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Author Topic: MPG and Jakes  (Read 3532 times)
Nick Badame Refrig/ACC
1989, MCI 102C3, 8V92T, HT740, 06' conversion FMCA# F-27317-S "Wife- 1969 Italian/German Style"
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« on: December 05, 2007, 10:58:19 AM »

Hi Guy's,

This morning I traveled 10 miles down the road to fuel up for an excursion to Cabella'a tomorrow with some friends.

After filling 88 gallons into the tank, "at $334.9 gal" I did my milage calculations and for the past 3/4 months I have

dropped in MPG to 5.8 to 6.2. I was averageing 6.5 to 7.5 depending on weather I was pulling the Yukon or not.

Question.... Does my newly installed Jake's effect my fuel milage?

I do notice that when the jakes are inguaged, there will be a bit of unburned fuel comming from the exhaust that

I can only see at night time when there are headlights shinning from behind.

Thanks in advance
Nick-
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« Reply #1 on: December 05, 2007, 12:05:38 PM »

Nick - If you leave your jakes on all the time your not going to get any coast out of your D-train - I only use mine on grades - FWIW
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« Reply #2 on: December 05, 2007, 12:22:51 PM »

Nick - If you leave your jakes on all the time your not going to get any coast out of your D-train - I only use mine on grades - FWIW

I agree with Niles. If you are leaving your Jake on all the time, it is like putting on your brakes every time you take your foot off the accelerator.

Richard
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« Reply #3 on: December 05, 2007, 12:53:53 PM »

PLUS, as you know, there is a huge difference in "around town and over the road" >>>Dan
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« Reply #4 on: December 05, 2007, 01:07:33 PM »

I do notice that when the jakes are inguaged, there will be a bit of unburned fuel comming from the exhaust that I can only see at night time when there are headlights shinning from behind.
Thanks in advance
Nick-

Nick, it is my opinion that there will always be a small amount of unburned fuel. There is always enough fuel from the injectors to maintain engine idle speed even with the Jakes engaged. Since the Jakes are opening the exhaust valves, this fuel is not burned and would appear as unburned fuel in your exhaust.

This is my opinion until someone comes up with a better answer or explains how the fuel could be turned completely off.

Richard
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« Reply #5 on: December 05, 2007, 01:13:16 PM »

Also what you may be seeing in the exhaust at night with the Jake working may be engine oil being sucked past either the rings or valve guide seals.  This can happen with mills in excellent condition.  The only other thing I can think of and it doesn't make any sense is that your fuel system is not completely shutting off when the Jake is applied and you are seeing unburned fuel like you said.

Yeah, the Jake is soosss neat that we tend to use it like service brakes all the time if it is left "on" all the time.  Saves your brakes, but the Jake can change our driving habits to the point that we do loose energy of the coach ordinarly coasting but instead being greatly slowed quickly by the use of the Jake.  Sorry for my lousy riting style today....I need more red wine.  He he he.  Smiley Smiley Smiley
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Nick Badame Refrig/ACC
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« Reply #6 on: December 05, 2007, 02:40:06 PM »

Thanks Guy's

I do have a bad habbit of leaving the jakes on....  I will work on that..

One thing I noticed, in cold weather the jakes work exceptionally well. I guess it's the thicker oil.

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« Reply #7 on: December 05, 2007, 02:50:57 PM »

Thanks Guy's

I do have a bad habbit of leaving the jakes on....  I will work on that..

One thing I noticed, in cold weather the jakes work exceptionally well. I guess it's the thicker oil.

Nick-

I think it has something to do with the exhaust valve clearances that were set during installation. I found that when the engine was cold the Jakes worked significantly better than when the engine was up to temperature. I suspect it has something to do with the clearances increasing as the engine warms up. I went in and changed my valve clearance from .059 to .055 and it really made a great improvement in the operation when the engine gets up to operating temperature.

Richard
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« Reply #8 on: December 05, 2007, 05:34:01 PM »

Richard, when the engine is turning quite a bit faster than the governor is calling for, the fuel should turn off completely at avery injector. There should be no fuel injected to maintain idle until the engine speed drops down to something near the governor setting.

