Are they all the same? O

 

I've been told that the DynoJet reads approximately 18% higher than the Mustang. But that the Mustang is a truer read. Just what I've been told.

 

Absolutely not. But you already knew that.

There are essentially 3 kinds of chassis dynos in use on tuning shops around the country:
Inertia Dynos in which your car spins a huge drum weighting 3000lbs or so and the time it takes to accelerate it is converted into Torque, then HP. Most DynoJets are like that.
Brake dynos in which that drum has an eddy current brake allowing load to be placed on the engine. Some Dyno Jets, DynoMites, and Mustang dynos are like tat
Hub dynos in which the hub is bolted on to an eddy current brake and load is put directly on the axles. Dynapacks are like that.

Dynapacks tend to read higher because they are measuring the power of the car with 100lbs of rotating mass removed. DynoJets are also notoriously high reading. Mustangs TEND to read lower than most.
There is no co-relation or percentage multiplier to calculate what your car would put down if it was dynoed on a different dyno. No two dynos ever read the same and most are not calibrated the same, so you can put down 400 on a DynoJet, put down 380 on a Mustang, 390 on another DynoJet, etc etc.

Dynos are tools meant to give you a safe place to load the car and put it through the engine's RPM range; thats it. Dyno numbers are only truly relevant when comparing power gains for the same car.
To get a "feel" for how high or low the dyno reads on average just ask the operator for what a stock LS2 / LS3 / LS7 puts down on that dyno...

 

The dynojet 248 is what most forum members use as a somewhat standard of reference. Anytime someone shows mustang or dyno dynamics many are quick to point out what it would be on the dynojet.
A good point to use the same dyno to measure improvements on the same car.

 

By doing things like multiplying the result by 18%, which is as arbitrary as, say, rolling ten dice , adding them all up and then adding that number to the dyno numbers...
For what its worth, in the 300WHP range most Mustang dynos will read within +-20WHP of a Dynojet. I've seen Mustangs that read high as well as some that read low; it is all in how they are calibrated.

 

Ding..Ding..We have a winner...Finally someone mentioned the calibration word..

 

Exactly. If I read one more time how a mustang is XX% lower than a dynojet I'll

Fact is, the eddy current dynos ALL have operator entered correction factors that can be used to skew the results. Alot of eddy current dyno owners talk about how their Mustang or Dyno Dynamics is "calibrated" to the local dynojets. Well, that right there should tell ya something.

Dynojets are much harder to fudge results on. The print outs have the operating parameters right on them. Temp, humidity, Baro. With that info, combined with the correction factor, it's almost impossible to fudge dynojet numbers.

SOME eddy current dyno owners use these operator entered correction factors to inflate numbers. Fact is, numbers sell. Some of us that own "stingy" dynos, just do what we do everyday, and whatever the numbers are, they are.

 

Don't concern yourself with comparison numbers between your car and anyone elses car or other dynos.

The greatest value of dyno numbers beyond the bragging is the relationship of the first pull to any of the subsequent pulls during a tuning session. A close second is the differences shown between sessions on the same dyno when changing parts and trying to determine the performance gains.

My only dyno session was on a Mustang dyno. The baseline pull netted 306hp. After tuning and several more pulls it rose to 320hp. Initially that sounds pretty weak for a LS3, but I run low 12's at 118-120 mph when some other guys who have dynoed nearly 400hp on other dynos can't get over 115 mph.

A dyno is just one more tool in the toolbox.

 

OK numbers sell, so why would the eddy current dyno(example Mustang ) dyno's operators want low numbers if they can adjust them.
Which most certainly they do read low in some cases.
I would think they would want them to read the same as the others.
Unless they sell products from the Dyno numbers and they depend on them.
Some Dyno's read high no doubt that so thats why a small grain is used in my mind when I read numbers. Finding out what a stock car c-6 runs is the best to gauge. To see if the numbers are in line. most everyone knows about what they should make. I use the dyno's to judge my gains. the numbers always show if you going the right way. A tool in the box!
My tuner just hooked up the lastest Mustang upgrade looks like it has it's
own weather station and complete new power box and computer.
He doesn't make any changes from car to car to raise numbers. They are what they are. I rolled out with only 513 453 both low for my mods.
They were what they were. That promted me to work on things!
I did some changes and brought the numbers up to where they should be. The Dyno wasn't loose that day. Tuner wasn't selling anything
wasn't even figuring a tune tell I said tune it!

 

I never had my car on a dyno. I just do street and freeway runs under controlled conditions and HP Scanner logging while working on my tunes or changing parts. I dont really need to know my HP, just what is in my logs and ET.

