A friend of mine came to me with a question while inspecting the following dyno sheet:

http://www.prizer-painter.com/personalpics/dynochart.jpg

His question was "Is the tq measured and the hp guessed?"

So we went on to diagnose the dyno run, and came up with the following:

At 7k rpm the car seems to be making exactly 263rwtq (green line). When plugged into the formula to get hp, though... (tq*rpm)/5252 we get 350rwhp, yet graph is showing something close to about 365rwhp.

So, thinking this might be a one-off fluke... I went through our own dyno library and picked out Shawn's dyno sheet (since it's a Dynapack), for your reference below:

http://forums.***************************************?attachmentid=87086&stc=1

For this one I picked a point at 4,000 RPM since it seems to make exactly 400rwhp at that point. Calculating backwards I determine that it would take 525rwtq at that RPM to make the designated rwhp. Yet the sheet is showing less than that... comething closer to 505-510rwtq. Lets be generous and assume 510rwtq@4k RPM... this would mean that the rwhp figure should be 388, yet the graph is showing roughly 10rwhp more than that.

Anyway, I'll leave the rest up to you... I'm not trying to come to any conclusions about the dynamometer, the software or the printouts... I just wanted to post an observation since you guys might have some ideas as to why it is so optimistic.

Boris

Plus, we haven't had a good dyno 'discussion' in a while :evil2 Just kidding :D

Verrrry Interesting! Clearly, if the plot doesnt abide by the defined relationship between Trq and Hp, it has to be discounted considerably. .The fact that youve shown us plots from two totally different Dynopak installations which both exhibit this abnormality proves that its not an anomaly from one particular dynopak site. I knew I liked Dynojets better ;) LOL..

Seriously, though, what would be cool is if you took your post, exactly as is, and submitted it to somebody in Tech Support at Dynopak for their response. Until they come back to us with one, Id think any dynopak result has to be suspect, assuming these two arent the only ones exhibiting this strangeness.

Boris Sherlock strikes again!! :D

Guess nobody here uses a dynapack or cares?

Paul, if you'd like, you can submit this to Dynapack ;)

Boris

I have been out of town. This is very interesting information and I am glad that you came forward with this Boris. I have heard of some things being added to the software of these Dynapaks from the factory for higher numbers but it was just heresay.

What happens when you do this with Mustang dyno plots?

tq is not corrected, hp is corrected in those graphs. that's where the differences come from. if you show the uncorrected hp, the lines cross where they should.

tq is not corrected, hp is corrected in those graphs. that's where the differences come from. if you show the uncorrected hp, the lines cross where they should.


AXC.. can you expound on this please? If trq is not corrected, and if whp being shown comes from those uncorrected trq readings, then how is that different than if we all started showing our Dynojet results Uncorrected, instead of using SAE correction as we all have been using. SAE is not perfect, and temperature differences Still will throw off readings, but I think its a far site better than just going with uncorrected readings. Using uncorrected Dynojet readings, for instance, makes it impossible to compare results from one set of conditions with another.. To the extent that it is Somewhat improved by using SAE correction, I see no benefit to going with uncorrected readings.. IF people were to show uncorrected readings, we might as well stop posting dyno results here at all, as theyd be meaningless for comparo purposes.

To the extent that this is true, if you know, please explain to us why its at all useful to be showing results based on uncorrected Trq readings from the Dynopak..

I agree with AXE in that you should only compare this to the raw data. If the raw data follows the formula, then I would feel better.

Once you have analyzed that, look into the correction factors that may cause this discrepancy.

I agree with AXE in that you should only compare this to the raw data. If the raw data follows the formula, then I would feel better.

Once you have analyzed that, look into the correction factors that may cause this discrepancy.


I dont understand.. Do we agree that in its raw data form, its impossible to use it to compare one persons run with anothers? Just like its impossible to produce any meaningful comparisons between one dynojet result with another, if Uncorrected results are shown? Just as if one person showed Standard corrected results and another showed DIN corrected results, with another who showed SAE corrected results??

I dont understand.. Do we agree that in its raw data form, its impossible to use it to compare one persons run with anothers? Just like its impossible to produce any meaningful comparisons between one dynojet result with another, if Uncorrected results are shown? Just as if one person showed Standard corrected results and another showed DIN corrected results, with another who showed SAE corrected results??

Paul,
I meant that to perform the calculations (tq*rpm)/5252 ), you need to use the raw data, not the corrected.

I understand where you are coming from. To compare between people and even days on the same car, you need to look at the SAE Corrected numbers.

tq is not corrected, hp is corrected in those graphs. that's where the differences come from. if you show the uncorrected hp, the lines cross where they should.

