Question on VTEC

BioHazard the Reaper

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#1
ok so i know htis is going to sound like a huge noob question, and that i dont have a "good" reputation with some members and will probably be made fun of for it for years to come but still, i ask this question. So i just got into the honda scene about 3 months, was in the 4wheel era for awile. My question is that im confused upon the difference between VTEC and non-VTEC. Now at first i thought it was one is DOHC and one is SOHC but then i found out you can get non VTEC DOHC motors so now im confused, will someone please clarify this for me?
 

slambed_civic

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#2
Variable valve Timing and lift Electronic Control

its basically a second cam but in one for SOHC....its 2 cam profiles on one cam

HERE SCC says it better than me..

Basically each pair of valves has three cam lobes, two that operate the valves at low-rpm, and a third that takes. over at high rom. During low-rpm operation, the two rocker arms riding the low-rpm lobes push directly on the top of the valves. In most cases, the cam profiles of the two intake valves will be slightly different, promoting swirl in the combustion chamber for better driveability. At high rpm (usually 4500 rpm to 6500 rpm range, depending on the engine) the ECU sends a signal to an oil control valve that allows oil pressure to flow into the low-rpm rocker arms. A third, high rpm rocker arm sits between the two low-rpm arms and follows a much more aggressive lobe. When oil pressure arrives, two hardened steel pins pop out of the sides of the low-rpm rocker arms and slide into sockets in the high-rpm arm, and the valves start folowing the larger cam profile. Just when you thought the engine was going to run out of power, output starts climing againas the engine "comes up on the cam" for the second time.

dude, dont worry about not knowing about hondas, we all had to start somewhere
 


BioHazard the Reaper

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#3
ok well that clears up very good i appreciate it =) and basically what makes VTEC better is that it "changes" the cam depending upon rpm and a non vtec always uses the "same" cam?
 

BioHazard the Reaper

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#4
err i had to edit my first post i had puttion some DOHC in place of SOHC...dont know if that confused ya but now its better
 


slambed_civic

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#5
i know my vtec kicks in round 4k.... thats when the second cam profile comes in and keeps power...
 

O.Z Chi

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#6
I don't trust magazines, so I'm quoting Slurp56 from CHB.com:



Okay, will its 3am and I finally finishe my write up. It ended up being 7 pages long in MS word Well, feel free to correct me if I made any stupid mistakes, its late... But this is in a little greater detail, How VTEC works

*Pssst! I told you that membership fee might be worth something later*

VTEC Explained... sort of…


VTEC is kind of cool in Honda’s. I believe it first started on Honda’s motorcycle’s and well, we all know what it is today. I am going to start pretty in depth, so I will include Lots of Pictures.



This is a picture of your Cylinder head in a DOHC VTEC engine. The rocker arms are basically what make VTEC work. There are a lot of pieces that compose the system, but the trick seems to be in the rocker arms. We know that when the camshaft spins, the lobes of the camshaft depress the rocker arms, which then depress the valves, opening them. If we take a look at a camshaft here:



We see that the two outside lobes are similarly sized and then the center lobe is a little bit bigger. You guessed it; the center lobe is your VTEC lobe. When your car is not in VTEC, the 2 outside lobes are pressing down 1 valve each (2 intake valves and 2 exhaust valves per cylinder). That’s how non-VTEC cars work, it has a lobe per valve, and there is no center VTEC lobe…. Follow me so far? Here’s the cool stuff….

These are your rocker arms:



There are 3 rocker arms per cam, per cylinder. The 2 outside arms are each being pressed on by the two outer camshaft lobes and are in turn each pushing down 1 valve each. This is how normal engines work. The center rocker arm (not present in non-VTEC applications) is your VTEC rocker arm. When it is activated, the 2 pistons slide out of it and lock all 3 rocker arms together. Then the VTEC arm is taller than the other two and is the only rocker arm making contact with the camshaft, the VTEC lobe on the camshaft to be exact. So now only 1 of the 3 Cam lobes is in contact with only 1 of the 3 rocker arms. Now, you’re in VTEC.

