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The Digital Fuel Adjuster, Part 1

A brilliant breakthrough in do-it-yourself car modification

by Julian Edgar

Click on pics to view larger images

At a glance...

  • DIY digital fuel adjuster
  • Allows remapping of fuel curves
  • Suitable for when larger injectors or larger airflow meter are fitted
  • Use it to lean or enrich mixtures
  • Superb driveability and easy tuning
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We don’t think that it is possible to overstate the importance of this project: an electronic kit that when built, allows full control over your car’s mixtures by means of a digital hand controller. And you want to know the real clincher? The kit costs just AUD$79.95. Don’t have the electronics skills to build it? It is also available through the AutoSpeed shop, fully built and tested.

If all you want to do is alter your car’s air/fuel ratio, expensive interceptors are pretty well all immediately blown out of the water. Want to change your high-load air/fuel ratio? This project can do it. Swapped in a new airflow meter or upsized the injectors? Again this project can provide the solution.

If your car uses a voltage-outputting airflow meter – hotwire or vane - this interceptor will work with it. We’re talking something that is easy to tune, absolutely seamlessly transparent in driveability, and easy to wire into place.

The Background Story

So how did this kit come about? It’s worth briefly backgrounding its genesis, if only so that you know what has gone into it.

The Digital Fuel Adjuster was developed and designed by Silicon Chip electronics magazine. The kit, along with many others, is covered in the Silicon Chip publication – Performance Electronics for Cars – which is available from Silicon Chip Publications or the AutoSpeed shop. The book is a must-have for DIY modifiers.

The kit for the Digital Fuel Adjuster is available from Jaycar Electronics [www.jaycar.com.au] or through the AutoSpeed shop for AUD$79.95, with the required Hand Controller kit AUD$59.95.  Built and tested versions of the Digital Fuel Adjuster and Hand Controller are also available from the AutoSpeed shop.

The electronics design and development of the Digital Fuel Adjuster was carried out by the brilliant and modest electronics engineer John Clarke, while I came up with the concept and did all the on-car development. (During this period I wore a different hat to an AutoSpeed contributor, working for Silicon Chip Publications as a freelance contributor to Performance Electronics for Cars.)

The unusual mixture of ingredients – development funding from an electronics magazine, design and development by an electronics guru with no particular interest in modifying cars, and concept development and project management by me – has resulted in something of which I am very proud. It’s a unique product that works superbly and undercuts by a huge amount similar commercial products... none of which, AFAIK, can do all that this one can, anyway!

So by no means should the Digital Fuel Adjuster be seen as an AutoSpeed-developed project, but at the same time I am happy that AutoSpeed endorses it and promotes it.

Julian Edgar

How It Works

Click for larger image

The Digital Fuel Controller (DFA) is an interceptor. It takes the voltage signal coming from the airflow meter and allows adjustment of this voltage up or down in very small steps. It interpolates between the steps (ie smooths the curve of the adjustments) and when no changes are made to the signal, the input exactly equals the output.

The latter means that there are no ‘digital jumps’ that might give poor drivability.

In fact, on the subject of driveability, it is brilliant – we spent over 24 months and thousands of kilometres trialling the DFA in different cars (including the ultra-smooth 1998 Lexus LS400, a 2002 Impreza WRX, an STi Impreza WRX, a Nissan S15 200SX, a 1988 Nissan Maxima V6 Turbo, a mid-Eighties BMW 735i and a 1999 Toyota Prius) and if the modified air/fuel ratios are set correctly, the drivability remains absolutely factory.

Click for larger image

The DFA allows real time tuning, so that you can make changes with the digital hand controller (either on the dyno or the road) and immediately assess the results. Additionally, you can scroll through the map in non-real-time mode, making changes as you go. Changes can be made at 128 load points that normally correspond to 0-5 volts, the output range typical of airflow meters. (Note that the DFA can also work on 0-12V and 0-1V signals, the latter especially good for modifying oxygen sensor outputs. We’ll cover these aspects at another time.)

