Dyno operator's comment has me puzzled.
 

I am making a small number of mods on my LS3 and getting a set of dyno pulls after each to chart progress.

My performance shop uses a Dynojet model 248. The operator is a young man, and to this latest point that has not been an issue.

But just yesterday, I called to ask if he had an early appointment this week. I explained I would like this early appointment because the weather forecast would place the temperature close to the same as the last pull.

The operator exclaimed that that was not necessary, that the Dynojet system compensated for ambient conditions.

I don't think this is true, my understanding was that the operator had to manually input atmospheric conditions before the pull, if it was to be computed. Furthermore I note there is no ambient data on the bottom of the pull chart I receive from the pull.

I either the Dyno operator doesn't know the full operation of his equipment or is just lazy and shining me on.

What sez the hive mind here at ls1tech?

i think he is meaning comparing the STD numbers to the SAE numbers. SAE numbers take the weather into calculation and give a comparible dyno number in any weather condition.

The dynojet stack has a weather station in it, however the humidity reads low on most of the dynojets I have used in Florida including mine.

The temperature, baro and humidity are all gathered from the built in weatherstation in the stack.

Ryan

either STD and SAE numbers will do, as long as the environmental conditions get normalized. comparing raw, non-normalized values makes no sense.

Even SAE corrections won't be the same in different weather conditions.

huh? how? that's the whole reason for these standards existence.

The dyno operator is correct in saying that it will compensate for the weather. If put together correctly, it will have a weather station that automatically adjusts the measured HP to SAE (if enabled).

Also, you don't have to print the weather conditions on the dyno charts. It is a print option that a lot of shops simply elect not to put on the dyno graph.

FWIW, I think SAE is reasonably accurate on naturally aspirated cars. We often tune late into the evening with a big temperature swing, so I get to see a moderate comparison. I don't think it is that accurate for F/I cars - especially turbos.

Man not to be rude but there is some bad info out there...

Both SAE and STD correct for weather, however it isn't perfect. If you want the best results it would be a good idea to get as close to possible on the weather but it isn't something that is totally required. For instance, we are testing a new intake manifold or something like that we try to do the before and after the same day and as close as possible.

If you take the same car and run it once in the winter, then again in the summer the numbers will be for the most of the time different even thought he dynojet measures weather conditions and applies a correction factor.

Yep. My LT1 & LS1 test cars are always baselined before testing something new. I have pulls on both cars in baseline form, (well over 100 total pulls on each car) that vary as much as 30 rwhp from very cold (like 15 deg F, with the doors open for ventilation) to well over 100 deg. Everybody on here seems to think SAE is the holly grail, but actually Std corrects closest of them all. I showed one guy his pulls from the winter before to pulls on the summer day he was here, Std difference was exactly half the difference using SAE. People seem to think shops that use do it for bigger numbers, which would be silly. All the dyno is acurate for is measuring changes on that vehicle, on that day.

What works in theory doesn't always work in reality on Dynojets. Here is an example. This is a stock C6 Z06 that dynoed less than 30 days apart. No changes to the car between dynos. In Oklahoma we can have some pretty wide swings in our spring time weather. Here were the weather conditions as recorded by the Dynojet weather station and uncorrected, STD corrected and SAE corrected graphs. While the SAE correction is close, it is still different between the two.

Run 3
3/23/2006
Temp 48.6 F
Absolute Pressure 29.9 inHg
Humidity 40%
Vapor Pressure 0.1 inHg

Run 4
4/20/2006
Temp 81.3 F
Absolute Pressure 28.9 inHg
Humidity 36%
Vapor Pressure 0.4 inHg

Out in the real world that's the way it is.

Not everything works like engineers and class room instructors would like them to. Evidently the engineers calculate X percent gains per degree change in ambient temps. In the real world it's a case of diminshing returns. The colder it gets, the smaller the gains per degree.

Not only Dynojets, but other brands of chassis and engine dynos.

Ditto flow benches.

Same for injector flow rates at different pressures. Raise the pressure in the real world and the flow rates don't continue to increase as much per psi pressure as it goes up. Makes the numbers from injector those spread sheets only a rough starting place.

Don't forget... the dyno's not going to compensate for the timing that the PCM is pulling due to really high intake temperatures. Anything above 77º, and the PCM starts backing out timing on most vehicles.

Some vehicles, not most. Most common would be over 100 degrees F. You can stop all temp corrections and they will still vary. It's not the PCM changing things. Carbureted engines will show the same variations. It's the weather stations and correction software.

Not to mention the fact that the engine is actually making more or less power depending on ambient weather. Add in the fact you have a computer program that's trying to skew or average the numbers based on an imperfect formula. Results on one car can change from 80 deg at 8am to 100 deg at 3 pm, pretty drastically.

Yeah, thanks, I should have included that.

Forgot to mention: The very cold condition pulls when corrected numbers show lower, uncorrected numbers are way up. Evidently predicting too much gain from lower temps.
SAE error is usually double that of Standard correction.

I've gotten more information than I bargined for! :)

Thank you to everyone that contributed!

My car's IAT timing modifier table was getting aggressive by the mid 80s as far as temperature. Every C6 I've seen has crazy invasive IAT timing tables. I'm aware that the power output will still vary, but I'm just stating that it's a variable introduced into the system that the dyno is not even going to try to compensate for.

Not if the vehicle has been tuned. We are talking about dyno/weather station corrections & variations. The big losses in corrected numbers versus uncorrected are in colder air, not hotter.

The best info you get from a dyno is not necessarily peak horsepower. Where the peak occurs, and where and how sharply the curve falls are often more useful than peak numbers. I’m not a dyno expert but my guess is that the shape of the horsepower/torque curve is less sensitive to weather than peak numbers.

The OP's car hasn't been tuned. I know, I know... it wasn't posted, but I happen to know that anyway. :nod:

At the end of it all, I take dyno numbers with a grain of salt. I realize the topic at hand, and I'm merely stating that I wouldn't put confidence in any of the "corrected" values doing an absolute adjustment that is 100% reliable. I'm not talking about where the numbers go in terms of up or down. I try to ignore dyno numbers and just use it as a tool to finalize spark values.

If I remember right, the OP's car was dyno'd on a pretty hot day. If the car is tested again soon, it's not going to be a drastic drop in air temperature... as we are still well above 80 (and usually 90) degrees right now. The weather correction will probably have as much error as normal variance. I'd be more apt to believe the numbers will change significantly if it were 40º outside compared to the original run.