Vtec

[tenzoracerevovii]

ok.....i know this question might seem stupid to some of you........but i don't give a shit.......i want to know how vtec works.......i know that the cams have 3 lobes or whatever.....primary, secondary, and vtec.......and there is a vtec solenoid.......when you hit a certain rpm.....the cpu sends a signal and a "pin" slides through the lobes and connects them all..........the vtec lobe is bigger than the other lobes......now........since vtec is dohc.....does that mean one cam for intake and one cam for exhaust?......when vtec is not engaged, then the valves open up one at a time. so when vtec kicks in, it opens up both valves at the same time?......so more air in and air comes out faster........so does that "extra air flow" make the "boost" in power?.......am i correct........if not......please correct me.....thanks

[CornerCarver]

VTEC can be SOHC or DOHC. I think all of the SOHC VTEC motors have been intake only, while with the DOHC motors some have been intake only and some are both intake and exhaust.

Some versions of VTEC change cam lift and duration with the third lobe, some shut down the use of one valve during low rpm operation (as seen in the fuel efficiency and motorcycle versions).

The performance car versions of VTEC use a mild cam profile for lower rpm/cruising mode, for best fuel efficiency and emissions, and a hotter cam profile for high rpm/WOT situations, which allows for maximum flow at high rpm.

Basically any motor which uses conventional cams is a compromise. The best cam profile for low end/low rpm performance is very different from the best cam profile for high end/high rpm performance. High rpm calls for long duration and high lift to cram as much charge as possible into the combustion chamber. The problem is, at low rpm, optimal air velocity is lost with this long duration/high lift setup. Lower rpm requires the valves to be open less and for less time to keep the charge flowing optimally.

Consider sucking air through a straw. If you suck x amount of air through a small diameter straw, it will travel nice and fast. Suck the same amount of air through a big huge honking straw, and the same amount of air will travel slowly. Engines need the air to travel fast for the mixture to swirl around and pack into the combustion chamber for best results. The problem is, the small straw will only suck so much air. You need a small straw to suck small amounts of air (low rpm), and a big straw to suck large amounts of air (high rpm). Small straw - mild cam profile. Big straw - hot cam profile. Non-VTEC motors choose a in-between straw size that suits both as best a possible, but will never suit either one optimally.

VTEC is basically two sizes of straw for one engine... That said, any manufacturers variable valve timing and lift system is basically the same principal. Variable cam timing, that is another story...

[drift]

http://www.leecao.com/honda/index.html

everything you need to know about all VTEC forms, with pictures to help figure it out

[praxis]

cornercarver....I like the straw anology.

[praxis]

Check out the post by VTEC-boi in this section (technical) entitled 'tech heads-check my work'...he has an article on his website (linked from the thread) that has a basic desription of how vtec works.