Ever wonder how VTEC works?

[Red EM2]

This is for the ones who do NOT know how VTEC works, and for the ones who DO know, please correct me if it can be proven that I have made a mistake on this anywhere. Seeing as this is my first Honda, I think I did ok on this, but please, correct me if I'm wrong.

This thread is designed to teach you the operation of Honda's, and possibly imports all together, greatest innovation.

Ever wonder how Honda SOHC VTEC (Variable Valve Timing and Lift Electronic Control) works? The DOHC VTEC works generally the same, but you will get the idea.

With a little research, here is the best I can explain it.

I am assuming you all know how a camshaft operates the vales on a car? A concentric lobe rotates on a central shaft, hitting the valve, opening it, allowing air to enter the combustion chamber. It turns out that there is significant relationship between the way the lobes are ground on the camshaft and the way the engine performs in different rpm (rotations per minute) ranges. The grind on a camshaft is often referred to as its 'profile'. The larger the profile, generally, the more power you can make.

VTEC was developed by Honda as a mechanical and electrical system to allow a Honda engine to essentially have two cam profiles, in one camshaft, for optimal engine performance through all RPM's.

Like its DOHC counterpart, SOHC VTEC, which is explained in this article, optimizes the flow of fuel/air mixture for high RPM operation while maintaining smooth and economical low RPM operation.

In a SOHC VTEC motor, the camshaft sits centered in the head, so both of the intake and exhaust valves are engaged on the same camshaft.
There are 3 camshaft lobes on a VTEC camshaft. The three cam lobes in the middle are the intake cam lobes. The two low RPM (the shorter of the 2 profiles) lobes actuate two valve rockers, which in turn pushes the intake valves open.



The high RPM lobe actuates a follower, which is shaped like a valve rocker, but doesn't actuate any valves, under normal conditions. While there are different intake cam lobes for high and low RPM operation, the same two exhaust cam lobes are used for all RPMs. The lack of cam profile changing for exhaust valves is the primary difference between DOHC VTEC and SOHC VTEC engines.

Below is a picture of the rocker arm assembly in a DOHC VTEC motor. Notice the valve operating lobes on the out side, with the VTEC engagement rocker in the middle. When VTEC actuates, a rod connects the 3 together, "making" VTEC.



To make VTEC happen, the ECU has to realize the following parameters from the sensors on the motor:
1 Your MAP (Manifold Absolute Pressure) sensor must read 0hg (aka 0 inches of vacuum). When it reads this it will send a 3volt signal to the ECU.
2. Your throttle has to be open 100% because your TPS sensor has to send a 4.5-volt signal to the ECU.
3. Your coolant temp has to be at running temp.
4. Vehicle speed has to be above 20mph.
5. You have to have 65-80 oil psi.
6. The engine must be running at a minimum of 4500 RPM.

During low RPM operations, the two outer (smaller profile) intake cam lobes directly actuates the two valve rockers, Just like in a normal engine. These low RPM intake lobes are optimized for smooth operation and low fuel consumption. The high RPM intake lobe rides on and actuates the high profile follower, But since the follower isn't connected to anything, it doesn't cause anything to happen, yet.



At the higher RPM ranges, at a point determined by the ECU, the 2 rockers for the valves, and the middle follower for the high profile LOCK together, allowing the 3 pieces to ride on the big profile on the camshaft, enabling the motor to ingest more air, and in turn, more fuel, for more power. since the middle camshaft lobe is higher, when VTEC engages, it lifts the 2 valve rockers off of the camshaft all together.



Now, on to the performance aspect of VTEC.

On a stock motor, equipped with a factory fuel and ignition system, there is no need to change the VTEC point. Because with the introduction of more air, more fuel is needed to prevent the motor O2 levels to go lean, in turn burning a valve, or otherwise damaging the motor.

To optimize VTEC, it is wise to introduce into the equation a larger fuel pump, larger injectors, and a controller, to change the fuel and ignition parameters of the ECU. Apex Integration, along with other performance parts manufacturers, make control boxes suited to just that purpose. You can see a image of the APEXi VAFC (VTEC/Air Fuel Controller below.



