[JohnNH]

I have always heard different stories of when the VTEC engages to the secondary cam profile for improved performance at higher engine speeds/load. Some people say you can feel it when it kicks in (I can't say I have), others say it is at 3000RPM, etc.

Now that I have a scan tool that reads back the VTEC solenoid status, I thought it would be fun to find out how often the VTEC is actually enabled during normal driving conditions. What I discovered is that VTEC engages very regularly even in normal (not aggressive at all) driving conditions. At anything but the most modest acceleration from a stop (for example turning on to a 45MPH type road) the VTEC engaged. And going up most medium size hills (even at minimal speeds, say 30MPH) VTEC often engaged. So, unless you have a very soft foot during acceleration and live in a very flat area, then you are probably engaging VTEC very regularly during your normal driving.

Here is an example of accelerating from 6MPH to 26MPH in about 6 seconds. This is a pretty modest acceleration, probably typical for most drivers. In this case, VTEC engaged at 2200RPM and 16MPH:

It appears that the VTEC activation is often based largely on engine load (not RPM). The following graph shows the same acceleration with calculated load value plotted (I believe derived from mass air flow data):

Here is another, slightly faster acceleration from about 10MPH to 45MPH. VTEC engages at 16MPH, 2345 RPM and 91% calculated load:

The VTEC disengaged at 44MPH, 2108RPM, 71% calculated load:

It is also interesting to note that this vehicle has both VTEC and VTC (variable valve timing control). The VTEC adjusts the intake valve lift and timing by engaging the secondary lobe pattern on the camshaft. In contrast, the VTC only changes the timing of the intake valves. The VTEC is an on/off control. It can only be fully on or fully off whereas the VTC is a continuous adjustment (the intake valve timing can be adjusted over a range of different values).

VTEC and VTC each use separate solenoid valves which control oil pressure to make their respective adjustments. The "spool valve assembly" or "VTEC solenoid valve" at the back of the engine is the solenoid that controls VTEC (engages the secondary intake camshaft lobes). This solenoid is controlled by the PCM so that it is either fully off or fully on. The valve assembly also has an oil pressure switch which is used by the PCM to ensure that the solenoid opened. The oil pressure switch is on the camshaft side of the solenoid. So when the solenoid is closed, there is low pressure at the oil pressure sensor. When the solenoid opens, the oil pressure at the sensor is high. This provides a check that the solenoid is working and that the oil pressure is there to engage the secondary cam lobes. If the oil pressure doesn't change when the PCM opens the solenoid valve, it knows there is a problem and flags a DTC.

The VTC solenoid is controlled to the target opening via a pulse width modulated signal (a continuous string of on-off commands where the amount of on and off times are varied to set the average opening time of the solenoid) from the PCM. This allows the VTC solenoid to be opened (on average) the correct amount to generate the desired intake valve timing.

Here are the locations of the two oil control valves and their respective filters:

 

[RickBlaine]

@JohnNH: As always, thank you for a well-written and researched post. I was literally just thinking about this earlier this morning- If the VTEC is not working, would a Check Engine Light (Malfunction Indicator Lamp) come on, or not?

I am wondering about filters #5 and #11 in your last illustration- if those filters are clogged, will a CEL come on?

My old Civic was like a rocket when the VTEC kicked in. My Accord, I just don't "feel" it. All joking aside, I am wondering....

Other members: Does YOUR scan tool read VTEC signals? If so, what do you have?

 

[t-rd]

Frequent VTEC engagement might be the case with the I4, but not so much with the V6. With the V6, there is a distinct difference between non-VTEC and VTEC. mode. The engine is considerably louder also. In normal driving, the V6 is very smooth and quiet. I've driven my cousin's 2003 I4 and it's a louder engine to begin with. But VTEC has not changed since it's inception. Controlled by oil pressure then a set of secondary pins engage which allow the lifts of the secondary larger cam lobes for higher valve lifts. It's still genius in this day of electronic valve controls, VTEC is still mostly a mechanical operation. It gives you power when you need it and saves gas when it's not in operation.

We need someone to do the same test on the V6.

What tool(s) are you using?

 

[JohnNH]

: If the VTEC is not working, would a Check Engine Light (Malfunction Indicator Lamp) come on, or not?

Hi Rick,

In almost every case, I believe so. For example, if the VTEC filter was clogged, then when the VTEC solenoid was opened the pressure on the downstream side of the VTEC solenoid valve would not be sufficient to set the pressure switch and the PCM would know that the VTEC didn't activate and would certainly flag a DTC. The same thing if the VTEC solenoid didn't activate. Of course there are probably some more unusual (contrived?) cases where it might not. For example, if there were a blockage downstream from the oil pressure sensor, that would obviously not be flagged by the oil pressure switch. I am not sure if the PCM would be able to detect such a failure from its other data, but I would guess not. But that would certainly be a much less likely failure mechanism...

For the VTC control, there is even better error sensing, since the PCM directly monitors intake cam position via the cam position sensor and can immediately detect if the target valve timing is not reached.

 

[JohnNH]

What tool(s) are you using?

