I would like to understand exactly by what mechanism VTEC operates on my 1.5. I can't find an explanation in the shop manual or anywhere else thus far. I know what VTEC means and that my second intake valves open at around 4K rpm. I just don't know what the electronic-mechanical action is and what affects it. Thanks.

 

there is a VTEC solenoid that is mounted to the side of the head, at least there is on DOHC motors that is what triggers the vtec engagement, not sure how exactly it works on the sohc motors

 

Howstuffworks "What does the VTEC system in a Honda engine do?"

 

This site will explain how the current generation of VTEC works in our L15A engines, it also mentions how the iDSI variant works. The Truly Amazing Honda Fit/Jazz

This site is also interesting because it talks about the next gen L15A thats supposedly coming to the US next year.
Honda L-Series SOHC i-VTEC Engines for the new GD Honda Fit/Jazz

Hope this helps.

 

So – - I have read the articles offered and have a few more clarifying questions:

1) Does VTEC in the L15A then operate by a third cam lobe and rocker and actuating pin that connects the two intake rockers to a third rocker, or does it operate by simply connecting the two existing intake rockers using only one cam lobe to actuate both? What is the specific mechanism?
2) How does the hydraulic pressure system work and what is the electronic part? Does an electronically-actuated mechanism change the hydraulic pressure somehow to move the actuating pin? How does that work?
3) Would changing oil viscosity affect it? Would the VTEC change the valve actuation differently if there was 15W-50 oil in the engine as opposed to 5W-20?

Thanks!

 

2007 Honda Fit Powertrain - Honda - Press Releases - Selected Story

 

the vtec in the L15a is a little different than previous vtec systems, in the past you had 2 valves sitting on 2 rockers sitting on 2 identical cam lobes, when vtec was activated a 3rd rocker and new higher profile lobe, that when not in vtec just coast, lock onto the 2 normal rockers, and now both use the higher profile 3rd cam lobe,

whats cool about the l15a is that the 2 valves are normally staggered in opening, and one has less lift, (i haven't seen any other motor do this), and it causes the mixture to swirl when entering the cylinder for more efficient combustion,
when you hit vtec, a hydraulic pin locks the second lower lift valve, to the higher lift valve, therefore synchronizing both valve's timing, and creating more lift from the second valve.

your oil viscosity will not effect anything (unless it is a very very incorrect weight possibly)
when vtec is activated a electronic solenoid moves a spool valve (think automatic transmission), allowing oil to flow to the locking pin mechanism.

cool stuff

 

this should explain everything:

 

I have also seen a couple of computer generated videos of the inner workings of various vtec mechanisms, if you look on youtube you will see many, I have yet to find one that shows the 12/16 valve type setup but there are many that show the various two and three mode variations.

 

Mr. debater, I am unable to receive your link. Mugen Man, that was the missing piece! One confusing thing remains - the Temple of VTEC Asia article on it describes the engine as a "12V/16V" engine and makes it very black-and-white clear that one intake valve is simply inoperative at low rpm and the dynamic of the single offset open valve is what makes the swirl. Then when the VTEC is actuated, the second valve opens with the first. The Honda article says that the timing of the intake valves is "staggered" and "skewed" in favor of the primary valve. It sounds contradictory, as though Honda means that both valves are opening, with the secondary less and later than the other, and you/they make it sound that when VTEC is actuated, the secondary valve then adopts the same timing and lift as the primary. Or does Honda mean "staggered" and "skewed" timing is one valve opening, then both as TOVA says? I could interpret it either way. What do you make of that?

 

the temple of vtec is wrong, both valves are always operational, and staggered untill vtec is engaged.

 

my little brother says that when v-tec kicks in.....

the engine will go VRRRRRRRRRRRRROOOOOOOMMMMMMMM!

 

Search search search. Even go look on youtube, there is videos explaining it.

 

Yeah, the explanations I have seen are quite confusing. But I know the secondary valves ride on a very low lift/duration combination to create a swirl in the combustion chamber and also to reduce pumping losses, and then when the VTEC kicks in a selonoid allows oil to flow through passages (Just like other VTEC types) to cause a retaining pin to hold the secondary valves rockers into place with the primary valves rocker at which point both valves will be operating on a higher lift/duration cam lobe.

 

Well then, I am enlightened. Thank you, gentlemen! What a wionderful little gem of an engine! I will tell TOVA they are wrong and see what they say...

 

The Temple of VTEC moderator answered me promptly and then actually forwarded my message to the author of the "Truly Honda Fit/Jazz" article. He kindly gave me a thoughtful answer that alluded to the fact that the sentiment sometimes is to keep things simple to avoid confusion, and that in fact the secondary intake does open slightly and then pins to the primary when VTEC is on. He also gave a link to more great photos:

http://asia.vtec.net/Series/FitJazz/lseriesphoto/

I'd recommend getting to know the US and Asia TOV sites and checking out the articles by WongKN. It's a great resource and that gentleman is a great source of info for Honda enthusiasts.

As for me, now, it's time to listen to the engine hum, happy to know what's going on inside...