Step modes are rarely used on a chassis dyno because, unlike an engine dyno, you have to stabalize the inertia of the dyno's rollers and vehicle's drivetrain between each step, which tends to create too long of pulls. Whether it is testing cam timing or any engine mod, the more realistic you can test the change, such as a true loaded vehicle simulation on a dyno, the better you can evaluate the results of the change. Example, advancing a cam may increase cyl pressure in a lower RPM range that could induce detonation at a given RPM and load, thus requires changing the timing table and/or fuel. The benefits of a given change in a 3000 Lb vehicle could give different results in a 6000 Lb vehicle. Step modes work best for truck or industrial engine testing where steady state is the simulation. Also, the step mode is the proceedure used by vehicle manufacturers, resulting in higher numbers then if they would have used the sweep mode. I've also seen aftermarket crate engines tested and advertised with unbelievably high HP/TQ numbers because they used the reverse sweep method, example, take the engine to 7000 RPM and have the dyno's load control rapidly reverse sweep it down to 3000 RPM. Here's where heavy flywheels shine.

 

Like I've said many many times, chassis dyno numbers are bullshit.

 

Not you again.

 

LOL, just passing through

 

My chassis dyno numbers are 900, my real world track numbers are 8.6. Pretty close in my book!

I think dynos ( chassis, mustang, dynojet, engine dyno) are just a tuning tool. If you go to the same one all the time you can find out if your making progress with your new mods.

Dynos are for tuning, tracks are for RACING!

 

[QUOTE=GOaT Cheese]Like I've said many many times, chassis dyno numbers are bullshit.[/QUOTE

In regards to the Power and Torque crossing at 5252RPM on a Chassis Dyno, this is how the numbers are displayed for ease of understanding to end user (car owner), and works for the US as you use HP and Ft/Lbs. On a Dyno Jet, as many have stated, Power is measured via accelerating a known mass, then, because a Tacho lead is connected, Torque can then be derived from the Power, so the Torque is a derived value.

On all other non Inertia Chassis Dyno's, Roller Torque is measured, Roller RPM is measured, and so we can Calculate Roller Power, and in this case, Roller Power and Roller Torque will not cross at 5252 Engine RPM, due to many different combinations of overall gearing. So in a perfect world, if we held a car at the same RPM in any gear, with different tyre sizes, different Roller sizes, we would see the same HP, but Torque would vary depending on what ever the overall gear ratio is. To explain this a little easier, if we had 2 dyno's side by side, one has a roller twice the size as the other, and we held a test vehicle at say 2000 ROLLER RPM (at WOT) on the small roller dyno, we would see X HP and Y Torque, let us also assume that the vehicle was doing 60MPH at this point.
If we then put the vehicle on the Large Roller dyno and held the vehicle at 60MPH at WOT, the large Roller would be doing 1/2 the revs, 1000RPM, but because of the diameter being twice the size of the smaller roller, we would end up with twice the Torque, or X HP and 2Y Torque, see how the HP is the same.

You were onto something with your axle Torque, but you over looked the Tyre to Roller ratio as far as how that effects the Roller Torque, which is not reported on Dyno's in the US (I think). If we assume your engine has 600Ft/lbs of Torque, we use a 1:1 gear in the gearbox, we then have an example 4:1 diff ratio, we would have 2400Ft/lbs at the axle. If we use a Tyre of 26 inch Diameter, on a Roller of a Diameter 8.5 Inches, this is a Torque Reduction, divide 2400 by the ratio if 26/8.5 (3.05), we now have 786Ft/lbs at the Roller, this example assumes no driveline loss, it is just showing you the maths.
Now no one in the US would take any notice of a dyno that showed a Torque figure of 786Ft/lbs, when you know at the Engine you had 600, so once again, if a Tacho is connected to the test vehicle, a Derived Torque figure is calculated from the Roller Power, and gives the operator a user friendly Graph with Power and Torque crossing at 5252RPM. Of course, if a Tacho lead is not connected, you can use things like Derived Engine RPM from Roller RPM, to calculate your Torque. The thing to remember is, the Roller Torque, or "Derived Torque" has the exact same shape, it is just on a different scale.

Elsewhere in the world were we use kW and Nm, which cross at 9543RPM, it is not such a big deal, so we don't usually display Derived Torque, but we can, so you might see Roller Torque, Derived Torque, Motive Force or Tractive Effort displayed along with Power.

Did that help or confuse?

 

Here is how our Mustang MD-1750 SE basically works, and other computer load controlled chassis dynos to my knowledge. For 99% of our forum applications they basically combine the information from two separate devices, the inertia from the rollers and the torque from the load torque arm/strain guage's reading for sweep pulls. Unlike an engine dyno most chassis dynos have rollers so they are all partial inertia dynos, with the known acelleration wieght of the rollers, it computes HP from the rollers, then with the reading from the load control's torque arm, it measures TQ, which is converted to HP by computing the speed of the rollers. It adds these two figures together and we have total HP of the vehicle, which we see numerically or graphically displayed. If we have an engine tach pickup it computes engine TQ from the total HP and now we can display TQ numerically or graphically, thus, HP/TQ always cross at 5252. If the pull is steady state, there is no inertia from the rollers so all HP/TQ readings are computed from the torque arm/strain guage. The dyno can show us many other things such as roller TQ, force, parisitic force, etc, which we seldom use but like stated before, this explanation covers 99% of its applications, as used on this forum, to my knowledge.

 

I wasn't gonna get my self pulled back into this whole deal. My position is that there are too many variables that are directly influenced by the dyno operator for the figures to be accurate. Dyno operators that are usually trying to sell something (tune, parts, reputation, etc.). Read some of the previous threads, the dyno formulas are open to mass manipulation by the operator, to PUSH whatever number suites them the best. The next clown I meet face to face that claims 400+ RWHP because he has all the "bolt ons" is gonna get a verbal thrashing! I've been around the block a bit, I know what 400 RWHP is, I know how to make it, and 75% of the jokers that claim it aren't anywhere near that kind of power output. But little Jonny buys a thousand dollar stick and some Kooks long tubes, and by God he better see some huge gains or else! It's rediculous! I'm not saying you can't get an ACCURATE RWHP figure, what I'm saying is you CAN'T GET an accurate RWHP figure. There was one guy on here ( I'll try and find his post) That said he went to the dyno shop, didn't like the numbers his car put down, so he made the dyno operator do it again and again until he got the RWHP that he was hoping for!!! Give me a goddamned break! Take a stock 305 h.p. Z-28 ( let me guess they were all under rated, that is bull **** too) It will make an honest 240-260 RWHP, little Jonny again (that kid is trouble) adds all the "bolt-ons", now he's at 415 RWHP, 160 h.p. over stock at the wheels! 200+ at the crank!! BULLSHIT!

 

OK GOat Cheese, don't take this personal but I have a suggestion that could save you a lot of typing and us much forum space, rather then tell us often that it's all BS, just sit tight and only let us know when something is not, like a, "NO B.S. ALERT". Could we try this so we know what is or is not BS. Thanks.

 

Okay, I'll do that. But promise me you will remember that I think everything is bullshit unless otherwise noted.

 

It's a deal, exept when I post, be quick with the noted alert, OK?