280z

AFM Upgrade (from stock to turbo-ish)

Details on how to upgrade AFM housing from smaller stock size to a larger size.

Be sure to verify A/F ratios using a wide band Oxygen Sensor to ensure the new AFM is set up correctly.

Pros:

Cons:

 

Comparison Via Photos (Larger AFM is from a Toyota Cressida) :


Outlet side (note it is the rectangular cross section inside that restricts the flow) stock AFM on right


Intake side ( the restrictive rectangular cross section is easy to see. Also shown is the temperature sensor)


AFM electronics compartment detail (note the circuit board held in place with 3 screws)


Measuring the temp sensor's resistance (~2270ohms at room temp ~ 21C)

 

Flow Restriction Comparison and Temp Sensor Comparison:

Parameter Stock 280z AFM Cressida AFM
 outlet flange diameter (ID) 67mm 74mm
 outlet flange cross sectional area 3526mm sq 4300mm sq
     
intake flange dimensions 50mm X 46mm 63mm X 49mm
 intake flange cross sectional area 2300mm sq 3087mm sq
     
Relative size (restrictions) X1 X1.34
     
Temp Sensor Resistance @ 21C 2,270ohms 2,280ohms

 

How to swap:

The only part that has to be moved is the small circuit board. It moves from the stock AFM to the larger turbo AFM. (Assuming temp sensors are the same)

  1. Remove the connector (4 screws and silicon)


    I had to "dremel" slots in the rusty screws to remove them.


  2. The contacts from the connector to the circuit board are actually compression fit so the connector simply pulls out with the contacts

    Disconnect the two wires from the temperature sensor (red) and the wire from the wiper (black).

Disconnected connector being removed.



Disconnected Connector

It is easy to see the two connectors are not 100% swappable

  1. Remove the three screws holding the circuit board in place.



    Circuit Boards Removed (Note the clipping needed for the Nissan unit to fit the Cressida AFM) 
  2. Swap circuit boards and reassemble.

    Nissan circuit board in Cressida AFM


    Finished!


  3. Since the flow resistance decreases as the cross sectional areas increase, spring tensions on the turbo AFM will not match those on the stock AFM. The AFM calibration process here will not properly set the correct Turbo AFM's spring tension. It should, however, get you close. You can then tweak a few teeth at a time to optimize.
    More AFM info here.

  4. For your reference, here is some data I collected from my stock '77 280z AFM with outside temp=6C:



    AFM Measurements taken on a 77 280z referenced to chassis ground.
    test 
    point
    39 36 6 9 8 7 27 Vreg temp
    rpm
    800 13.93 13.93 0.01 11.59 7.48 3.22 5.09 14.04 6
    1000 13.92 13.92 0.01 11.64 7.52 4.14 5.14 14.08 6
    1200 13.98 13.98 0.01 11.7 7.56 4.68 5.23 14.15 6
    1400 14.03 14.03 0.01 11.76 7.59 4.99 5.21 14.18 6
    1600 14.35 14.35 0.01 12.06 7.78 5.41 5.25 14.49 6
    2300 14.32 14.32 0.01 12.05 7.75 5.82 5.25 14.45 6

Normalized data (to 12V standard) from above table

test point

39

36

6

9

8

7

27

Vreg

temp

rpm

800

11.91

11.91

0.01

9.91

6.39

2.75

4.35

12.00

6

1000

11.86

11.86

0.01

9.92

6.41

3.53

4.38

12.00

6

1200

11.86

11.86

0.01

9.92

6.41

3.97

4.44

12.00

6

1400

11.87

11.87

0.01

9.95

6.42

4.22

4.41

12.00

6

1600

11.88

11.88

0.01

9.99

6.44

4.48

4.35

12.00

6

2300

11.89

11.89

0.01

10.01

6.44

4.83

4.36

12.00

6

 

The ECU sends a voltage ratio to the ECU. The ratio is U/Ub as per the schematic below.

V Ref Resistance   (9-8) Vu (8-7) Vub (9-6) Vu/Vub
3.51 3.64 9.90 0.37
3.51 2.88 9.91 0.29
3.51 2.44 9.91 0.25
3.53 2.20 9.94 0.22
3.54 1.96 9.98 0.20
3.57 1.60 10.00 0.16

The following model shows Vu/Vub vs RPM 

Harris Model: y=1/(a+bx^c)
Coefficient Data:
a = -62.727835
b = 46.860151
c = 0.049964787

Applying the model to other rpms yields (normalized and assuming Vub=10 and Vpin9=10):

RPM Vu/Vub 7
3000 0.139 5.048
4000 0.122 5.220
5000 0.111 5.328
6000 0.104 5.403
7000 0.098 5.460

 

 


AFM wiper at 1500rpm


AFM wiper at 3000rpm


AFM wiper at 4000rpm

 


AFM Schematic

 


Here you can see the AFM bypass adjustment screw. It is used to close a passage under the primary AFM passage.
This passage is primarily used to vary the air to fuel ratio at idle. The screw controls how much "extra air" bypasses the AFM.
Closing passage is accomplished by turning the screw CW.
btw When the intake "pops" aka front fire usually due to running lean;. the energy passes through this passage. 
If it is closed off, the energy will damage the flap.