Home » Public Forums » GMCnet » [GMCnet] BRAKE PERFORMANCE AND ALTITUDE
| [GMCnet] BRAKE PERFORMANCE AND ALTITUDE [message #81169] |
Tue, 20 April 2010 07:03  |
Ken Coit
Messages: 151
Registered: November 2005
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I picked up this from another thread I started and thought it worthy of its
own space:
Ken Henderson asked Ron if his brakes had bedded in any better. His answer:
Ken..
Yes, They did seem to bed.. BUT, they still aren't what I had hoped for.
Actually better at low altitude.. Ran much better too..
Maybe I just need some SOB Air Brakes?
- Hide quoted text -
Ron
--
76 Eleganza II
Conifer, CO
--
My intuition tells me that if engine and brake performance are affected by
altitude that maybe the vacuum system has a problem. Does this make any
sense to anyone else?
Ken Coit, ND7N
Raleigh, NC
Parfait Royale
1978 Royale Rear Bath, 403, 3.07
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| Re: [GMCnet] BRAKE PERFORMANCE AND ALTITUDE [message #81209 is a reply to message #81203] |
Tue, 20 April 2010 13:29  |
Ron
Messages: 250
Registered: February 2004
Location: Conifer, Colorado
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Fred..
Yes some time ago..
People talk about sliding the rear wheels. Hell I cannot slide any wheels on the gravel road I live on.. Scary sometimes..
Ron
now a P.O.
Conifer, CO
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| Re: [GMCnet] BRAKE PERFORMANCE AND ALTITUDE [message #81233 is a reply to message #81231] |
Tue, 20 April 2010 17:18 |
Ron
Messages: 250
Registered: February 2004
Location: Conifer, Colorado
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Ken..
Well Maybe you are right.. When I am home without the pump my brakes are even worse.. I will look at the vacuum system.. Although there really isn't too much to it.. When I am coasting or going downhill the pump doesn't run..
Ron
now a P.O.
Conifer, CO
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| Re: [GMCnet] BRAKE PERFORMANCE AND ALTITUDE [message #81243 is a reply to message #81238] |
Tue, 20 April 2010 18:37 |
Ron
Messages: 250
Registered: February 2004
Location: Conifer, Colorado
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Billy..
At high altitude.. It runs almost EVERY time you use your brakes.. That is true of almost everyone I talk to out here in Gods country.. The altitude does that.. I bet I get 5 or 6 inches vacuum less engine vacuum that all of you at lower altitudes..
I am sure that contributes to the problem..
Ron
now a P.O.
Conifer, CO
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| Re: [GMCnet] BRAKE PERFORMANCE AND ALTITUDE [message #81254 is a reply to message #81247] |
Tue, 20 April 2010 20:01 |
jhb1
Messages: 303
Registered: February 2004
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Ron
I ran across I 8 don't know how high it goes between Yuma and San Diego after JimB rebuilt my braking system and it was fine. Came back from Vegas to Phoenix thru Flagstaff still no problems.
John H. Bell
77 Royale; QuadBag,Manny OneTon,Honda EV4010, FITech
Montreal Qc.
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| Re: [GMCnet] BRAKE PERFORMANCE AND ALTITUDE [message #81258 is a reply to message #81254] |
Tue, 20 April 2010 20:55 |
Ron
Messages: 250
Registered: February 2004
Location: Conifer, Colorado
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John..
I just came through Flagstaff and all..
Maybe I am expecting too much of my brakes.. But I just don't feel they are quite right.. Just don't know what else to do!
Ron
now a P.O.
Conifer, CO
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| Re: [GMCnet] BRAKE PERFORMANCE AND ALTITUDE [message #81261 is a reply to message #81243] |
Tue, 20 April 2010 21:16 |
C Boyd
Messages: 2629
Registered: April 2006
Karma: 18
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| Ron wrote on Tue, 20 April 2010 19:37 |
Billy..
At high altitude.. It runs almost EVERY time you use your brakes.. That is true of almost everyone I talk to out here in Gods country.. The altitude does that.. I bet I get 5 or 6 inches vacuum less engine vacuum that all of you at lower altitudes..