 

I'm going to modify your list of dynos and make one correction.
Inertia Dyno Spins a roller of known inertia.
Absorption/Inertia Dyno Spins a roller of known inertia in combination with an eddy current power absorber to dissipate HP.
Absorption Dyno Connects directly to the hub/shaft to absorb/dissipate HP.

The correction is to the Dynapack dyno above...it absorbs/dissipates HP via hydraulics, it's not an eddy current absorber.


To add some to the discussion, an inertia dyno is actually the most accurate real world measurement of HP. It's accurate because the inertia of the drum is always known and there is only one measurement made to determine HP...the RPM of the drum. With a known inertia, the kinetic energy of the drum can be calculated at any RPM. The small change in KE resulting from a small change in RPM divided by the time it took to change RPM yields HP with Torque calculated by an engine RPM pickup via the HP equation. It's real world because as HP increases, acceleration increases both on the dyno and on the street meaning any additional HP that goes to accelerate the drivetrain/tires on the dyno is representative of what happens on the street. There is no fudging on a Dynojet unless you input bogus ambient conditions, but that will show up on the dyno sheet. You can also go from one Dynojet to another and get very consistent readings which is why they are considered to be the "standard" when dyno racing...I mean comparing HP.

An Absorption/Inertia dyno adds to dyno error by introducing an additional place for errors to occur in the eddy current aborption circuit. That circuit uses a strain guage to measure torque then calculates HP using drum RPM which is then added to the HP calculated from the inertia circuit. Add to that the ability to intentionally introduce "fudge" factors like on Mustang dynos to "simulate" being on the road, and you have a recipe for huge errors. On the plus side is the ability to control the acceleration of the engine via the absorption portion to enable better tuning as opposed to pure inertia dynos that result in shorter and shorter dyno runs as HP goes up...the opposite of what you want when tuning a high HP engine sensitive to a tune.

The Dynapack absorption dyno referenced above is riddled with places for compromised accuracy. They use "hydraulic HP" as the basis for measuring HP...the wheels drive a hydraulic pump with "known" displacement and the pressure is controlled via an electronically controlled valve to load the engine. From there, HP is calculated based on displacement and pressure. You can see from this link:
http://www.hydraulic-systems.com/pages/ref_formulas.htm
The formula for hydraulic HP (in the left column) is HP=GPM*PSI/1714...but that's the HP delivered by the hydraulic fluid to the system. To get the HP required to drive a pump (which is what Dynapack is doing), you have to go to the right column under "Rules-of-Thumb" to get 1HP will pump 1 GPM@1500 PSI or HP=GPM*PSI/1500 or HP=GPM*PSI*.00066667. I've specialized in hydraulics my whole career and we use HP=GPM*PSI*.0007 which is the same thing just rounded more...the point is, it is just a rule of thumb not an exact science. There are so many variables WRT to hydraulics it isn't even funny. The internal leakage of the pump, leakage at valve, temperature of the hydraulic fluid, friction in the pump, etc. Dynapack uses Parker pumps in their dynos but they disassemble every pump they get from Parker to "blueprint" them before assembling the dyno. While on the surface, this seems to appear they are going to great efforts to provide accuracy, it is in reality an admission of the inexact science being used and more of an exercise to provide acceptable consistency from one dyno to another. In practice, these dynos have their highest accuracy (relatively speaking) on day one and it's all downhill from there. As the pump wears, internal leakage increases. As the pump ages, internal friction changes. As the hydraulic fluid gets older, viscosity changes (that's why they recommend 5 year oil change intervals). If I went to one of these dynos that was 7 years old, I would take the HP reading with a grain of salt... who knows if the owner of the dyno has ever even changed the oil. The Dynapack dyno is probably the best for tuning purposes...it eliminates wheel spin and can control the engine load/RPM precisely for tuning purposes.

There is not one chassis dyno that is the absolute best, they all have their good and bad points. As Sam said, they are best when used as a tool to show HP increases.

One other thing, I've seen a lot of posts talking about Mustang dynos or Dynapack dynos being "load bearing" or "true loading" dynos as opposed to inertia types that "aren't load bearing/true loading" and it always makes me cringe and shake my head. I always want to ask them: "if an inertia dyno doesn't load the engine, why does it take the engine so long to get to 7000 RPM???" The simple answer is the roller/drum has inertia and inertia is defined as the resistance to a change in motion...or inertia provides the loading.

 

so, what would you do to the calibration of a dynojet to read lower (on purpose) and or read higher (on purpose) and would the end user (the customer) know about it by looking at the sheet.

 

I have written pages and pages of posts on this topic on various forums. Here's a little write up I did a few weeks ago in the C5 section.

