Home » Public Forums » GMCnet » Trip to High Altitude
| Trip to High Altitude [message #214623] |
Tue, 16 July 2013 15:49  |
larry.whisler
Messages: 356
Registered: August 2005
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We returned from our first trip in the GMC to the western part of the United States.
One item that I noticed was the lower octane gasoline available at the pump, 85 octane.
I had some issues with using 87 octane in the flatlands that resulted in spark knock and possible vapor lock.
So, I am looking at this 85 octane option and thinking WTF. If
I am having problems using 87, why would anyone buy 85.
Then we traveled on toward the Tetons, and at altitude I am losing power. I am wondering what is wrong with the engine?
Then as we top over the peak, the elevation is reading 10K.
Okay, higher altitude=lower air pressure=power loss. We get back to 'flatlander' altitudes it should not be a problem.
And then it dawned on me that is why they sell the lower octane
fuel at altitude. Higher altitude = lower effective compression ratio and you can get better performance with out spark knock using the lower octane fuel.
So is this a correct assumption from a flatlander in central Indiana?
larry whisler
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| Re: Trip to High Altitude [message #214645 is a reply to message #214623] |
Tue, 16 July 2013 19:03  |
Craig Lechowicz
Messages: 541
Registered: October 2006
Location: Waterford, MI
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Yes you are correct. It's about 3% reduction in power for every 1,000 feet (unless turbo or supercharged,) so 10,000 feet is a serious reduction.
Craig Lechowicz
'77 Kingsley, Waterford, MI
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| Re: Trip to High Altitude [message #214652 is a reply to message #214651] |
Tue, 16 July 2013 19:39 |
LarryInSanDiego
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Registered: September 2006
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I forget the numbers, but higher octane fuel has less energy content (BTUs or whatever units you prefer). At higher altitudes (lower air density), you can benefit from lower octane (higher energy) fuel with less sensitivity to detonation.
Larry Engelbrecht
San Diego, CA
'73 26' ex-Glacier
TZE063V100319 03/07/73
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| Re: Trip to High Altitude [message #214663 is a reply to message #214623] |
Tue, 16 July 2013 21:39 |
Craig Lechowicz
Messages: 541
Registered: October 2006
Location: Waterford, MI
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Larry,
I think so, although there may be some subtlety between power output and compression ratio that I don't know about, but Matt Colie would.
Craig Lechowicz
'77 Kingsley, Waterford, MI
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| Re: [GMCnet] Trip to High Altitude [message #214665 is a reply to message #214663] |
Tue, 16 July 2013 22:52 |
jimk
Messages: 6734
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Location: Belmont, CA
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Larrry,
I encounter this issue every time I cross the Rockies.
When I know ew will be heading for lower elevation, I try not to tank up
with the low octain.
My twin turbos works best at high altitude, at low altitude, I can only run
with low p
On Tue, Jul 16, 2013 at 7:39 PM, Craig Lechowicz <
craig.lechowicz@sbcglobal.net> wrote:
>
>
> Larry,
> I think so, although there may be some subtlety between power output and
> compression ratio that I don't know about, but Matt Colie would.
>
> --
> Craig Lechowicz
> '77 Kingsley, Waterford, MI
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Jim Kanomata
Applied/GMC, Fremont,CA
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| Re: [GMCnet] Trip to High Altitude [message #214679 is a reply to message #214651] |
Wed, 17 July 2013 06:35 |
jhbridges
Messages: 8412
Registered: May 2011
Location: Braselton ga
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Actually, the compression ratio won't change. You'll still com[press the charge to one eigth its volume. But, if you compress a 14.7PSI charge to one eigth its volume, you'll have more fuel and air than if you compress 11.7 PSI to one eigth its volume.
--johnny
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On Wed, 7/17/13, larry.whisler <larry.whisler@hotmail.com> wrote:
Subject: Re: [GMCnet] Trip to High Altitude
To: gmclist@temp.gmcnet.org
Date: Wednesday, July 17, 2013, 12:31 AM
So, I am equating a 3 percent per 1000 ft. altitude
loss in power to an equal loss of compression ratio.
Therefore, a 10K altitude would make an 8.5 compression
engine into a 6.0 compression engine
I don't think the function is linear but at these calcs it
should be close.
