robinpeck
This appeared on the XR1200 forum and I thought it might be of some interest to SD owners.
http://www.xr1200ownersgroup.com
Things to remember about dynos...
--------------------------------------------------------------------------------
"Here's the new "Horsepower" section of the forum, and certainly everybody's interested and excited to start seeing all sorts of dyno charts detailing baselines, modifications, etc. That's natural, but... Well, let me start this another way.
I have been a bit hesitant to post something like this, but the more I thought about it the more I thought it was important before all sorts of dyno results did get posted... and perhaps confused the issue more than they enlightened it. Essentially one has to ask a few questions, and remember a few points, before ever trying to evaluate, and more importantly compare, dyno results. We already have some dyno sheets posted, but with each of those I have yet to see the single most important piece of information that should go along with them.
What kind of dyno were they done on?
Were the runs done on a water-brake/hydraulic dynamometer? Or were they done on an electric AC (alternating current), or a DC (direct current), or eddy current rig? Or more commonly these days, were they done on a inertia/flywheel/accelerometer dyno?
This is way, way more important than most people realise, because each type will give you significantly different results... i.e. for instance a "Mustang" water brake will always show you lower numbers than a "DynoJet" accelerometer rig will, but may actually be more accurate.
To add even more confusion there is not just differences between brands and types of dyno's, but there are often even extreme differences between two individual dyno's of the very same type and brand. The installations are almost always different shop-to-shop, and even the size of the room they are installed in, how they get their airflow, how they get rid of the engine exhaust, how solidly they are mounted, how old they are, how well they are maintained, etc. can all make a difference.
Then there is their calibration... And their software from the given manufacturer, and what version it is.
And, of course, there is temperature, altitude, air density, etc. that have to be considered.
So far no one has ever been able to come up with any sort of truly reliable set of compensation, comparison, or equivalency formulas to compare dyno's to each other, and so the only real way to compare dyno results is to run on the same dyno, and preferably on the same day. Needless to say this is rarely practical for when you are doing mods, but it is still best to go back to the same dyno you ran on before. Most dyno shops do have correction factors for their own dyno's that will work quite well accounting for running on different days of varying atmospheric conditions, but it really is almost impossible to accurately compare figures obtained on different dyno's in different places.
That may not be what anyone wants to hear, but... it's pretty much the way it is.
Another thing to remember is that "horsepower" is really just a man-made mathematical construct to try and measure work over time. Torque is actually what a dyno measures...
Here's a trivia question for you... Why is it that the torque curve and the horsepower curve on a dyno run ALWAYS cross at 5252 RPM?
(That is, if they do cross... some engines don't turn as fast as 5252 RPM, and with those engines the two curves will NEVER cross...)
The answer is simple. The mathematical formula for computing horsepower is:
Torque x RPM / 5252 - (torque times RPM divided by 5252)
Torque is the force exerted in trying to twist the crankshaft. Some much torque is exerted in each rotation of the crank. So to attempt to figure the amount of "work" that force (measured torque) can do over a period of time you take the number of times the crank rotates over, say, one minute, and multiply the force by those number of times in that minute and divide by a constant.
As you can see, one can directly measure the force (torque), but there is really no way to directly measure how much that force (torque) can actually get done (work) over time, so a mathematical formula was constructed to try and do so.
There are lots of other ways to measure engine output, and some that even work much better to compare different size engines to each other (like BMEP), but somehow torque and horsepower together seemed to become the standard for motorsports engines.
I won't even mention the differences between DIN and SAE horsepower figures... As this has already gone on long enough and is probably boring too many of you.
However, there are still a lot of other factors one should keep in mind when discussing power output of our engines. We may all want to discuss some of those factors so we can all decide on what standards we want to all set here for what's posted and what's not, etc. No doubt some will scoff or laugh, or maybe even try to say dyno differences, correction factors, calibration methods and standards, etc. should be ignored, rather than address the matter... But until we all settle on what details should be included in the dyno posts then by and large those posts and charts will not give anyone here data they can use.
At the very least the type of dyno, the brand of dyno, what software is being used, and what conditions were the runs made under should be included with any dyno post.
But that's just my opinion... What do each of you think???
