I get a lot of people in the shop that have moved from lets say COLORADO and being now in Texas there boat requires about 4 more inches of a higher pitch.

I, asked Baja Daze on TeamBaja to explain elavation and the effects,AND boy did he do it,I wonder Daze if you would explain it to all the Offshoreonly members as you did on the other site .I thought it was great and also showed your intellegence.

Thanks again Baja Daze ,You are A SMART Man.

Talk to you later Darin.

the lakes here in new mexico are at about 4500 feet when we go to havasu we have to go up 1 pitch size like if you use a 28 then you go to a 30 havasu is at 500'
the thing that suks is when you come home you almost feel like something is wrong with your boat after being almost at sea level. someone told me one time that for every 1000' you drop you loose aprox. 10 psi of cylinder compression maybe some of the experts can enlighten us i know when i do compression test in havasu it reads higher than it does at home in albq. 4500'

test

Hey Darin. Thanks for the mention.

Basically, what it boils down to is air presure.

At lower levels or sea level, the air pressure is at it's maximum. The pressure at sea level is 14.7psi and will slowly decrease with elevation, approximately 0.5psi per 1000 feet. It's not a perfect linear relationship, but it's a very close approximation for the lower atmosphere.

For instance, at 5000' elevation, the atmospheric pressure is only 12.2psi. This is the same reason, some people may feel out of breath in the mountains and water boils faster at higher altitudes; Thus, there is less incoming air available for combustion. And by being surrounded by a lower pressure medium, cylinder compression will also suffer.

This directly effects the ability of an engine to make horsepower and torque. The lower the pressure, the less horsepower you will make and vice versa.

If you're accustomed to boating in say, Colorado at 5000' elevation, and then take a trip to the ocean, your boat will act as if you put a blower on your engine making 2.5psi boost.

So, in essence you must conpensate with different pitch props. At sea level you will make the most horsepower and therefore can turn your biggest prop. When gaining altitude, you will have to make up for the loss in horsepower with smaller props.

Darin in your example, changing 4" in pitch from Colorado to sea level is not uncommon. You could be losing as much as 30-50 horsepower in a typical stock big block in Colorado at 5000'

Here is a simple chart giving pressures at different altitudes.

0 14.69
500 14.44
1000 14.19
1500 13.94
2000 13.69
2500 13.44
3000 13.19
3500 12.94
4000 12.69
4500 12.44
5000 12.18
5500 11.93
6000 11.68
6500 11.43
7000 11.18
7500 10.92
8000 10.67
8500 10.41
9000 10.16
9500 9.91
10000 9.65

thanks bd that was very well explained