These data are from the Autel MaxiSys MS906. It has a few little quirks (and a few functional deficits, in my opinion) but it is a very handy tool. It supports a lot of manufacturer specific codes. On my 2007 4CYL Accord, some of the data I really value (and couldn't find on most other scanners) are:

oxygen sensor heater currents
idle target
egr command and actual lift feedback
fuel injector timing (pulse widths)
knock sensor output
evap system
catalyst monitor
misfires (by cylinder) -- This is pretty key
ABS wheel speed sensors - so you can look at each sensor output to diagnose problems

plus, it does have some "activation" or functional tests. For example, you can do things like force the cooling fan on or off, or force the fuel pump on or off, which are great for debugging. You can turn off individual fuel injectors. It also has pretty good support for body electrical (you can get feedback in lbs of the weight on the passenger side seat weight sensor, control feedback of electronic locks, etc.) I think you can program TPMS codes (into the ECU, but you still need a reader to read the IDs off the TPMS units) and do idle learn procedure, but I haven't explored these yet to know if they work...

 

[bobbo]

I think people forget that today's iVTEC is very different from the late 90's VTEC ... I mean the engineering wizardry may still be very similar, but the iVTEC on my K24 is really meant for gas savings - whereas the VTEC on my B18 was outlandish boy racer goodness.

It probably doesn't make the situation any more clear that (someone correct me if I'm wrong) our K24's only have a VTEC cam lobe on the intake, and not intake and exhaust. A "true" VTEC, kick you in the pants experience, has both - like an RSX Type-S's K20.

As someone who's daily driven both a B18 and H22, the feeling of proper VTEC engagement from 6500 to 8500 rpm was unmistakable - the most deafening and push you into your seat experience I've ever had in a 4 cylinder coupe.

I've never felt anything remotely close to that sort of VTEC crossover while driving Betty.

 

[mtts60]

I have never been able to tell when it is on or off.

 

[Amar]

well in my v6 I can hear it engage at like 5 or so thousand rpm (honestly when I floor it I'm looking out for police not at my rpms lol) but when I was reading the manual it said theres 2 vtec's one at 2100 or so rpm and one at like 5k one for fuel efficiency and one for performance

 

[carlson03]

I have never been able to tell when it is on or off.

Same here

 

[demonics12]

Economy IVTEC is a more advanced and more higher defined type of VTC. Cars like ours phase Vtec only around 30 degrees, while more aggressive Vtec, like in the RSX type s and Civic Si, are closer to up to 60 degrees advance. This, and the fact that the intake and exhaust cam are being phased, is why Vtec in other cars may be more prominent. My numbers may be off, going off of what I recall.

This is why the type of Vtec "push" like in a newer Si or Type s just doesn't even compare to ours, as the cam is being advanced much further, increasing the power and sound that you hear when it fully engages.

 

[bobbo]

@JohnNH - amazing write up mate

 

[specV2006]

Vtec on the 2013-2015 6-6 accords comes on on the intake cam, and does so at 4900 rpms. You can feel a extended pull at high rpm however, the j35 has very good low to mid range power as well so the difference in feel won't be the same as a 2.0L k20 because off cam performance is terrible compared to on vtec performance on a 4 cylinder.

 

[boomercarguy]

Recently, Wheeler Dealers had an episode in which they restored a Honda S2000. They had great pictures of the inside of the engine and the dual cam operation when the VTEC "kicks in". It is simpler than I thought and fantastic engineering. It reinforces the reason for keeping clean oil in your engine, even though that was not the original problem with this particular car's VTEC not working.

 

[jfuredy]

When I had my 96 Civic EX Coupe I was curious about VTEC engagement as well. What I did to monitor it was connect a piezo-electric speaker (like this one) to the VTEC activation signal coming out of the ECU. I connected the red wire to the activation pin and the black wire to a grounded pin. This way, even while concentrating on driving, I could HEAR when the ECU was turning on the VTEC system. If I remember correctly, it came on any time there was a significant engine load and the RPM was above 4X00 RPM. So if you were cruising at a constant speed or decelerating, the VTEC wouldn't activate, even if it was above the RPM setpoint. (This method would only work with the VTEC (on-off control), and not the VTC (modulating control).)

I haven't bothered to do this with my V6 yet, but maybe someone else wants to test it out?

 

[teezythakidd]

Very cool read! Thanks! @JohnNH

 

[03EX]

I think people forget that today's iVTEC is very different from the late 90's VTEC ... I mean the engineering wizardry may still be very similar, but the iVTEC on my K24 is really meant for gas savings - whereas the VTEC on my B18 was outlandish boy racer goodness.

It probably doesn't make the situation any more clear that (someone correct me if I'm wrong) our K24's only have a VTEC cam lobe on the intake, and not intake and exhaust. A "true" VTEC, kick you in the pants experience, has both - like an RSX Type-S's K20.

It depends on the type of K24.

The K24A4 found in our Accords only has a mild 2-lobe VTEC on the intake cam, and the motor also operates in 3-valve mode at low RPM. This results in 160hp @ 5500 RPM

The K24A2 found on the TSX is a "true" VTEC motor with VTEC on both intake and exhaust cams = 205 hp @ 7000 RPM (in the 06-08 TSX)

The current-gen Civic Si has the K24Z7, which is a direct-injected motor that has only VTEC on the intake cam, but still manages to produce 205hp @ 7000 RPM

 

[Coach Steve]

The only time I really ever feel or notice the VTEC engage is when I'm getting on the freeway, although it's very subtle. It's sorta like the way a turbo kicks in or if you imagine a very small shot of nitrous. Very subtle but when it does engage, the acceleration rate increases noticeably.