Cool, so now we know how VTEC works, almost. What happens with the middle rocker arm when it’s not engaged? Well it sits free and rests on the lost motion assembly. The lost motion assembly is installed under the middle rocker arm and absorbs any movement in the middle rocker arm to avoid excess chatter and it slamming around. The lost motion assembly looks like this:



Pretty basic, it just sits and absorbs movement in the middle rocker arm. Cool! Now one of the last pieces to the puzzle, what is a VTEC Solenoid and what does it do? Also, how do the pistons in the middle rocker arm get pushed out to lock the assembly together?

Well, if you look at the first picture, you will see oil orifices that go into the rocker shafts. There is oil being pushed through the rocker shafts at all times. It keeps things cool and lubricated. When not in VTEC, there should be about 7psi of oil pressure being pushed through the rocker shafts. Hold that thought, oil goes through the rocker shafts at about 7psi.

A VTEC solenoid is the following:



Inside the VTEC Solenoid there is a valve:



In that illustration you can see there is 1 wire coming off of the VTEC Solenoid, when there is no voltage, the valve is open. When you supply 12V to the wire, the valve closes. In the first VTEC solenoid diagram, you can see where it bolts up to the cylinder head. The oil from the rocker shafts pass through the VTEC Solenoid as it exits the rocker shafts. When you supply 12V to the solenoid, the valve closes and oil pressure builds in the rocker shafts. It builds to above 57psi. This oil pressure pushes the pistons out of the center rocker arm, locking all three rocker arms together.

Awesome! To sum it all up, VTEC is oil operated. At higher RPM’s the ECU supplies the VTEC solenoid with power which closes a valve. Then oil pressure builds in the rocker shafts, which flows to the middle rocker arm, and pushes out the 2 pistons locking the set of 3 rocker arms together. When the 3 rocker arms are locked together, only the center rocker contacts the center lobe on the camshaft. The center lobe is more aggressive and gives you the power boost that you can feel!!!


As complicated is VTEC is, I made it look really simple. So, if VTEC is so cool, why isn’t it on all the time?! And why cant I just buy an APEXi VAFC and set VTEC to kick in at like 2,000 RPM? Well, I’m sure you’ve all seen those muscle cars at the track. When they are sitting on the line at idle, the motor runs really rough and sometimes will sputter and die. Aggressive camshafts love to have a ton of air. They work great in high RPM’s because we’ve got the air started down the intake and it has a nice velocity that feeds the engine. It wouldn’t be suitable for Honda or any car manufacturer to produce a car that ran bad at idle.

On the Other hand, camshafts that will idle really good, suffer at high end. They don’t have the extended duration that can aid performance in a vehicle. But hey, they idle great! Honda made the best of both worlds, it’s like having a conservative camshaft down low that idles well and gets good gas mileage, and an aggressive cam for high RPM’s.

The last thing I want to cover is IAB. So just what is this 2nd intake runner everyone talks about?! Well, on the B18C1, H22, and I believe the H23 (if not more applications), there are 8 intake runners. If you look at the intake manifold for a B16 or Type R motor even, you will see that there are 4 runners for the intake. I am not to dispute which is a better design, but how it works is like this:



It’s really hard to see, but if you can locate the “Intake Air Bypass Valve Body” (IAB) you can see the 2nd set of holes for the intake runners. I am sure you’ve peered inside a throttle body, as you step on the throttle the plate swings open and closed… same concept. There is a little black vacuum tank located under the intake manifold. When the ECU sends a power signal to the IAB Solenoid, it opens a valve located on that black vacuum tank and vacuum is removed from that little snail looking thing on the “Intake Air Bypass Valve Body.” When that snail thing is not under vacuum, the plates swing open and allow more air into the engine.

