hypoid gearset in it. Every hypoid gearset is a matched pinion and ring gear. These two gears are 'matched' by putting them into a machine, squirting some abrasive into the gear teeth and having them machine each other. This is always done on the 'drive' side of the gears. This, and the geometry of the gearset in general, result in the hypoid gearset being more efficient at driving than coasting.

However, in 2WD you are always pushing the front axle gearset from the tires, using the 'coast' side of the gears. Since everything is rotating in either case, this is the basis for the argument that the drivline in general has slightly higher efficiency in 4 Full Time than in 2WD.

Every time this discussion comes up at work, someone references "a test they ran years ago." But I've never seen the test report.

In God I Trust. All others must bring data. **edit** Some odd censorship going on here? This original phrase was 'In (insert name of diety here) I Trust.' Looks different after I post it...

My personal data is that 4 Full Time is noisier than 2WD. Since all that NVH comes from rotational energy that's not being used to go forward, and ultimately out of my fuel tank, I conclude that I'll run in 2WD when it's dry and above freezing.

tcoilburner · **Posted:** Wed Aug 05, 2009 12:53 pm

[quote="nursecosmo"]Well, a smoother tip out sounds like a good idea, but what are the "driveablity governors"? Edit: when reading their description, it looks like the driveability governers are simply a more gradual increase and decrease from 0%TPS signal to load and back to 0%. That still should not address the parasitic loss of the drive train while coasting. When I want to coast a long way without using fuel I simply take it out of gear. I can usually get about twice the coasting distance that way.[/quote

By not have as much engine braking affect coasting is how GDE get the libby to coast better. Almost the same thing you are doing by taking it out of gear. The only parasitic loss you are losing is the engine, everything else is still turning in the drivetrain.

danoid · **Posted:** Wed Aug 05, 2009 3:10 pm

Leaving the transmission in gear will actually save fuel. If you use vehicle momentum to keep the engine spinning above idle speed you are not using fuel. If you shift to Neutral then the engine uses fuel to idle.

linewarbr · **Posted:** Wed Aug 05, 2009 3:20 pm

danoid wrote:

Leaving the transmission in gear will actually save fuel. If you use vehicle momentum to keep the engine spinning above idle speed you are not using fuel. If you shift to Neutral then the engine uses fuel to idle.

Isn't that part of the reason all NAFTA CRD's were autos - because of small things like that, the manuals were not EPA compliant?

Joe Romas · **Posted:** Wed Aug 05, 2009 4:48 pm

nursecosmo wrote:

Just wondering, how on earth the engine tune could possibly improve the coasting ability of the CRD? No tune is giving any more fuel than another @ 0% pedal depression, whether it is the GDE, Inmotion, or OE. The same goes for the boost parameters.

Think Engine Brake :lol:

DarbyWalters · **Posted:** Wed Aug 05, 2009 5:39 pm

That is weird because my CRD coasts forever...even in DRIVE the revs drops and it just keeps on rolling. I do keep my tires at 40psi.

nursecosmo · **Posted:** Wed Aug 05, 2009 6:57 pm

danoid wrote:

Leaving the transmission in gear will actually save fuel. If you use vehicle momentum to keep the engine spinning above idle speed you are not using fuel. If you shift to Neutral then the engine uses fuel to idle.

So you are saying that no fuel is injected during deceleration or coasting down hills etc?

danoid · **Posted:** Wed Aug 05, 2009 8:49 pm

If the transmission is keeping engine rpms above idle there is no need for fuel. Air passes through, but there is no need to keep a three way catalyst in stiochiometric ratio like inferior engines. The low idle limit doesn't need to feed fuel to stop from stalling and there is no request from the driver for torque when coasting. So zero fuel.

OK, it's not that simple, but it's close. There is a period of time that the fuel ramps down to near zero to avoid sudden driveline lurches. And coasting down while above your set speed in cruise control still trickles a little (set your cruise to 50, then accelerate to 80, coast to 60, press cancel and you'll feel it). Others are more knowledgable (spelling?) than I, I'm just a transmission guy.

Sir Sam · **Posted:** Wed Aug 05, 2009 9:19 pm

username wrote:

Tj's are about as aerodynamic as a mailbox so drag contributes alot more to the overall inefficiency. 1mpg difference is washed out by any change in driving speed or wind direction for example.

I commute 250miles one-way twice/week. I think I will test out the mpg w/o the axels averaged over a few trips to compare with my current average. I'll post back on this thread for anyone interested in a quantitative analysis.

Been there done that. But I look forward to you confirming my findings.

tjkj2002 · **Posted:** Wed Aug 05, 2009 11:05 pm

danoid wrote:

There's also a persistent rumor that 4 Full Time gets better fuel economy than 2WD. Here's the logic...