Essentially, what that does, is introduces user programmable fuel maps, and VTEC engagement points. A stock motor really doesn't benefit from one of these, as there is no way to introduce as much fuel at the nessecary RPM to combat detonation and the engine running lean.

Companies like GReddy, AEM, Hondata, and TurboXS produce user programmable interfaces, to allow for tuning of the ECU fuel maps to compensate for the lower VTEC engagement point.

Here are some videos to demonstrate in a 3d model how VTEC actually works.
Video 1
Video 2

A varible of VTEC I do not know too much, but am learning steadily, is i-VTEC, utilized on 02+Si's, RSX's (Base model and Type S), the Acura TSX, newer Accords and CRV's and the Honda Pilot.

i-VTEC introduced continuously variable camshaft phasing on the intake cam of DOHC VTEC engines. Valve lift and duration are still limited to distinct low and high rpm profiles, but the intake camshaft is now capable of advancing between 25 and 50 degrees (depending upon engine configuration) during operation. Phase changes are implemented by a computer controlled, oil driven adjustable cam gear. Phasing is determined by a combination of engine load and rpm, ranging from fully retarded at idle to maximum advance at full throttle and low rpms. The effect is further optimization of torque output, especially at low and midrange RPMs.

Below, you can see what the cylinder head on a K series i-VTEC looks like. Notice the timing CHAIN, instead of a belt. That is an integral part of the i-VTEC function.






I hope this helps, and if you have any questions, please IM me, or post it here.

[AudioGuy93DelSol]

Excellent write-up. One correction though:

Originally Posted by Red EM2

Your throttle has to be open 100%

It's actually 50%.

[00accord44]

Did I just witness a noob post a meaningful, in depth, useful, thought out post? Hath hell frozen over?

Way to rep for Team Noob!

[BLU CIVIC]

he got it from www.howstuffworks.com although everyone that wanted to know has gone there...or any orther Honda based website...

[hondacivic99sivtec]

whoopie do. you can copy and paste. i'm with blu on this. way more ppl have done a search and found this on their own.

[00accord44]

I guess you're right. But I'd still rather see a noob post a thread like this than "Whut is a terbow" or "I beat a Veyron with my Civic" or the dreaded "Don't you guys think insert domestic car heres suck!"

[Gohan Ryu]

Hey RedEM2 here's a link for you.

http://en.wikipedia.org/wiki/Plagiarism

I hope this helps, and if you have any questions, please IM me, or post it here.

[Red EM2]

So you guys are trying to convince me that I cut and pasted that?

Ok then... Find that document WORD FOR WORD with pictures and I will believe it.

I got alot of the information and pics off the net... But the typing is original... It is my work. So blow off.

I did not spend 3 years in school, and 5 years more learning to have people call me a liar, so if you don't have anything to contribute to my thread, don't post. Since this is my first Honda car, I decided to try and help people understand a pretty cool technology such as VTEC. So to all the haters,

At least when I did this on Honda-Tech, people didn't call a guy a liar and crap. And I thought H-T was a little off when it came to stupid posts, but thanks anyways.

[somick]

Chill out Red Em2!

You did a pretty good job! I did enjoy your article!
Everyone is entitled to his own opinion. And there is no need for cursing.

They did not create that kind of post. You did.
Thank you,

Sam

[BLU CIVIC]

No to sound like an ass or something, but have you looked at the questions in the Honda section? you said you spent 3 years in school and 5 years more learning right...imagine if i started explaining to you how a combustion chamber worked...you'd look at me like i was retarded or something...the same way we are looking at your post

there are over 75,176 threads and 584,847 posts in this Honda section alone (i say "this" b/c there is an unmoderated section) and surely someone has posted "How vtec works"...but i value your contribution and hopefully someone gained some knowledge from you post...that is all

[Gohan Ryu]

It's still plagiarism - slightly rewording the work of several other people and compiling it into one document doesn't get you off the hook.

I'm not hating, most of the info you provided is accurate. But you should provide links or at least make reference to the original sources of information.