Am I out of tune with this line of thought ?
larry
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Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
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| Re: Trip to High Altitude <revised> [message #214686 is a reply to message #214663] |
Wed, 17 July 2013 08:06 |
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Matt Colie
Messages: 8547
Registered: March 2007
Location: S.E. Michigan
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| Craig Lechowicz wrote on Tue, 16 July 2013 22:39 |
<I just realized that I did not explain Expansion Ratio.>
Larry,
I think so, although there may be some subtlety between power output and compression ratio that I don't know about, but Matt Colie would.
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Yes, Matt would know...
The problem come out in terms that even most motorheads can't easily resolve.
The first is a "Charge Pressure". This is the pressure in the cylinder before the fire starts. It has a whole lot to do with how that fire goes from there. You may have the same compression ratio as you had at sea level, but you sure don't have the WOT charge pressure available. The lower pressure charge does burn more slowly.
The second thing that is thought of in design is "Expansion Ratio". This is very much like compression ratio in most cases and is closely tied to the engine's thermal efficiency. <This is the ratio of the usable volume from the time the cylinder gets to peak pressure to the time that pressure is lost due to either the piston travel or the opening of the exhaust valve.> This is what actually gets you net horsepower.
Now, why does this all matter?
Because if the flame front is slower, you either need earlier spark (remember Emery's pictures of the easy to adjust distributor?) or faster burning fuel (like 85 octane).
Let's put two bad situations together. Hold ON...
Take an old engine, reduce the squeeze (CR) so it can run on what is pump gas today and then, because you got the timing wrong (maybe it is the specified, but still wrong), it pings, so you fill it with premium. The ping stops. But, about 1~1.5K later, so does the engine. Diagnosis shows that the valves are gone which is confirmed when then heads are removed. The exhaust valves and seats are toast. (Tot for those of German extraction, but toast is really more accurate in this case.)
What the (expletive deleted) happened? It was sounding so good....
You lowered the expansion ratio and you used fuel that would burn more slowly. That means that there is less change in volume of the combustion chamber so the slow burn gets to stay burning longer and the fire is still raging when the (unfortunate) exhaust valve opens. The exhaust valve in most engines are hanging by a thread in the best of times and they only get to survive because the worst of the fire is over before it becomes their problem and they get to sit back in the "cool" seat when their job is done. If either of those gets disturbed...
This is just one of the reasons why the engines with a fully implemented active control system that include knock monitor do so well. It is also why a C.Boyd can run tweak the engine and run on premium - he can hear and feel the combustion process. I know how long it took him to learn this and I do not want to venture the cost (borne by he and others).
Now, if you understand all that - Great.
If not, can you phase a question that might clarify things for all the readers?
Matt - the dyno lab refugee
Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit
[Updated on: Wed, 17 July 2013 08:11] Report message to a moderator |
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| Re: [GMCnet] Trip to High Altitude [message #214709 is a reply to message #214686] |
Wed, 17 July 2013 12:09 |
jhbridges
Messages: 8412
Registered: May 2011
Location: Braselton ga
Karma: -74
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OK, how does BMEP relate to all of this? I know it's a calculated value... but when we were going for approval from the FAA to weld flat motors airworthy, they told us to run the test (new engine certification run) on the highest BMEP engine we wished to be certified for, using each type of weld (Case, bearing web, spark plug hole, valve seat) and demonstrate that the welds held; and we would then be certificated to weld airworthy anything from that BMEP on down. It turned out to be a Lycoming supercharged engine as fitted on Aero Commander twins. GSIO-something, I think maybe 4i90 CID (?). The FAA aid the represented the highest stressed engine.
--johnny
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On Wed, 7/17/13, Matt Colie <matt7323tze@gmail.com> wrote:
Subject: Re: [GMCnet] Trip to High Altitude
To: gmclist@temp.gmcnet.org
Date: Wednesday, July 17, 2013, 1:06 PM
Craig Lechowicz wrote on Tue, 16 July 2013 22:39
> Larry,
> I think so, although there may be some subtlety between
power output and compression ratio that I don't know about,
but Matt Colie would.
Yes, Matt would know...
The problem come out in terms that even most motorheads
can't easily resolve.