Ok, I see what you're saying, but then this leads us to 'improper' readings since the torque is uncorrected it's only accurate under the conditions it was made, hp on the other hand is corrected (UP?). I wish they showed the correction factors (and conditions) since it seems the SAE correction seems to add power on top of the uncorrected numbers.

Is it possible to also show this with corrected torque?

Boris

>>I understand where you are coming from. To compare between people and even days on the same car, you need to look at the SAE Corrected numbers<<

Exactly... Why else post dyno results? They cant be used for anything if in their raw form.. And in so far as day to day results will vary based on conditions which can be 'corrected out' if the correction factors are used, if uncorrected results are used, a person cant even compare before and after results with himself even!

I think I see what youre saying though; to get the plots to fit into the formula, you need to use the uncorrected results with dynopak, which is not the case with Dynojet. But beyond that, Im still not sure how to interpret the dynopak results as they are shown.. ie, it was said the trq results are not corrected, but the whp results are.. ? Does that mean we can use the whp results to compare, but never the trq results?

I am not familiar with the dynapack software, but let me offer two possibilities:

1) When comparing torque and horsepower figures, the same correction factors must be applied to each (whether raw, standard, or SAE).

2) The torque that is *actually* measured is what is applied to the dynapack hub (not at the engine crank). Using the measured torque, horsepower is calculated from this torque value multiplied by wheel rpm divided by 5250. BUT...the torque *plotted on the graph* is not the torque applied to the wheel hub that was measured. Instead, it is the computed torque that the engine was producing factoring into account the gear ratios of the transmission and final drive. If that ratio is not computed correctly, the plotted torque value will be off. I'm not sure how an operator enters this in the dynapack software...or if it is derived automatically based on the engine-to-wheel rpm ratio....but it could explain the descrepancy as well.

I am not familiar with the dynapack software, but let me offer two possibilities:

1) When comparing torque and horsepower figures, the same correction factors must be applied to each (whether raw, standard, or SAE).

2) The torque that is *actually* measured is what is applied to the dynapack hub (not at the engine crank). Using the measured torque, horsepower is calculated from this torque value multiplied by wheel rpm divided by 5250. BUT...the torque *plotted on the graph* is not the torque applied to the wheel hub that was measured. Instead, it is the computed torque that the engine was producing factoring into account the gear ratios of the transmission and final drive. If that ratio is not computed correctly, the plotted torque value will be off. I'm not sure how an operator enters this in the dynapack software...or if it is derived automatically based on the engine-to-wheel rpm ratio....but it could explain the descrepancy as well.

Dynapacks require you to manually enter, gear ratio, final drive, overall tire diameter. I have worked with a Dynapac for 2 years and love Dynojet for it's idiot proof setup, you don't have to enter anything. I have seen operators enter in the wrong values and let me tell ya, it ain't pretty at the end of the day.

Brian

you can pull up the correction factors that were used for SAE correction on the dynapack. i don't know why they only show uncorrected torque, but corrected and uncorrected hp, but it's just a function of how the software displays the data.

if you really want a "corrected" torque number to compare, just work the hp formula backwards to get that figure.

as far as entering in variables into a dynapack, the only thing that needs to be entered is final gearing (tranny gear ratio x diff ratio), start/stop rpm, and ramp time. only major difference over other loading dynos is the addition of the gearing input, but this is just to get away from the inductive pickups used by other dynos to used get a rpm signal.

you can pull up the correction factors that were used for SAE correction on the dynapack. i don't know why they only show uncorrected torque, but corrected and uncorrected hp, but it's just a function of how the software displays the data.

if you really want a "corrected" torque number to compare, just work the hp formula backwards to get that figure.

as far as entering in variables into a dynapack, the only thing that needs to be entered is final gearing (tranny gear ratio x diff ratio), start/stop rpm, and ramp time. only major difference over other loading dynos is the addition of the gearing input, but this is just to get away from the inductive pickups used by other dynos to used get a rpm signal.

My point is that these types of dyno sheets with corrected/uncorrected results can mis-represent the numbers. Needless to say, not a fan of the software.

It's hard to work back to get peak numbers for example, since the scaling doesn't allow for it easily.

