This, is VTEC explained.

[Miracle]

UPDATED 8-15-06 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. 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

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[gitrdone]

so the vtec controler aint worth it? what if i changed my fuel pump and injectors?

[Miracle]

The most benificial 'all motor' Change you can do to your car, is to change the fuel system to a returnable system, which really isnt worth it anyways on the D17, but if youre willing to go all out, and spend some futile money on the D17, then thats your perogative.

But if you wanted to, then maybe a walbro fuel pump, the swap to a returnable system, aftermarket fuel rail, bigger injectors, a better Intake Manifold, an adjustable FPR, Camshaft, valvetrain, cam gear and VAFC would yield some decent gains. But thats if you want to dump 2 maybe 3 grand into the motor. Which youre going to have to do anyways if you go boost'd. Or you could wait for a K to show up, and then youre really free to do anything.

Its really all up to you.

[gitrdone]

i see,

[Pnoymugen]

Good write-up on the sohc vtec. Good Job!

[Yoshimi]

Nice one, Red.

[MidShipCivic]

Originally Posted by catch_me_if_u_can

so the vtec controler aint worth it? what if i changed my fuel pump and injectors?

To need any of that you need to have your engine taking in more air than normal.

[EgSiCknEsS]

can you explain to me how the vtec controlller works?
i have a ls-vtec all motor runnin a p28 jun chip....will that benefit my car to run better?

[03EM2guy]

nicely done

[k24em2]

Just FYI, the D17A2 VTEC system only uses two intake lobes. It basically runs 12 valve operation until the crossover point then goes to 16.

[HondaRacing3]

I have a D17A1 and I might look into installing a Vtec head. I've read what people have to say on other topics, however my friend told me that if I did this I would need a full swap kit, new oil lines need to be drilled for the head, a new ECU, and my check engine light will always be on b/c it would be "chipped". I've noticed no one has adressed this issue yet. If I can find a used headswap for around 300 would you recomend this, I'd like to hear if anyone has done this to their car. This is my first car so I dont wanna screw it up guys lol


Modified by HondaRacing3 at 5:24 AM 7/18/2006

[k24em2]

It isn't worth the effort or the money.

[prelude_vtec99]

Where exactly is the VTEC crossover point on a D17A2? I'm hearing different things like 2 stage where 2000rpm all intake valves kick in and then at 4500 VTEC engages? It's too bad there's no distinct VTEC note with the D17...

[HondaRacing3]

yes 4500 i believe

[k24em2]

Originally Posted by prelude_vtec99

Where exactly is the VTEC crossover point on a D17A2? I'm hearing different things like 2 stage where 2000rpm all intake valves kick in and then at 4500 VTEC engages? It's too bad there's no distinct VTEC note with the D17...

No there is only one crossover, where all the valves are engaged.

[Doebmx1]

very good write up, wish would have been able to read this when i was learning..

[MidShipCivic]

There's always a different sound, and k24em2 is right.

[asdfsad]

I thought that i would add a little to this thread while asking a few things. I have found a few videos illustrating how Vtec works. These two videos basically reinforced what you have explained.

http://videos.streetfire.net/s...a.htm

http://videos.streetfire.net/s...a.htm

OK well here goes my newbie questions. This is my first vtec car/honda i have ever owned and I was trying to understand how Vtec works a little better. I have a h22a4 DOHC prelude and I was wondering one thing, how is the sound Vtec "makes" created?

[Miracle]

Originally Posted by asdfsad

I was wondering one thing, how is the sound Vtec "makes" created?

The sound youre hearing is the engine uder a heavier load of air and fuel. Its when the locking rods engage in the rockers that VTEC begins, and allows more air in the motor, which is the noise you hear.

BTW, VERY good find on the videos. Im going to put those in the OP if you dont mind.

[asdfsad]

Originally Posted by Red_EM2

The sound youre hearing is the engine uder a heavier load of air and fuel. Its when the locking rods engage in the rockers that VTEC begins, and allows more air in the motor, which is the noise you hear.

BTW, VERY good find on the videos. Im going to put those in the OP if you dont mind.

haha thanks for the explanation Btw how many posts does one need to have before the messaging function works?

nvm works and yea put it in the OP so people can see how it works

[xenoturkey]

The only thing I would like to add to this sticky is that honda most certainly did not invent VVT (In essence the mother of vtec). GM had the first version of VVT in 1975. Otherwise, good technical explanation.

Just FYI.

[Rich EX]

great info for a noob on how Vtec works, but we have Vtec-E (economy) engines and more helpful to know how much Vtec-E sucks before we get our hopes up and then disapointed when a stage 3 cam gives us about 12 hp. 7thgencivic.com has a stickie on how all SOHC Vtec engines work. from 0 to 4500 one intake valve rides the primary and 2nd valve on secondary, after 4500 they both switch to primary, exaust stays the same.

[ghost_ryder35]

Very nice writeup

[xquiset]

nice post

[Miracle]

Originally Posted by xquiset

nice post

Originally Posted by ghost_ryder35

Very nice writeup

Originally Posted by Doebmx1

very good write up, wish would have been able to read this when i was learning..

Originally Posted by Pnoymugen

Good write-up on the sohc vtec. Good Job!

Wow, all positive comments, Im kinda suprised on my first write up.

Looking good. If you guys have anything to add, like a different type of VTEC, or whatever, throw it up here. It needs to be Honda VTEC related though.

Lets keep this rolling.