Gliding in The Beast - I'm thinking its not worth it under 45mph. - Fuelly Forums

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Old 06-18-2008, 04:44 AM   #1
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Arrow Gliding in The Beast - I'm thinking its not worth it under 45mph.

Well, I tried neutral gliding on neighborhood streets on my way home last night, and this morning on my way to work. I'm beginning to think its not worth it at neighborhood speeds. This is what I noticed:

It glides nice and easy, and I guess because it weighs over 3 tons & gets a lot of momentum going, & at low speed wind resistance is not a large factor... It glides quite far. In a couple of places I was able to shift into N and glide up and down a few hills for a total distance of almost 1/2 a mile, and stay within +/- 5 mph of the speed limit. That's the good part.

Here's the bad part...

In most of the streets the speed limit is 35 mph. Traveling at 35 my tach reads about 1400 - 1500 rpm. Stopped, in N it idles at about 725 rpm. Logic would dictate that shifting into N would cut my rpm's roughly in half, making for considerable savings, right? Not exactly. At the same point if all I do is take my foot off the accelerator and let it coast in gear the tach reads 1000 - 1100 rpm. Shifting into N at that point only drops the rpm's to 900 - 1000. A savings at best of only 100 rpm, and at that point IMHO not worth the wear & tear on a 10 y/o 150,000 mile tranny to shift - even though the dealer looked at it this spring for the 150,000 mile checkup & scheduled maintenance and said they could not find anything wrong with the truck.

Then I really got to thinking. I was coming up on a red light, and I was traveling at 35 mph, 1350 rpm. I shifted into N and coasted to the light, watching my tach. Immediately upon shifting into N the RPM dropped to 950. The RPM slowly dropped as my speed decreased, and then quickly fell to 750 rpm when I hit 5 mph. I'm thinking obviously the computer is increasing the minimum idle speed for higher road speeds, and probably does not know and/or care that the vehicle is not in gear. Probably to keep the vehicle from loosing too much speed if you just take your foot off the accelerator. Its got to be an enhanced driveability thing, or is it just a truck thing? If the truck was fully loaded at 70 mph, and I took my foot off the accelerator, maybe it would start engine braking, and that's why the idle increases? To prevent the tranny from getting jerked around too much?

Anyway I'm thinking that under 45 mph I'm just going to take my foot off the pedal and coast in gear (just like I have been doing for years).

I welcome anyone else's comments/thoughts/suggestions on my findings, and would be curious if a passenger vehicle of the same vintage (1998) would behave in a similar fashion.

-Jay
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Old 06-18-2008, 08:12 AM   #2
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Well, I can tell you about my 2002 GMC full size truck. The line was updated for 1999, so it doesn't apply perfectly, but here goes.
- My truck normally idles very low, I think something like 550rpm.
- With GM's really nice torque converter, neutral is not going to make a big difference at speeds under 35mph, since it gets to drop to near idle speed anyway.
- It doesn't drop quickly; there's a bit of a delay, then it drops slowly.
- However, using N does make a slight difference, since my truck does not use DFCO easily, and yours may not use it at all; so a few more RPM, even with the throttle closed, still uses more gas than if you can lower the RPM at all.
- Also, using N when stopped does make a difference.
- I don't think you're putting any significant wear on the transmission by shifting between N and D.
- At higher speeds, it may be a good idea to rev-match when going from N to D. Just start to get lightly on the gas pedal at the moment that your shifter clicks into D; in my truck, this results in a perfect rev-match.
- The increased idle when at speed is probably to keep the transmission fluid moving fast enough to cool the transmission, since it is pumped by the input shaft. It's probably the same in most 1998 GM vehicles (except maybe Saturns, which I think have very different transmissions).
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Old 06-18-2008, 07:57 PM   #3
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The difference is your engine is not under a load so even at 950rpm is will use less fuel compared to being in hear at 950rpm, the injectors duty cycle would be as small as possible with no load, compared to a labouring engine at 1400rpms it would tend to provide richer fuel, I maybe wrong?
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Old 06-18-2008, 08:10 PM   #4
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Quote:
Originally Posted by ShadowWorks View Post
The difference is your engine is not under a load so even at 950rpm is will use less fuel compared to being in hear at 950rpm, the injectors duty cycle would be as small as possible with no load, compared to a labouring engine at 1400rpms it would tend to provide richer fuel, I maybe wrong?
Sounds like more experiments are warranted once I get a scangauge... My birthday is coming up soon, maybe I can drop some hints...

-Jay
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Old 06-19-2008, 05:47 AM   #5
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Quote:
Originally Posted by ShadowWorks View Post
The difference is your engine is not under a load so even at 950rpm is will use less fuel compared to being in hear at 950rpm, the injectors duty cycle would be as small as possible with no load, compared to a labouring engine at 1400rpms it would tend to provide richer fuel, I maybe wrong?
You are 100% correct, as I've observed using my fuel rate/fuel injector duty cycle meter. Duty cycle might be 1.5% at 550rpm in neutral, 2.7% at 950rpm in neutral, and 10% at 950rpm WOT. It's still more fuel than a lower idle but still far less fuel than under load.
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Old 06-20-2008, 06:07 AM   #6
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Yeah, I'm trying to figure that out on Marvin, idle seems to float at 1100 while you're moving, it's only when stopped in N or P it goes to 800-850ish.
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Old 06-20-2008, 09:41 AM   #7
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because you are turning the drive shaft,,,its turning the trans ,,shows you there isn't a complete neutral
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Old 06-20-2008, 10:03 AM   #8
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I still think that there is a complete neutral, and the reason for the increased RPM is to keep the transmission oil moving fast enough to cool it.
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Old 06-20-2008, 10:13 AM   #9
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I still think that there is a complete neutral, and the reason for the increased RPM is to keep the transmission oil moving fast enough to cool it.
if the engine is running the pump is flowing/pumping fluid
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Old 06-20-2008, 11:31 AM   #10
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sooo.....how is it at any speed I can put it in neutral and drop the pedal and watch the rpms rocket to rev limit? Thereby proving, there is a neutral.

Something to try, eliminate the accel to speed neutral drop. Start on a hill and push the vehicle from a dead stop, when idle is at its lowest. See if it increases. That may help reduce the variables as to why the rpms don't drop for a coast.
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