I wonder if some of you tech types could explain a few things to me:

1) HOw does variable valve timing improve performance?

2) Why wait until 6,000 RPM for VTEC to "kick in?"

3) How does the functioning of i-VTEC differ from VTEC (I know that the end result is a more usable torque curve - but how does it work?)

 

Search button haha, but i'll indulge a bit and give you a brief overview.

Original vtec stands for variable valve timing and electronic lift control. It's what you would call a "cam changing" system. At a given point in the RPM range a mechanism changes the cam to a different set of lobes. Basically it's like having an extra set of cams in the drive train. I can go into the details but the basics are that since cams control vavle overlap/stagger, and lift duration you can have a cam setup for low rpm power and one for high rpm power instead of having ot have just one setup for blended performance. when coupled with an ecu to control air/fuel it's obvious why there are big gains to be had. Cam changing leads to very big power gains but only in certain portions of the power band.

I-vtec is new to honda but is a much more common valve control methodology. It's also known as cam-phasing. It allows continuous adjustment of the valve stagger and ignition timing to give a small but solid gain across the entire rpm band. Usually on the order of 5-15lb feet depending on the size of the motor in question.

Right now the only car makers to use both cam phasing and cam chaning in north america are honda and porsche. Toyota/nissan also have both but only in JDM cars and aren't likely to bring them to NA.

The question of why cam change at 6000 rpm is a complex one. It really has to do with the engine tuning, air fuel maps etc and what the honda engineers thought was the best rpm for making power and also being safe. If you were to switch the cams too early to the high lobe you would lose power, if you do it too lat eyou also lose power. That being said most honda cars benefit from a slight lowering in vtec crossover of a few hundred rpms. I'm betting the tsx is no exception and a lower switchover would get rid of that nasty vtec dip.

 

To put it simply, the optimum timing used to open and close valves at rpm x is different than at rpm y. So a cam tuned for low rpms would be very bad at high rpms, and a cam tuned for high rpms (like a race cam) would be very bad at low rpms.(if you hear cars with very aggressive cams, they almost sound like they are going to stall at idle. I'm sure you have heard this in the big muscle cars, etc).

So what vtec does is it allows the engine to essentially change to more aggressive timing at higher rpms, thus giving us the best of both worlds.


Check this link for more.

http://auto.howstuffworks.com/question229.htm

 

VTEC has only ONE changeover between cam lobes. That changeover very distinct in the B, H, and F series Honda motors. I believe the stock RPM changeover for VTEC motors is 5k-5.5k RPMS. I-VTEC however, has 3 progressive lobes that makes the changeover smoother and less distinct. There's alot more to it than that though.

 

This is incorrect. It seems many people think there are multiple vtec points. But there is only one, whether its vtec, or ivtec. (There was another thread on here a while ago discussing the same thing. ) The difference in i-vtec is that it change the phase of the cam...basically moves it slightly...to make minor changes along the rpm range untill eventually the vtec cams kick in.


EDIT: here is the other thread...

http://www.acura-tsx.com/forums/show...&threadid=1791

 

fdl... i think he meant there are multiple lobes and not multiple rpm changeover points. Meaning the lobes get phased in after the vtec rpm crossover point is reached.

 

Wow. This is soooo helpful. Thanks guys!

 

This is also incorrect.

 

there are 3 lobes but 2 are low RPM profiles and thr 3rd in the center is the high lift high RPM profile.
the low RPM lobes have separate rockers 1 for each lobe.
so there are 3 lobes but only 1 changeover point.

the intake came is valiable with respect to the crank and can vary the timing of the intake cam between 0 and 25 degrees advance and 25 retard for a total of 50 degrees of adjustment

 

Exactly. In other words no different than the old vtec. The thread I linked above goes into alot of detail on the subject.

 

All VTEC and I-VTEC motors have multiple lobes. The cam phasing in the I-VTEC is being controled at the cam gear (or cam sprocket). Have you seen some of the cars with the cut valve cover to expose the timing belt/cam gears? The reason is that they can tune the cam timing with the adjustable cam gears. However, they adjust them once (on the dyno) to get the optimal gain, and usually leave them as it (manually adjustable). In the I-VTEC, this adjustment is being controlled by the ECU. This is was Honda engineers call it "Intelligence", therefore the "I" in the I-VTEC.

Regarding the comment on only one VTEC crossover point on all the B, H, and F series motors, yes, that's correct. However, the B18C1 found in NA Integra GS-R have a secondary butterfly which opens the butterfly valve in the intake manifold at around 6000 rpm to draw more airflow into the motor. VTEC crossover for this motor is at around 4400 rpm.

 

As a side note to what vwong was saying the rsx-s and j30a4/j32a2 also have multiple runners. If they were going to make an tsx with more power a multiple intake runner system would be a way to make a good bit more power both down low and up top.

 

um...doesnt Toyota's 2zz have both cam phasing and cam changing?? toyota's cars all have VVT-i which is just continous cam phasing..but the 2zz adds in lift as well for VVTL-i... its the engine found in the celica, matrix and the corolla XRS that is comming soon...i think you got mixed up with Dual VVT-i which is exclusive to toyota's japan line up...

 

I could definitely be wrong. I'll check it out. Dual VVT-I is like the dual vanos system BMW uses?

 

i believe so..it varies both intake and exhaust...here is SOMEONE else's explanation..much better job than i can do.

VTEC = variable lift/timing/duration system with two different profile cams for each valve which change the lift, duration, and timing. VTEC is NOT continuously variable, it is a staged system based on RPM.

