| From my post on 1/6/2003: 've been reading the threads about dynos and see a lot of confusion. I'm sure most people here will notice that the cars that dyno at high altitudes (such as near the Denver area) seem to have unrealistically high HP numbers. There's even a recent thread where people are wondering how a guy is able to push so much HP with the stock turbos. So I did some research and here's the deal: At higher altitudes engines put out less HP. The air is less dense and engines do not have as much available oxygen to burn so they cannot put out the power they can at seal level. To better compare dyno results taken in different conditions the results much be compensated. So a "correction factor" is introduced. This is a number which is multiplied to the readings of the engines dyno'd at higher altitudes and different temps. This factor is pretty accurate for most engines (which are naturally aspirated), so the dyno is set up to apply this correction factor to all cars that dyno. But here's where things get confusing... Turbocharged/intercooled engines do not lose as much HP due to the altitude as naturally aspirated engines do. All will lose some HP, but on average a NA car will lose about 3% of engine output per 1000 feet, so at 6000 feet you've lost about 18% of your output on a NA engine. But on a turbo engine the turbos' wastegates will try to maintain a pre-set amount of pressure *above* the atmospheric pressure. For example, if you're running 10 psi boost at sea level, you're running 10 psi on top of 14.7 psi for a total of 24.7 psi, and a naturally aspirated car will be running 14.7 psi. But if you go where the ambient pressure is only 12.3 psi, such as is the case in Denver, the air is only 83% as dense as it is at sea level. The naturally aspirated car is breathing the atmospheric air at 12.3 psi, which is 83% of the pressure it had at sea level. The turbo car running 10 lbs of boost on the other hand will have 12.3 psi+10psi=22.3 psi, which is 90% of the pressure it had at sea level. So as you can see the turbo engine is not losing power with altitude at the same rate as a NA engine is. The problem that we are seeing with the dyno results occurs when a turbocharged car (which doesn't lose as much power at altitude as a NA car) pulls up onto a dyno that's at a high altitude. Its results will still be multiplied by a correction factor which is set for the loss NA cars experience. The end result after correction is a dyno reading that shows the power and torque to be inaccurately high. [ http://www.twinturbo.net/net/viewmsg.aspx?forum=general&msg_id=752270 ]
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