This is another good idea by Honda. What it allows for is the long runners are only supplying air to the engine at low speeds. This helps with the low end torque, longer runners means greater velocity, equals good for low end. In high RPM’s (about 5,750 RPM for the B18C1 motors) the IAB swings open and now you’ve got shorter runners which can help supply more air. The shorter runners will make less velocity but higher pressure, this makes for a good mid-range and top end.
 

BioHazard the Reaper

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#7
wow that just cleared mine and so many more questions up...i appreciate it, and i loved the diagrams, looking at pictures is far easier than trying to picture them in my head...i appreciate your guys help
 

Intelica350R

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#10
Sorry this is so long but I think the whole story will help.

OK maybe you guys can help me on this one.

I just had a 2000 JDM type R (B18C)(OBD2) motor and tranny installed into my 1994 Integra GSR (OBD1). The ECU that was installed was an OBD1 reprogramed/revamped to the JDM OBD2 settings and it worked for just one week, but with a juddering problem (like Rev limit or out of gas) in VTEC that slowly worked its way from 8500 RPM's down to 5000 RPM's over the week untill my car died when I last hit 5000 rpm's, and dead is where it is now. The car would not read me a diagnostic code, just a solid check engine light. I thought it may be a fuel problem, so I replaced the Fuel Pump with a higher volume Walburo one, and then the Main relay, but the car still wouldn't start, it just keeped cranking, so I checked at the fuel rail for fuel and it was getting it fine. I then moved to spark and ended up replacing my very worn Cap, Rotor, and Plug Wires, and found that the distributer ground wire was loosely connected to its ground and I tightened it. The Plugs looked a bit fouled but I think it was from me stepp'n on the gas to much when trying to start it, but I checked for spark at each plug and they were sparking fine on all cylinders. The car now sputtered out the exhaust very quietly on every rotation of the engine as it cranked, but would not start. I checked the diagnostics again after disconecting the battery and resetting the ECU, and it ran code 21, which is the VTEC Soleniod Valve. I checked this out and one of the two wires comming off of the lower valve housing harness (Green Plug) was split real close to the plug and only 3 strands conected it. It was so close to the plug that I could not resplice and butt connect it, so I pulled the wires out of the harness (accidently breaking it) and slid each wire onto the corresponding prongs. This did not change a thing, and the code still flashed. Do I need the harness to ground somthing or what, because I figured I could get it to atleast start, I am trying to find a new harness now. Also would the VTEC Solenoid Valve or connection be preventing the engine from starting???? It makes scence that it was causing the vtec problem and maybe the reason why it died when I hit 5000 RPM's the last time, but I don't see why it would prevent starting. If it is the Solenoid Valve and not just the connection, can just the Valve be replaced or does the whole Vtec solenoid need replaceing (or are they the same thing)? How much does one of these run and how hard is it to replace? I would not be asking all of these questions but my shop manual does not cover this section. Thanks for any help Dan
 

v-teced

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#13
VTEC!!???........WTF!!??.......I thought it was VtecH!!!.....wow!! ou learn sumthin new everyday
 

Rick James

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#14
Re: Re: Question on VTEC

Originally posted by slambed_civic
i know my vtec kicks in round 4k.... thats when the second cam profile comes in and keeps power...
so then does that mean that in the DOHC it acts as a third cam
 

vi3tboi714

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#15
Originally posted by v-teced
VTEC!!???........WTF!!??.......I thought it was VtecH!!!.....wow!! ou learn sumthin new everyday


pwahahahahahaha ^^^^^ quote of the day mang, lol
 

TurboZinc

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#16
Re: Re: Re: Question on VTEC

Originally posted by eminemslm0
so then does that mean that in the DOHC it acts as a third cam
Each cam would have a seperate profile, so it wouldn't be like a third cam, but more like a second "set" of cams.
 

Beelzebubba

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#17
How about a Cliff's Notes Explaination:

VTEC makes your engine schizophrenic. Sorta' like Dr.Jekyll and Mr. Hyde...except for vtec-e which is like Dr. Jekyll and Dr. Jekyll with a Red Bull.
 




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