EPA/Alliance/Global 4WD Dyno 

Correlation Factor Program

Preliminary Data Review

6-13-13
Overview: 

4WD-to-2WD Correlation Factor Test Program

Goal: Evaluate need / quantify 4WD dyno correlation factor. Such a factor 

would be used adjust FE & CO2 for any tests performed using 4WD 

dynamometer to ensure that any “general” 2WD vs. 4WD dyno testing 

bias is not inappropriately attributed to the specific vehicle types that are 

targeted for 4WD dyno testing.

• 12 vehicles, 10 labs

• 3 road load derivations in each mode (average used for FE testing) 

• 4 FE tests in each mode (FTP, Hwy, US06). 
Alliance Preliminary Analysis

• Analysis still underway; presented data is preliminary

• Error checking not complete

• Incomplete BMW Data Set was not included

• No individual B1, B2, B3 Data (FTP) 

• No individual US06 City or US06 Highway Data

• Missing/erroneous drive data

• Recommend that EPA Impala data set be re-examined

• Intra-lab 2WD-to-4WD correlation significantly different (vs. EPA Explorer)

• Impala shows divergent highway result vs. Explorer and overall dataset
Road Load Derivations
2WD vs. 4WD Dyno Coefficients

80 90 100 110 120 130 140 150 160 170

140

120

100

80

60

40

TRL50 Mean

R

L

5

0

M

e

a

n

2WD

4WD

DynoStat

Wrangler

Wrangler

OUTBACK

OUTBACK

Impala

Impala

G25

G25

Fit

Fit

Explorer

Explorer

Escape

Escape

CX-5

CX-5

ACCORD

ACCORD

4Runner

4Runner

Scatterplot of RL50 Mean vs TRL50 Mean

2WD 4WD

90

80

70

60

50

40

30

20

10

0

Mean of RL50_

Mean

59.0032

79.2927

Worksheet: Copy of Worksheet 1

Chart of Mean( RL50_Mean ) lbs

 2WD dyno ABC’s ~ 25% higher
RL Curve Comparison 

• Point-to-point comparison (i.e. RL Force @50)

• “Power Integral” (PI)

• Integral of power-vs-speed curve from 10 – 70 mph

• Proportional to the road-load energy required to drive a constantacceleration trace from 10-70 mph.

• Easily calculated from polynomial coefficients:

 

70

10

2

2

3 1

3

4 1

4

1

70

10

3 2 PI  CV  BV  AV dV  CV  BV  AV 
2WD 4WD

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

2WD 4WD

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

RL50_CVariation1

M

e

a

n

PI_CVariation2

1.64346

0.676318

1.61894

0.688451

Road Load Coefficient of Variation % ( RL50, Power Integral)

Compares "ABC" curves @50mph point Compares "ABC" curve power over 10-70mph

2WD vs. 4WD RLD Variability

 Overall, 4WD RLD variability > 2x 2WD
2WD 4WD

4000

3000

2000

1000

0

P

I_

S

t

De

v

2

Wrangler

OUTBACK

Impala

G25

Fit

Explorer

Escape

CX-5

ACCORD

4Runner

Wrangler

OUTBACK

Impala

G25

Fit

Explorer

Escape

CX-5

ACCORD

4Runner

2174.2

1348.77

Road Load Power Integral Standard Deviation 

2WD vs. 4WD RLD Variability
2WD 4WD

4

3

2

1

0

P

I_

CV

a

ria

tio

n

2

Wrangler

OUTBACK

Impala

G25

Fit

Explorer

Escape

CX-5

ACCO 4RunneRDr

Wrangler

OUTBACK

Impala

G25

Fit

Explorer

Escape

CX-5

ACCORD

4Runner

1.61894

0.688451

Road Load Power Integral - Coefficient of Variation (%)

2WD vs. 4WD RLD Variability
FE Results
B1 B2 B3 C ity _C omp Hw y US06 US06C US06H