What's wrong with RPM pickup? Are you saying Dynapack doesn't use this? So I guess it determines RPM (and torque) by you entering in a final gear ratio? Wow. :confused :confused

Funny thing about that is you gotta have all gears on file and I'm sure MANY operators make assumptions on which gear is 1:1, etc. For example the first graph that I posted was done on an RX-7 with 4.1 rear in 4th gear (1:1)... yet the ratio says 4.33 (inaccurate). I'd rather it used an inductive pickup to determine ratio and allow you to override it if it's slightly off or something. :dunno

Boris

you don't need gearing charts to do this for car you don't know the ratio for. the software has a built in function that allows you to determine the gearing ratio for the car based on a speed/rpm test that compares to your tach or other rpm readout from your car. it's pretty simple to use and has been very accurate the times i've had to use it.

there are things i don't like about the dynapack software either, but all products have their ups and downs. i just like the dynapack better overall because it's a better tool for tuning since it's a loading dyno vs inertia. it's also more sensitive to changes in power and safer to use. i'm not concerned about comparing numbers all the time, just want to make sure my machine is reliable and consistent.

as far as comparable numbers to dynojets, there is a dynojet down the road from us and we have tested cars here and there on the same day, back to back, and results differ by less than 1%. for these tests, we used stock cars with a generally accepted range of output to make sure both dynos are reading within a norm. one of the latest cars we did this with was a 03 z06, corrected hp was 348-351 dynapack, 350-353 dynojet. 3 runs on each machine, same day, same car, same gas, less than 30 mins from time of last run on dynapack to first run on dynojet.

*edit* tire diameter input on the dynapack does nothing except allow you to log vehicle speed more accurately, doesn't affect the hp/torque values.

My point is that these types of dyno sheets with corrected/uncorrected results can mis-represent the numbers. Needless to say, not a fan of the software.

It's hard to work back to get peak numbers for example, since the scaling doesn't allow for it easily.

What's wrong with RPM pickup? Are you saying Dynapack doesn't use this? So I guess it determines RPM (and torque) by you entering in a final gear ratio? Wow. :confused :confused

Funny thing about that is you gotta have all gears on file and I'm sure MANY operators make assumptions on which gear is 1:1, etc. For example the first graph that I posted was done on an RX-7 with 4.1 rear in 4th gear (1:1)... yet the ratio says 4.33 (inaccurate). I'd rather it used an inductive pickup to determine ratio and allow you to override it if it's slightly off or something. :dunno

Boris

Whats the final drive ratio?

ALL dyno results "misrepresent" numbers, and are useful only for gross comparisons between cars/dynos, and halfway decent comparisons between the same car, on the same dyno, with controlled changes...

I sent a link to this thread to John Card at Dynapack a couple days ago asking for his input. Here is his response:


Hello John,

I tend to try and stay out of those discussions because I end up coming off as an outsider that is only there to defend the dyno - which is true, but I am naturally assumed to be biased and just there to spout propaganda - whether it is based on fact or not. I will be happy to give you as much information as you need if you want to continue the discussion however. To answer some of the points below:

Our dyno measures torque and rpm directly off of the axle and calculates power directly from those numbers. Other dynos only measure what the roller did, and then try to back calculate everything from there, while making assumptions that things like tire slippage, growth, alignment problems, rolling resistance, etc. don't exist, and still do not tell you actual measured drive wheel torque numbers - they give you the numbers that they think you want to see.

Many people freak when they see that their car is making 1200 lbft of torque on the axle, and say - "hey that can't be right - the other dyno doesn't say that". Think about it. If you are making 300 lbft at the crankshaft, and have a 4:1 gear multiplication in the drivetrain by the time it gets to the axle, the axle torque IS 1200 lbft (not factoring drivetrain loss - which isn't a lot). If the dyno only showed you 300 at the rear wheel, then that would mean that the actual crank torque would be about 75. Most dynos just give you that 300 number - even though it is wrong. We show you the actual measured torque as a default, and then have another screen that will divide it by the gear ratio for you if that is what you want. Which one sounds more "trustworthy" to you - real direct measurement, or back calculated numbers based on roller acceleration?

The reason for the discrepancy in the power and torque numbers cited, is due to atmospheric power correction being applied. Atmospheric power correction (such as SAE power correction) is only supposed to be applied to power numbers - not torque numbers. Torque is an actual force - power is a calculated number. Earlier versions of our software would apply the SAE power correction to the power numbers, but not the torque numbers - this is why you can back calculate the power numbers to an equivalent amount of torque and it doesn't add up - the difference is the power correction being applied.

If you look at the raw uncorrected numbers in the dyno data, the math will add up. While this is the way that the SAE correction is supposed to be applied, it is different than what most people are used to - so the torque is now corrected as well in later software versions in response to customer requests. There are a number of machines in the field that are still using the older software however - which is technically more correct in this regard. So it really isn't an "optimistic" dyno, its a case of not understanding the dyno data that he was looking at.

If you have any other questions please let me know. If you want to copy and post these explanations, feel free. If it becomes necessary, I'll log in and get involved, but people in discussion forums tend to be kind of territorial and defend their turf to the death. I really don't want to get into internet arguments with people - especially when they are usually making their arguments based on assumptions and not facts.

