Motocross Action Magazine - The worlds leading publication about motocross and supercross.
2023 DUBYA WORLD VET MOTOCROSS CHAMPIONSHIP VIDEO: JUST LIKE BEING THERE China Diesel Engine
THIS WEEK AT GLEN HELEN: OPEN PRACTICE THURS; OTHG ON SAT. & SOCAL VET MX ON…
FIRST LOOK! BETA’S FIRST-PRODUCTION 2024 450 RX
2023 WORLD VET CHAMPIONSHIP PHOTO GALLERY & RESULTS
2023 DUBYA WORLD VET MOTOCROSS CHAMPIONSHIP VIDEO: JUST LIKE BEING THERE
PARADIGM: “SHIFTING TOWARDS GREATNESS” – A JETT LAWRENCE FILM BY…
THIS WEEK IN MXA: 24 HOURS OF GLEN HELEN SOLO
LAST YEAR’S DUBYA WORLD VET CHAMPIONSHIP TRACK FROM ABOVE
MXA TECH SPEC: HOW TWO-STROKE POWER VALVES OPERATE & WHY
TEN THINGS ABOUT MOTOCROSS BOOT PHILOSOPHY
MXA TEAM TESTED: SCAR RACING TITANIUM FOOTPEGS
MXA TEAM TESTED: TM DESIGNWORKS FACTORY EDITION FX CHAIN GUIDE
(1) Concept. Power valves are found at the exhaust port on all two-stroke engines. They are used to modulate the power of the motorcycle by constantly changing the exhaust timing to work with the engine’s rpm. Four-strokes utilize valves on top of the cylinder that open and close the intake and exhaust ports to neatly allow air and fuel in and out of the combustion chamber; two-strokes don’t have these valves.
(2) Exhaust ports. In layman’s terms, combustion engines use the “suck, squeeze, bang, blow” method to create power. Dirt bikes suck air and fuel into the cylinder, compress it with the piston, light it with a spark, and blow the fumes out the exhaust port and into the exhaust pipe when making power. To adjust the power characteristics of a two-stroke engine, aftermarket tuners and manufacturers raise and lower exhaust ports, creating more top-end power by raising them or making more bottom-end grunt by lowering them.
(3) How it works. Nowadays, power valves are utilized to open and close a two-stroke cylinder’s exhaust port as needed when the engine’s rpm changes. At low rpm, the exhaust port is closed off by the power valve, only allowing small amounts of fumes through. Yamaha uses a cable from a computerized mechanical control unit that detects the ignition frequency and determines the required angle of the valve as needed for the engine. Yamaha’s YPVS system uses a valve that is raised and lowered into the exhaust port at an angle to allow air through the port. KTM uses a spring-loaded valve that rotates to open and close. The KTM valve has a much broader range for modulating power. It begins to open at 5000 rpm and is fully opened at 7500 rpm. A power valve that opens quickly creates a snappy and more aggressive throttle response off the bottom end. A power valve that opens slowly builds power slower and creates more torque with a smoother hit.
(4) History. Before Yamaha created the first power valve for two-strokes in the early 1980s, the window for power was much smaller. Plus, when an engine builder was looking to make horsepower, he would have to give up power somewhere else on the curve, typically on the low end. The innovation of the power valve allowed race bikes to have torque on the bottom and on top by modulating when the power valve opens up. It creates the best of both worlds.
(6) Yamaha. The YPVS power valve found on Yamaha two-strokes can’t be adjusted externally. Since the 2021 and older-generation Yamaha two-strokes are lower on horsepower than KTMs (the YZ125 makes 33.99 horsepower compared to the 125SX’s 37.52, and the YZ250 has 46.08 horsepower compared to 49.33 for the 250SX), their power valve doesn’t need to be modulated with the same care. The lower horsepower and torque on the YZ models make them easier to ride. In contrast, with the additional power of a KTM comes the need to control that power.
TEN THINGS ABOUT MOTOCROSS BOOT PHILOSOPHY
TEN THINGS ABOUT MOTOCROSS BOOT PHILOSOPHY
CLASSIC MOTOCROSS IRON: 1972 CZ 400 YELLOW TANKER
2023 DUBYA WORLD VET MOTOCROSS CHAMPIONSHIP VIDEO: JUST LIKE BEING THERE
JALEK SWOLL OUT OF 250 EAST SUPERCROSS WITH SPIRAL FRACTURE…
THIS WEEK’S BIRTHDAY BOYS: A-MART, JORDI TIXIER, KEN…
THE REBIRTH OF CAN-AM OFF-ROAD MOTORCYCLES VIDEO
SCAR RACING’S 20TH ANNIVERSARY “GOLDEN…
MXA VIDEO: WE TEST THE ALL-NEW 2023-1/2 GASGAS MC450F FACTORY EDITION
SIGHTS & SOUNDS: 2023 DAYTONA SUPERCROSS PRESS DAY RAW
THIS WEEK IN MXA: E-BIKE VS. 125 TWO-STROKE & NASHVILLE…
2023 DUBYA WORLD VET MOTOCROSS CHAMPIONSHIP VIDEO: JUST LIKE BEING THERE
THIS WEEK AT GLEN HELEN: OPEN PRACTICE THURS; OTHG ON SAT. & SOCAL VET MX ON…
FIRST LOOK! BETA’S FIRST-PRODUCTION 2024 450 RX
2023 WORLD VET CHAMPIONSHIP PHOTO GALLERY & RESULTS
2023 DUBYA WORLD VET MOTOCROSS CHAMPIONSHIP VIDEO: JUST LIKE BEING THERE
PARADIGM: “SHIFTING TOWARDS GREATNESS” – A JETT LAWRENCE FILM BY…
THIS WEEK IN MXA: 24 HOURS OF GLEN HELEN SOLO
LAST YEAR’S DUBYA WORLD VET CHAMPIONSHIP TRACK FROM ABOVE
MXA TECH SPEC: HOW TWO-STROKE POWER VALVES OPERATE & WHY
TEN THINGS ABOUT MOTOCROSS BOOT PHILOSOPHY
MXA TEAM TESTED: SCAR RACING TITANIUM FOOTPEGS
Stud Exhaust Valve MXA TEAM TESTED: TM DESIGNWORKS FACTORY EDITION FX CHAIN GUIDE