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Snufflepuffster t1_iwe3wrq wrote

Reply to comment by evilleppy87 in How an engine works. by ooMEAToo

often valves close or open early to take advantage of acoustics (shock waves) to ram air into the cylinder or throw it out. So timing is very nuanced and can be adjusted drastically from engine to engine. In general the exhaust valve isn’t going to match the stroke, it might be early or late, extended or short due to design.

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evilleppy87 t1_iwe8c32 wrote

I'm aware that there's going to be some variability, it just seemed to me that closing 90° before TDC was a little extreme, especially considering in most cases the exhaust valve closes a little after TDC

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Drunken-samurai t1_iwge1ej wrote

Bringing up variability, there is VVT (variable valve timing) which takes advantage of this in real time to use different valve timings at different RPM's to provide more efficiency or power.

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evilleppy87 t1_iwgrf7o wrote

Like Honda VTEC, for example?

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Drunken-samurai t1_iwkt2fq wrote

Yes your right, this is part of what the VTEC acronym is (Variable Valve Timing & Lift Electronic Control).

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