Thanks,
John Card
Dynapack USA
dynapackjohn@earthlink.net
Phone (559) 292-3800
Fax (559) 2924900

"especially when they are usually making their arguments based on assumptions and not facts."

Boy,isn't that the truth.

Shifter_0: Sorry, "final drive ratio" is actually your diff ratio (final drive), I said final gear ratio which is the combination of your final drive (diff) and transmission. It's the number you see on the dyno sheet which is what's used in calculating torque to the wheels if it were a 1:1 output (obviously all the gearing multiplies the actual output).

Thanks for that post Croak, I wish John would make it on to the board for the discussion. I personally don't understand how horsepower can be SAE corrected while the torque can't if horsepower is a derived field. They both have to mathematically match up to one another. If you SAE Correct one side of the function, you have to do it to the other, else the peak numbers we see are 'inaccurate.'

In addition, I don't see how measuring actual torque at the hub is more accurate than measuring it at the wheel, with a roller. Lets assume a non-innertial dynamometer, such as a Mustang or a loaded Dynojet, which in theory work the same as a Dynapack (I don't know the software/math they use). I do see, however where the results would be much more repeatable since your alignment, tires, slip, even outside tire diameter (changes gear ratio) are controlled.

Interesting...

Boris

"especially when they are usually making their arguments based on assumptions and not facts."

Boy,isn't that the truth.

And this is based on facts? Your generalizations don't convey your point.

Boris

And this is based on facts? Your generalizations don't convey your point.

Boris
I think they do.

I think they do.
Jim,

I believe Boris is asking you to clarify your post with facts rather then with generalizations.

perhaps you can actually type a post worth reading rather then your typical vague and unsupported rambling.

-Ron

ALL dyno results "misrepresent" numbers, and are useful only for gross comparisons between cars/dynos, and halfway decent comparisons between the same car, on the same dyno, with controlled changes...

Exactly.

Jim,

I believe Boris is asking you to clarify your post with facts rather then with generalizations.

perhaps you can actually type a post worth reading rather then your typical vague and unsupported rambling.

-Ron
Ouch. :rolleyes

Ouch. :rolleyes
I agree, lift your socks dude and make a positive contribution, it's tiresome.

Ouch. :rolleyes
yet again, you have nothing useful to post and are a glorified post whore. the fact that you cannot say anything rather then ouch and roll your eyes or even laugh (nice edit by the way) simply strengthens my statement that you are an unproductive member of this community.

BFC is not a stage for your one line comedy act nor is it a place for you or anyone else to carry a grudge against anyone. every single post you have made in this thread has been intended as an attack on other members of this forum.

your constant attacks and jabs at other forum members will not be tolerated and you will be removed from this forum if you do not change your attitude.

-Ron

Go huff and puff on someone else's door.I took direct quote out of what a member posted to emphasize there seems to be a whole lot of opinions and not facts posted.Call me any name you want,it is very easy with a keyboard.

Go huff and puff on someone else's door.I took direct quote out of what a member posted to emphasize there seems to be a whole lot of opinions and not facts posted.Call me any name you want,it is very easy with a keyboard.
Jimmy,

your tough guy act won't get you anything besides a bad rep on an internet forum. as discuised as they are, your attempts to make physical threats is childish and a clear representation of your undesireable presense on this forum.

this is a technical thread of which you have nothing positive to add. instead, you choose to wave your hands for attention, this is rather sad on your behalf. If you'd like to carry this conversation further, feel free to PM me or if you'd like to do with without keyboards, I can forward you my address and telephone number.

-Ron

Dynapacks require you to manually enter, gear ratio, final drive, overall tire diameter. I have worked with a Dynapac for 2 years and love Dynojet for it's idiot proof setup, you don't have to enter anything. I have seen operators enter in the wrong values and let me tell ya, it ain't pretty at the end of the day.

Brian


Brian, you are wrong.
Dynopaks:
A) Don't EVER ask about tire info as there are no tires on the Pak, just wheel hubs.
B) You don't have to "Manually" enter anything. It will calculate those values for you during a calibration setup run, right down to the EXACT final drive for what ever gear your in.

I do not know the DynoPaks inside and out but I run my cars on one all the time, probably have made more runs than anyone here that doesn't own one personally.

I am completely guessing but my cars are run in 4th gear, ONLY because thats what everyone else runs at. 4th gear is not a 1:1 ratio, 5th gear is. It DOES show a difference in torque if you run in 5th or 4th, or even 3rd.

