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The Uppity 12v Admin
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Discussion Starter #1 (Edited)
Doing some testing the other day for my injection pump conversion, I learned something about how the P7100 works that can save some fuel.

When descending a hill at cruising speeds, some prefer to pull the truck out of gear and letting it idle under the assumption that less rpm equals less fuel injected if the rack is at the "idle" position. With the P7100, you will actually see reduced fuel consumption by leaving the truck in gear. When coasting with no throttle input at cruise RPM, the governor weights will move the rack past "idle" to it's parked position, stopping injection altogether.

Seeing this for yourself is easy. Next time you're coasting down a hill with your foot completely off the throttle and no if other motorists will be affected, key your engine off. You will notice that your rate of deceleration doesn't change. If the pump had been injecting fuel, your rate of deceleration would have increased. Clutch in to restart the engine as the pull coil in the FSS requires bumping the starter, and continue with your journey. Auto trucks will have a harder time doing this test due to the lockup/OD solenoids losing power, but it may still work. Couldn't say.

I don't know exactly what RPM the governor will begin to park the rack at no throttle, it probably varies depending on your GSK, adjustment, idle speed, and other factors, but mine will do it as low as 1250 rpm.

 
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i also noticed this, because going down a hill with revs at 800 its a lot quieter than at 800rpm idle, the turbo sounds really nice too with no other noise

1994 RCLB 12 valve nv4500 4x4 160hp pump afc mods
 

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This works for almost any vehicle. It's how governor's work.
 

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Why would anybody put their transmission in neutral while going down hill to begin with? All you do is pick up speed and use the hell out of the brakes.
 

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seeing this for yourself is easy. Next time you're coasting down a hill with your foot completely off the throttle and no other motorists will be affected, key your engine off.
What!!!!!!!!!!!!!!
 

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The Uppity 12v Admin
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Discussion Starter #6
Some people do not assume that the engine controls reduce fuel delivery below idle regardless of the system and whether it's electronic or mechanical. Particularly in the diesel crowd, coasting in neutral seems to popular because they have comparatively limited engine braking compared to a gasser without a compression brake, retarder, or exhaust brake.

Next time you're coasting down a hill with your foot completely off the throttle and if no other motorists will be affected, key your engine off.
I would like to assume you skimmed over this part, but you clearly included it after trimming the rest of the post out.

Life is not a Michael Bay movie. The truck will not suddenly careen out of control, flip over, and burst into flames. The engine will continue spinning and you will still have power assist for the steering and brakes. Any competent motorist should be able to recover from a loss of engine power, particularly one that was initiated on purpose.
 

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I wondered about the injection events when going down hill or even slowing down, when I installed a Tiny Tach on my Fummins to check the dash tach's accuracy I noticed that whenever I let up on the fuel at speed or going down hill the Tiny Tach quits reading, when I touch the throttle it goes back to reading rpm's.

FYI, Tiny Tach's get their signal from the injection pulse, a sensor clamped to one of the injector lines picks up the signal.
 

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My engine developed an air leak and shut down. I was still able to steer the truck and through a few turns with no power steering, then tune 90* into my driveway and stop the truck.

I'm 5'6 and 170lbs, not exactly the biggest or strongest guy around.

I wonder if a manual truck would still have power steering because the engines are connected to the road.





Also in a new car you see the instant mpg goes to 99 when you let go of the gas. If you pop the clutch in or out in neutral it goes to zero.
 
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I wonder if a manual truck would still have power steering because the engines are connected to the road.
It would as long as you didn't push in the clutch and kept the motor spinning.
 

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The 10th Man
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OK, I know this thread is old, but I just seen it today... So yeah. I also over analyze everything, so come prepared.

Tony I am glad brought this up! The whole governor being constantly active thing was something @OK12V and I were trying to convince the world years ago to little avail.

However, I'm not sure I completely agree with the fuel saving aspect of it.

2 case scenarios:

1st. You're going a down hill and you don't hit the clutch.

Your engine will be running at the same RPM you were cruising at, say 1800RPM which obviously takes power to spin whether that power is being created by fuel or or drained from your momentum. The power to spin the engine will also be going through the differential, T-case and Transmission, so there will be friction losses there as well.

So at the bottom of this hill, you won't have used any fuel, but you will have lost speed due to the engine drag.


2nd scenario. You're going down a hill and you do hit the clutch.

