StakeMan

Mexstan,<br><br>I am wrong often. Thanks for clearing that up.

Vaughn MacKenzie

MexStan, where abouts do you live? I am hoping to take a road trip into central Mexico this fall. I want to tour some of the colonial cities, Aztec &amp; Myan ruins, cruise through the mountains, and maybe clear to Veracruz. If I decide to brave Mexico City I'd like to see Chepultapec Park &amp; Museum of Antrhopology among other things. Haven't been there for a long time.<br><br>Back in '87 I drove from Chiapas to Guatemala City. Terrible road, slow going, but very scenic and enjoyable drive.<br><br>Vaughn

Mexstan

I live in the City of Queretaro which is about a 2 hour drive northwest of Mexico City. Have not driven there yet but Veracruz is about a 6 or 7 hour drive from here. We are talking about taking a drive there soon.<br>I drive in MC too often. Don't like it. On Friday took an unexpected rush drive to MC and took two hours to get there and three hours back. The traffic is horrendous!!!! It can easily take an hour or more from the city limits to the airport.<br>Let me know if/when you are coming down and if I can help you in any way. The roads are all good now as they are mostly toll roads. I can help you with insurance, car permit etc.<br>I may be taking my new truck for long drive myself around the end of May. It looks like my wife and I will drive from here via the east coast to Toronto and then return inland. When I do, I hope to meet some of the guys that contribute to this site. Will post my route and dates when I know.

The Boss Hog

Vaughn,<br> In the interest of having a good, open discussion, I would like to offer a different view point. I completely agree with your analysis but only as applied to gasoline fueled cars. The air/fuel ratio on a modern fuel injected, computer controlled, gasoline engined automobile is controlled very tightly around stoichiometric (about 14.5 to 1). As you point out, the absolute air pressure (psia) does decrease at high altitude even though boost (as measured by psig) remains the same. The mass air meter reads the lower mass of air entering the engine and cuts back the amount of gasoling being injected. The result is the a/f ratio remains the same (good emissions) but the total amount of gas burned is less resulting in less BTU's and less torque produced.<br> Now lets talk about a diesel and here I admit I am a novice but, a diesel nomally runs way on the lean side of stoichiometric. It can be as high as 85 to 1 at idle. Even under load, it still runs well over to the lean side (oxygen rich). As long as the the turbo is providing enough air (oxygen) to burn all the fuel, boost is not the limiting factor. The amount of fuel injected is what determines the torque output. Therefore, at high altitude, the diesel should be running a little closer to stoichiometric which would increase EGT's and maybe a little increase in smoke but the torque output should remain the same. The only caveat I see is that I don't know what the MAP and AIT sensors located at the air cleaner are telling the computer. If they tell the computer to cut fuel then obviously all bets are off.<br> I'd be interest to hear if I am way off base here or not. I'll be taking a trip to Colorado this June so I'll do some precise measurements (1001, 1002, 1003 . . .) and post the results.<br><br>The Boss Hog<br>(still skoolen in Floridah ;D )<br>

cp

Actually, a turbocharged engine (diesel no exception) will make more power at altitude due to the decrease in exhaust back pressure. It suffers no loss in intake pressure (30&quot; on the gage is 30&quot; pressure, at altitude or at sea level. It is the sum of pressures, atmospheric plus whatever added by the turbo) and has less exhaust pressure to fight against.<br><br>Learnt this back in college....

cp

[quote author=Mexstan link=board=20;threadid=12216;start=0#116802 date=1047866894]<br>... as a pilot who has flown turbo-charged aircraft, have never seen a plane with anything to monitor turbo speed. On some of the older A/C there is a manual waste gate control.<br>[/quote]<br><br>Turbine engines (turboprops and jets) have engine speed instruments, usually marked in % of rated speed.

Mexstan

CD, you are correct as I failed to mention that I was talking about piston engines.

cfm3110

at higher altitude the thinner air forces the turbo to spin faster and on a stock truck you should be fine, but if in a &quot;hot&quot; truck you could possibly &quot;spin a bearing&quot; in your turbo. this is not info from personal experience, just what i have been warned about.

Push Rod

[quote author=cp link=board=20;threadid=12216;start=15#117209 date=1047950823]
Actually, a turbocharged engine (diesel no exception) will make more power at altitude due to the decrease in exhaust back pressure.[/quote]

I'm going to disagree with this one. You may see lower exhaust back pressure, but that lower back pressure (which simply reduces turbo drive pressure and allows better cylinder scavenging) will not be enough to compensate for the loss of absolute boost which normally occurs at higher elevations (see Vaughn's explanation). I've seen this proven time and time again by dyno runs at varying altutudes and my own simple seat-of-the-pants-ometer. This is much the same as exhaust manufacturers promising horsepower gains from 4&quot; and 5&quot; exhaust. They just aren't there. The most I've ever seen is 5 ponies. Remember, there are only 2 ways to make power with a diesel:

1) More fuel
2) Better combustion of existing fuel. Improved cylinder scavenging (through better boost/drive pressure ratios) will increase combustion efficiency, but altitude change and exhaust size increases will be neglible.