I will forward this link to Kevin, the Tuner at Knowledge Performance that may be able to better answer the question here.

Also, just to quit the hating between Dynos when I get the chance I'll have the new Roadster Turbo (GT42R) and M3 Turbo Dynoed on a DynoPak and then a Dynojet on the same day.

Also, the Dynopak IS SAE corrected. I might also add that typically the Dynopaks read a LOWER reading than DynoJets...

I believe John Card of Dynapack has answered the original question very clearly, and this thread no longer has an obvious purpose. To summarize, John indicated that: early versions of the Dynpack software did not apply SAE corrections to torque, but did to horsepower. Later versions of the software apply SAE corrections to both values, and hence the newer versions of the software display the expected relationship between hp and torque (hp = Torque x rpm/5250)

Yea, I should have read the rest of the post before replying! I'm glad John spoke up and explained it.

If folks would like I can now start posting the Hub Torque Values as well. They are WAY bigger than the corrected version, LOL.

Sorry if I sounded defensive, it just bugs me that this is such a drawn out discussuion. If the Dynos are with X% of each other lets leave it alone. I actually am curious to see what my cars will make on one vs the other though so I'll still test that when I can.

Good to see that things appear to have sorted themselves out.

The main issue in getting comparable numbers to a Dynojet has to due with the dyno operator. The Dynojet has little or no user adjustability, which is good for comparison purposes, not so good for tuning flexibility. The Dynapack has a lot of user flexibility, which is what makes it a great tuning tool, but you have to make sure that tests were completed in the same manner if you want them to be directly comparable. If you program a Dynapack to operate in the same manner as a Dynojet for a particular car, you will get comparable numbers - earlier sombody quoted a 1% difference - that is definitely possible. But if the Dynapack is programmed to run in a completely different manner (mainly much faster or slower rate of acceleration) then you will see different numbers of course. This is also the main reason that you will see different results on other dynos as well - people dont take the time to do comparable tests, and the load levels and acceleration rates are different - yielding different results. If you change the load level on the SAME dyno, you will see different results, so it is mainly a case of people assuming that all things are equal when they are not, and not knowing how changing the parameters of the test can affect the measured data. I'm registered now, so if you have more questions, I'd be glad to answer them.

Thanks, John @ Dynapack USA

Thanks for your post John. People seem to not want to acknowledge that accelerating at a different rate on a dyno will yield a different result, and that the dynojet rate is not at all variable or controllable, unlike any non-inertial only dyno. The load is fixed on the Dynojet which makes them great for car to car comparisons, at least relatively, and somewhat useful for WOT tuning, but not quite useful for much else.

Thanks for clarifying the SAE correction on the Dynapack!

People seem to not want to acknowledge that accelerating at a different rate on a dyno will yield a different result, and that the dynojet rate is not at all variable or controllable, unlike any non-inertial only dyno. The load is fixed on the Dynojet which makes them great for car to car comparisons, at least relatively, and somewhat useful for WOT tuning, but not quite useful for much else.

Not quite... depends on which Dynojet you use... http://www.dynojet.com/automotive_dyno/224xLC_dyno/index.php ofcourse that's not what everyone here dyno's on, but regardless, that wasn't even the point.

I wish though, that John would answer my previous followup questions to the post that Croak made on his behalf:

Thanks for that post Croak, I wish John would make it on to the board for the discussion. [i]I personally don't understand how horsepower can be SAE corrected while the torque can't if horsepower is a derived field. They both have to mathematically match up to one another. If you SAE Correct one side of the function, you have to do it to the other, else the peak numbers we see are 'inaccurate.'

In addition, I don't see how measuring actual torque at the hub is more accurate than measuring it at the wheel, with a roller. Lets assume a non-innertial dynamometer, such as a Mustang or a loaded Dynojet, which in theory work the same as a Dynapack (I don't know the software/math they use). I do see, however where the results would be much more repeatable since your alignment, tires, slip, even outside tire diameter (changes gear ratio) are controlled.

Welcome to the board John... :buttrock

Thanks,

Boris

Not quite... depends on which Dynojet you use... http://www.dynojet.com/automotive_dyno/224xLC_dyno/index.php ofcourse that's not what everyone here dyno's on, but regardless, that wasn't even the point.