DynoJets are inertia dynos, and have been around for years, much longer than any type of load cell dyno. Inertia dyno's work on the principle of the acceleration of a known mass over time. Their rollers are the known mass. Weighing in at over 2500lbs or so. Your car gets strapped down to the machine, and the dyno collects it's data. It is able to calculate horsepower by measuring the acceleration in rpm of the rollers in regards to RPM. This is why gearing can affect the dyno results, more on that in a bit. Now that the dyno has recorded the horsepower curve, it can take the integral of that curve and get the torque curve. Since the dyno’s power calculations are based on the acceleration of mass over time in regards to RPM, gearing is very important. Since a vehicle with a lower gear ratio can accelerate the mass to a higher speed using less engine RPM, it will show a higher horsepower number than a car with a higher gear ratio. If a car is able to accelerate the dyno’s rollers from 200rpm (roller) to 300rpm (roller)in 1500rpm (engine), then the dyno is going to record more power than a car that did that in 2000rpm (engine).

Now we go to Mustang dyno’s and other loaded dyno’s. Our Mustang MD-1100SE dyno’s rollers weigh 2560lbs. That is the actual mass of the rollers, much like the DynoJet. That’s about where all the similarities end. When we get a car on our dyno, we enter two constants for the dyno’s algorithms. One being the vehicle weight, the other being what’s called “Horsepower At 50mph”. This is a number that represents how much horsepower it takes for the vehicle to push the air to maintain 50mph. This is used as the aerodynamic force. Mustang dyno’s are also equipped with a eddy currant load cell. Think of a magnetic brake from a freight train. This magnetic brake can apply enough resistance to stall a big rig. Off one side of the eddy currant load cell, there is a cantilever with a 5volt reference load sensor (strain gage). As the rollers are spinning this load sensor is measuring the actual torque being applied. So as the rollers spin, the load sensor is measuring the force being applied, sending that information to the dyno computer, taking into account the two constants entered earlier, computing the amount of resistance needed to be applied to the rollers to load the car so that the force of the rollers resistance is as close to the force the car sees on the street. The dyno is then able to calculate the total force being applied to the rollers in torque, and then taking the derivative of that torque curve to arrive at the horsepower curve. Since torque is an actual force of nature, like gravity and electricity, it can be directly measured. Horsepower is an idea that was thought up by man, and cannot be directly measured, only calculated.

I like to state it like this. . . I start by asking how much your car weighs, lets say 3500lbs. Now you take your car and you make a make a WOT rip in your tallest non overdrive gear, how much mass is your engine working against? 3500lbs right? Now you strap your car on a DynoJet and you make a WOT in the same gear, how much mass is your engine working against? 2500lbs right? Now you strap your car on a Mustang dyno, how much mass is your engine working against? 2500lbs. Plus the resistance being applied by the eddy current generator. We’ve seen anywhere for 470lbs of resistance to over 700lbs of resistance as measured in PAU force in the data logs. So which one is more accurate? Well they their both accurate. If a DynoJet dyno says you made 460rwhp, then you made 460rwhp. If a Mustang dyno says you made 460rwhp, you also made 460rwhp. Now which one of those numbers best represents what your car is doing when its on the street. That’s a different question.

The most important thing to remember is that a dyno is a testing tool. If the numbers keep increasing, then you’re doing the right thing. We try to look over at NET gain, instead of Peak HP numbers. A 30rwhp increase is a 30rwhp increase regardless of what dyno it is on.

Now I can address how to calculate the difference between one type of dyno and another. Simply put, you can’t. Because Mustang dyno’s have so many more variables, it’s not a simple percentage difference. We’ve had cars that made 422rwhp on our Dyno, two days later make 458rwhp on a DynoJet the next day. We’ve also had cars that made 550rwhp on our dyno, make 650+rwhp on a DynoJet a few days later at another shops Dyno Day. For instance, my 2002 Z28 with a forged internal LS6 Heads/Cam/Intake, makes 460rwhp on our dyno. I thought that was a little low, since I’ve had cam only LS6 Z06 vettes make 450rwhp. So I overlaid the dyno graphs. Guess what, the PAU force for my car was almost 200lbs more than the C5Z06 that made 450rwhp with cam only. So I entered the weight and horsepower at 50 number for a C5Z06 and did another horsepower rip with my car. The only reason I did that was to compare Apples to Apples. This time my car made 490rwhp, no other changes. Now I don’t go around saying my car made 490rwhp, I say what it actually did with the correct information entered into the computer. It made 460rwhp. Now if I ever get a chance to take it on a DynoJet (which I plan to in the spring), I have no doubts it’ll be over 500rwhp. I know this based on airflow and fuel consumption on the data logs.

