dyno dynamics shootout mode comparison

[Mignon]

Perhaps AW111 can enlighten us on what the values programmed into the various shootout modes are. I have done some calculations to see how their programming works. Using your photos of how rotary value changes with linear value I can tell they assumed the engine inertia was in a flywheel of radius of gyration of 10.91 cm. The radius of gyration is kind of like the centre of gravity but in inertia terms. It's the radius at which all of the mass can be considered to be acting in a complex shape. For a flat uniform disk, which is not dissimilar to many flywheels, the radius of gyration is (actual radius / sq rt of 2). Real car flywheels have actual radii between 5 and 6 inches so 10.91 cm (4.29") is not far off for the radius of gyration. I use a value of 3.5" in my own calculations.

So that begs the question why did they use a default value of a flywheel weighing 100 kg? Real car flywheels weigh between 7 and 12 kg usually. 100 kg is ridiculous.

I then did the calculations to work out how much extra torque the software would think the engine was producing with a flywheel weighing 100 kg instead of say 10 kg. This depends on the gearing and I saw a value of 33 rpm per kph for the Tuscan in your photos. At this gearing the dyno should over read wheel torque by about 27 ft lbs at a ramp rate of 10 kph/s. The dyno does not show quite this much so I'm wondering if they got the maths right.

As a purist I find it disappointing that they have deliberately chosen values to make their dyno read high.

[spitfire4v8]

The irony is that one of the reasons I chose to go with dyno dynamics over a decade ago is that in the industry it's generally regarded as a pessimistic dyno! I like the fact that if I say a car's got 400hp if that same car gets run elsewhere they'll probably get a graph with higher power on it. Customers generally like that. If I said it was 450hp, then they run it and get a graph with 400hp on it, they'd be back demanding to know why!

I have limited experience of dastek, but I think my tuscan race car (5 litre rover v8) may have been on one before I owned it. I will try and find the graph to compare if it was dastek.

[Mignon]

I think the chances of a customer being unhappy, or even querulous, with a high reading power graph is about as likely as his car being driven in by his mechanic and himself arriving on his flying pig.

[GregK2]

spitfire4v8 said:

Our poster in the other thread who's car was run in the wrong mode might be interested in the rotary result and how low it is.

I only just noticed this as it was pointed out in the other thread. Fascinating reading, hats off to you for the effort you've put in here.

[Stan Weiss]

Mignon said:

Perhaps AW111 can enlighten us on what the values programmed into the various shootout modes are. I have done some calculations to see how their programming works. Using your photos of how rotary value changes with linear value I can tell they assumed the engine inertia was in a flywheel of radius of gyration of 10.91 cm. The radius of gyration is kind of like the centre of gravity but in inertia terms. It's the radius at which all of the mass can be considered to be acting in a complex shape. For a flat uniform disk, which is not dissimilar to many flywheels, the radius of gyration is (actual radius / sq rt of 2). Real car flywheels have actual radii between 5 and 6 inches so 10.91 cm (4.29") is not far off for the radius of gyration. I use a value of 3.5" in my own calculations.

So that begs the question why did they use a default value of a flywheel weighing 100 kg? Real car flywheels weigh between 7 and 12 kg usually. 100 kg is ridiculous.

I then did the calculations to work out how much extra torque the software would think the engine was producing with a flywheel weighing 100 kg instead of say 10 kg. This depends on the gearing and I saw a value of 33 rpm per kph for the Tuscan in your photos. At this gearing the dyno should over read wheel torque by about 27 ft lbs at a ramp rate of 10 kph/s. The dyno does not show quite this much so I'm wondering if they got the maths right.

As a purist I find it disappointing that they have deliberately chosen values to make their dyno read high.

Dave,
Using your numbers but the same acceleration rate as an engine dyno.

Weight = 220.4622# --- Radius = 4.2953 Inches --- Rev per second acceleration rate = 300 --- Torque = 27.58

Stan

[Mignon]

Stan Weiss said:

Dave,
Using your numbers but the same acceleration rate as an engine dyno.