3.00%

2.00%

1.00%

0.00%

-1.00%

-2.00%

-3.00%

-4.00%

-5.00%

FE_

p

e

r

_4

-

2

Toyota

SUBARU

NISSAN

Mitsubishi

Mazda

Honda F

Honda A

Ford

EPA I

EPA E

Chrysler

Toyota

SUBARU

NISSAN

Mitsubishi

Mazda

Honda F

HoFordnda A

EPA I

EPA E

Chrysler

Toyota

SUBARU

NISSAN

Mitsubishi

Mazda

Honda F

Honda A

Ford

EPA I

EPA E

Chrysler

BMW

Toyota

SUBARU

NISSAN

Mitsubishi

Mazda

Honda F

Honda A

Ford

EPA I

EPA E

Chrysler

BMW

Toyota

SUBARU

NISSAN

Mitsubishi

Mazda

Honda F

Honda A

Ford

EPA I

EPA E

Chrysler

BMW

Toyota

SUBARU

NISSAN

Mitsubishi

Mazda

Honda F

Honda A

Ford

EPA I

EPA E

Chrysler

Toyota SUBARU

NISSAN

Mitsubishi Mazda

Honda F

HoFordnda A

EPA I

EPA E

Chrysler

Toyota

SUBARU

NISSAN

Mitsubishi

Mazda

Honda F

Honda A

Ford

EPA I

EPA E

Chrysler

-1.0

9

%

-1.3

9

%

-0.9

6

%

-0.6

7

%

-1.0

7

%

-0.8

0

%

-0.9

5

%

-1.8

5

%

95% CI for the Mean

FE Delta (4WD-2WD) % - All Data
B1 B2 B3 C ity _C omp Hw y US06 US06C US06H

3.00%

2.00%

1.00%

0.00%

-1.00%

-2.00%

-3.00%

-4.00%

-5.00%

FE_

p

e

r

_4

-

2

-1.0

9

%

-1.3

9

%

-1.2

4

%

-0.7

5

%

-1.1

6

%

-0.8

0

%

-0.9

5

%

-1.8

5

%

Toyota

SUBARU

NISSAN

Mitsubishi

Mazda

Honda F

Honda A

Ford

EPA I

EPA E

Chrysler

Toyota

SUBARU

NISSAN

Mitsubishi

Mazda

Honda F

HoFordnda A

EPA I

EPA E

Chrysler

Toyota

SUBARU

NISSAN

Mitsubishi

Mazda

Honda F

Honda A

Ford

EPA I

EPA E

Chrysler

Toyota

SUBARU

NISSAN

Mitsubishi

Mazda

Honda F

Honda A

Ford

EPA I

EPA E

Chrysler

Toyota

SUBARU

NISSAN

Mitsubishi

Mazda

Honda F

Honda A

Ford

EPA I

EPA E

Chrysler

Toyota

SUBARU

NISSAN

Mitsubishi

Mazda

Honda F

Honda A

Ford

EPA I

EPA E

Chrysler

Toyota SUBARU

NISSAN

Mitsubishi Mazda

Honda F

HoFordnda A

EPA I

EPA E

Chrysler

Toyota

SUBARU

NISSAN

Mitsubishi

Mazda

Honda F

Honda A

Ford

EPA I

EPA E

Chrysler

95% CI for the Mean

FE Delta (4WD-2WD) % - Complete Data Sets (No BMW Data)
Impala Results

Concern:

 Directionally opposite highway result from overall dataset

 Large difference in 4WD-to-2WD correlation: (1) within same lab (same 

site, restraints, personnel?), (2) between “passive” 2WD vehicles

2WD 4WD

28.50

28.25

28.00

27.75

27.50

Dyno Mode (2WD or 4WD)

FE_

bag (m

p

g)

27.7675

27.7345

Individual Value Plot of FE_bag (mpg)

Lab = EPA E

2WD 4WD

37.0

36.5

36.0

35.5

35.0

34.5

Dyno Mode (2WD or 4WD)

FE_

bag (m

p

g)

36.2177

35.2105

Individual Value Plot of FE_bag (mpg)

Lab = EPA I

FE Offset (4-2) = 0.12% FE Offset (4-2) = 2.78%
B1 B2 B3 C ity _C omp Hw y US06 US06C US06H

