Cycle World - - News - By Kevin Cameron

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.

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