And now something to throw into the mix for us 03 guys. 02 and under trucks used to take a single air pressure reading at startup from the MAP sensor. This reading was used by the ECM to determine optimum fueling levels for a given boost and throttle position sensor reading. This is why the 02s and down would begin to smoke as you increased your altitude during driving (if you didn't shut the truck down) and why power would be down (at the bottom) if you started the truck at high altitude and drove it down the mountain.

The 03s now take dynamic readings from the IAT/AP sensor mounted to the air cleaner. The ECM will now dynamically adjust fueling parameters based on ambient air pressure. As such, the influence of altitude on our trucks performance should be minimized (but not eliminated. The fueling changes appear to be relatively insubstantial).

Rod

Mexstan

Now THAT is very interesting. Rod, thanks for this type of info on this site. It makes sense to me. Maybe you or someone else can explain something I have seen on just a few occasions with my truck that may be related with this. At around 7,500 foot altitude on a day of about 90 degrees and a hot engine, when I shut the engine down for around 30 to 60 minutes and then start it up it is hard starting. When it does start there is a large cloud of black smoke. If this was a gas engine I would saw it was flooded by the symptoms. The only difference is that it runs normaly and does not hunt or 8 stroke for a while.

Push Rod

Just off-hand, I wonder if its possible for the common rail to vapor lock? It would take several cranks to get the rail up to pressure in a case like this, and when it did, you'd liable to get an excess of fuel until rail pressure stabilized. Keep in mind, this is a theory only. For all I know, the rail isn't susceptible to this.<br><br>Rod

Mexstan

Vapor lock in the common rail?? Mmmm. Yeah, suppose that could be possible. As an argument against this I would say that diesel fuel is not as prone to to vapor as is gasoline. However the common rail IS over a hot part of the engine and could heat soak when the engine is off. As for excess fuel I don't see how this could happen with injectors. If there was vapor lock would there not be the opposite and the injectors inject a very small amount of diesel? Unless this small amount of diesel is too lean a mixture to support ignition? But that small amount of diesel should not be accumalitive due to the purging of the exhaust stroke. Or am I all wet?<br>Hey, hope you enjoy me arguing against myself.<br>But I like your theory.

cp

[quote author=Push Rod link=board=20;threadid=12216;start=15#117380 date=1047980852]<br>[quote author=cp link=board=20;threadid=12216;start=15#117209 date=1047950823]<br>Actually, a turbocharged engine (diesel no exception) will make more power at altitude due to the decrease in exhaust back pressure.[/quote]<br><br>I'm going to disagree with this one. You may see lower exhaust back pressure, but that lower back pressure (which simply reduces turbo drive pressure and allows better cylinder scavenging) will not be enough to compensate for the loss of absolute boost which normally occurs at higher elevations (see Vaughn's explanation). [/quote]<br><br>Feel free to disagree; it's not me you're disagreeing with. I'm just the messenger, repeating what I was taught in college classes in aircraft propulsion and later in flight school. There is no loss of 'absolute' boost--30psi of air in the intake manifold is 30 psi, no matter the altitude, the engine cannot tell the difference. That part of the performance equation is equal. The gain comes from having less exhaust pressure. Given two identical engines, each with 30psi indicated manifold pressure, the one at the higher altitude produces higher horsepower.<br><br>Next: Vapor lock occurs on the suction side of pumps and diesel has a very high vapor pressure, so that is unlikely.

Mexstan

No need to disagree as you are right.

Nevada

[quote author=cp link=board=20;threadid=12216;start=15#117744 date=1048033460]The gain comes from having less exhaust pressure. Given two identical engines, each with 30psi indicated manifold pressure, the one at the higher altitude produces higher horsepower.[/quote]

Okay, I have been following this thread with great interest, but you are starting to loose me now. 30psi of INDICATED boost would have to be higher ABSOLUTE boost at sea level. 30psi indicated plus 14.7psi atmospheric versus 30psi indicated plus something LESS than 14.7psi atmospheric. Secondarily, will boost completely overcome differences in DENSITY which will be reduced with altitude?

Intuitively boost pressure would affect horsepower in a more dramatic way than the reduction in exhaust back pressure that would come from reduced atmospheric pressure.

Inquiring minds want to know.