Variable Intake Velocity

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By John Burns

Dear MOby,

What are variable intake manifolds? How do they work and why?

Walter Barlow


Dear Walter,

If you’d not been paying attention in high school and reading Hot Rod magazine, you’d know that low-end power is boosted by longer intake and exhaust tracts, and high-rpm power generally increases with shorter ones. Tuning the lengths of the intake bellmouths and exhaust header pipes is a science and black art they’ve been doing on engines forever.

Now we’re talking torquey: Can-Am cars in the ’70s used big-block V-8s tuned for torque in part by using long intake runners.

In smaller motorcycle engines, wouldn’t it be nice to be able to have intake runners with adjustable lengths, so you could boost torque at low rpm and let the engine breathe at high rpm? Automobiles, in which costs and complexities are relatively easy to hide, have used this technology for years. The first time I recall seeing this, in my motorcycle memory, was on the first MV Agusta F4 in 1998 (lead foto), which used a vacuum deal to move its intake bellmouths up and down depending on rpm.

Yamaha liked the idea so much, it adopted it for the 2007 Yamaha R1 and gave it a catchy acronym, YCC-I, Yamaha Chip Controlled Intake, at which link you’ll find an excellent explanation of how and why it all works, including videos.

Funnels up! At a certain predetermined rpm, the R1 was ready for high-rpm horsepower production.

Funnels up! At a certain predetermined rpm, the R1 was ready for high-rpm horsepower production.

Meanwhile, not to be out-acronymed, Suzuki says it’s …read more

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