Thanks!
Dallara"
http://www.xr1200ownersgroup.com
Things to remember about dynos...
--------------------------------------------------------------------------------
"Here's the new "Horsepower" section of the forum, and certainly everybody's interested and excited to start seeing all sorts of dyno charts detailing baselines, modifications, etc. That's natural, but... Well, let me start this another way.
I have been a bit hesitant to post something like this, but the more I thought about it the more I thought it was important before all sorts of dyno results did get posted... and perhaps confused the issue more than they enlightened it. Essentially one has to ask a few questions, and remember a few points, before ever trying to evaluate, and more importantly compare, dyno results. We already have some dyno sheets posted, but with each of those I have yet to see the single most important piece of information that should go along with them.
What kind of dyno were they done on?
Were the runs done on a water-brake/hydraulic dynamometer? Or were they done on an electric AC (alternating current), or a DC (direct current), or eddy current rig? Or more commonly these days, were they done on a inertia/flywheel/accelerometer dyno?
This is way, way more important than most people realise, because each type will give you significantly different results... i.e. for instance a "Mustang" water brake will always show you lower numbers than a "DynoJet" accelerometer rig will, but may actually be more accurate.
To add even more confusion there is not just differences between brands and types of dyno's, but there are often even extreme differences between two individual dyno's of the very same type and brand. The installations are almost always different shop-to-shop, and even the size of the room they are installed in, how they get their airflow, how they get rid of the engine exhaust, how solidly they are mounted, how old they are, how well they are maintained, etc. can all make a difference.
Then there is their calibration... And their software from the given manufacturer, and what version it is.
And, of course, there is temperature, altitude, air density, etc. that have to be considered.
So far no one has ever been able to come up with any sort of truly reliable set of compensation, comparison, or equivalency formulas to compare dyno's to each other, and so the only real way to compare dyno results is to run on the same dyno, and preferably on the same day. Needless to say this is rarely practical for when you are doing mods, but it is still best to go back to the same dyno you ran on before. Most dyno shops do have correction factors for their own dyno's that will work quite well accounting for running on different days of varying atmospheric conditions, but it really is almost impossible to accurately compare figures obtained on different dyno's in different places.
That may not be what anyone wants to hear, but... it's pretty much the way it is.
Another thing to remember is that "horsepower" is really just a man-made mathematical construct to try and measure work over time. Torque is actually what a dyno measures...
Here's a trivia question for you... Why is it that the torque curve and the horsepower curve on a dyno run ALWAYS cross at 5252 RPM?
(That is, if they do cross... some engines don't turn as fast as 5252 RPM, and with those engines the two curves will NEVER cross...)
The answer is simple. The mathematical formula for computing horsepower is:
Torque x RPM / 5252 - (torque times RPM divided by 5252)
Torque is the force exerted in trying to twist the crankshaft. Some much torque is exerted in each rotation of the crank. So to attempt to figure the amount of "work" that force (measured torque) can do over a period of time you take the number of times the crank rotates over, say, one minute, and multiply the force by those number of times in that minute and divide by a constant.
As you can see, one can directly measure the force (torque), but there is really no way to directly measure how much that force (torque) can actually get done (work) over time, so a mathematical formula was constructed to try and do so.
There are lots of other ways to measure engine output, and some that even work much better to compare different size engines to each other (like BMEP), but somehow torque and horsepower together seemed to become the standard for motorsports engines.
I won't even mention the differences between DIN and SAE horsepower figures... As this has already gone on long enough and is probably boring too many of you.
However, there are still a lot of other factors one should keep in mind when discussing power output of our engines. We may all want to discuss some of those factors so we can all decide on what standards we want to all set here for what's posted and what's not, etc. No doubt some will scoff or laugh, or maybe even try to say dyno differences, correction factors, calibration methods and standards, etc. should be ignored, rather than address the matter... But until we all settle on what details should be included in the dyno posts then by and large those posts and charts will not give anyone here data they can use.
At the very least the type of dyno, the brand of dyno, what software is being used, and what conditions were the runs made under should be included with any dyno post.
But that's just my opinion... What do each of you think???
Thanks!
Dallara"