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VTEC Jus How Does It Work?
Xtreme Kermit - 31/7/12 at 06:51 PM

I thought I knew how variable valve timing worked, by applying oil pressure to the cam/sprocket to vary the relationship between them.

But looking at a Honda VTEC head, they don't work like that.

Anyone know how they work? And are they the only two methods?

Daddylonglegs - 31/7/12 at 07:00 PM

Spooky! Just been talking to my eldest about V-Tech, he's got a Civic Type-R.

Try this link

JB

Xtreme Kermit - 31/7/12 at 07:13 PM

No wonder the head construction looks a little odd, you have the cams and an idler shaft for each set of rockers.

I wonder if the Toyota one is the same [dives into google]

Xtreme Kermit - 31/7/12 at 07:21 PM

So it looks like the Toyota one uses the rotation of the cam against the sprocket and not the extra cam lobe.

http://www.youtube.com/watch?v=6AXh8O7hWU4&feature=youtube_gdata_player

MikeRJ - 31/7/12 at 08:43 PM

The one that (IMO) is the cleverest by far is the MG-Rover VVC mechanism. It's the only system I know of that gives step-less control of cam duration (from 220 to 295 degrees). It does this by continuously varying the speed of the cam as it turns - by slowing the cam down as the valves approach full lift you effectively increase the duration.

Xtreme Kermit - 31/7/12 at 08:56 PM

Well there are many ways to skin the cat...

I guess the extra shafts in the vtec head contribute to it being a tall engine.

Interesting

loggyboy - 31/7/12 at 09:30 PM

IIRC its a little gnome that makes more noise but no acctuall power gains...
lol




Xtreme Kermit - 31/7/12 at 09:34 PM

I knew there would be magic in there somewhere

COREdevelopments - 31/7/12 at 09:48 PM

Toyota has a regular vvti and their sporty version is a vvtl-i (variable valve timing and lift). It uses the regular oil pressure sprocket along with a complex cam arrangement to give the lift after 6000rpm.

Rob

StrikerChris - 1/8/12 at 12:15 AM

I've had the vvt sprocket apart on my nissan engine.The nissan sprocket is in 3 parts,the bit that locates on the cam,the outer which drives the chain,and a helically splined shuttle in the centre.When the solenoid opens the oil pressure moves the shuttle down the pulley and thus slightly advancing or retarding the timing because of the helical splines.
No idea how honda do it,their vvt sprockets are half the size (and weight!)









Chris

Xtreme Kermit - 1/8/12 at 06:12 AM

quote:
Originally posted by StrikerChris

No idea how honda do it,their vvt sprockets are half the size (and weight!)



That seems to be because the Honda sprockets are passive, the vtec bit happens in the rockers brining the extra cam lobes into play. It looks like vtec cams have an additional four lobes per cam.

MikeRJ - 1/8/12 at 07:53 AM

quote:
Originally posted by Xtreme Kermit
quote:
Originally posted by StrikerChris

No idea how honda do it,their vvt sprockets are half the size (and weight!)



That seems to be because the Honda sprockets are passive, the vtec bit happens in the rockers brining the extra cam lobes into play. It looks like vtec cams have an additional four lobes per cam.


The i-VTEC system (e.g. used in the K20A) uses a variator in the cam pulley AND dual cam profiles. I was investigating the aftermarket ECU modification (K-Pro) that allows you to alter the fueling and ignition, and by god does the ECU have a lot of calibration! The Variator allows 50 degrees of cam timing change and the ECU has a fueling table for every 10 degrees. Then you have the dual cam profiles (high speed and low speed) so that's ten sets of tables for fuel, and then another ten tables for ignition timing!

The VTEC system uses 3 lobes for two valves - at low speed each valve is operated by a separate lobe, the same as most conventional 16v engines. When VTEC is actuated, the middle lobe which is larger with more lift and duration operates both valves together.

[Edited on 1/8/12 by MikeRJ]

MikeRJ - 1/8/12 at 07:55 AM

quote:
Originally posted by loggyboy
IIRC its a little gnome that makes more noise but no acctuall power gains...
lol


It makes some very impressive power gains, and more to the point the engine makes just as much power at low RPM as the majority of normally aspirated production engines.