Dyno Maths Library

[FWombat]

I tried this on some of my runs from the summer, and I'm getting a low HP reading as well. I have a Supra and entered all the appropriate data. My #'s must have been off by at least a factor of 2.

Hmmm, maybe the sample rate was low.

I've looked at a couple of people's logs that were "off", and every time it's been due to a low sampling rate or an incorrect parameter.  Does it "look" like a dyno chart?  Usually when the sampling rate is too low, it doesn't look like a dyno chart.  

If you send me your log, and a copy of the dyno library with your constants filled in, I'll take a look and figure out what's wrong.  I've done this for a couple people now and I've yet to see the math not work.

- Brian

[FWombat]

A couple of notes on mistakes I've seen using the library:

If dyno functions don't LOOK like a dyno chart, then your sampling rate is probably too low.  Remember, the library's noise filters are setup for fastest possible sampling.  Slower rates will be over-smoothed.

If the functions do look like a dyno chart, but the numbers are off from what you expect, then you probably have an incorrect constant.  Make sure the vehicle weight is accurate on a truck scale.  Drastic differences are usually due to an incorrect gear ratio or wheel/tire size.

Here's a technique to verify that you have your gear, tire, and wheel information correct.  One of the internal calculations of the library is Vehicle_Speed_MPH.  Log Vehicle Speed, and compare it to Vehicle_Speed_MPH.  They should be within 1 MPH if all your gear ratios are correct.  

- Brian

[BLKMGK]

I noticed today - when the library is loaded the name will appear at the bottom of the window in the pane surrounding the application.

[Hau]

Is there a way to filter the data so that the  graph of RWHP doesn't drop before the end of the run?  It's looks kinda wierd to see rpm's still climbing yet RWHP is a huge negative number.  The calculations are good and my VSS is within 2-3 mph of vehicle_speed_MPH.  Also from one pull to the next, is anyone seeing big variations in RWHP?  I'm not sure if it's tire slip or what but back to back pulls don't come up with similar numbers.

Thanks,
Hau

[Mike tsai]

I think I'm doing something wrong here but when I look over the graph, I notice that it's giving me hp readings before the vehicle speeds start moving.  This is within the area where you rev up to the point where u drop the clutch.  What am I doing wrong?

[FWombat]

I think I'm doing something wrong here but when I look over the graph, I notice that it's giving me hp readings before the vehicle speeds start moving.  This is within the area where you rev up to the point where u drop the clutch.  What am I doing wrong?

Ah...I guess I didn't make the technique clear for how to do a dyno run.  It's NOT like doing a quarter mile run.    Here's what you do:

1) Find a long, flat, straight strech of road.
2) While driving, put the car in the gear you want to use for the run.
3) Coast down or brake IN GEAR, until you're at about 1500-2000 RPMs.  
4) Floor it to redline.

All the power calculations are based off RPMs, so if you rev in neutral you'll show REALLY high power.  

- Brian

[FWombat]

Would you happen to know what values is correct to input below?  Thanks.

DRIVE_RATIO = ?
CD_DRAG = ?
CD_AREA = ?

Thanks.

DRIVE_RATIO = Final Drive Gear.  4.266 for your car.

For the CD numbers, these should be close enough for jazz:
CD_DRAG = .34
CD_AREA = 20

[FWombat]

We still haven't figured out why the Dyno library won't import on some people's machines.  So, I guess I'll go ahead and post the internal calculations.  If it doesn't import on your machine, you can enter these calculations manually into a math library.

Equation descriptions will be of the following form:

EQUATION_NAME
EQUATION_CONTENTS
# Units
# Comments, if applicable

Capitalization is important.  Equations with names in all CAPS act as constants, and will need to be changed to suit your vehicle.  Equations in mixed case will be identical for every car.  The two "final" calculations (_Wheel_HP and _Wheel_Torque) have underscores in front of them to raise them to the top of the channel list when configuring overlays.  

Here we go!


_ZERO
Engine Speed * 0
# Always equals zero.  
# Used as a workaround for the fact that AEMLog doesn't allow equations to be constants.

WHEEL_SIZE
17 + _ZERO
# Wheel size in inches.

TIRE_WIDTH
245 + _ZERO
# Tire Width in millimeters

TIRE_ASPECT
45 + _ZERO
# Tire Aspect Ratio

GEAR_RATIO
1.686 + _ZERO
# Gear reduction ratio for the target gear being logged.

DRIVE_RATIO
3.133 + _ZERO
# Final drive reduction ratio.  

CD_DRAG
0.32 + _ZERO
# Coefficient of drag.  
# Used to estimate power loss from aerodynamic drag.

CD_AREA
22 + _ZERO
# Frontal area in ft^2.  
# Used to estimate power loss from aerodynamic drag

WEIGHT
3900 + _ZERO
# Total vehicle weight in lbs.  
# Should include driver, passengers, fuel and cargo.

Wheel_Radius
((WHEEL_SIZE / 2) + (TIRE_WIDTH * TIRE_ASPECT / 2540)) / 12
# Radius of wheel in ft.

RPS
(FILTER 50 Engine Speed) / 60
# Engine speed in rev/sec.  
# Filtering is employed to filter out signal noise and sensor limitations.

DeltaRPS
DIFFERENTIATE 25 RPS
# Engine acceleration, in rev/sec^2.
# Some filtering is employed again here.

Wheel_Speed
RPS / (GEAR_RATIO * DRIVE_RATIO)
# Wheel speed, in rev/sec.

