Var­i­o­ram

Crit­ics still de­bate which is bet­ter, Var­i­o­ram or non-var­i­o­ram? To­tal 911 ex­plains the me­chan­ics and the­ory be­hind the vari­able in­duc­tion sys­tem in­tro­duced on the Porsche 993

Total 911 - - Wrightune -

Porsche’s Var­i­o­ram in­duc­tion sys­tem was de­signed to im­prove low- to midrange torque by op­ti­mis­ing air­flow at dif­fer­ent rpms. In­tro­duced on the 993 Car­rera RS’S M64/20 en­gine in 1995, the tech­nol­ogy was then rolled out to the stan­dard 993 Car­rera range with the in­tro­duc­tion of the 1996 M64/21 en­gine. The pre­vi­ous M64/05 en­gine made use of a sin­gle throt­tle body and a res­o­nance cham­ber tuned to make use of Helmholtz res­o­nance (where pres­sure pulses caused by the open­ing and clos­ing of the in­take valves cause air to be bounced off the walls of the res­o­nance cham­ber and forced into the com­bus­tion cham­ber).

Var­i­o­ram adds a sec­ond, up­per throt­tle body that flows through to a cen­tral plenum. From this cen­tral plenum, six vac­uum-op­er­ated in­duc­tion run­ners are mounted. Be­low 5,000rpm, the ECU elec­tron­i­cally trig­gers th­ese six run­ners (one for each cylin­der) ex­tend­ing them to nearly twice their nor­mal length, us­ing rub­ber seals to cut off the pri­mary throt­tle body and res­o­nance cham­ber.

The re­sult­ing in­crease in run­ner length makes use of the Ven­turi ef­fect – by re­duc­ing the cross-sec­tional area the speed of the air­flow is in­creased. The in­creased air ve­loc­ity aids cylin­der charg­ing, while low­er­ing the op­ti­mal rpm for res­o­nance in­duc­tion.

By im­prov­ing low-range air­flow, Var­i­o­ram en­gines en­joy around 20 per cent more torque at 5,000rpm than non-var­i­o­ram units. Once the en­gine reaches 5,000rpm, and the throt­tle is more than 50 per cent open, the in­duc­tion run­ners are re­tracted to their nor­mal length. The pri­mary throt­tle body is reen­gaged, and one of the con­nect­ing tun­nels be­tween the two banks of the res­o­nance cham­bers is opened via a vac­uum-op­er­ated valve.

Above 5,800rpm the sec­ond con­nect­ing tun­nel is opened, again through the use of a valve. Th­ese fi­nal two stages op­er­ate in the same way as non-var­i­o­ram en­gines, us­ing the res­o­nance cham­ber to im­prove the vol­u­met­ric ef­fi­ciency of the en­gine at high rpms.

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