Altitude and boost?

Since atmospheric pressure is 14.7 at sea level and more like 11.7 at 5,000 Ft elevation, would'nt the first 3 PSI of boost go to waste? Well, not go to waste, but if you're somewhere where the atmosphereic pressure is 11.7, would'nt the first 3psi of boost do nothing but fix the elevation differeance (while adding power)

I have another question, theres this 'rule of thumb' I heard about that says you lose %3 of power for every 1000FT of altitude, do you guys think this is true?

My third question, let's say you have a 100 HP all motor engine and you go to a really high altitude, high enough that your engine makes more like 70 HP... if you boosted your motor to add only 30 HP and ended up having 100 HP at that high altitude, would the engine be reliable as stock since it's making only 100 HP?

EDIT: I'm not 100% sure if atmospheric pressure at 5000FT is 11.7, so if you know the correct pressure, please correct me.

well if your at 5,000 ft elevation and if the air is usually alot colder than sea level areas you could be making more power because of air density... but yea you do loose power at higher altitudes, and a turbo can make up for the atmospheric pressure difference and then sum. I would suspect reliability would be the same.

The first 3 psi of boost would bring you back to sea-level performance. However, all turbos limit their boost pressure. This limit is dependent on a pre set limiter (in the old days it was a 'wastegate') So long as the turbo spins fast enough to develop full boost then you will not experience any performance loss due to altitude.

This is why piston powered aircraft work very well with turbos.

Yes a lot of the time turbos function as equalizers on aircrafts. At 5000ft, barometric pressure is 24.9 in. HG according to Hugh MacInnes. Using the pressure ratio method of

(14.7+PSIG)/14.7

I did 13.7/14.7 and got 0.93. Looks like you lose like 7% for each psi of atmospheric pressure loss.

It may be slightly less reliable due to heat created by compression. With an intercooler, you're looking at about the same reliability.

Power is lost when air pressure goes down. At an altitude of 4000 m i think you lose something like 30% of the power.

Most modern turbocharged engines compensates for this loss by increasing boost pressure. This does however increase turbocharger speed so there are limits for this compensation.

isn't this why saab and volvo started using "low pressure" turbos?

huh?
"Boost" is defined as the pressure above atmospheric pressure, no matter the elevation.

isn't this why saab and volvo started using "low pressure" turbos?
Low pressure turbos are mainly used to increase torque, but they also increase top end power slightly. The idea was to deliver a drivable engine with a low fuel consumption.
Early low pressure turbos wasn't equipped with any electronic boost controlling devices.

Today the engines use a torque demand engine management system which can see to that the engines are delivering the same torque output even if the car is driven at low or high temperature in cold or hot weather. The systems also compensate for things like alternator and AC compressor load.