Why The Suter 500 GP Replica Has Throttle Body Injection

Thinking about Suter Racing’s MMX500 offering—its $130,000 replica 500cc Grand Prix 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 highside 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.

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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 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 fuel-air 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.

Does this mean two-strokes could make a comeback? Technically yes, but in practice only maybe. Because DFI has to create an evaporated mixture in the 60 to 70 crank degrees between exhaust closure and ignition, special (read: expensive) injectors capable of rapid injection of ultra-fine fuel droplets (Mother Nature prefers droplets of 10 microns or less) are needed, and such injectors can’t keep up above maybe 10,000 rpm. That means you need something very different from the auto replacement injectors down at the NAPA store. The problems are less for transfer port injection, but remember that things happen at double speed in two-strokes; 10,000 rpm in a two-stroke is like 20,000 on a four-stroke.

There are other problems. When Honda was developing its two-stroke PGM injection, initial throttle response was so instant (abrupt) that it broke midcorner wheel grip. Complicated software had to be written to make injection “burbly”—like carburetors—to bring the engine in more gradually and controllably. A photographer I spoke with years ago was at Honda’s test track one day in the ’90s on other business and saw a distant crew with an NSR, performing one roll-on after another, then plugging in the laptop and making a change, then more roll-ons. Even with carburetors teams tended to stay with slower-responding round-slide carbs rather than the new flat-slides whose sharper response (developed for MX) acted more like fuel injection.

When I saw the injection on the NSRs at Eastern Creek in 1993 (Australian GP) the only way I could tell was because injection needs to “know” exhaust pipe center-section temperature to avoid cold-pipe seizures. The engine’s induction system was invisible inside its airbox, but there in view, sticking out of the pipes, were the temp probes and their leads.

The Suter V-4 has two contra-rotating cranks to cancel the engine’s gyro effect, as previously practiced by Yamaha. Intake is by carbon reed valves located in the cylinder vee and the two butterfly throttle shafts are geared together. Exhaust port height is varied by the usual gizmos to improve bottom-end (9,500) without compromising top-end. The engine is made a bit over 500cc by stroking from 50.5 to 58.5, giving a displacement of 576.3cc. This was done to allow the expected 200 hp to be achieved with less abrupt porting than would be the case at 500cc. This is actually somewhat more power than the racing 500s were making toward the end, as their power was extensively compromised in the interest of rideability that translated into competitive circuit lap times.