Each of the 128 load points can be adjusted up or down in 127 steps, which gives a very fine range of adjustment indeed. The removable digital hand controller shows both the load point being accessed and the up/down adjustment. Tuning changes are made with pushbuttons on the hand controller – no laptop is required.

It is brilliantly effective at altering mixtures, it’s available now, and it is incredibly cheap.

So, how do you use it?

Tuning

The digital Hand Controller uses a two-line LCD display, eight ‘direction’ pushbuttons, a VIEW/RUN pushbutton and a RESET button. (Incidentally, the pics of the Hand Controller shown here are of the prototype – the final kit version differs a little in appearance.)

The hand controller can be set to three different modes. RUN and VIEW modes are selected with a pushbutton on the hand controller, while LOCK mode is selected with a switch on the main unit

RUN mode:

  • view the tuning map real-time as it is accessed by the car
  • make real-time tuning changes

VIEW mode:

  • move up or down through the map, reading what tuning adjustments have been input
  • make non-real-time tuning changes

LOCK mode:

  • tuning changes are prevented but the map can be viewed
Click for larger image

In RUN mode the display shows which load site is being accessed by the running car, and what up/down changes have been made at that load site. But what are these load sites all about, anyway?

Typically the DFA is set up for 0-5 volt signals. In these cases, an input voltage from the airflow meter of 0 is indicated by a load site of 1 (shown on the hand controller as “INPUT 1”), and an input of 5 volts is shown as a load site of 128 (“INPUT 128”).  From this it’s not too hard to work out that all the in-between signal voltages will show up as in-between load sites – for example, a signal input voltage of 2.5V will show as a load site of 64.

For example, a particular airflow meter might have a minimum voltage output of 1.5V at idle and a maximum voltage output of 4.6V at full load. This means that when the car is being driven and the Hand Controller is in RUN Mode, INPUT numbers ranging from 38 to 118 will appear on the LCD. In this example you then have 80 load sites at which the airflow meter voltage can be adjusted (there’s no point in adjusting load sites below 38 or above 118, because in this case they’re never accessed).

So the INPUT load sites are just another way of showing airflow meter output voltage.

In RUN mode you can real-time tune each load site as you’re on it. For example, if you’ve got an assistant driving the car up a long hill and the Hand Controller is showing load site 87 (“INPUT 87”), pushing the white ‘up’ key will cause the mixtures to be immediately richened at that load site. Each time you press the white ‘up’ key, the OUTPUT number shown on the display will change upwards.

For example, the display might look like this:

OUTPUT    +2  (dV)

INPUT        87    /RUN/

This shows that at load site 87, you’ve increased the voltage output of the airflow meter by 2 units (which will cause the mixture to be richer). If you want the mixtures to change even faster, press the black ‘up’ key, which will cause the adjustment to jump up in blocks of four units. Pressing the ‘down’ buttons will reduce the airflow meter voltage at that load site, leaning the mixture at that load. The maximum adjustment is plus/minus 127 units.

(The ‘dV’ to the right of the top line means ‘delta voltage’, ie change in voltage.)

Tuning in real-time RUN mode can be useful but VIEW mode tuning is often used. VIEW allows you to scroll left/right through all the load sites and then make tuning adjustments.

So for example, a moment ago in real-time tuning RUN mode we set load site 87 to +2, and now we’d like to tune load site 86. By pressing the RUN/VIEW button we can set the controller to VIEW mode, and then by pressing the left/right keys we can move up and down the load sites, putting in any tuning figures we want. If the car is running, these tuning figures will immediately take effect. So after moving to load site 86 in VIEW mode and pressing the black ‘up’ button twice, the display will look like this:

OUTPUT     +2     (dV)

INPUT         86    <VIEW>

A complete tuning map might look something like the following:

Load Site

Tuning Adjustment

38

5

39

8

40

11

42

7

42

7

43

16

44

17

45

16

46

17

47

17

48

17

49

17

50

15

51

12

52

12

53

12

54

12

55

12

56

11

57

8

58

6

59

2

60

2

61

1

62

0

63

0

64

1

65

1

66

1

67

3

68

3

69

4

70

4

71

4

72

4

73

8

74

12

75

12

76

12

77

13

78

13

79

13

80

18

81

17

82

17

83

17

84

17

85

17

86

17

87

13

88

13

89

13

90

13

91

13

92

13

93

13

94

13

95

14

96

14

97

14

98

14

99

14

100

14

101

14

102

14

103

14

104

14

105

12

106

12

107

12

108

12

109

12

110

12

111

12

112

12

113

12

114

12

115

12

116

12

117

12

118

12

Click for larger image

It’s easier to see what it looks like when it’s graphed.

In this case all the tuning adjustments are positive – that is, the air/fuel ratio at all loads (except load point 62 and 63) needed to be richer. You can see that less enrichment was needed at medium loads, but fuel was added at both low and high loads. (This is the actual tuning map for a 1985 BMW 735i, which doesn’t use closed loop. The tuning was done on the road using a MoTeC air/fuel ratio meter.)

Note that unlike a programmable management system - where you’d expect the map to be more regular in shape - the resulting air/fuel ratios from the DFA interception are the outcome of both what the factory had already programmed and the changes that you want to make. For example, if you want richer mixtures in the midrange than the factory had set (eg in a turbo car as it comes on boost), there’s likely to be a hump in your tuning chart at that point.

Conclusion

Tuning is easy:

  • There’s a range of load sites accessed either real time (RUN) or non-real-time (VIEW)
  • Up/down tuning adjustments at each load site are made with the up/down keys
  • In VIEW mode only, the left/right keys takes the tuner through the different load sites

In use it’s even simpler than this explanation indicates – it takes only a few minutes for even novice tuners to get the hang of things and start tuning the air/fuel ratios.

Next week – Set-up and install of the Digital Fuel Adjuster

The Shortcomings

Let’s be realistic – all products and modification approaches have shortcomings. So what are the DFA’s?

Firstly, it is an interceptor. All interceptors – no matter how fancy their names, initials or advertising – have negatives. The most major is that the ECU needs to be confused into doing what you want. For example, if you reduce the load signal coming from the airflow meter, the mixtures will go leaner – but the ignition timing is also likely to advance a bit. For this reason it is important that the engine be monitored for detonation when the mixtures are being altered.

Secondly, you can’t change the mixtures away from 14.7:1 when the car is in closed loop (that is, when the ECU is using the feedback of the oxygen sensor(s) to determine mixtures). Again, that’s the same with any interceptor. (However, as mentioned above, the DFA can be used to intercept the oxygen sensor signal – but that’s another story. Also, when major engine management changes like injector or airflow meters are made, the DFA can be used to bring the mixtures back into the range of closed loop adjustment. The ability of the DFA to be able to change the airflow meter signal across the full load range is therefore very important in all cars.)

Finally, the DFA comes in kit form and so needs to be built. As with all electronic kits, if you make a mistake in the construction process, it’s likely that the DFA won’t work. (But in that case spend more money and buy a pre-built kit.)

These Pre built kits are available here in the Autospeed Shop

The Specs

 

  • Voltage input: any voltage from 0 to 14.4V
  • Voltage Output: 0 to 1V, 0 to 5V or 0 to 12V plus offset
  • Offset Adjustment: +/-127 steps corresponding to 19.6mV for 5V range
  • Maximum Offset adjustment: +/- 0.5V with 1V range +/-2.5V with 5V range or +/-6V with 12V range (optional finer resolution reducing adjustment range by 5)
  • Input adjustment points: 1 to 128 corresponding to 39mV steps from 0 to 5V for 5V range
  • Input to output response time for offset change: 5ms
  • Display update time: 250ms
  • Step up and down with 1 step per button press or at 4 changes per second if button held
  • Skip offset adjustments: step up and down with 4 steps per button press or at 16 steps per second if button held

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