I wish though, that John would answer my previous followup questions to the post that Croak made on his behalf:



Welcome to the board John... :buttrock

Thanks,

Boris

Actually, it's pretty simple. Torque is measured directly, RPM is measured directly. A table of torque values is generated a multiple data points through the sampling period - this can be under acceleration, steady state speed, whatever you want. The Torque vs. RPM graph is generated directly from that table. The Horsepower is calculated from the Torque and RPM data, and then a correction multiplier (derived from the PC formula) is factored into that - i.e. HP x 1.05 = Corrected HP. It is simply a case of the torque numbers are raw, the HP numbers are corrected - or at least they are on the older software versions. Both sets of numbers are correct. I think I understand the point you are trying to make, which is that both sets of numbers have to jive, but they don't "have" to if you understand the data you are looking at. I you want to make comparisons and you want raw numbers, look at the torque curve. If you want corrected, look at the HP curve. There really isn't any need to analyze both. More torque at any RPM point equals more power at that same RPM point, so really, either graph can tell you all you need to know - and both are accurate in their own context. As I said before, newer versions of software have both sides corrected. Not because it needed to be changed, or the users didn't like it, but because the users were tired of explaining it to their customers.

On your other point, the Dynapack, Dynojet, Mustang, do not work on the same principle at all. The Dynojet and Mustang are similar, but neither is the same as a Dynapack. The issue of "accuracy" is a trick one, because a lot of people don't really understand all of the dynamics involved in a test. All three of these machines can show different numbers and all three of them can still be "accurate". How? - because you are performing a different test on all three machines, yielding different results.

An inertia machine (Dynojet 248, some Mustang Dynos) does not measure torque directly, it measures acceleration of an inertial mass. If you know the mass, and you know the rate of acceleration, then you can back calculate the amount of power needed to accelerate that mass at that rate. Simultaneously, engine RPM is measured, and then the rpm is divided back into the acceleration data, and the torque is back calculated from that. Simple right? You can do this exact same thing with a G-tech pro. People say that the G-tech would be less accurate because of tire spin, etc. - but the same things can happen on a roller dyno. The roller mass is fixed, so the load level is fixed, so a 2900 pound Honda gets tested as if it is a 3600lb car - which is one reason why some people have trouble getting their inertia dyno tuning to match what happens on the street.

A loaded roller dyno (small roller DJ, some Mustang dual roller models - others) measure the torque of the roller against an eddy-current brake to measure the torque of the roller, and calculate it against the RPM of the engine - Torque of one thing, RPM of something else. The torque being measured, is what the roller did - not necessarily what the car did - so maybe your results are good, maybe not. The dyno is accurate though, because the roller was accurately measured. Too bad we don't drive the roller.

Tire slip, growth, pressure, alignment, strap tension, and vehicle position on the dyno can all skew the losses in the tire and change the measured numbers on a roller dyno - in some cases dramatically. I have seen two clicks on each side of a ratcheting tie-down make a difference of 10hp on a roller dyno - due to the tire drag. For example: One guy with a stock M3 goes to an inertial roller dyno and lays down a peak of XXX HP. Another guy has the same M3, but has suspension mods - more rear toe in, wider tires, larger, heavier wheels, lowered suspension (Worse CV alignment) etc. and then he install a chip that is supposed to be good for 10HP and heads off to the dyno and lays down a number that is 5HP down from what the stock guy posted on the internet. Now he's pissed, and "something has to be wrong with the dyno - because so and so's car made XXX and I have to have more than that. He confronts the dyno operator and doubts the seller of the chip. The reality is, he has more drag from the wider tire, more weight in the rim/tire, more drag in the CV's, and the guy that put it on the dyno strapped it down tighter than the other guy - more drag still. The thing was down 15hp worth of drag without the chip - the chip did its job and got him 10 of it back. The dyno was still accurate both times because it measured what the roller did. See how things can go bad?

With a Dynapack, there are no tires to slip or drag, alignment doesn't matter, there are no straps, so tension doesn't matter, so the chance of having theses variables affect your test is reduced greatly - in some cases, entirely. But we are now measuring the car a little differently without all of this drag so you'd expect it to be a little different, but at least it is consistent and repeatable. The torque is measured directly from the axle through a solid mechanical coupling - no loss, and direct measurement. RPM is measured directly off of the axle - same as the torque. If you want to measure something precisely, it is best to measure it directly - we do this - others don't.

So when it comes to accuracy, theoretically all of the machines can be accurate for what they are doing, but which one do you think will most consistently give you give you solid repeatable data? They are different though, and the users are different, and the vehicles are different, and the vehicles are installed on the dyno differently, etc. etc., so don't be surprised when you see varying numbers on the internet and wonder which one is "accurate". It's amazing that they are as close as they are.

That was the most informative post I have ever read. :worship:

Awesome reply, just what I expected... thank you!