<snip>
This is just one of the reasons why the engines with a fully
implemented active control system that include knock monitor
do so well. It is also why a C.Boyd can run tweak the
engine and run on premium - he can hear and feel the
combustion process. I know how long it took him to
learn this and I do not want to venture the cost (borne by
he and others).
Now, if you understand all that - Great.
If not, can you phase a question that might clarify things
for all the readers?
Matt - the dyno lab refugee
--
Matt & Mary Colie
'73 Glacier 23 Chaumière (say show-me-air)
Now with 4 working Rear Brakes
SE Michigan - Twixt A2 and Detroit
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Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
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| Re: [GMCnet] Trip to High Altitude [message #214724 is a reply to message #214709] |
Wed, 17 July 2013 14:30 |
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Matt Colie
Messages: 8547
Registered: March 2007
Location: S.E. Michigan
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| Johnny Bridges wrote on Wed, 17 July 2013 13:09 |
OK, how does BMEP relate to all of this? I know it's a calculated value... but when we were going for approval from the FAA to weld flat motors airworthy, they told us to run the test (new engine certification run) on the highest BMEP engine we wished to be certified for, using each type of weld (Case, bearing web, spark plug hole, valve seat) and demonstrate that the welds held; and we would then be certificated to weld airworthy anything from that BMEP on down. It turned out to be a Lycoming supercharged engine as fitted on Aero Commander twins. GSIO-something, I think maybe 4i90 CID (?). The FAA aid the represented the highest stressed engine.
--johnny
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Johnny,
This should not be that hard to explain to someone that already knows how engines work.
B Brake - the dyno other force measuring machine
M Mean - as in average
E Effective - what it works like
P Pressure - do I have to?
As the engine output torque can be calculated back to an average pressure pushing on the piston as the piston goes down the stoke with crank throw providing a varying lever arm to push on....
So, Peak BMEP is going to be at the torque peak of any engine. If you want more torque, you have to get more BMEP. That torque and therefore BMEP relate directly to the forces inside the engine. That is why they want to know what will happen there.
The calculation for BMEP is not all that tough for an engineer with a slide rule or now a computulator. If I were at home, I could turn around and one of several books that have it spelled out in detail. If you ask again in about a week, I will get one down.
That do you ? If no, I will gladly try again.
Matt - being glad that Mary is the one driving up-sun
Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit
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| Re: [GMCnet] Trip to High Altitude [message #214751 is a reply to message #214724] |
Wed, 17 July 2013 17:46 |
jhbridges
Messages: 8412
Registered: May 2011
Location: Braselton ga
Karma: -74
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That's fine. I knew the acronym, and McCune showed how to calculate it. Using a sliderule (1969 or so). Essentially maximum stress on the welds, which was the intent.
And as an aside, after standing behind that engine hammering away on the test stand for the prescribed torture tests, I never worried about aviating around behind anybody's certificated engine until a couple of Tiaras unloaded. They were a Bad Idea.
--johnny
'76 23' transmode Norris
'76 palm beach
--------------------------------------------
On Wed, 7/17/13, Matt Colie <matt7323tze@gmail.com> wrote:
Subject: Re: [GMCnet] Trip to High Altitude
To: gmclist@temp.gmcnet.org
Date: Wednesday, July 17, 2013, 7:30 PM
Johnny Bridges wrote on Wed, 17 July 2013 13:09
> OK, how does BMEP relate to all of this? I know
it's a calculated value... but when we were going for
approval from the FAA to weld flat motors airworthy, they
told us to run the test (new engine certification run) on
the highest BMEP engine we wished to be certified for, using
each type of weld (Case, bearing web, spark plug hole, valve
seat) and demonstrate that the welds held; and we would then
be certificated to weld airworthy anything from that BMEP on
down. It turned out to be a Lycoming supercharged
engine as fitted on Aero Commander twins. GSIO-something, I
think maybe 4i90 CID (?). The FAA aid the represented
the highest stressed engine.
>
> --johnny
Johnny,
This should not be that hard to explain to someone that
already knows how engines work.
B Brake - the dyno other force measuring machine
M Mean - as in average
E Effective - what it works like
P Pressure - do I have to?
As the engine output torque can be calculated back to an
average pressure pushing on the piston as the piston goes
down the stoke with crank throw providing a varying lever
arm to push on....
