Hey! If you don't smoke, drink, do drugs, or chase wimmins you gotta have SOME sort of a habit!

 

OK. But the last couple of weeks around here have also been 105, so there may not be much temp difference. Not humid though...

 

All things being the same except altitude, you will notice a bit more oomph in the seat of your pants, provided you don't change your diet. That is a different sort of oomph

 

Dan,

Just changing elevation usually changes hp 3% per 1000 feet, but your right that
temp and humidity does effect hp (not that Bigs asked directly). Temp seems drop
the hp by 1.3% per 10 degrees increase, and Humidity seems to drop the hp .74%
per 10% humidity increase. So depending on the variables, you could gain or loss
hp. Given today's conditions where the air pressure was higher in Provo then in
Dallas even with the 4000 ft difference, and the temps were about the same and
the humidity was 26% difference, Bigs is making about 3% more hp in Provo then
he would in Dallas...

 

My take is, regardless of the humidity, air pressure, and temp, you WILL feel an improvement in you 928. Maybe it's only throttle response or probably more HP (seat of the pants) feel, but either way.......... you WILL fell a diference!

 

I live at 4200' ft. here in Carson City. When I take my 928 to my folks house in the Bay Area I can feel more power at the low altitude. Not a lot of power but I can tell the difference.

 

George - your data is king as is an understanding of the variables at any given altitude . The biggest unkown is what all the metering and subsequent adjustment that is going on at the same time. Because the systems that manage and affect their change upon the fuel/air/ignition operate differently it is really difficult to know if there is a noticable difference due solely to one or even two variables let alone the exponential effect of the metering and adjustments. That is why much of this data is based upon static carbureted engines. Their fuel flow, ignition timing, etc., remains the same regardless of the change in altitude or humidity.

Another variable, certainly at high speed, is the aerodynamic effect of air density at any given altitude. Not so noticeable in a stoplight to stoplight WOT charge, but certainly at the speeds you traverse (+200mph) it makes a difference too.

All of this can make one's head spin trying to undertand the effect of all variables. What would be necessary is to put the car and dyno into an altitude chamber like those used to train military flight crews. That way you could manage the environmental variables for each subsequent run. That would give you the real engine output performance at any given altitude. Would also show you how the computer adapted for the differences. The only variable you could not measure would be the effect of drag at a given altitude but then that could be easily calulated and applied.

George, I marvel at your abilities to to extract the most out of your car for the ORR events. I would very much someday like to meet you and see your car. Your knowledge and sharing of it here has been inspirational and informative. My posts on this thread are done in a sense of sharing an opinion in a dialog that makes me think. I enjoy that and your posts. Will you be at the OCIC?

 

Thanks Dan. I will not be at OCIC given the current state of my car:

 

Keep in mind that the LH engine management system in the later cars adapts very well to changes in air density/altitude. A 10% reduction in mass flow through the hotwire mass airflow sensor is treated the same whether it's caused by a change in air density or a change in throttle position. It can't tell what causes the difference, and really doesn't care. The reduction in air density means that the car will be less responsive to changes in throttle position, and full-load power will be reduced.

Carburated cars suffer much more from the effects of altitude, since fuel metering is based on velocity through a venturi. Temperature and humidity are ignored. A big change in altitude might warrant a change in jetting to compensate for the lower air density. When racing boats at higher-altitude lakes, I would often jet down to compensate for the change in total air density. At the same time I would prop up to let the engine rev, ultimately looking for top speed and horsepower peak RPM's to happen at the same time. Same theory of props or gearing that we use at sea level, but with the lower actual top speed available with the power reduction.

 

George - I would say, regardless of the altitude, your car's present ability to perform would be the same.

Sorry you will not be at OCIC. I wish you goodspeed in getting your great car back on the road.

Dr Bob - you are correct in your statements. My premise for my posts were to address Bigs' initial question "Will I notice a distinct "seat-of-the-pants" difference when I get down there?". I just wanted to note that unless all the variables are constant it really is not possible to predict. George gave a very good example of how the differences yesterday between Utah and Dallas in barometric pressure. temperature and humidity, plus the effects of stated hp decline per 1000 ft of elevation and 10 degrees of temperature, etc., resulted in a net 3 hp advantage for the car in Utah. It is incorrect for us to assume just because we drive from a higher elevation to a lower one that the performance of our cars will improve. Especially when the variance is only 3,900 feet. Too many variables make it very hard to predict any hp performance advantage.

So Bigs, unless you have some way to quantitatively measure the hp output of your car and can recognize the compensation of the computer, I'd say you probably won't notice much difference. Now if you lived up on the Continental Divide, I'd say definitely yes.

 

 

George -

Now you've got me thinking...

Are you saying that atmospheric air pressure is just as important (or nearly as important) as air density?

To my feeble automotive mind, I can envision that in the combustion chamber of an engine, increased air density would still be in play. IOW, at a lower altitude, more dense air would still be more dense air as it flowed into the combustion chamber.

But once inside the combustion chamber of an engine - where it was no longer open to the outside atmosphere, would that air still be subject to higher atmospheric pressure? It would seem to me that the air would now be within a closed system where outside atmospheric pressure would no longer be in play.

Or even if it was in play, it would seem to me that inside an engine, the outside air pressure would be meaningless when compared to the air pressure generated by the advancing piston.

 

This is why turbos are so nice. They will push in the same amount of air and see less loss at altitudes.

 

No chance of doing that in Seattle ... leaving a car out like that will find it filled with rust, rats nests and general destruction. You guys have it SO GOOD in Cali

 

Don't superchargers do the same Colin?