Actually, it's pretty simple. Torque is measured directly, RPM is measured directly. A table of torque values is generated a multiple data points through the sampling period - this can be under acceleration, steady state speed, whatever you want. The Torque vs. RPM graph is generated directly from that table. The Horsepower is calculated from the Torque and RPM data, and then a correction multiplier (derived from the PC formula) is factored into that - i.e. HP x 1.05 = Corrected HP. It is simply a case of the torque numbers are raw, the HP numbers are corrected - or at least they are on the older software versions. Both sets of numbers are correct. I think I understand the point you are trying to make, which is that both sets of numbers have to jive, but they don't "have" to if you understand the data you are looking at. I you want to make comparisons and you want raw numbers, look at the torque curve. If you want corrected, look at the HP curve. There really isn't any need to analyze both. More torque at any RPM point equals more power at that same RPM point, so really, either graph can tell you all you need to know - and both are accurate in their own context. As I said before, newer versions of software have both sides corrected. Not because it needed to be changed, or the users didn't like it, but because the users were tired of explaining it to their customers.

I completely understand your point, but I think what you missed part of mine; what I meant was that if one side is corrected and the other isn't than the peak numbers that we always publish are based on an uncorrected torque figure and a corrected horsepower figure (makes it look impressive or weak depending on correction factors). In my opinion they should both be either corrected or uncorrected (changed by operator as they need it such as on the Dynojet), but it sounds like this is "fixed" now, so no need for this point anymore.

On your other point, the Dynapack, Dynojet, Mustang, do not work on the same principle at all. The Dynojet and Mustang are similar, but neither is the same as a Dynapack. The issue of "accuracy" is a trick one, because a lot of people don't really understand all of the dynamics involved in a test. All three of these machines can show different numbers and all three of them can still be "accurate". How? - because you are performing a different test on all three machines, yielding different results.

Understood, and I agree. I wouldn't know which one to call more accurate, but in my opinion a Mustang or Dynojet simulating road load conditions with wind would be the most 'accurate' simply because it gives you the numbers at the WHEELS, what you'd be getting on the street, not the hub (more on that later).

An inertia machine (Dynojet 248, some Mustang Dynos) does not measure torque directly, it measures acceleration of an inertial mass. If you know the mass, and you know the rate of acceleration, then you can back calculate the amount of power needed to accelerate that mass at that rate. Simultaneously, engine RPM is measured, and then the rpm is divided back into the acceleration data, and the torque is back calculated from that. Simple right? You can do this exact same thing with a G-tech pro. People say that the G-tech would be less accurate because of tire spin, etc. - but the same things can happen on a roller dyno. The roller mass is fixed, so the load level is fixed, so a 2900 pound Honda gets tested as if it is a 3600lb car - which is one reason why some people have trouble getting their inertia dyno tuning to match what happens on the street.

Yup, understand this too, and if my tires weren't spinning I'd try this with a GTech that I have already, but unfortunately it can't be as accurate since I don't have a weather station (for correction) and don't know my exact weight, unlike on a dynamometer and due to road conditions probably can't keep it as repeatable. (Just trying to make a case for Dyanamometers vs. GTech... ;)).

A loaded roller dyno (small roller DJ, some Mustang dual roller models - others) measure the torque of the roller against an eddy-current brake to measure the torque of the roller, and calculate it against the RPM of the engine - Torque of one thing, RPM of something else. The torque being measured, is what the roller did - not necessarily what the car did - so maybe your results are good, maybe not. The dyno is accurate though, because the roller was accurately measured. Too bad we don't drive the roller.

I don't see how this is a problem... isn't this the way the Dynapack operates minus the roller? What's wrong with pushing a roller? By the same token I could easily say "Too bad we don't drive the hubs."

Tire slip, growth, pressure, alignment, strap tension, and vehicle position on the dyno can all skew the losses in the tire and change the measured numbers on a roller dyno - in some cases dramatically. I have seen two clicks on each side of a ratcheting tie-down make a difference of 10hp on a roller dyno - due to the tire drag. For example: One guy with a stock M3 goes to an inertial roller dyno and lays down a peak of XXX HP. Another guy has the same M3, but has suspension mods - more rear toe in, wider tires, larger, heavier wheels, lowered suspension (Worse CV alignment) etc. and then he install a chip that is supposed to be good for 10HP and heads off to the dyno and lays down a number that is 5HP down from what the stock guy posted on the internet. Now he's pissed, and "something has to be wrong with the dyno - because so and so's car made XXX and I have to have more than that. He confronts the dyno operator and doubts the seller of the chip. The reality is, he has more drag from the wider tire, more weight in the rim/tire, more drag in the CV's, and the guy that put it on the dyno strapped it down tighter than the other guy - more drag still. The thing was down 15hp worth of drag without the chip - the chip did its job and got him 10 of it back. The dyno was still accurate both times because it measured what the roller did. See how things can go bad?