I am sure that contributes to the problem..
Ron
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Ron: JIT: while you are touring on flat ground maybe plug off the aux vacuum pump system and see if anything changes? Could have a leak in it?? Are you running a vacuum gauge?
C. Boyd
76 Crestmont
East Tennessee
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| Re: [GMCnet] BRAKE PERFORMANCE AND ALTITUDE [message #81265 is a reply to message #81243] |
Tue, 20 April 2010 21:40 |
fred v
Messages: 999
Registered: April 2006
Location: pensacola, fl.
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| Ron wrote on Tue, 20 April 2010 18:37 |
Billy..
At high altitude.. It runs almost EVERY time you use your brakes.. That is true of almost everyone I talk to out here in Gods country.. The altitude does that.. I bet I get 5 or 6 inches vacuum less engine vacuum that all of you at lower altitudes..
I am sure that contributes to the problem..
Ron
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does your cruise control work? there is a vacuum hose that goes up to the brake pedal to trip off the cruise control when you hit the pedal. could be a broken hose killing your vacuum.
and what is your vacuum reading at idle?
Fred V
'77 Royale RB 455
P'cola, Fl
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| Re: [GMCnet] BRAKE PERFORMANCE AND ALTITUDE [message #81266 is a reply to message #81169] |
Tue, 20 April 2010 22:01 |
Gary Cottingame
Messages: 85
Registered: January 2005
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Mac Donald,
If you'll get in touch with The Tulsa Lunch Bunch, I'm sure you can find a place to check out your brakes.
http://www.gmcclassics.com/rallies/tulsa.html
Gary
73 26 CL
TZE21
Denison, Tx
--- On Tue, 4/20/10, D C *Mac* Macdonald <k2gkk@hotmail.com> wrote:
From: D C *Mac* Macdonald <k2gkk@hotmail.com>
Subject: Re: [GMCnet] BRAKE PERFORMANCE AND ALTITUDE
To: gmclist@temp.gmcnet.org
Date: Tuesday, April 20, 2010, 9:29 PM
Hi, Dan.
Any recommendations for brake work here
in the middle of Oklahoma? I have new
shoes for the rears and had the front
worked on so that I could bring Money Pit
home from New Braunfels, TX.
* Mac Macdonald *
* Oklahoma City *
** "Money Pit" **
* '76 ex - P.B. *
* In my many years, I have come to the conclusion that one useless *
* man is a shame, two is a law firm and three or more is Congress. *
* -- John Adams (1735- 1826) *
----------------------------------------
> To: gmclist@temp.gmcnet.org
> From: gregg_dan@hotmail.com
> Date: Tue, 20 Apr 2010 21:01:53 -0500
> Subject: Re: [GMCnet] BRAKE PERFORMANCE AND ALTITUDE
>
>
>
> No Ron, you are not expecting too much from your brakes. they should work. That back wheel should lock up in a panic stop. You should not have to anticipate a stop 1/8th mile ahead. I know how it is. When I bought our coach I drove it 900 miles with brakes like your's. Also needed two monkeys to help me steer the thing above 55 mph. Now, our's drives and brakes like a car. Sure it takes longer to stop it but I feel really good when I hit the pedal. You have something messed up on your coach. The external vacuum pump should not come on every time you hit the pedal, mine doesn't. If you ever get anywhere close to Fremont stop in at Jim's and let Donnie get to the root of your problem. The boy is good on GMC coaches.