Vehicle_Speed
Wheel_Speed * (2 * PI * Wheel_Radius)
# Vehicle speed, in ft/sec.

Vehicle_Speed_MPH
Vehicle_Speed * 60 * 60 / 5280
# Vehicle speed, in mi/hr.

Wheel_Accel
DeltaRPS / (GEAR_RATIO * DRIVE_RATIO)
# Wheel acceleration, in rev/sec^2.

Vehicle_Accel
Wheel_Accel * (2 * PI * Wheel_Radius)
# Vehicle acceleration, in ft/sec^2.

Vehicle_Accel_Gs
Vehicle_Accel / 32.17
# Vehicle acceleration, in Gs.
# This may be useful for comparing to G-Tech or similar device readings.

Raw_HP
WEIGHT * Vehicle_Accel * Vehicle_Speed / (32.17 * 550)
# Power, in HP.

Raw_Torque
Raw_HP * 5252 / Engine_Speed
# Torque, in ft/lbs
# This is signal is much too noisy to be useful.  For internal use only.

Filtered_HP
FILTER 75 Raw_HP
# Power, in HP
# Adds another filtering step to the power calculations.  If you feel that your power curves are too coarse or too smooth, this is the place to adjust.  Higher filter numbers will smooth the curve, lower numbers will make it rougher.  If the filter is too low you'll begin to see oscillations in the power graphs.  If this happens, raise the filtering.

Filtered_Torque
Raw_HP * 5252 / Engine_Speed
# Torque, in ft/lbs
# See Filtered_HP.

HP_Drag_Loss
(CD_AREA * CD_DRAG * 0.00256 * (Vehicle_Speed_MPH PWR 3)) / 375
# Power, in HP.
# Estimate of power loss due to aerodynamic drag.

HP_Rolling_Loss
WEIGHT * 0.014 * Vehicle_Speed_MPH / 375
# Power, in HP.
# Estimate of power loss due to road/tire friction.  Does NOT take into account driveline losses.

_Wheel_HP
Filtered_HP + HP_Drag_Loss + HP_Rolling_Loss
# Power, in HP.
# Calculated power at the wheels.  This is the channel you want to monitor when viewing logs.  If the top of the power graph is not being displayed, you may need to revise the display range.  Do NOT use Auto Range.  

_Wheel_Torque
_Wheel_HP * 5252 / Engine_Speed
# Torque, in ft/lbs.
# Calculated torque at the wheels.  This is the channel you want to monitor when viewing logs.  If the top of the power graph is not being displayed, you may need to revise the display range.  Do NOT use Auto Range.

[FWombat]

Nope, definitely doesn't look right.  Did you set engine speed to log at the fastest possible rate?  If you did, PM me a copy of the log and your math library, and I'll adjust the filtering to display the right numbers.

In general, if it doesn't LOOK like a dyno sheet, something's not working right.  Usually it's the sampling rate.

- Brian

[Hau]

Brian,

Are your dyno graphs for horsepower and torque dropping off before engine speed does?  If so, is it because of the filtering?

[FWombat]

Yes, my power graphs fall off a little before the end of the pull.  This is normal; it's due to the filtering I had to employ and the averaging inherent in the derivation function.  Mine usually falls off about 100 RPM before the end of the pull.  How soon it falls off depends on the sampling rate.

- Brian

[FWombat]

Looks like we're gonna have to change this a little.  I've now looked at several people's logs, and every one has required different filter settings.  The math always works though.  

If you want to use the library, get a log, adjust the library constants to your car, and see how it looks.  Then PM or email me your log and math library, and I'll adjust the filtering to work for your car.  Once the filtering is set it will continue to work as long as you use the same sampling rate.

Faster sampling rates are the best, as they give you the most accurate picture of what's going on.  The internal logger set to the fastest level, with engine speed on a fast channel, seems to give the best results.  I'd suggest using this.


- Brian

[JeffB]

I ran my car on a loading dyno in vehicle simulation mode... I entered in the weight into Brian's calcs that the dyno was using for my vehicle and the results came out pretty close...

at low boost the dyno showed 374rwhp and the math lib showed 385rwhp... at high boost the dyno showed 530rwhp and the math lib showed 553rwhp...

so it looks like the library reads somewhere in between a mustang dyno and a dynojet...

Brian, how would i go about adding a correction factor variable that would let you tweak it until the numbers match... in my case i would enter .958 to turn my 553 into the 530 that the dyno showed... should i add that to an existing formula or add another term?

[FWombat]

Brian, how would i go about adding a correction factor variable that would let you tweak it until the numbers match... in my case i would enter .958 to turn my 553 into the 530 that the dyno showed... should i add that to an existing formula or add another term?

Before we go adding a scalar correction, any chance you could do a run OFF of the dyno, and compare it to the dyno's numbers?  The power numbers are quite close, and I'd bet that much of the discrepancy is due to the fact that the load was simulated by the dyno.  

The correction wouldn't be hard to implement.  But this library was really intended to be accurate on the road.  I'm not crazy about the idea of tweaking it to work on the dyno, since that defeats the purpose of getting on the dyno in the first place!!    

- Brian

[JeffB]

there's nothing in the world that i'd rather do right now... unfortunately, i spent 3 months building my car, trailored it to the dyno, and have never driven it... damn snow won't go away... i'll let you know once i get out on the road... my biggest concern is that i won't have any traction and with wheelspin the calcs go out the window...  i should be able to do a comparison on the low boost stuff anyway...