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High Altitude Dyno Posted by Kevin K [Email]  (#374) [Profile/Gallery]
(more from Kevin K) on Mon, 15 Sep 2003 22:01:27
In Reply to: More Dyno Runs, Matt, Mon, 15 Sep 2003 15:23:10 Members do not see ads below this line. - Help Keep This Site Online - Signup |
Likely were given standard SAE corrected numbers, that used corrections that are for non-turbos only. Correction is to get what would happen at about 68F at sea level.
Dyno runs are corrected for ambient pressure, temperature, and humitity. only the humidity is directly valid for turbos.
temp ratio for correction is in degK ... 460 over the degF reading. so takes more than a few degs ambient change for this to be a big correction from a dyno chart.
ambient pressure is often the biggest factor among the standard corrections for non-turbos. At 5000 ft, it is 30/24.82 = 1.21. A modern turbo car will crank up the gage boost to maintain torque rating in the mountains, and the ambient air correction will pump up the dyno values way too much.
To get close to exact, proper pressure correction for a turbo run would be based on absolute pressure in manifold, at both max hp and max torque rpms.
So if you run at sea level at constant 23 psi, the true pressure correction for a 5000 ft run at 23 psi is (23+14.7)/(23+12.2)= 1.07 . You can get the exact humidity correction from your dyno guy.
The temperature correction is a bit unsettled, since it depends on IC heat soak, fan use, and other stuff. Best to start with the standard ambient correction. Then if charge temp at max hp and tq points is available, an additional correction for charge temp vs ambient could be use as a basis for comparison. This accounts for transient intercooler efficency, compressor efficency, and IC pressure drop related temps.
For a basis of comparison, at the usual sea level location, use the NA corrected hp and tq ratings, but corrected for boost, for equivalent sea level values at same xx psi.
To correct an SAE corrected run for boost, multiply the total na correction by the absolute boost pressure ratio, then divide by the ambient pressure ratio. This assumes no change in gage boost with altitude.
Now, if the ecu elevates boost at high altitudes in a consistent way, and you are just comparing other like saabs, you do the same correction, but need to know and use what the sea level boost would be for the 'absolute boost pressure ratio'.
posted by 165.121.8...
, Mon, 15 Sep 2003 22:01:27 <-- Viewing This Message
, Mon, 15 Sep 2003 18:40:46
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