When electronic engine controls first appeared, they seemed to be sounding a death knell for the kinds of power-enhancing modifications Corvette owners had come to know and love. But by the late 1980s, pre-programmed performance PROMs (Programmable Read Only Memory) and do-it-yourself software became readily available and it was once again "game on" for high performance engine modifications and tuning.
The PROMs (actually EPROMs, the "E" means erasable) used in the ECMs (Engine Control Module) that control L98 and 1992-93 LT1 engines are removable and reprogramming them requires that they first be erased using ultra-violet light. Once a PROM's data has been erased, it can be programmed using a "burner", but the process doesn't always move to completion as planned. Sometimes existing data can't be completely erased, and other times what was a perfectly good PROM, won't allow new data to be written.
By comparison EEPROMS (electronically erasable programmable read only memory), also called "flash" memory require neither ultra-violet light for erasing existing data nor a special PROM burner to write new data. Erasing and writing are both done electronically, which is one of the reasons EEPROMS can be repetitively re-flashed without problems.
Until recently, an uncooperative PROM was simply a minor inconvenience that could be eliminated by purchasing a new one. A few years ago, what was a minor inconvenience turned into a major pain in tender parts of the anatomy. First GM switched to non-erasable PROMs, and then they eliminated them completely from the replacement parts inventory.
Difficulty in obtaining replacement PROMS combined with limitations in the original control systems prompted Weswood Performance to develop an entirely different solution to custom programming L98 engines. Weswood's Digispark system uses a 1994-95 LT1 PCM (Powertrain Control Module) in place of the original ECMs used in 1985-1993 Corvettes. As opposed to a complete control system replacement, the Digispark system uses the LT1 PCM to control engine (and transmission, if desired) functions, but retains the original ECM to control all emissions equipment and handle communications with the body computer, which in turn operates the digital portions of a C4 dashboard.
Obviously, you don't fall off a turnip truck one day and design a system like this the next. On the other hand, if, like Wes Burch of Weswood, you're a retired electronic engineer with 30-plus years of experience designing sophisticated electronic systems for the US Navy, building a custom engine control system is a relatively straightforward task--but definitely not an easy one. General Motors doesn't disclose many of the deep, dark details of engine control system functions, so Burch had to do a fair amount of research to determine the precise electronic details needed to assure proper operation and compatibility.
Burch installed the prototype Digispark system in January, 2008 on his '72 Corvette, in which he had previously installed an L98 engine using a '90 ECM and "mail order" chip. After he had the Digispark system operating satisfactorily, he built another system and installed it on his '91. He's been using both systems as long term validation tests to verify that no durability problems exist, and to explore the capabilities of off-the-shelf tuning software. To date the two cars have accumulated over 10,000 trouble free miles including several heat-soaked 400-plus mile trips and a few drag strip passes.
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."
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.
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.)
Part of a Digispark installation involves programming the PCM for proper engine control. All systems come with a custom pre-programmed PCM to get the car running. The kit can be further expanded to include licensed TunerCat software, licensed Datamaster scanning software, and a diagnostic port-PC interface cable. Since the PROM that's hard-wired into the LT1 PCM is electronically erasable, all reprogramming operations can be accomplished by connecting a PC to the PCM through the interface cable, which plugs into the diagnostic port (also called ALDL, which stands for Assembly Line Diagnostic Link) that's connected to the LT1 PCM.
In addition to controlling engine functions, a PCM also has the capability of controlling an automatic transmission. With a 700R4, Digispark only controls torque converter lock-up. If a Digispark system is installed, a vehicle owner then has the option of replacing a 700R4 trans with a 4L60E. Although the latter is essentially an electronic version of the former, it offers the advantage of altering upshift and downsift shift points, shift firmness and torque converter lock-up to be altered by a few keystrokes and reprogramming the PCM, as opposed to making mechanical modifications to the transmission.
Although installation of a Digispark system isn't terribly difficult, it does require a 21st Century mentality that's comfortable with electronic equipment (like cell phones, personal computers and the like). As Burch puts it, "On a scale of 1 to 10 (most difficult), Digispark is probably about a 6 or 7. A Corvette owner who can competently replace a distributor and is skilled enough to read email should have no trouble with this kit. Use of a dyno will be necessary to obtain max power, but drivability tuning only requires the scanning software and a fairly level road where a steady speed can be maintained for 2 to 3 minutes. And if someone gets in trouble, help is only a phone call or email away."
Although the Digispark system was designed to simplify and expand tuning capabilities, it may also become a lifesaver for C4s. As replacement ECMs and PROMs continue to become more difficult to find, a Digispark system may become the best option available to keep L98 engines alive.