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Old 12-03-2012, 02:43 AM   #15
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Gary....not trying to be argumentative, honest. But I am not sure I agree with you.

Air intake systems, even on today's remarkable computer-managed air/fuel systems, can handle only a certain amount of air intake volume....even at pedal-to-the-metal situations. It's not just air volume that contributes to power....it's air density and its combination with the correct computer-controlled fuel amount that generates combustion energy.

The new systems dial back fuel volume (and ignition timing is also adjusted accordingly) to compensate for changing air density ("thin air") at higher elevations. An offsetting benefit of this is also to better control emisions since the new computer-controlled systems don't allow the engine to "load up" like the old carbureted systems did. But anytime thinning air density coming into the engine mates with leaner fuel supply that's compensating for that thinning air, the resulting combustible power, or energy, diminishes. That's what goes on as we go up. While the computer and sensors can manage this condition to keep emissions down and keep perfomance reasonable, they cannot make up for the decreased fuel/air density mixture, which is really the source of power. Power loss on newer vehicles is much less noticeable at elevation these days 'cause the engines are running cleaner than they used to. But it's still there....

Example. NHRA Top Fuel Dragsters and Top Fuel Funny Cars (and other classes as well) run significantly lower speeds and elapsed times (E.T's) at Denver, CO than they do at Pomona, CA....and most every other dragstrip at lower elevations. Same cars, same classes, same engines, same nitro methane fuel....but much different air density. Hence, less horsepower. Their computers adjust for it, but they just can't make the same power....and the cars are slower. That's a fact.

I will assure you that power loss is quite noticeable even at levels below 5000 feet out here. I live at 5000 feet, and we get "used" to a certain level of get up and go at this elevation. Down in Sacramento, for example, it feels like somebody put an extra turbo or someting on the motor compared to what we are used to up here. But coming back up - at about 3,000 feet or so - we keep going OK; but it feels like the turbo, or whatever we had in Sacramento, just pulled over at the last rest stop. It is quite noticeable. And it gets more noticeable the closer we get to home. True.

Just my thoughts....FWIW. I won't belabor any of these points anymore. "Hooray!" they all said with a smile!!! BTW, I know you just gotta' be a whole lot warmer where you are than we are. Br-r-r-r. Take care....and thanks again for the conversation.
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Old 12-03-2012, 03:53 AM   #16
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Gary.....and all others that have read my way-too-long posts. My comments re reducing GCWR by 10% for every 1,000 feet elevation are dead WRONG! WHAT was I thinking?? Very embarrasing!. My proof reader (me) didn't catch that. A good guide in the past is to consider reducing GCWR by 2% or so for every 1,000 feet. I cannot recall where the 10% came from....must have been the keg I opened!! Sorry about that. And I violated my promise NOT to belabor these points anymore. Oh well.....sigh.
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Old 12-04-2012, 08:02 PM   #17
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A modern engine maintains the stochiometric air/fuel ration (about 14.7:1) under all conditions, including high altitudes. It's true that it cuts back on fuel to do that when the air is thin (reduced air mass), but that only means the go-pedal has to be pushed further down to draw in more air and thus allow more fuel and maintain speed. So there is a loss of power, but it shows up as a loss at the top end (wide open throttle). As long as the engine has reserve capability, you won't notice the "loss". Few of us tow at wide open throttle, if only becasue we don't want to hear the noise and see the RPMS climb up into the range needed to produce the max rated horsepower.

So the only situation in which you come up short is where maximum horsepower is required. But GCWR is not about horsepower - it doesn't promise you can tow fast or up a any conceivable grade. GCWR basically stipulates the amount of load that can be moved without damage to the drive train or unsafe handling. Yeah, the new SAE J2807 towing standard has performance requirements, but it's still not about towing at wide open throttle.

Bottom line is that you may lose some performance at the top end, but you do not have to reduce the GCWR to compensate for it.
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Old 12-04-2012, 08:18 PM   #18
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Quote:
Originally Posted by Gary RVRoamer View Post
A modern engine maintains the stochiometric air/fuel ration (about 14.7:1) under all conditions, including high altitudes. It's true that it cuts back on fuel to do that when the air is thin (reduced air mass), but that only means the go-pedal has to be pushed further down to draw in more air and thus allow more fuel and maintain speed. So there is a loss of power, but it shows up as a loss at the top end (wide open throttle). As long as the engine has reserve capability, you won't notice the "loss". Few of us tow at wide open throttle, if only becasue we don't want to hear the noise and see the RPMS climb up into the range needed to produce the max rated horsepower.
True on normally aspirated engines, add a turbo and that changes. For instance: Cummins rates their turboe'd engines for full power up to 10,000 feet.
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Old 12-05-2012, 09:44 AM   #19
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Yeap, a turbo provides increased air flow so that the point where the air supply is inadequate is deferred. Diesels also have a wide open air intake (no throttle plate as on a gas engine), so they have full access to the air supply.
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