> Dan
> --
> Dan & Teri Gregg
>
> danandteri.blogspot.com
>
>
>
> ///Halon Automatic Fire Extinguishers
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| Re: [GMCnet] BRAKE PERFORMANCE AND ALTITUDE [message #81322 is a reply to message #81247] |
Wed, 21 April 2010 10:41 |
mlincoln
Messages: 107
Registered: August 2006
Location: Salt Lake City
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"Vacuum" pumps are not necessarily inherently less efficient with altitude. However, the total vacuum that can be achieved will be less, simply because atmospheric pressure is less. A perfect vacuum (no gas molecules) at sea level would be 29.92 inches of mercury: the weight of a column of atmosphere extending from the barometer into outer space would displace a column of mercury that high against a vacuum on the other end of the column. Let's suppose that perhaps a stock GMC (using the interaction of the throttle plates and venturi system in the carburetor as the "vacuum pump") can achieve about a 60% evacuation of air molecules in the vacuum system, thereby achieving a maximum "vacuum" of about 18 inches of mercury (below atmospheric), or an absolute pressure of about 12 inHg compared to a true vacuum.
Now consider the situation at 3,000 meters (ca. 9200 feet), where the atmospheric pressure on a standard day is just over 21 inches of mercury, or about 72% of the sea level value. If the inherent function of the carburetor venturi was the same (and I assume it would be, given ideal gas laws and the fact that the venturi is cast in metal and doesn't change diameter, etc), you might expect this venturi to again achieve a maximum of 60% of a perfect vacuum given the best position of the carb butterfly and throttle linkage. But that would be 0.6 * 21 inches Hg = 12.6 inches of mercury at best, or only 70% of the sea level value of "vacuum". In that case, anything vacuum assisted would have less assist. If the vacuum advance was set up to mechanically advance the timing fully at a value of 16 inches of mercury--below atmosphere--and note that I'm just making this 16 inch figure up out of thin air, then you could never achieve a full advance at 9200 feet, where maximum vacuum
would be only 12.6 inches. Likewise, if the brake function was assisted by the difference exerted across a diaphragm in the brake booster between atmospheric pressure and (partial) vacuum pressure, the most differential assist you'd get at 9200 feet would be 12.6 inches Hg as opposed to perhaps 18 inches Hg at sea level on a standard day. At any altitude, you'd get less assist (and advance) at wide open throttle with a low vacuum situation. At any altitude, a vacuum leak (more like an inwards leak of gas into the "vacuum" lines, not a leak of vacuum outwards!) would reduce advance and braking boost. In the case of brakes, I assume this means you'd either have to push the pedal harder to get the same brake line pressure, or else settle for less brake line pressure and a longer stopping distance.
Now about assist pumps. Remember every pressure measurement must be referenced to some other pressure, typically either an approximation of absolute vacuum or else some ambient, external pressure. Whether the pressure reference (electrically coupled to the pump function) is an aneroid chamber (ca. zero absolute pressure) or a differential with atmosphere would be important to know. If it is an aneroid, then the pump is likely set up to turn on when the absolute pressure in the engine vacuum system is "too high". If it is a pressure differential compared to atmosphere, then the pump is likely set to turn on at some absolute difference, to try to maintain a certain differential. If the pump function was aneroid-coupled, then increasing altitude and decreasing atmospheric pressure wouldn't cause it to turn on any "sooner" (in space, the pump would never need to turn on). If the pump function is coupled to a pressure differential with atmosphere (e.g., always maintain 8 inH
g by turning on whenever pressure differential falls below that value), then the pump would work at any altitude to maintain this minimum difference. Now that minimum difference might well be less than what a well functioning engine would achieve, at sea level, with a closed throttle. As stated above, your engine may well achieve more than this minimum, at least at sea level. I also suppose in this case that auxiliary vacuum pumps measuring pressure differential from atmosphere might well turn on at wide open throttle going uphill in the GMC (but you probably can't hear them then).
So in a stock GMC, I can certainly see someone noticing less vacuum advance and less brake effectiveness at altitude. Whether the pump is enough to make up for this in greater or lesser degree depends on the type of pump (how it measures "vacuum") and the set point at which it turns on.
Mike "who has spent far too much time in the MICU and pulmonary laboratory calibrating and puzzling over mechanical ventilators and cardiopulmonary catheters" Lincoln
On Apr 20, 2010, at 7:55 PM, rhusak wrote:
>
>
> Carleton..
> I live at 9200 feet..
> --
> 76 Eleganza II
> Conifer, CO
> _______________________________________________
> GMCnet mailing list
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Mike
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