UPDATED 8-15-06This 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.
Mods, if you could, please sticky this and leave it open for further discussion/questions. Now, on to the meat of this post.
Ever wonder how Honda SOHC VTEC (Variable Valve Timing and Lift Electronic Control) works?
Well, 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 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 much about is i-VTEC, utilized on 02+Si's, rsx's TSX, newer Accords and CRV's.
Here is a link to a VERY good write up on i-VTEC:
http://www.superhonda.com/foru...24797
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.
Modified by Red_EM2 at 2:05 AM 8/15/2006
Modified by .Red.EM2 at 10:57 AM 10/20/2006
Sola gratiaFlickr|DeviantArt
i see, 
© 
