SAE Dyno Correction Factors

[ST]

Something to keep in mind when looking at power figures and tuning on a dyno (ie not all dyno in the bay area use this)...


Dyno Correction Factor and Relative Horsepower


So what's all this correction factor stuff anyway??

The horsepower and torque available from a normally aspirated internal combustion engine are dependent upon the density of the air... higher density means more oxygen molecules and more power... lower density means less oxygen and less power.

The relative horsepower, and the dyno correction factor, allow mathematical calculation of the affects of air density on the wide-open-throttle horsepower and torque. The dyno correction factorÂ* is simply the mathematical reciprocal of the relative horsepower value.

What's it good for?

One common use of the dyno correction factor is to standardize the horsepower and torque readings, so that the effects of the ambient temperature and pressure are removed from the readings. By using the dyno correction factor, power and torque readings can be directly compared to the readings taken on some other day, or even taken at some other altitude.

That is, the corrected readings are the same as the result that you would get by taking the car (or engine) to a certain temperature controlled, humidity controlled,Â* pressure controlled dyno shop where they measure "standard" power, based on the carefully controlled temperature, humidityÂ* and pressure.

If you take your car to the dyno on a cold day at low altitude, it will make a lot of power. And if you take exactly the same car back to the same dyno on a hot day, it will make less power. But if you take the exact same car to the "standard" dyno (where the temperature, humidity and pressure are all carefully controlled) on those different days, it will always make exactly the same power.

Sometimes you may want to know how much power you are really making on that specific day due to the temperature, humidity and pressure on that day;Â* in that case,Â* you should look at the uncorrected power readings.

But when you want to see how much more power you have solely due to the new headers, or the new cam, then you will find that the corrected power is more useful, since it removes the effects of the temperature, humidity and atmospheric pressure and just shows you how much more (or less) power you have than in your previous tests.

There is no "right" answer... it's simply a matter of how you want to use the information.

If you want to know whether you are going to burn up the tranny with too much power on a cool, humid day, then go to the dyno and look at uncorrected power to see how exactly much power you have under these conditions.

But if you want to compare the effects due to modifications, or you want to compare several different cars at different times, then the corrected readings of the "standard" dyno will be more useful.

How's it calculated?

The Society of Automotive Engineers (SAE) has created a standard method for correcting horsepower and torque readings so that they will seem as if the readings had all been taken at the same "standard" test cell where the air pressure, humidity and air temperature are held constant.

The equation for the dyno correction factor given in SAE J1349 JUN90, converted to pressure in mb, is:


http://wahiduddin.net/calc/images/density/cf.htm1b.gif
Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â*



Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* where:Â* Â*cf = the dyno correction factor
Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â*Pd = the pressure of the dry air, mb
Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â*Tc = ambient temperature, deg C

The pressure of the dry air Pd, is found by subtracting the vapor pressure Pv from the actual air pressure. For more information about pressures and calculation of the vapor pressure, see Air Density and Density Altitude.

The relative horsepower is simply the mathematical reciprocal of the correction factor.



Horsepower and Torque:

Power is the rate at which work is done. When the engine torque is turning the crankshaft and power is being delivered, the resulting horsepower may be expressed as:

Â* Â* Â* Â* Â* Â* Â* Â*

which can be simplified as

Â* Â* Â* Â* Â* Â* Â* Â*

Â* Â* Â* Â* Â* Â* Â* Â* where:Â* hp = horsepower, hp
Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* t = torque, ft-lbs
Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* rpm = engine speed, revolutions per minute


This is a great formula. Basically it says that if you can keep the same amount of torque, then the more rpm you can turn, the more horsepower you get!

That's why Formula One and CART and IRL engines all turn incredible rpm. The faster the engine turns, the more power it can make (when it's properly tuned to operate at that speed).

Consider for example: a normally aspirated internal combustion engine typically produces about 1 to 1.5 ft-lbs of torque per cubic inch when it is properly tuned to operate at any specific rpm. With a 2 litre (1 litre is about 61 cubic inches) engine, producing 1.5 ft-lbs of torque per cubic inch, you would expect to get about 180 hp at 5200 rpm... but you will get a whopping 415 hp if you can get it to run at 12,000 rpm.

The 3.5 liter IRL engine is reported to produce about 650 hp at 10,700 rpm. That would be about 1.5 ft-lbs per cubic inch.

