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What does VTEC sound like?

Joined
23 November 2001
Messages
728
Location
Lancaster, PA USA
My friend has an Integra Type R. When he hits VTEC, It feels and sounds like he threw a switch; a very pronounced change in sound, and a shove in the back.

With my NSX, I do not feel or hear a pronounced change as I climb through the revs. Power seems fine, but I've not been in any other NSX to compaire it with. Is the transition on the NSX much smoother, or should I really notice it?

At what RPM does it kick in?

How can you tell if its working?

Thanks. I'm still a newbie exploring his car.

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keep the shiny side up
MikeC 01 #46
 
I believe the profile for VTEC is much more subdued than other more modern VTEC engines. I feel a significant difference in power in the S2000 vs. the NSX, and the dynos of the two cars reflect this.

-- Chris

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http://www.NSXClassifieds.com - free NSX classifieds site!
 
The high-RPM cam profile kicks in on the NSX from 5800-6000 RPM depending on various factors.

It is much more subtle on the NSX than many other Hondas like the Preludes and Integras.

An easy check is to watch your oil pressure as you climb through 6000 RPM, then maintain a steady 6500 RPM and you should see the oil pressure drop a bit.
 
I use Pyroil Oil Treatment (a viscosity index enhancer), and now when I pass into VTEC, the drop in oil pressure is notably less than it was before just running Mobil 1 10w30 or Castrol Syntec 10w30...

But, yes...your oil pressure will drop a bit when you get into VTEC, since the VTEC is operated by oil pressure.

-Andie
 
Ok you guys peaked my interest. What do any of you think the difference is? Different cam profiles, or different V-Tec design, or both.

[This message has been edited by nsxxtreme (edited 07 March 2002).]
 
MikeC
At what RPM does it kick in?

The TIDH(I LOVE this book and read it whenever I have time to learn and be fascinated by the engineering of the NSX) states that the secondary intake plenum is activated at 4800 rpm so I assume VTEC(Variable Valve Timing and Lift Electronic Control) is activated at the same time.


How can you tell if its working?

If you can rev to 8k rpm, it's working.
smile.gif



NSXXTRME,
What do any of you think the difference is? Different cam profiles, or different V-Tec design, or both.

One major differnce between the NSX system and other Honda VTECs is the NSX use of the Varialbe Volume Induction System. "This system uses a separate magnesium plenum, located beneath the main intake manifold. This second plenum is separated from the primary manifold by six butterfly valves. These valves open at approximately 4800 rpm and are activated by manifold vacuum."

Of course the cam profiles are going to be very differnt however VTEC is on the intake and exhaust side of the cams for both the Integra and the NSX.

Acura so generously provided the cam specs in the TIDH as shown in image 2 below.

tidh26.jpg


tidh31.jpg


HTH,

-Nader
 
Originally posted by nsxxtreme:
Ok you guys peaked my interest. What do any of you think the difference is? Different cam profiles, or different V-Tec design, or both.


Less difference in the cam profiles probably. The larger NSX motor probably requires less variation to get optimal high and low RPM operation.
 
Originally posted by Nsxotic:
The NSX actually has a two stage v-tec where the others do not. I believe it first kicks in at 4600 rpm and then the second stage is around 5600 rpm. Due to the two stages, they are more mild in transition and that's why you don't feel the change like in an Integra.

Uh... The NSX does not do anything special with valve timing, duration or lift at any other RPM. The VTEC system in the NSX is the same fundamental design as the Integra GSR, Prelude VTEC and DelSol VTEC. They all have a high and low RPM cam profile and switch between the two. On the NSX this switch takes place between 5800 and 6000 RPM as I stated previously. This is in the service manual if you would like to check.

I believe you are thinking of the Integra Type-R which has an additional "stage" at low RPM. I put "stage" in quotes because it is not not really VTEC per se, and it does not change cam profiles in this "stage." It is simply activating the second intake valve at 2500 RPM. This is not really VTEC since it is not varying the timing, lift or duration, it's simply turning "on" the second intake valve. The ITR engages the third cam lobe at 6000 RPM which is really VTEC.
 
I wish I had better pictures to show the Variable Volume Induction System (VVIS) but I’ll show what I’ve got:

Below is the picture of the magnesium intake chamber (lower portion of the picture). This chamber becomes part of the intake path when the butterfly valves open as manifold pressure rises above (vacuum decreases) a certain value (~4800 rpm on a stock motor).
aai1.jpg


In the upper portion of this picture, you can see the butterfly valve array. This piece bolts between the intake chamber and the intake manifold.
aaj1.jpg


So the VVIS tunes the engine for better high-rpm breathing AND better low-rpm breathing. It makes this switch around 4800 RPM. VVIS has nothing to do with cam timing or valve lift (VTEC)--only intake path geometry.

Between 5800-6000 RPM, depending on a few factors like throttle position and a few others I think, VTEC will engage the high-profile cams. Note: if the engine management system measures a water temperature below a certain value then the high-profile cams are not engaged and the redline fuel cutoff is lowered to safeguard the engine.

Some (most) of us use the word “VTEC” to indicate that the engine is using the higher cam grind—this is a misnomer. An engine either has VTEC (Variable Timing and Lift Electronic Control System) or it doesn’t. It doesn’t go in and out of VTEC. It’s understandable why this naming convention exists but if someone really understands how VTEC works they will probably not use the word VTEC to describe a specific cam grind. That is, VTEC is the system that switches between two cam profiles; it is not the name of a cam profile. But, it’s more sexy to say “when VTEC kicks in this car moves!” than to say “when the higher cam profile is selected by VTEC, this car moves!”