So, Peak BMEP is going to be at the torque peak of any
engine. If you want more torque, you have to get more
BMEP. That torque and therefore BMEP relate directly
to the forces inside the engine. That is why they want
to know what will happen there.
The calculation for BMEP is not all that tough for an
engineer with a slide rule or now a computulator. If I
were at home, I could turn around and one of several books
that have it spelled out in detail. If you ask again
in about a week, I will get one down.
That do you ? If no, I will gladly try again.
Matt - being glad that Mary is the one driving up-sun
--
Matt & Mary Colie
'73 Glacier 23 Chaumière (say show-me-air)
Now with 4 working Rear Brakes
SE Michigan - Twixt A2 and Detroit
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Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
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| Re: [GMCnet] Trip to High Altitude [message #214761 is a reply to message #214760] |
Wed, 17 July 2013 20:10 |
jhbridges
Messages: 8412
Registered: May 2011
Location: Braselton ga
Karma: -74
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You'll get more power, which will sag by about the same percentage as the fitted engine. However, 80% of 300HP might be enough where 80% of 200 isn't. So you'd have more power. Doesn't Ms Mondello still sell hot rodded 455s for Oldsmobile fans? Or get one of the Jims to have their engine builders run you one up. You will, however be using up some of the margin built into the 455. You're already using up a lot of it pushing a motorhome around with it. Also you'll be generating more heat, and the cooling system will need to handle it. And applying more push to the trans, etc. It's all interrelated, and 'you can't do just one thing' as we say in my business.
All this is easy for me to say, I'm in the Southeast, I think the highest place mine is likely to see is the Blue Ridge Parkway which is nowhere near the Western altitudes, and the grades are fairly gentle. Monteagle TN is about the steepest log haul mine ever sees.
--------------------------------------------
On Thu, 7/18/13, larry.whisler <larry.whisler@hotmail.com> wrote:
Subject: Re: [GMCnet] Trip to High Altitude
To: gmclist@temp.gmcnet.org
Date: Thursday, July 18, 2013, 12:50 AM
Johnny,
I was referring to the compression ratio at high altitude as
an 'effective compression ratio.' I realize that the actual
compressio ratio of the engine has not changed at altitude.
Turbo/supercharging would have the same opposite effect as
JimK mentions in his post.
So, what's the consensus of installing a higher compression
engine in the GMC and using higher octane fuel at the lower
altitudes? Somewhere in the 9-10.5 range.
Has anyone done this?
Any thoughts
larry
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Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
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| Re: [GMCnet] Trip to High Altitude [message #214808 is a reply to message #214769] |
Thu, 18 July 2013 06:37 |
jhbridges
Messages: 8412
Registered: May 2011
Location: Braselton ga
Karma: -74
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Well, I lost a good friend behind a Tiara (in a Pawnee) which lunched on takeoff and stuck him in a gravel pit... and like so many folks of his generation, he couldn't swim.....
I suspect turning everything twices as fast was a recipe for the disaster that engine became. Superficially, it was a good idea, but...
--johnny
.
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On Thu, 7/18/13, Matt Colie <matt7323tze@gmail.com> wrote:
Subject: Re: [GMCnet] Trip to High Altitude
To: gmclist@temp.gmcnet.org
Date: Thursday, July 18, 2013, 2:12 AM
Johnny Bridges wrote on Wed, 17 July 2013 18:46
> That's fine. I knew the acronym, and McCune
showed how to calculate it. Using a sliderule (1969 or
so). Essentially maximum stress on the welds, which
was the intent.
> And as an aside, after standing behind that engine
hammering away on the test stand for the prescribed torture
tests, I never worried about aviating around behind
anybody's certificated engine until a couple of Tiaras
unloaded. They were a Bad Idea.
>
> --johnny
Johnny,
The Continential Tiara was an amazing engine. The
design was amazing and it was even more amazing that they
were able to get any to run the 50 hours to get
certified. I knew three of the engineers on the
program. One design and two test. The test guys
would just shake their heads when asked what happened. '
Matt
--
Matt & Mary Colie
'73 Glacier 23 Chaumière (say show-me-air)
Now with 4 working Rear Brakes
SE Michigan - Twixt A2 and Detroit
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Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
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