The front axle has a hypoid gearset in it. Every hypoid gearset is a matched pinion and ring gear. These two gears are 'matched' by putting them into a machine, squirting some abrasive into the gear teeth and having them machine each other. This is always done on the 'drive' side of the gears. This, and the geometry of the gearset in general, result in the hypoid gearset being more efficient at driving than coasting.

However, in 2WD you are always pushing the front axle gearset from the tires, using the 'coast' side of the gears. Since everything is rotating in either case, this is the basis for the argument that the drivline in general has slightly higher efficiency in 4 Full Time than in 2WD.

Every time this discussion comes up at work, someone references "a test they ran years ago." But I've never seen the test report.

In $$$ I Trust. All others must bring data. **edit** Some odd censorship going on here? This original phrase was 'In (insert name of diety here) I Trust.' Looks different after I post it...

My personal data is that 4 Full Time is noisier than 2WD. Since all that NVH comes from rotational energy that's not being used to go forward, and ultimately out of my fuel tank, I conclude that I'll run in 2WD when it's dry and above freezing.

That only holds true for a "reverse cut" gears which are only found in a high pinion front axle which the D30A is not.The D30A is a low pinion diff so when going forward the torque is applied to the "coast" side of the gears,in reverse the torque is applied to the stronger side of the gears.It's reverse for a reverse cut diff(high pinion) which also make a high pinion diff stronger then the comparable low pinion counterpart.

Sir Sam · **Posted:** Thu Aug 06, 2009 12:15 am

tjkj2002 wrote:

That only holds true for a "reverse cut" gears which are only found in a high pinion front axle which the D30A is not.The D30A is a low pinion diff so when going forward the torque is applied to the "coast" side of the gears,in reverse the torque is applied to the stronger side of the gears.It's reverse for a reverse cut diff(high pinion) which also make a high pinion diff stronger then the comparable low pinion counterpart.

yet again I find we agree on this.

Even then, when I had a XJ with a HP D30 and a 242 the gas mileage was always higher when in 2wd than any 4hi.

tjkj2002 · **Posted:** Thu Aug 06, 2009 12:24 am

Sir Sam wrote:

Even then, when I had a XJ with a HP D30 and a 242 the gas mileage was always lower when in 2wd than any 4hi.

Yep and that's from the parasitic loss due to powering more components instead of just pushing them along but without actually sending power to them.

Sir Sam · **Posted:** Thu Aug 06, 2009 12:32 am

tjkj2002 wrote:

Sir Sam wrote:

Even then, when I had a XJ with a HP D30 and a 242 the gas mileage was always lower when in 2wd than any 4hi.

Yep and that's from the parasitic loss due to powering more components instead of just pushing them along but without actually sending power to them.

yup, just leave it in 2wd for best mileage

nursecosmo · **Posted:** Thu Aug 06, 2009 1:09 am

danoid wrote:

If the transmission is keeping engine rpms above idle there is no need for fuel. Air passes through, but there is no need to keep a three way catalyst in stiochiometric ratio like inferior engines. The low idle limit doesn't need to feed fuel to stop from stalling and there is no request from the driver for torque when coasting. So zero fuel.

OK, it's not that simple, but it's close. There is a period of time that the fuel ramps down to near zero to avoid sudden driveline lurches. And coasting down while above your set speed in cruise control still trickles a little (set your cruise to 50, then accelerate to 80, coast to 60, press cancel and you'll feel it). Others are more knowledgable (spelling?) than I, I'm just a transmission guy.

Like you say, it's not that simple. Not even close. Picture this: you are coasting down a long mountain, the temperature is -10*, your engine is being pushed round and round at 2200 RPM, 3080 liters of very cold air is pumping through your cylinders every minute, the viscous heater is moving too fast for the clutch to be engaged (not that it would be able to keep up anyway), oil is getting sucked past the rings and becoming very stiff, your rings are trying hard not to break. How long do you think it would be before your fingers fell off?

Just like gas engines, oilers also need heat when coasting or bad things happen. The problem with diesels is that they don't idle at stoich, and so can't heat the air stream nearly as well. that is why you will see slowly dropping temps when coasting in cold weather. Try turning off the engine, even in warm weather, while going down a mountain in gear and you will see what I'm talking about when the temp needle drops to zero in like a minute.

It is well established in the hyper miler community that dropping out of gear, while coasting down is the better way to save fuel in a gasser or in a diesel. If the engine is spinning at 2200 RPM, it injects the same amount per power stroke as the minimum amount injected at idle.

Libby's are "stiff"

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