Your engine will be spinning at ~800RPM, and there will be very minimal friction loss going through the transmission. You are however burning fuel to maintain your idle RPM.

So at the bottom of this hill, you will have used some fuel, but you won't have lost speed due to engine drag.


So it becomes a balancing act between fuel used, and momentum lost. It would be interesting to do testing on this, measuring fuel usage on a hot engine, and then seeing how much speed you lose with and without clutching. This would be an entertaining experiment, However simply using the Law of Conservation of Energy, we can come to a basic conclusion.

In scenario #1, the engine is spinning faster thus creating more friction, and so is the drivetrain. So by not hitting the clutch, more energy gets wasted to friction and thus heat. Now there's the argument of the efficiency that the fuel is being burned at, and whether that makes up for the extra friction. However we must remember that this is the same fuel that must be burned to make up for speed lost while coasting, making it somewhat of a null-argument.

Now, if this hill is rather steep, so riding the brakes is required, or if you'll be braking shortly after the hill for a turn or stop sign, then the speed lost while coasting down the hill has no effect on the overall fuel usage of your trip and it becomes a no-brainer to coast the engine without hitting the clutch, might as well slap the exhaust brake on as well.

We are also truck people, so it's worth mentioning that if you're hauling a heavy load up this hill, than crest the top and are going back down. If you hit the clutch letting the engine return to idle, this CAN cause the engine to overheat due to your cooling system performance being directly proportional to engine RPM.

I guess the proof is in the pudding, so do I clutch down every hill??? Heck No lol. I'm not a MPG junky, nor do I believe that the extra hassle or clutch wear is worth the cost of fuel.

Now, if you've made it this far, you're probably wondering where all this crap came from, how can this poor soul devote so much thought into such a pointless topic. The answer is simple. You try driving around in circles all day on a tractor going 8 mph. The mind wanders. And this tractor has a mechanical fuel shut-off, which makes playing around/testing really easy.

Back to the governor involvement, Tony's test makes it pretty clear that it decreases fueling when the RPM is over the RPM set by the throttle pedal, even to the point of cutting off fueling completely. Here's a vidier showing that the opposite is true as well, that the engine will increase fueling if the RPM drops below the RPM set by the throttle pedal.

 

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The Uppity 12v Admin
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Discussion Starter #12
Losing speed due to engine drag is kinda the point. If the rate of speed loss is too great, I add just enough throttle to maintain my desired speed. I am not disagreeing with you, but I think fuel consumption at idle versus fuel consumption at ever-so-slight throttle and 1800 rpm is probably splitting hairs.

My FADEC system will have the ability to measure fuel volume consumption (gal/hr) with .5% accuracy and fuel mass consumption (lb/hr) with about 1.25% accuracy. I'll be sure to add this test to the list.
 

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The 10th Man
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Of course we're splitting hairs! MPG Junkies are known for splitting hairs, and "what to do while coasting" is towards the bottom their list of tricks, so this topic by definition is splitting hairs regardless of the conversations content. I think we can agree that if you're applying ever-so-light throttle to remove the engine drag, you are effectively idling the engine to 1800 RPM, if the fuel you are giving the engine is not enough to idle the engine to 1800 RPM, then the difference will still be drained from your momentum. The difference in fuel consumption between idling the engine at 800 RPM versus 1800 RPM is at least double. I think that's a safe guess, but it will be your job to confirm with your electro-fueling wizardry.

My point in all this is simple. This rack parking phenomenon is valuable to know regardless of what you do with the information, but If someone is still willing to pop the tranny out of gear when coasting, than that is still the most MPG friendly option.

I know you're The Uppity 12V Guy, but you must remember that I'm The 10th Man, it's my job to disturb crap.
 

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The Uppity 12v Admin
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Discussion Starter #14 (Edited)
I don't know if it is or not. I would think you'd have less rack at 1800 rpm with partial downhill throttle than sitting in the driveway, as the powertrain is still providing a light braking action. I mean, the slope of the hill is a huge factor so our arguments may be diverging.

I haven't practiced my own takeaway of Israeli military doctrine in a while, but your 10th Man takeaway compels me to post this:



:na:
 
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The 10th Man
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You pose an interesting topic, the difference in transmission drag when rolling versus sitting. After thinking about it further, you would seem to have a point. When sitting, you're powering the transmission from the engine, when rolling, the transmission is being backfed by the wheels and thus powered by your momentum. Now It would be really easy to overthink all this, so instead, I want to give you a little backstory that also ties in with my 10th man comment.

