Quote:
Originally Posted by synolimit
Correct me if I'm wrong but correction factor is automatic and only has to do with correcting back to the target temp and humidity etc. In SAE the dyno pretends its really like 77 degrees I believe. If its 85 in the dyno room it's just calculating back to 77 which is why uncorrected is always lowest since it's the true number and was a hotter run calculation. Unless of course its 30 degrees in the dyno room, then uncorrected should be higher than SAE since it will drop power since at 77 a car should make less power than at 30. In std the target is like 66 degrees which is why it always reads higher, a cooler day would produce more power. If 100% is 77 degrees then the 0.05% you're seeing is just getting the uncorrected number back to what it would be IF it was 77 and perfect conditions in the dyno room. I believe this is why dynojets are so accurate when someone in the desert gets 300hp SAE and someone in Florida or high in the mountains also gets 300hp with same mods. It's because the targets the same. Only uncorrected would show who's city is best, but it shows with two people showed up same day same track they'd be 100% equal.
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I can't recall the specifics, but that sounds about right as far as temperature.
SAE is definitely more conservative correction than any of the other ones, which is why I treat it as a lower bound estimate (and STD as an upper bound).
The corrections for SAE (or any other correction) can be < 1.0 if the temps are especially cold and so forth -- but whether they bump the numbers up or down, they are far from perfect.
I posted several dynos, same car, same tune, same shop, same unit, same elevation, but different weather conditions, and while SAE correction brought the values closer together they were still off by quite a bit.
The reason for consistency in dynojets is because it calculates power based on the speed at which the known (and held constant) weight and diameter of a drum is spun, rather than adjusting load based on roller or hub resistance, which will mean more variance from unit to unit, shop to shop, based on what settings were selected (although, arguably, load holding dynos are more precise for purposes of tuning because you can more easily target a desired load range in, say, a fueling map).
Dynojets actually back calculate torque based on power, I believe, which is kind of opposite to the way it would ordinarily compute these values, but it still results in considerable consistency because of the drum's values being held constant.
Anyway, where things get really tricky is when the ambient conditions assessed by the dyno's sensors don't quite match the actual conditions. So for example, you might have a sensor that reads a consistently hotter or colder than actual temperature and wind up with constant under or over-correction. So long as the sensor location isn't moved around, the variance will be consistent, so it all works out, but this is probably why some shops have dynojets that tend to be consistently generous vs. not.
Also, factors like air temps in cylinder, deviations in the garage vs. outside, how quickly various closed loop trims converge on target values, etc. can all create (effectively random, in this case) variance in measurement.
I'd have to look at the formula, but I've found that SAE seems to do most of it's correction based on measured air pressure -- you tend to see massive correction (1.05+) being applied to motors dynoed in higher elevation/lower air pressure.
I think cars dynoed in FL tend to get a bit screwed by SAE because we're at pretty much sea level here, but consistently have really high humidity and hot temps, which I've (anecdotally, admittedly) notice don't seem to impact the CF value as much as pressure.
Now of course, temperature and humidity will affect air pressure, but all things being equal, SAE seems to be more generous when air pressure is down even by a bit -- that definitely helped the TSM dynos -- relative to temperature or humidity as separate factors.
****, now I need to go look this stuff up to confirm... I think there's a fairly new SAE correction calculation as well, so that will probably result in some confusing results if one shop's dynojet software uses the older correction and another shop's dynojet uses the newer one for some reason.
Quick comment of note:
"The magnitude of the power correction for tests run at non-standard conditions should not
exceed 3% for inlet air or 3% for inlet fuel corrections. If the correction factor exceeds these
values, it shall be noted as a nonstandard test in accordance with 8.1. "
In other words, corrections greater than 1.03 (or less than .97) should be taken with a pinch of salt
From this SAE document
http://www.mie.uth.gr/ekp_yliko/SAE_...E%95%CE%9A.pdf
Also informative:
Corrected Power
One more... this guy's a bit, er, eccentric, but he seem to know fluid and thermodynamics, and has a number of very useful calculators to play with
https://wahiduddin.net/calc/cf.htm
Final edit: I think the dynojet uses inches of mercury to measure air pressure, and apparently that is not ideal for absolute air pressure calculations... not my bailiwick, but something else to keep in mind. That wahiddun link talks about it...
https://wahiduddin.net/calc/pressure.htm
You can also use those calculators and historical national weather service data to arrive at more precise corrections for dynojet readings in order to eliminate sensor readings that may not accurately model ambient conditions (example: My last dyno claimed Humidity was 19%... according to NWS it was more like 40%...