Weight = 220.4622# --- Radius = 4.2953 Inches --- Rev per second acceleration rate = 300 --- Torque = 27.58

Stan

Thanks Stan. My vehicle simulation program doesn't calculate torque from inertia, it calculates an effective extra amount of vehicle weight as I describe in the flywheel article on my old website so I had to plough through the maths again last night to do the torque calculation. I f'ing hate rotational dynamics; they never seem particularly intuitive and I can never remember the equations, especially after 25 years so my tired old brain got a bit of a workout for a change. Glad you agree the numbers. You've used the whole 100 kg at 300 rps/s and I've used 90 kg (the notional 100 minus the actual 10) at 330 rps/s so in effect the same thing.

[Mignon]

Stan, if you go back to page 1 of this thread and look at STEST1.20 the difference between 10 kph/s ramp rate and 20 kph/s is about 15 ft lbs of torque. I'm pretty certain from other things mentioned that this test was using the default 100 kg flywheel inertia. In which case it should be showing about twice the torque error. Do you agree? Methinks whoever programmed this dyno had the right idea but the details and the maths wrong.

If you can think of any reason why the dyno error should be half what I think it should be please let me know.

[Stan Weiss]

Mignon said:

Stan, if you go back to page 1 of this thread and look at STEST1.20 the difference between 10 kph/s ramp rate and 20 kph/s is about 15 ft lbs of torque. I'm pretty certain from other things mentioned that this test was using the default 100 kg flywheel inertia. In which case it should be showing about twice the torque error. Do you agree? Methinks whoever programmed this dyno had the right idea but the details and the maths wrong.

If you can think of any reason why the dyno error should be half what I think it should be please let me know.

Dave,
Yes, by doubling the ramp rate it should double the torque,

Stan

[Mignon]

Stan Weiss said:

Dave,
Yes, by doubling the ramp rate it should double the torque,

Stan

Every cloud has a silver lining eh. They f'ked up the inertia values but it's only half as bad as it might have been because they f'ked up the calculations too.

[b73]

I acknowledge this thread hasn't seen any activity for a few years.
I stumbled across it while looking for similar info regarding how DD handles drive train inertia calcs.
If anyone is still interested, I have some info to add.
Here's a post made on the now-defunct efi-university form by Steve Nichols of Dyno Dynamics on this topic.
It may go some way to explain the logic behind the inertia numbers they use.







Also, here is the contents of the Shootout config files for 4, 6 and 8cyl N/A cars.
I can add the others if anyone is interested.

VERSION = 1

TITLE = \2SHOOT_4
DATE = 2002/10/05

INERTIA = 80
INERT_DTRAIN = 8

RAMP = 10
DWT = 1.15

COAST = 30, 235
COAST = 50, 218.2
COAST = 70, 201.4
COAST = 90, 184.6
COAST = 110, 167.8
COAST = 130, 151
COAST = 150, 134.2
COAST = 170, 117.4
COAST = 190, 100.6
COAST = 210, 83.8
COAST = 230, 67







VERSION = 1

TITLE = \2SHOOT_6
DATE = 2002/10/05

INERTIA = 100
INERT_DTRAIN = 10

RAMP = 12.5
DWT = 1.15

COAST = 30, 432
COAST = 50, 401.2
COAST = 70, 370.4
COAST = 90, 339.6
COAST = 110, 308.8
COAST = 130, 278
COAST = 150, 247.2
COAST = 170, 216.4
COAST = 190, 185.6
COAST = 210, 154.8
COAST = 230, 124







VERSION = 1

TITLE = \2SHOOT_8
DATE = 2002/10/05

INERTIA = 120
INERT_DTRAIN = 12

RAMP = 15
DWT = 1.175

COAST = 30, 432
COAST = 50, 401.2
COAST = 70, 370.4
COAST = 90, 339.6
COAST = 110, 308.8
COAST = 130, 278
COAST = 150, 262.6
COAST = 170, 247.2
COAST = 190, 231.8
COAST = 210, 216.4
COAST = 230, 201