Blow is a picture of the 3 intake rocker arms for the #4 cylinder. These rocker arms can operate independently of each other OR they can be “locked” together as one. VTEC is in the business of locking and unlocking these arms—that’s it. When the head is assembled, these rocker arms are sandwiched between the cams and the valves. The cams actuate the pads on the top of the rocker arms and the rocker arms in turn actuate the valves. The two outside rocker arms are the ones that move the valves up and down—the center arm does not directly move a valve. In a similar non-VTEC engine, there would only be two rocker arms, but the design of VTEC is all about the center arm. It’s the high-lift profile cam lobe that operates this center arm. When the arms are operating independently of each other, the valves are opened and closed according to the cam profiles of the lobe above the rocker pad, these are the low-profile or smaller diameter cam lobes. BUT when the rocker arms are locked together, the higher-profile cam grind that actuates the center rocker arm is left to operate the valves.
aau1.jpg


Here are the cams. See how the center cam lobe is "taller" than the two on either side (ok this picture doesn't show it too well but the center is a little bit taller). This is the high-profile cam grind that the valves see when the rocker arms are locked together.
aas1.jpg


The arms are locked and unlocked by pins in a similar way to keyed door locks (well, kind of). Local oil pressure (controlled by VTEC) actuates these pins. If VTEC wants to lock the arms together then it increases the oil pressure that these pins see which pushes them away from their natural unlocked positions. In the picture below, just above my thumb is a little doughnut shaped thing where the pin of the middle rocker arm goes. This doughnut shaped thing is connected to a spring which is overcome by the oil pressure when the arms are locked. When VTEC drops the oil pressure, then the spring returns the pins to their unlocked position. The amount of oil required to move these pins is very small and in my opinion cannot come close to accounting for the sudden drop in oil pressure some claim to see on their gauge when revving into the VTEC switchover range.
aar1.jpg


Here’s a shot of a pin extending from the center rocker arm.
aaq1.jpg



Man, this got a whole lot more involved than I intended. Hopefully this post helped to clear up what VTEC does and doesn’t do, but I’m not so sure I accomplished this task. Please note the engineering precision required to lock and unlock these arms at 6000 rpm! Pretty cool, eh?

DanO

[This message has been edited by DanO (edited 08 March 2002).]
 
Originally posted by Lud:
Uh... The NSX does not do anything special with valve timing, duration or lift at any other RPM. The VTEC system in the NSX is the same fundamental design as the Integra GSR, Prelude VTEC and DelSol VTEC.



I just spoke with my NSX tech again today and he swears that the the NSX cams have three different lobes signifying the two stages of v-tec and 'normal' operation. The second lobes used at around 4600 and the third or highest lobes used during 'full' v-tec at around 5800 rpm. Hmmmm, I guess he's just a moron.
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Well, I don't know they guy so I have no idea whether or not he is a moron. But I do know that either you are misunderstanding him or his is wrong about this issue. If it's the latter I'd start looking for a new tech because if he doesn't understand this he doesn't understand a very fundamental part of how ALL DOHC engines function.

Three cam lobes do not equal three profiles. You would have two lobes even if the car didn't have VTEC because each one is controlling a valve. There are four valves per cylinder and two cams (the "dual" in DOHC) meaning each cam controls two valves per cylinder (two intake, two exhaust).

Below the 5800-600 RPM VTEC crossover point, the two outside lobes activate those valves like they would in any DOHC non-VTEC car. This is why I say if your tech does not understand that those two lobes are not "different stages of VTEC" then he just doesn't get how a DOHC engine works.

The difference with VTEC is that at that 5800-6000 RPM (on the NSX) the center arm is engaged by a sliding pin. This pin locks the outside arms to the center arm. The center cam lobe profile is more aggressive so the outside arms basically float above their les-aggressive cam lobes since they are locked together following the profile of the center lobe. Thus the center lobe changes the profile of the two outside arms, but does not itself directly control a valve, just the profile of the two outside arms which control one valve each.

DanO's writeup is quite good - read it. The diagrams and text from the TIDH book are also pretty good. The service manual also has a decent writeup with diagrams.

[This message has been edited by Lud (edited 09 March 2002).]
 
DanO,

I must say, VERY nice visual representation of NSX internals with some good TECH in their as well.

Was that your NSX motor?

Thanks for the pics and the more involved the better!
smile.gif


-Nader

[This message has been edited by Prancing Horse (edited 09 March 2002).]
 
The more I read the TIDH, the more GA-GA I get over the NSX.

Just to confirm the comments here in beautiful display...

tidh30.jpg



Lud,

I would like to thank you for taking the time out to scan all the pages from the TIDH! This book wonderfully depicts the R&D that went into this car. Truly a work of art...

What other manufacturer (other then the Prancing Horse mark
smile.gif
) goes into such meticulous, compulsive detail and shares it with their owners?
eek.gif


-Nader
I need to stop visiting this site otherwise I will have to resort to a meal a day and me moving out from my appt. to a futon at work to purchase a NSX. Hmmm...

[This message has been edited by Prancing Horse (edited 10 March 2002).]
 
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