The Ferrari 3.0 liter Formula One engine is rumored to produce about 860 hp at 18,500 rpm. That would be about 1.33 ft-lbs per cubic inch.

And at the other end of the rpm spectrum, one model of the 360 cubic inch four cylinder Lycoming IO-360 aircraft engine produces 180 hp at 2700 rpm, which is 0.97 ft-lbs per cubic inch.

In general, production automobile engines that have a broad torque band will produce about 0.9 to 1.1 ft-lbs per cubic inch. Highly tuned production engines, such as the Honda S2000 or the Ferrari F50 are in the range of 1.1 to 1.3 ft-lbs per cubic inch. Highly tuned race engines such as NASCAR, IRL and Formula One are often in the range of 1.3 to 1.5 ft-lbs per cubic inch.

[O3EVO]

Always wondered why this was never talked about much. I know on the Mustang forums I used to frequent, this was all they talked about...

[ST]

Always wondered why this was never talked about much. I know on the Mustang forums I used to frequent, this was all they talked about...


When Dynojets were the rage, many people "cheated" with SAE corrected and Raw hp a lot. I wasn't aware MD's utilized it until recently and did more investigation on it. It could explain a lot for the descrepencies in hp readings.

[O3EVO]

I remember going through that with my Cobra.

521whp / 536wtq Uncorrected #'s

506whp / 521wtq SAE #'s

So in your investigations ST, what are most shops giving numbers in?

[ST]

Kyle - thats a good question that needs further research. I know only the data from 2 shops thus far ( 1 does and 1 does not).

[VINO EVO]

^ hehehe i know what your talking about as well

[earlyapex aka jack ass]

For information for the community, we do not have or use a weather station on our mustang dyno. Weather stations can add or subtract output with the exact same tune on the car depending on if the room temp has changed run to run or not because it uses a changing correction depending on a set standard temp. Usually 77F. You can imagine that this could be pretty frustrating while tuning a car and the car increases power with hotter temps or decreases power with cooler temps or vice versa with no changes to the tune run to run.

This is not to be confused with normal power output changes from the actual car depending on the temperatures day to day. Just like driving down the road,track,strip day to day. Which I personally think is more indicative to an environment the car should be tuned in.

We do use a static SAE correction that we never change so it is the exact same run to run, day to day, year to year.

[ST]

Static SAE correction factor? O.o

How does that work when proper SAE SAE J1349 JUN90 correction takes into account:

http://wahiduddin.net/calc/images/density/cf.htm1b.gif

where:Â* Â*

cf = the dyno correction factor
Pd = the pressure of the dry air, mb
Tc = ambient temperature, deg C

Might as well turn off all correction since you're just multiplying by this "static" figure, no? I mean what good is it to have RWHP x Z, when Z is just an arbitrary number and doesn't correlate to real conditions, thus defeating its orginal purpose?

[earlyapex aka jack ass]

Static SAE correction factor? O.o

How does that work when proper SAE SAE J1349 JUN90 correction takes into account:

http://wahiduddin.net/calc/images/density/cf.htm1b.gif

where:

cf = the dyno correction factor
Pd = the pressure of the dry air, mb
Tc = ambient temperature, deg C

Might as well turn off all correction since you're just multiplying by this "static" figure, no? I mean what good is it to have RWHP x Z, when Z is just an arbitrary number and doesn't correlate to real conditions, thus defeating its orginal purpose?







You would have to inquire about all that with Mike as he set the dyno up. I just use it. :)

[nebolic]

Static SAE correction factor? O.o

How does that work when proper SAE SAE J1349 JUN90 correction takes into account:

http://wahiduddin.net/calc/images/density/cf.htm1b.gif

where:

cf = the dyno correction factor
Pd = the pressure of the dry air, mb
Tc = ambient temperature, deg C

Might as well turn off all correction since you're just multiplying by this "static" figure, no? I mean what good is it to have RWHP x Z, when Z is just an arbitrary number and doesn't correlate to real conditions, thus defeating its orginal purpose?







not really static if Pd and Tc varies. It could be the case that the formula doesn't account for every variable in "real world" condition but maybe the accounted for variance in the formula is close enough. Whether application of the adjustment factor is necessary or not, probably up to the end user. Regardless if it is applied, I think to most ppl as long as there is consistency to determine the delta would suffice. If you're looking at it from one perspective: the argument would be who's got the highest HP on the dyno and adding these adjustment factors in will change the result. But I suppose most ppl here don't care too much who's got the highest HP no?

