Tuning the DigifantTM ECU
We get a lot of questions about racing a DigifantTM ECU GTI. If you don't know, DigifantTM is VW's version of the Bosch L-JetronicTM Engine Management system. Most VW enthusiasts are under the impression that DigifantTM is no good for tuning or performance let alone all out racing. They are partially correct.
The DigifantTM system as delivered on late 80's and early 90's VWs are susceptible to going lean at high RPM and frequently exhibits off-idle hesitation. So for many street tuners this is a noticeable and significant disadvantage. Furthermore common modifications that move your torque peak around may make the hesitation problem even worse at a higher, normally usable RPM.
So why do we race the late model Golf II? Well frankly, it was a challenge and the car was already in our possesion. After studying what other cars were doing with modified fuel injection it looked like we could make the car at least as competitive as an early CIS-E GTI which is a frequent winner in ITB. When a more careful look at the engine systems was taken we realized a few things which could potentially be an actual performance advantage over the earlier CIS cars.
Cylinder Heads:
The cylinder heads on a late model DigifantTM Golf are know as 'AA' heads due to the suffix on the part number. These heads have an improved port design over the earlier CIS heads. While comparisons have not yet been done, in general machining tolerances improved with the later heads and valve seat depths and seat angles are reportedly more consistent. The injector bosses of the AA heads do not impede the airflow as much as the earlier mechanical injectors.
Intake:
The intake tract from throttle body to airflow meter is less restrictive than the CIS equivalent. Unlike the CIS metering unit the air flows through a DigifantTM Unit in a straight shot, there are no 90º corners.
How do you make power with limiting SCCA rules?
When dealing with DigifantTM engine management system, we do what you would do with a Bosch L-JetronicTM system which is to add or remove resistance to the various sensors which tweeks the DigifantTM ignition timing and fuel mixture for high RPM engine operation.
ECU Sensors:
There are a number of inputs which can be adjusted as per SCCA rules in order to alter the output of the computer. These inputs include:
Air Flow sensor output
OXS sensor voltage
Throttle position switches
Coolant Temp Sensor
Knock Sensor
Distributor Hall Sender
Intake Air Temp Sensor
Fuel Injection Mods
Note: We no longer make Digifant ECU chips for these cars.
Ignition Tweaks
To get an adjustable timing curve for racing we've used two different
approaches. One is to simply lock in a total timing advance position into
the distributor. This makes the car hard to start, but is very accurate, reliable
and simple. The other approach is to use an older distributor and re-curve the advance curve to be more aggressive.