3.00%

2.00%

1.00%

0.00%

-1.00%

-2.00%

-3.00%

-4.00%

-5.00%

FE_

p

e

r

_4

-

2

-1.3

4

%

-1.6

1

%

-1.4

7

%

-1.1

0

%

-1.2

5

%

-0.9

7

%

-1.0

0

%

-1.9

5

%

Toyota

SUBARU

NISSAN

Mitsubishi

Mazda

Honda F

Honda A

Ford

EPA E

Chrysler

Toyota

SUBARU

NISSAN

Mitsubishi

Mazda

Honda F

HoFordnda A

EPA E

Chrysler

Toyota

SUBARU

NISSAN

Mitsubishi

Mazda

Honda F

Honda A

Ford

EPA E

Chrysler

Toyota

SUBARU

NISSAN

Mitsubishi

Mazda

Honda F

Honda A

Ford

EPA E

Chrysler

Toyota

SUBARU

NISSAN

Mitsubishi

Mazda

Honda F

Honda A

Ford

EPA E

Chrysler

Toyota

SUBARU

NISSAN

Mitsubishi

Mazda

Honda F

Honda A

Ford

EPA E

Chrysler

Toyota SUBARU

NISSAN

Mitsubishi Mazda

Honda F

HoFordnda A

EPA E

Chrysler

Toyota

SUBARU

NISSAN

Mitsubishi

Mazda

Honda F

Honda A

Ford

EPA E

Chrysler

95% CI for the Mean

FE Delta (4WD-2WD) % - Recommended Data Set

(No BMW, No Impala)
Regression Modeling

• Need to account for drive-style impacts

• Demonstrate significance of dyno-mode

• Model FE = ƒ (Vehicle, EER-j2951, Dyno Mode)
Regression Modeling - Example

Regression Analysis: FE_bag (mpg) versus EER (j2951), Dyno Mode_4WD, ... 

* Lab_Toyota is highly correlated with other X variables

* Lab_Toyota has been removed from the equation.

The regression equation is

FE_bag (mpg) = 29.6 - 0.347 EER (j2951) - 0.372 Dyno Mode_4WD

- 6.33 Lab_Chrysler - 1.75 Lab_EPA E + 8.80 Lab_Ford

+ 21.2 Lab_Honda A + 19.5 Lab_Honda F + 15.9 Lab_Mazda

+ 7.59 Lab_Mitsubishi + 11.6 Lab_NISSAN + 11.6 Lab_SUBARU

78 cases used, 1 cases contain missing values

Predictor Coef SE Coef T P

Constant 29.5651 0.1236 239.28 0.000

EER (j2951) -0.3474 0.1411 -2.46 0.016 

Dyno Mode_4WD -0.37184 0.07608 -4.89 0.000

Lab_Chrysler -6.3344 0.1665 -38.04 0.000

Lab_EPA E -1.7509 0.1736 -10.08 0.000

Lab_Ford 8.8009 0.1780 49.45 0.000

Lab_Honda A 21.2309 0.1785 118.96 0.000

Lab_Honda F 19.4684 0.1711 113.76 0.000

Lab_Mazda 15.9353 0.1664 95.74 0.000

Lab_Mitsubishi 7.5904 0.1807 42.01 0.000

Lab_NISSAN 11.6232 0.1702 68.30 0.000

Lab_SUBARU 11.5888 0.1678 69.08 0.000

S = 0.332523 R-Sq = 99.9% R-Sq(adj) = 99.9%
Modeling Results

Model Fit 

(R^2)

4WD 

Coef

4WD Coef

Significance

Drive 

Coef

Drive Coef

Significance

FE(4-2)

(Model/Drive Corrected)

FE(4-2)

(AVG)

FTP 99.9% -0.3174 100.0% -0.1374 90.1% -1.2% -1.3%

Hwy 99.9% -0.3718 100.0% -0.3474 98.4% -1.0% -1.1%

US06C 99.0% -0.2901 100.0% -0.1229 76.1% -1.8% -1.6%

US06H 99.7% -0.3137 99.9% -0.3133 94.7% -1.1% -1.3%

 4WD Dyno mode is a significant factor

 4WD Dyno mode reduces FE in all models/phases (high correlation variation 

across vehicles)

 Correcting for drive style (i.e., regression model) has minimal impact on 

resulting 4WD-to-2WD FE offset (see model FE delta vs. data set average)
Summary

• Note: Analysis is preliminary.

• Average road load variability higher on 4WD dyno’s

• Quantification of impact TBD

• Dyno mode is significant factor

• 4WD correlation factor is needed

• Drive rating also a significant FE factor but did not bias dyno factor 

• Variation in 4WD-to-2WD correlation using simple 2WD 

vehicles is a concern

• Magnitude of correlation differences suggests that factors other than 

vehicles differences ( i.e., differences in the “passive” non-drive axle of the 

test vehicles) are responsible.

• Additional study/best practices are needed! 4WD dyno control systems 

(load splitting), restraints/tie-down/skew, brake drag, etc. are all possible 

contributors.

• Bottom Line: If 4WD dyno correlation is not predictable/well understood for 

simple, “passive” 2WD vehicles, we cannot assume that a lower FE result on 

a 4WD dyno test is attributable solely to AWD/HEV “active” attributes 

(AWD system, regen braking).