But since we’re asked this question constantly we're fairly conservative, and hence tell our customers that the difference is closer to 6-7%, but as you make more power, and the more your car weighs, the difference increases as well. You must remember, Dyno's regardless of the type are tuning tools, and are in no means meant to tell people how fast their car is. Now which one is more "real world" is a totally different question. I like to explain it like this..... If you drive your car in a situation in which you have no mass and you're in a vacuum, so basically if you do intergalactic racing in space, use a DynoJet. If your car sees gravity, and has an aerodynamic coefficient, and you race on a planet called Earth, then use a Mustang Dyno

 

I love dyno discussions, they hurt my melon.

While we are on the subject, I have a question. On the dynojet with a weather station, does the operator enter the correction factor, or does the weather station depending on the da?

I always get accused of having fake numbers, and I could care less, I just want to be smarter. I'm over 6000 ft so my correction factor is >1, which it should be. I just want to know if its the operator taking a stab at it, or if the wx maching is getting it.

 

It depends on how they set things up. Our dyno gets real time weather date entered in automatically by the software. Other places need the data manually entered. You'd have to ask the opperator to be sure.

 

The one I use enters in all the WX data automatically. I just didn't know if the correction factor was calculated auto from those numbers or not. Either way, its never apples to apples.

 

Great info, glass slipper! I had always though Dynapacks were eddy current dynos... Seeing as they are Hydraulic brake dynos, wouldn't the viscosity of the oil lower as the pull proceeds and everything heats up, leading to a larger and larger error the longer the car is dynoed for? Or is there a temperature correction (I imagine there would have to be)?

One thing I would like to expand upon is the problem with tuning a high HP car on a DynoJet: The pull happens so fast that the engine is never really held at any given load point for very long, particularly at the highest parts of the powerband.
As a result, you can run a much more agressive tune on a DynoJet than you could on the street; you can dial in several degrees of timing more, and then if you were to take that car and, say, attempt a top speed run the heat load on the engine would be massively larger and it would tend to detonate.
I found that out first hand when I tried at top speed run on my STI-Swapped Subaru RS. The car ran very reliably on the street and was perfectly suited for quick bursts of speed, but at 177MPH it melted a hole on the side of the piston and trashed a $4500 engine
The best tune is one that is made on the dyno, then verified on the street.

 

To purposely read higher a tuner would have to input D/A calibrations which maximize the horsepower output... Example,, 5% humidity,elev ation at sealevel and a temperature of 20 degrees..

Lower readings would be input opposite.... Example 99% humidity, 7000 foot elevation, and 100 degrees

Results should say SAE corrected... I always laugh when I see dyno number posts because the poster never gives us any info on type of dyno. corrected numbers or uncorrected numbers and/or density altitude if these corrections were not made by the tuner..
I've seen stock C5 Ls1 numbers range from 280rwhp-340rwhp on this forum... It really is the biggest joke.. Most posters think that a dyno number is like getting on a scale to weigh something and that number will be the same no matter what.. Bottom line is to find a good tuner and make sure you stick with them them from bone stock readings through all of you future mods. Talk to the tuner/tech.. Ask questions..

 

I could not agree more.

As for what different cars make stock, and dyno accuracy, C6 Z06s make almost exactly 443 hp on mine. I've dynoed several and they're all within a couple hp of each other.

DJ weather stations are automatic. As the saying goes "It is what it is". Strap it down and run it. When you're done, hit print.

As for load, the big drum DJs are a pretty substantial load. Can you hold a car in one cell? With proportional air control, kinda, but not as well as an eddy current. Is that big drum just a freewheel on the motor? Not a bit. It'll make ya blow right through a questionable clutch on a higher hp motor.

As for too much timing because of the lack of load. My theory on that is the tune "window" which every motor has. There's an area, both A/F and timing, where they don't or won't make any more, or not much more, power, as you add timing, or take away fuel. This is that gray area that needs to be left a little on the conservative side.

 

But you have the inputs backwards to make the dyno read higher. This is the formula for correcting to SAE:

cf=1.18*[(990/Pd)*[(Tc+273)/298]^1/2] - 0.18
Where cf is correction factor, Pd is the ambient pressure of dry air in hPa, and Tc is ambient temperture in degrees Celsius.

As temperature goes down and pressure goes up, the engine makes more actual HP and the cf becomes less than 1 via the formula above. You want a cf greater than 1 to make the dyno increase HP therefore you would input a higher temperature and lower pressure. Higher humidity will increase the cf also. Elevation isn't used and neither are barometer readings, absolute pressure is used. Barometer readings are normalized to sea level so barometric pressures in Denver can be compared to Miami and isobars on weather maps will mean something.