How are HP comparisons made accurate regarding air temp?

[TalkToTheFiST]

So im currently in between tune updates, awaiting my next e30 version from Dizzy. Took the FiST for a little drive tonight and just like to keep an eye on my AP numbers to make sure everything looks ok. Tonight was the coolest ambient so far with the new turbo and tune, at 72F and 3rd gear was pulling harder than 2nd used to and it got me thinking. Whether it's MFG engine HP numbers or dyno WHP runs, how do people (or do they) take into consideration air temps when citing numbers? For example a guy who goes to my gym talks cars with me and he had his Mustang dyno'd at 412whp. I told him my FiST will likely make around 300 but that 112whp difference could be very inaccurate depending on temps for each of us no?

And we hear numbers all the time int he car world, this guy's car makes this much or that much power but one person might dyno at 40 degrees in November and another at 95 in July. Is this something that is discussed a lot and im just new to automotive or is it overlooked by most people discussing power numbers? I watch a lot of videos for fun too of street cars drag racing and am wondering how much the cited HP numbers could be off just due to air temps. Just curious as to what you guys think with this.

 

[Intuit]

Too many variables. Fueling is an obvious one but engine temp and oil viscosity are other factors. Lab conditions would be required to get precise comparisons. Labs are anything but practical. Wouldn't be surprised if there are even differences between the dyno and driver. The main difference between OEM vs aftermarket tunes is, OEM tunes are based around year(s) of testing under a very wide range of parameters/conditions and scenarios. Linear changes to the environment may have non-linear impacts to reliability and performance. My motorcycle, based around very old MAP fuel injection technology (no oxygen sensor - can't self-tune) can have very noticeable performance and MPG swings just based on ambient/engine temp and/or fuel-source alone.

 

[gtx3076]

Elevation is another factor when people across the country are trying to compare numbers. Most people getting dyno'd are aware there are discrepancies based on temp, elevation, type of dyno, etc. But when you're just shooting the shit it's not necessary to get into all that. Plus, more HP doesn't even determine which car is actually faster on the street.

 

[Dialcaliper]

To make it even more complicated, any dyno run you have ever gotten data from already has some correction factors applied to it to account for ambient pressure, air inlet temperature and humidity. There are a few different standards used, and theoretically you can ask for the data sets along with your “pretty plot”. These are not the same as a “corrected” crank horsepower estimate, and without them, a dyno is essentially useless as a tuning tool since conditions change slightly even throughout a single dyno session.

Assuming the operator bothered to update the associated calibration info at the beginning of the run, the error will be managable, but some shops don’t bother doing it every time or every customer especially on “dyno days” - YMMV

Also note, this only accounts for the “theoretical” difference in air density. If your car is pulling timing or changing other operating factors in your tune because of changes in temperature or pressure, the dyno does not compensate for that - especially an issue for turbo cars and ambient pressure especially at altitude, since with boost, manifold pressure and temperature don’t scale linearly with ambient conditions due to shifts in the compressor map efficiency.

But as long as you make sure the conditions are relatively close to previous dyno runs on different days like within 10-15 deg temperature and avoiding large differences in humidity if you can. Fortunately, the change in ambient pressure from day to day at a single location due to weather isn’t usually a problem as long as the correction is applied properly.