When I'm trying to learn or understand something, I try to pass it through 3 filters.

1st, break it down into smaller parts, understand each part, then reconstruct it.
2nd, over exaggerate the numbers to see if the answer is clearer, then work my way back and see if the results are still applicable.
3rd, once I think I'm right, try to prove myself wrong. This is where the 10th man thing comes in, as when I developed this method, I had no idea what the 10th man even was.

So let's do that here.

1st. With the tranny in neutral, the engine is spinning slower = less friction. The transmission is no longer having to spin the engine = less friction, and all the gears will be spinning slower = less friction. The differential is no longer having to spin the engine = less friction.

2nd. Right now we're talking 800 RPM idle, and 1800 RPM cruise. What if that was 800 RPM and 4000 RPM? Kinda makes the answer a no-brainer, you don't typically associate 4k RPM and MPG. Now it's easy to think "4000 RPM is an unrealistic example", and that's true. But what's the difference? The number is exaggerated and so is the results. If you scale that number back, how does it change in a way that makes 1800 RPM more fuel efficient than 800. Another way to exaggerate the numbers would be to change the transmissions gear, instead of OD, maybe 3rd. Again a no-brainer, however this result could be a little more flaky as it's effectively overdriving the engine by a lot, which will suffer from increased friction. Still serves as mind-candy.

3rd. I'm going to give this an honest effort. Ideas are welcome.

A) Idling down the hill uses fuel.
This is true, but like we said, it's the same fuel the engine will end up using to re-gain the speed lost by coasting down the hill.

B) While Idling, the Injection Pump is having to compress fuel for it to reach injection pressure. Compressing any substance takes work.
Answer is basically the same as the above.

C) Idling fuel efficiency is less than the efficiency while cruising down the highway, making up for the extra friction caused by coasting.
Diesels are known for there idling efficiency, burning 1/2 GPH, and because the engine is hot, and this is only a short idling sesion, efficiency will be even greater due to an increased burn temperature and warmer oil. So I don't have a definite answer, but I just don't see it. But I could be wrong.

D) Lower RPM causes the Alternator to work harder to charge the batteries increasing load.
.... Ummm, No.

E) Lower Drivetrain friction allows for the fluid temperature of the transmission, T-case and Dif to decrease thus increasing friction.
Ummmmm.... A very long hill maybe!

F) Pressing the clutch engages the throwout bearing increasing friction.
Well, first off, the throwout bearing is always "engaged" it's just not pressing that hard. Hitting the clutch for such a short duration combined with very minimal drag makes it into somewhat of a null issue.

G) With the engine coasting at 1800 RPM, the cooling fan is spinning faster and is thus pulling the truck through the air like a propped airplane also reducing aerodynamic drag.
Does it look like I know!

I think we ended up overthinking it after all. lol

My viewpoint on this is still the same. I'm not super-crazy about MPG, so it's not worth it to me to have to clutch on every hill. If someone is, then I believe that throwing the tranny in neutral on downhills is still the most MPG friendly option.
 

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I agree more or less, using neutral downhill can get you to your destination on less fuel, but it also depends on the lay of the land. how long/steep the hill is, is there an uphill on the other side to climb, is braking going to be required, (if it is, no point in throwing it in neutral). I tried the coasting downhill in gear off the throttle yesterday, while pulling the shutoff cable, no change in rpms whatsoever. I didn't realize the p pump defueled that way till reading this thread. Interesting.
 
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The Uppity 12v Admin
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Discussion Starter #18
I forgot to update this a few days ago, I did more testing and found that the rack will only park with absolutely zero throttle input while coasting. At least with my truck, if you toe the pedal at all, but not even enough to hear a change in engine tone while coasting, injection will begin. This includes the slightly applied throttle that cruise maintains when coasting.

So, it would seem Cowboy303 was on the right track with his idea.
 

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The 10th Man
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The fully laden canadian swallow can hit around 21 mph carrying half it's own weight, but struggles badly in headwinds higher than 18 mph. However in a vertical dive, said laden swallow can hit peak speeds of up to 45 mph, with near zero input (for the speed it's travelling) and wings in a neutral outstretched position. This extra speed gained on the downward dive can carry momentum needed to nearly re-gain altitude at will. So the laden swallows speed and wing position is gravity dependent so to speak.:hehe::nerd:
 
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