I have seen the haze or small amount of smoke that you refer to and I have wondered about it, too. The only thing that I could think of is that there might be a small amount of lube oil mist coming out the exhaust.

I could see that this might happen if the jake completely relieved the compression while the piston was up and the valves closed before it went down too far. That would lead to a vacuum forming over the piston before it uncovered the intake ports.

I don't see how a vacuum might form around the valve stems, however.

For what it's worth.

Tom Caffrey
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Tom Caffrey PD4106-2576
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« Reply #9 on: December 05, 2007, 07:06:28 PM »

Richard, when the engine is turning quite a bit faster than the governor is calling for, the fuel should turn off completely at avery injector. There should be no fuel injected to maintain idle until the engine speed drops down to something near the governor setting.

I have seen the haze or small amount of smoke that you refer to and I have wondered about it, too. The only thing that I could think of is that there might be a small amount of lube oil mist coming out the exhaust.

I could see that this might happen if the jake completely relieved the compression while the piston was up and the valves closed before it went down too far. That would lead to a vacuum forming over the piston before it uncovered the intake ports.

I don't see how a vacuum might form around the valve stems, however.

For what it's worth.

Tom Caffrey

You may be entirely right, but I was under the impression that the governor could not turn the fuel off entirely. It could only reduce the amount to what is required to keep the engine at idle speed.
Again, only an idea of mine.
Richard
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« Reply #10 on: December 05, 2007, 07:07:41 PM »

Nick, i suspect if you leave the jakes on and drive mostly downhill, your mileage will improve back to what it was.  you are obviously driving uphill too much.  go DOWN to the coastline and stay there.

or head down to florida at this time of year.
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Tom
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« Reply #11 on: December 05, 2007, 07:11:51 PM »

Nick, i suspect if you leave the jakes on and drive mostly downhill, your mileage will improve back to what it was.  you are obviously driving uphill too much.  go DOWN to the coastline and stay there.

or head down to florida at this time of year.

Or put Great big tires on the back end of your bus and itty bitty tires on the front.

That way you'll always be going downhill!  Grin

DF
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« Reply #12 on: December 05, 2007, 07:56:52 PM »

I use the Jake intermittently, when going down grades, in any gear, and to slow at a stop light or sign.  I note that at  highway speed, black smoke is often emitted, but when in lower gears, not necessarily so.  and yes....I do think it changes fuel mileage.  This opinion after 6 years of using the coach, mostly in the western part of the US, but also a number of trips east including Nova Scotia.

But....worth EVERY penny ( dollar ?)   Wink

FWIW

RCB
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« Reply #13 on: December 05, 2007, 10:02:57 PM »

I have heard truckers say that leaving the jakes on will decrease mileage because they will often get activated when the driver only wanted to coast, but now has to use fuel to regain momentum lost. Not the most efficient way to travel and mostly due to inattention. Not the main reason, but defiantly one of the reasons, I installed a momentary-on foot switch in parallel with my Jake toggle switch. I only activate it when I need it, and Iím not fumbling for the switch all of the time.

Could what you are seeing at night be moisture condensing in the cool air? The hot compressed air, when dumped into the cold atmosphere, will have its moisture condense right? I was also under the impression that the fuel was cut off until reaching idle but I really donít know all that much about the operation of the governor.

Laryn


Photo of when I was installing the jake foot switch in the hole where the clutch used to be.
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« Reply #14 on: December 06, 2007, 02:13:18 PM »

I mentioned valve guide seals as well as the rings being a potential source of nighttime exhaust pipe smoke/hazzing during Jake operation because I was told that since the exhaust valves are only being cracked open, a situations can/does exist that lets oil weep past the seals.   Something to do with pulse cycling or the seal material memory or valve stem wear or temperature cycling or something like that for whatever it is worth.  Anyway, that is what the factory Jake Brake engineers said long ago and far away.   Smiley Smiley Smiley
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