WHY THE SUTER 500 GP REPLICA HAS THROTTLE BODY INJECTION
Thinking about Suter Racing’s MMX500 offering—its $130,000 replica 500 GP two-stroke V-4—I realize why they gave this bike four-stroke-style throttle body fuel injection. They did it because selling the bike with carburetors would have left its buyers helpless in the face of how it used to be done—confronted by main jets, slides, needle jets, pilot jets, needles, air correction jets, and the various heights of jet shrouds. An experienced tuner carried literally hundreds of these items to every race. All this is meaningless to modern riders.
With the single exception of Honda’s PGM injection, used around 1993, all those hallowed slip, grip, and high-side 500 GP bikes were fueled by carburetors. That meant that jetting had to go up and down with atmospheric pressure and temperature, the main variables of air density. In extreme conditions account had to be taken of humidity as well, for as we all know, water vapor doesn’t support combustion.
Giving the Suter Ecu-controlled throttle body injection (one injector under each intake butterfly, plus a showerhead injector hovering over each intake bellmouth) saves the buyer from all that, just as it does all who ride modern fuel-injected four-stroke production bikes.
Carbureted streetbikes ran rich in warm weather (lower air density) because they had to be jetted rich enough stock not to run excessively lean in cold weather (higher air density), so in EPA terms they— two-stroke or four-stroke—were usually whirling storms of unburned hydrocarbons (UHC) on two wheels.
Modern bikes therefore carry sensors to measure air density, throttle angle, and what-have-you and have detailed fuel and ignition maps in memory to keep engine tune constantly corrected to ambient weather conditions. Electronic fuel injection gathers up all the knowledge of top tuners of the carburetor era and packages it as ECU, control software, sensors, and resident, el maps. And it can take care of even the long-standing assertion that extra oxygen in the wooded section of the Hockenheim track (trees “transpire,” right?) could lean out your bike. If that oxygen exists, the oxygen sensor on a modern production or racebike will detect it and the ECU will add any necessary extra fuel to keep mixture constant.
You have probably read recently of plans by OSSA and KTM to produce direct fuel injection (DFI) two-strokes for off-road competition. This is entirely different from the throttle-body injection on the Suter and on the 1993 Honda NSR500. Throttle-body injection simply replaces each carburetor with a throttle body and injectors. A fuelair mixture fills a cylinder’s crankcase during the piston up-stroke, and on the following downstroke that mixture is slightly compressed and jets into the cylinder through (typically) five transfer ports. Because the exhaust port remains open during all of transfer, some fuel-air mixture gets lost out the exhaust to become UHC emissions—the basic problem that put road-going carbureted two-strokes out of production around 1984.
DFI solves that problem by either 1) injecting the fuel directly into the combustion chamber, after the exhaust port has closed, or 2) by timing the injection of fuel into a part of the transfer stream so it can’t reach the exhaust port until after it has closed (this is called TPI by KTM, Transfer Port Injection). Because no fuel can be lost out the exhaust, this drops UHC to very low values. Because only air and no fuel is drawn into the crankcase of a DFI engine, the crankcase is not cooled by fuel evaporation. In some DFI or TPI snowmobile engines it has therefore been necessary to provide a liquid coolant loop to cool the case.