nebo

[ST]

not really static if Pd and Tc varies. It could be the case that the formula doesn't account for every variable in "real world" condition but maybe the accounted for variance in the formula is close enough. Whether application of the adjustment factor is necessary or not, probably up to the end user. Regardless if it is applied, I think to most ppl as long as there is consistency to determine the delta would suffice. If you're looking at it from one perspective: the argument would be who's got the highest HP on the dyno and adding these adjustment factors in will change the result. But I suppose most ppl here don't care too much who's got the highest HP no?

nebo


well a certain someone made a big long post about this "highest hp" not to long ago, and the way people like to post numbers in the power mods section makes this more than applicable i would think.

the main issue of course is in consistency of the data and not necessarily the absolute numbers. in an ideal world, dyno x to dyno z of comparable brand should be close, but with this data, it begs to question the day to day data accuracy even on the same dyno x.

[nebolic]

not really static if Pd and Tc varies. It could be the case that the formula doesn't account for every variable in "real world" condition but maybe the accounted for variance in the formula is close enough. Whether application of the adjustment factor is necessary or not, probably up to the end user. Regardless if it is applied, I think to most ppl as long as there is consistency to determine the delta would suffice. If you're looking at it from one perspective: the argument would be who's got the highest HP on the dyno and adding these adjustment factors in will change the result. But I suppose most ppl here don't care too much who's got the highest HP no?

nebo


well a certain someone made a big long post about this "highest hp" not to long ago, and the way people like to post numbers in the power mods section makes this more than applicable i would think.

the main issue of course is in consistency of the data and not necessarily the absolute numbers. in an ideal world, dyno x to dyno z of comparable brand should be close, but with this question, it even begs the question of dyno x on one day to dyno x another day data accuracy.


best guesstimate game. But really tho, who just goes to a dyno just to dyno and see what they're making... even then, it's a general number which you need to take with a grain of salt.

But i do agree with you that dyno x on day 1 vs dyno x on day 2 can be different.

nebo

[ST]

best guesstimate game. But really tho, who just goes to a dyno just to dyno and see what they're making... even then, it's a general number which you need to take with a grain of salt.

But i do agree with you that dyno x on day 1 vs dyno x on day 2 can be different.

nebo



who likes to see numbers? http://www.norcalevo.net/index.php?option=com_smf&Itemid=2&board=22.0 ;)

while i am as guilty as anyone looking at the concrete numbers, my main impetus for dyno figuresÂ* has always to be utilized as a tuning tool with the delta in numbers more important then the actual figures themselves.

It's just interesting though how/where the numbers are derived from..

[earlyapex aka jack ass]

my name impetus for dyno figures has always to be utilized as a tuning tool with the delta in numbers more important then the actual figures themselves.


Indeed, that's why I always try, when able, to show before/after power curves.

[ST]

my name impetus for dyno figures has always to be utilized as a tuning tool with the delta in numbers more important then the actual figures themselves.


Indeed, that's why I always try, when able, to show before/after power curves.


+1 glad we actually agree on something, for once...

ps...gawddamn i must be dislexit....i typed name, when it should of been main :X

[trinydex]

Static SAE correction factor? O.o

How does that work when proper SAE SAE J1349 JUN90 correction takes into account:

http://wahiduddin.net/calc/images/density/cf.htm1b.gif

where:

cf = the dyno correction factor
Pd = the pressure of the dry air, mb
Tc = ambient temperature, deg C

Might as well turn off all correction since you're just multiplying by this "static" figure, no? I mean what good is it to have RWHP x Z, when Z is just an arbitrary number and doesn't correlate to real conditions, thus defeating its orginal purpose?





what happens when you unplug the temp input? it won't vary on its own.

[smack]

just curious if there is a formula for forced induction engines since that first post specifically stated in two different places "normally aspirated" engine.

i'm assuming that the discrepancies would be less for forced induction based on the hp losses at high altitudes between n/a motors(greater losses) and f/i motors(less losses).
miller in utah is a pretty good example. the turbo cars have a nice advantage there over the n/a cars.