Zero-to-69,

Please don't take this the wrong way. I like your enthusiasm for learning... that's a good thing! So I will recommend that you go down to your local bookstore, or better yet a discount book store, and pick up a 1st year textbook on Physics. They are dirt cheap... people finish their course and give the book away because they "can't stand Physics". You know what they say about one man's junk.....

I think you'd get alot more out of that then anyone could possibly post here on the board. Hey.. it all relates to engines and going fast!

-Mindgame

 

Excellent suggestion!

I recently found a good college Physics text at a book sale held by the local public library. $2 for a $million worth of info.

 

I never take offense! I play here with the big boys to learn, not to be turned
away.

It wont be the first time I purchased a book from a suggestion. Scientific Design of Exhaust and Intake Systems proved to be an excellent source.

The strange thing is, my highschool Physics didn't go into depth, or even
touch on a few of the points I've posted in this forum.

I remember lessons on Mechanical advantage (pulley's, gears, levers), sound pressure, Bernoulli's principle, basic Ohm's law. In College, all of the Physics
related to Electric/Electronics.

I'll have a look through my old text out of curiousity; I doubt it will mention
anything we're discussing here.

 

Your MAP sensor performs two functions. First it tells the computer what the barometric pressure is. It can range anywhere from 81kPa at 6,000-ft altitude, to 101kPa at sea level. That's "standard" barometer.... depending on the weather, you will see numbers a bit above or below those.

When you start the engine, the MAP sensor is now reading the pressure inside the intake manifold, which is equal to the barometric pressure minus any pressure "losses" in the intake track. At idle, you are intentionally putting an obstruction (closed throttle blades) in the inlet air path to reduce air flow, decreasing the pressure available. When you fully open the throttle blades, MAP should approach barometric pressure, with the differences representing the inefficiencies (friction losses) of your inlet air ducting/filter/etc.

For a comparison of "standard" barometric readings vs. altitude:

ELEV - - - - -STD BAROMETER
Feet - - - -"Hg - - -PSIa - - kPa
0 - - - - -29.92 - - 14.7 - - -101.3
1,000 - - 28.86 - - 14.2 - - - 97.7
2,000 - - 27.82 - - 13.7 - - - 94.2
3,000 - - 26.81 - - 13.2 - - - 90.8
4,000 - - 25.84 - - 12.7 - - - 87.5
5,000 - - 24.89 - - 12.2 - - - 84.3
6,000 - - 23.98 - - 11.8 - - - 81.2

 

This table is quite revealing. Want to get a feel for how much power is lost going from sea level to 6,000ft? Think about about how much power a blower car would lose going from 14.7 to 11.8psi of boost, almost 3psi differerence. Look at the PSI column and you can see that same difference in air density from 0 - 6,000ft.

OTOH, it's very interesting that human performance, in acclimated very fit individuals like bike racers, can improve at high altitude. Human performance is also intimately related to "air flow" - how much oxygen can be delivered to the muscles by the cardiovascular/respiratory systems. And anyone sho has gone suddenly from the flatlands to high altitiude knows how debilitating that is due to the sudden decreased oxygen content of the air and lower barometric pressure. But OTOH, most bicycle speed records have been set at relatively high altitude, illustrating the iomportance of aerodynamic drag. Of course, the riders are extremely fit and also need to acclimate before trying for a record.

In the realm of internal combustion I know that piston driven aircraft have an optimum altitiude (air density) for maximum speed. I assume that is the altitiude where the loss of hp from lower oxygen content is balanced by the lower drag from the thinner air. I am not enough of an engineer to back this up, but I suspect that wrt automobiles, a similar phenomenon exists wrt top speed, where aerodynamic drag is much more important than in the 1/4m. For a given combo there would be an optimum air density for top speed. But in the 1/4m, hp is king and the denser the air the better.

Rich

 

Tino,
Everything from your intial post in this thread was covered in your first physics textbook. Gravity, pressure, density, altitude.... and the beat goes on.

It seems you are really searching for concepts in your responses (other threads too) and the only problem with that, as I see it anyways, is the lack of a good foundational understanding. I don't think I'm the only one who sees this.

The desire to learn can't be bought and you've got that in spades. Just need to channel your efforts in the right direction. Now, I'm gonna go grab my physics book and try to brush up on a few things I've forgotten.

-Mindgame

 

They basically give used text books away at the discount book stores. Especially when new books are adopted for a new curriculum. I find books all the time on thermo, materials science, statics, etc.. dirt cheap. Can't beat that.

-Mindgame

 

Very good points, Rich.

I suspect that at TF or FC speeds, the few tons of downforce results in so much induced drag (the drag that results from creating lift or downforce) that becomes the speed limiting factor. In order to produce the necessary downforce at altitude (Denver), lots of angle of attack is required on the wing and induced drag is even higher.

For land speed record cars, you pretty much have to accept the altitude of the few places there are to run.

Air density and the power vs. drag equation applies to jet aircraft dramatically. Many jets can exceed Mach 1 or Mach 2 at altitudes at 35-40,000 ft. where the air pressure is about 3 psi. Sure power is way down, but so is drag. The Concorde did M2 @ about 65-70000 ft, where there atmospheric pressure is about 1 psi! The SR-71 family of Blackbirds did M3.2 or so at 80,000+ where the pressure is about .3-.4 psi. Here the drag is minimal, but the few air molecules there are strike the aircraft at over 3000 ft per second and the friction that causes heats everything up to 250-650F.

Very few jets can fly at M1+ at sea level, where the aero drag means a lot. Power required to ovecome aero drag is proportional to the cube (^3) of the speed. As the air density drops at altitude, the drag drops at the same rate.

AFAIK, piston engine aircraft speed records have been set at near sea level. In the 500-600 mph range, power is probably more important than drag reduction from lower atmospheric pressure.

My $.02

 

Thanks for the info! That pretty much the idea I had but the table is some very interesting facts. It is amazing how much the altitude and air pressure can affect our cars performance. It would be nice to see some dyno numbers of different KPA readings and not have the numbers corrected to see how much this stuff is actually affecting some of the people like me that see a regular of 88 KPA.

 

Hi again,

Sorry if this came across as a classroom Physic's lesson, but I wasn't sure
how to word the question as it related to engines.

Grade 12, general Physics in Ontario doesn't seem to cut it (year 1990).
I regret not taking the advanced level course, but at the time it was not required
to enter Trade school, or College.

I've come a long way since buying my Camaro. I couldn't change a tire
back at the age of 15, now I'm making horsepower from an oil pump...who
would have thought?

One day, I'll post a reply to a thread a surprise all of you. Once I begin
connecting all the pieces, the picture is going to be beautiful.

Thanks for the suggestions. If you have any intermediate level reading
for Physics, Fluid Dynamics, Thermodynamics, please send them my way!