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Exhaust Scavenging

HOW DOES EXHAUST SCAVENGING WORK?

EXHAUST SCAVENGING IT THE METHOD OF USING EXHUAST PULSES FROM ONE CYLINDER TO HELP PULL MORE OF THE NEXT CYLINDER'S EXHAUST GASSES.


AS YOUR ENGINE IS RUNNING, THE EXHAUST FROM EACH CYLINDER IS COMING OUT IN PULSES- 1 EVERY 2 REVOLUTIONS FOR A 4-STROKE ENGINE.


AS THIS PULSE IS LEAVING THE CYLINDER PAST THE EXHAUST VALVE, IT HAS ALOT OF ENERGY AND MOMENTUM, AND PUSHES ITS WAY DOWN THE EXHAUST TUBE TO LOWER PRESSURE AREAS.


YOU CAN THINK OF THIS PULSE LIKE A CAR DRIVING PAST YOU ON THE ROAD.  AS THE CAR GOES PAST YOU AT HIGHER SPEEDS, YOU FEEL YOURSELF GETTING PULLED ALONG AFTER IT.


THIS EFFECT IS SAME AS WHEN THE PULSE GOES THROUGH THE COLLECTOR AND MAKES MINI VACUMMS IN THE OTHER TUBES.


THIS VACUMM EFFECT HELPS TO ACCELERATE THE NEXT PULSE FROM AN ADJACENT CYLINDER, AND THE CYLCE REPEATS AS THE SECOND PULSE PASSES THE COLLECTOR AND PULLS THE THIRD PULSE.

HOW CAN EXHAUST SCAVENGING BE MODIFIED?

FINDING THE CORRECT TIMING OF THESE PULSES LEADS TO DIFFERENT LENGTH HEADERS GIVING DIFFERENT POWER BOOSTS TO THE ENGINE. 


LONG TUBE HEADERS ARE BETTER FOR HIGH RPM APPLICATIONS WHERE THE EXHAUST GASSES WILL ALREADY HAVE ALOT MORE SPEED AND WILL BENEFIT FROM BEING INDEPENTENT LONGER.


SHORTY LENGTH HEADERS WILL GIVE A MORE SUBSTANTIAL BOOST TO MID RANGE POWER AS THE SCAVENGING EFFECT WILL TAKE PLACE SOONER IN THE SHORTER PIPES.

WHICH HEADER LENGTH IS BEST FOR MY ENGINE?


NEITHER IS A BAD OPTION FOR PERFORMANCE, BUT THE DESIGN OF THE COLLECTOR, DIAMETER OF THE PIPES, AND TRAVEL DIRECTION WILL HAVE MUCH BIGGER IMPACT.

 

LOG STYLE HEADERS ARE USUALLY SIMPLE PATHS THAT RAM DIFFERENT PULSES INTO EACHOTHER, AND DO NOT HAVE ANY NOTICIBLE SCAVENGING EFFECTS.

TO BENEFIT FROM EXHAUST SCAVENGING, A TUBULAR DESIGN IS THE MOST COMMON, BUT THERE ARE SOME CAST MANIFOLDS THAT SCAVENGE EFFECTIVELY.

EXHAUST SCAVENGING FOR BOOSTED ENGINES?

FOR BOOSTED APPLICATIONS WITH ADDITIONAL PRESSURES BEFORE THE TURBO, EXHAUST SCAVENGING IS LIMITED AND USUALLY NOT CONSIDERED.

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