With a Dynapack, there are no tires to slip or drag, alignment doesn't matter, there are no straps, so tension doesn't matter, so the chance of having theses variables affect your test is reduced greatly - in some cases, entirely. But we are now measuring the car a little differently without all of this drag so you'd expect it to be a little different, but at least it is consistent and repeatable. The torque is measured directly from the axle through a solid mechanical coupling - no loss, and direct measurement. RPM is measured directly off of the axle - same as the torque. If you want to measure something precisely, it is best to measure it directly - we do this - others don't.

So when it comes to accuracy, theoretically all of the machines can be accurate for what they are doing, but which one do you think will most consistently give you give you solid repeatable data? They are different though, and the users are different, and the vehicles are different, and the vehicles are installed on the dyno differently, etc. etc., so don't be surprised when you see varying numbers on the internet and wonder which one is "accurate". It's amazing that they are as close as they are.

I completely see your point here, and can see why a Dynapack might be better for a tuning tool as it's repeatable (I would hate to hit a pothole on the way to the dyno and lose 5hp, right? :)). But as with your example above for comparison purposes when you're comparing your car versus your friend... even though your car might be putting out 10hp more than I but your bling wheels are showing that you actually make 10hp less TO THE WHEELS than I do, does that make the dyno inaccurate? I guess it depends on what you want to measure: your axles (probably as close as you can get to measuring at the flywheel without actually pulling out your engine) or what the car is actually putting to the ground.

Either way, your point is well made and theoretically the Dynapack would seem to be the most consistent since it takes suspension, wheels, tires and alignment out of the equation and seems like a better tool for comparing modifications on the same car (although to be honest, I've never experienced gain or loss in power based on how I was strapped or any of that type stuff). I've even dyno'd multiple Dynojets and came away with the same exact numbers within 1hp! But once again, theoretically Dynapack seems to be best in this respect.

Thanks again, John, very informative.

Boris

The main point was that all of the machines can be technically accurate, but different at the same time - because of the differences in the method of testing. The whole issue of accuracy is mainly a discussion issue for internet racers that want to post power numbers. Real tuners just care about the before and after differences and the ability to precisely and repeatably measure small changes so that they can effectively quantify their work. If a machine is 10hp higher or lower than something else is mainly just a problem for the "mine's bigger than yours" crowd.

The Mustang or Dynojets aren't able to simulate road loads effectively, only approximate them at best. This isn't something that is easily done in this price range, which is why we don't do it either. Some machines say they do, but when you really look at it, it's a pretty weak attempt at simulation. If we can't do it precisely, we don't want to do it at all, because the data you gain would be based on guesstimates, and we aren't in that business - we measure.

I know what you mean about power at the hubs vs. power at the wheels, but it is theoretically the same thing. The only difference is the slippage and drag of the contact patch, which is difficult to control and unpredictable. If you had a perfect tire, it would be the same thing. Tires aren't perfect though, and you really never know just exactly how much of a problem it is - better to eliminate it entirely and not have to be bothered with it because nobody has a perfect tire. The "power to the ground" thing is actually called Tractive Effort - not power or torque. We have a screen that shows calculated tractive effort at the tire if you want to see that. Whether or not you will maintain traction on a real road at that level is a whole separate issue, but be assured that most good asphalt will grip better than the average steel roller, so you aren't getting a good approximation there either - especially after the rollers get some time on them and they get polished by the tires.

Thanks John :)

Boris

John -

Also want to offer my thanks. Your posts have been very helpful and I've learned a great deal. Really appreciate your participating in this forum.

Neil

John, I came across this thread, and had a couple questions regarding the dynopack dyno. I have used all three dyno's that you have mentioned in your posts (dynojet, mustang, dynopack) with varying numbers. My most recent dyno was a dynopack 4wheel dyno, that was separated to allow 2 2wd drive cars to use the dyno at once. When I got my numbers I thought they were excellent (comparing my numbers with others online, which I know I shouldn't put too much stock in) and was almost skeptical of the results.

Later in the day, the dyno started to overheat. This was caused by a broken fastener that lodged in one of the water passages used to cool the hubs. Would this "overheating" cause the dyno to read optimistic, or should these numbers still be considered "accurate" as long as the final drive ration was correct (transmission gear ratio x differential ratio)?

Thanks for your information so far, very informative.

You're welcome guys - glad to be of help.

96technoM3 - The temp shouldn't have affected the power numbers generated. We don't allow the temps to get into the range where it would cuase a change. If the temp alarm went off, it was WELL before it would have had an impact. All of the protection mechanisms on this machine are set very conservatively. I would be confident that the data is fine.