So i have been reading and i have found that superchargers have a boost diffrence at diffrent heights and i was wondering why it is like that?

 

The faster the rpms the faster the crank is moving which moves the sc pulley faster. I start around 6psi and max around 15.5 psi

 

If you mean at different altitudes, it's simply because the air density and air pressure is lower the higher the altitude. This will be much more noticeable with a MAP gauge reading (as you would have with an OBD reader and phone app.) than with a mechanical boost gauge (which measures gauge pressure, not absolute pressure), but both will show a drop with altitude.


Here's a simplified explanation: At sea level, atmospheric pressure is 14.7 PSI. At 10,000 ft. it is 10.1 PSI. A supercharger produces a pressure ratio, which stays fairly constant with inlet pressure (within a reasonable range of pressures). The Stock R53, is about a 1.8 P.R. at high rpm's. That means that the boost pressure is 80% higher than atmospheric. So at sea level, it will produce 14.7 x 1.8 = 26.5 PSI absolute pressure or (26.5 - 14.7) = 11.8 psi gauge pressure . At 10,000 ft. it's 10.1 x 1.8 = 18.2 PSI absolute or (18.2-10.1) = 8.1 psi gauge pressure.


I'd be interested in hearing from folks who live at high altitudes what kind of boost pressures they actually see.

 

Think about people who climb Mt. Everest. They carry a number of oxygen tank with them right? This is because the air gets thinner the higher you go in the atmosphere. It becomes very difficult to breathe because your lungs aren't strong enough to keep sucking in the air you need at greater altitudes. It's the same principle with an engine. There is less air, so the supercharger won't be able to produce as much boost and the intake won't be able to pull as much either.

 

I see, thanks much for the replies guys i get it. So, does that mean that my MINI makes more HP at sea level than where i live at about 1,000 ft above it? I don't think that would make much diffrence would it.

 

Yup, it will make a difference. Every car, boosted or not, will make more horsepower at sea level than 1,000ft of elevation.

 

Nice, looks like ill chose a dyno in florida

 

I found this equation that will allow you to calculate horsepower loss at different altitudes. Apparently you can expect to lose 3%hp/1000ft. The R53 MCS's base horsepower is 163, so you could expect to lose almost 5hp at 1,000ft.

 

Anybody worth a damn running dyno tests will give you numbers that are corrected to the SAE standard for temperature, barometric pressure and humidity. At 1000 ft, you'll see a loss of about 2% per the latest SAE standard. In reality, the difference between sea level pressure and 1000' is about 4%. So if you really test at sea level, the correction factor could be < 1 (corrected power less than measured).

 

I can't speak to boost pressure, as I haven't one, but yes, you lose about 4% real time hp for every 1000' of elevation (I live at 8800'). The power difference is amazing, when I drove my SAAB down to sea level, I swore that I had gained ~50 hp, it was that obvious to my ButtDyno. And yes, any Dyno worth their salt WILL adjust to STP(Standard Temperature and Pressure), so it shouldn't matter where you get it done.

 

Don't tune 81st sea level if you live higher up. I saw cars in colorado get tuned in Florida and run like crap in co. But we were a mile up

 

That's typical certainly for carbureted cars as they don't adjust for the lower air density. Its better now with fuel injection as the MAS has a direct correlation to the air density and thus the mixture. I remember having to PUSH our station wagon over a pass when it was still tuned for sea level right after our family moved here in the 70's-It was running so rich at 11,500 ft that it wouldn't pull itself over the pass

 

Coming from Colorado to Cali I could tell a difference in how my car sounded and boosted. Even when I moved down to Boulder at a little over a mile high from a town up in the mountains that was at 7,000+ ft above sea level. The difference from there to about a little over a mile high, the car was much different. When I moved here I retuned the car, had it tuned in Colorado first. Gained some good tq and hp numbers.

 

I do that transition EVERY DAY, live at 8800', work at 5200'-18 miles of canyon=3600' elevation change, X2 daily