The Digispark/LT1 PCM mounts...
The Digispark/LT1 PCM mounts in the space formerly occupied by the original ECM...
In addition to eliminating issues associated with replacement PROMs, Digispark also brings the advantage of sequential fuel injection to L98s. Although sequential systems don't offer horsepower improvements in all applications, they do significantly improve drivability, throttle response,fuel economy and exhaust emissions. Sequential systems also allow fuel distribution problems to be eliminated through individual cylinder fuel flow tuning. Improved fuel metering is also achieved through use of left- and right-side oxygen sensors (L98 systems only monitor a single oxygen sensor).
Prior to developing the Digispark system, Burch was running an aftermarket performance chip in his nearly stock '91. After installing the Digispark system, Chris Harwood of Xtreme Motorsports tuned the car and obtained an additional 17 peak horsepower and 3 lbs./ft of torque. Gains were even more substantial above 4200 RPM, where the horsepower produced by TPI engines falls off quickly. According to Burch, "I can't say that the Digispark system itself is responsible for the power gains, but it did allow the engine to be easily tuned to deliver more power. I believe in some RPM ranges, sequential injection does produce a bit more power than batch fire. But Digispark unquestionably delivers greater ease in tuning for drivability, smoother operation, and improved fuel economy."
...which moves to the front...
...which moves to the front left corner of the engine compartment. The black box just above the ECM is the Digispark interface module.
The biggest obstacle in the development of the Digispark system was creating a conventional distributor that would produce the signals that would keep the PCM happy. Burch accomplished this by installing an OptiSpark shutter wheel and optical sensor in a standard HEI distributor housing. Because the distributor is located in the traditional Gen I location, high and to the rear of the engine, it does not suffer from the reliability problems that plague OptiSpark distributors. The US Patent & Trademark Office has accepted a design Patent application for the Digispark distributor and has approved legal use of the term "Patent Pending."
But the distributor wasn't the only obstacle standing between Burch and a smoothly operating system. He also had to interface the new PCM and original ECM to operate on the same engine and had to create the correct Vehicle Speed Sensor signal to keep the LT1 PCM happy.
Weswood currently offers two variations of the Digispark system in either speed density or mass air control: A Street Rod (or Race) version designed for vehicles not originally equipped with an L98 engine (like C1, C2 and C3 models) and a late C4 kit for 1990-1991 Corvettes.
Whether an EFI system is of...
Whether an EFI system is of the batch fire or sequential persuasion, the plug wires have to be connected in the proper sequence. But with sequential fuel injection, the injectors must also be connected in the correct sequence, hence the numbers on the lead wires.
In the very near future Weswood will expand the Digispark system to include an Early C4 kit for 1985-1989 Corvettes and an LT-1 kit for 1992-1993 Corvettes. Obviously, Digispark systems can be used on vehicles other than Corvettes, and in the future, Weswood plans to offer kits for F-bodies, B-bodies, and ultimately any EFI engine.
The primary difference between early and late C4s is control system type; 1985-89 models utilize a mass air sensor whereas 1990-1991 models rely on speed/density calculations. 1994-95 LT1 systems incorporate a mass air sensor, but the PCMs can be programmed for speed/density operation. Installation of a Digispark system therefore allows an owner the option of selecting speed/density or mass air control, irrespective of original system type.
All Digispark systems include a PCM, wiring harness, and Digispark distributor. Buyers can purchase their own oxygen sensors (two required), ignition module, knock sensor and knock module or these items can be purchased from Weswood as an expanded kit. Likewise, a MAF sensor and flexible couplers are available from Weswood or many other sources. C4 systems also contain an interface module, (to handle communications between the LT1 PCM and original ECM) a freon pressure sensor and evaporator temperature sensor (which enables the PCM to control idle speed while preventing air conditioning compressor operation when freon pressure is too low or too high, or if the evaporator is blocked with ice.)