VVTi = continuously adjustable intake cam shaft which allows for the timing to be adjusted according to RPM. Valve duration and lift is completely unaffected as the same cam lobes are used throughout.

VVTLi = continuously adjustable intake cam shaft, as in VVTi, but with a second cam lobe which engages at a preset RPM in the ECU. The second lobe affects lift and duration. Timing is still being adjusted by the variable camshaft.

iVTEC = Read VVTLi

iVTEC and VVTLi adjust the cam shaft timing in EXACTLY the same manner. So far all cam shafts are adjusted the same way, whether it be VVTi, iVTEC, or BMW's Vanos, they all use a screw type mechanism to adjust the timing. I'll post a picture of the system if anyone requests. THE ONLY DIFFERENCE BETWEEN THE LIFT ON VVTLi AND VTEC(or iVTEC) IS THE WAY IN WHICH THE SECOND CAM LOBE ENGAGES ITSELF.

VTEC uses hydraulic pressure and metal rods to connect up a second rocker arm being driven by the high RPM cam lobe. When locked up the whole rocker assembly is now driven by the high RPM cam lobe, thus changing the valves behavior to the high RPM cam profile.

VVTLi uses a freely moving rod in the main rocker arm to allow the high RPM cam lobe to not affect the valves. When engaged a small stopper pin is pushed underneath the freely moving rod, thus taking away the space it had to move freely in. The rod then touches the pin which allows the high RPM cam profile to affect the rocker arm and valve behavior. Think of it this way, get a pencil and hold it 1cm above a button. Now move the pencil down 1cm and then up 1cm, repeat forever. You will never push the button in. Now what if someone stuck a 1cm high block between your pencil and the button. You would now hit the block and in turn make the block push the button. Same thing with VVTLi's lift.

 

Yep you are right, the celica/corolla xrs uses this and is dubbed vvtLi by toyota.

 

Man ... I feel like I just had a class in school in this thread Good stuff guys

 

I agree.

I've sazid it before and I'll probably say it again:

Acura-tsx.com rocks!

 

i agree, i considermyself to be pretty knowledgable and i am impressed to see people actually know what they are talking about here. other boards i have been to people seem to be living under rocks.

 

wow, very class.

 

Two questions that has been bugging me for a while now:

Why not just always have the intake valves open (with maximum lift) during the duration of the intake stroke, and the exhaust valves open during the duration of exhaust stroke? Does it have something to do with the inertia of the air entering the cylinders at high RPM (ie. intake air is still entering the cylinder after the piston has passed BDC and is coming back up)?

Why do cars still have camshafts? Why not have each valve on its own solenoid (or any other high-speed linear actuator)? Then you could vary valve timing/lift in any way imaginable and you wouldn't be limited by camshaft geometry. Not to mention it would be lighter, and performance customization through ECU swaps would be limitless.

Thanks oh wise car gurus

 

good question, the reason is that a low lift shorter duration cam will produce more torque than a high lift one will. also the valve overlap needs to be increased under high rpm to get max airfuel mix into the cylender. if you do this at low rpm it causes the engine to "lope" just like nascar and drag race engines do.
when you get valve overlap it means that the exhaust and intake valves are both open for a short duration just before the intake stroke.
the way it works is that the exhaust gasses going out of the cylender are already in motion, since the gas has mass it has inertia. by opening the intake valve sooner than the intake stroke the exhaust gasses leaving the cylenter will tend to pull or suck more airfuel into the cylender just before the exhaust valve snaps shut. at the same time the piston is headed down to bring in a new charge. this causes a slight supercharging effect into the engine.

as for the valves with a solinoid, ford was developing a technology i heard about a few years back. they found they had to modify the electrical system to run on 36 volts because of the current needed to control the valves.
the engine does run but unfortunatly makes a hell of a lot of noise. they were working on a way to keep the valves more quiet from slapping into the head. but the idea is really innovative. and your right you would have total control over the vlaves.
but personally i think the internal combustion engine is just about as refined as they are going to get. i think technology will be developed further into fuel cell technology.

 

ok... so how exactly am i incorrect when i am agreeing that there is only one change over point yet there are multiple lobes... not just 2 sets.

and if you read the articles that you list it say the same thing.

 

Ferrari has a really neat way of doing this. The camshafts on some Ferrari engines are cut with a three-dimensional profile that varies along the length of the cam lobe. At one end of the cam lobe is the least aggressive cam profile, and at the other end is the most aggressive. The shape of the cam smoothly blends these two profiles together. A mechanism can slide the whole camshaft laterally so that the valve engages different parts of the cam. The shaft still spins just like a regular camshaft -- but by gradually sliding the camshaft laterally as the engine speed and load increase, the valve timing can be optimized.

Several engine manufacturers are experimenting with systems that would allow infinite variability in valve timing. For example, imagine that each valve had a solenoid on it that could open and close the valve using computer control rather than relying on a camshaft. With this type of system, you would get maximum engine performance at every RPM. Something to look forward to in the future...

 

Good info guys. Here's my question:
Could it do any damage or undesirable wear-and-tear to cross into the vtec just briefly before shifting? For example, sometimes I shift at 6100 or so, and it occurs to me that it might be better to either keep it completely out of the vtec range or else give vtec a chance to run its course all the way up to 7000. Is there harm in being a vtec tease?

 

it may not be long before the automakers of the world all but abandon the conventional internal cumbustion engine in foavor of this
http://world.honda.com/FuelCell/

also check this out
http://www.hondacorporate.com/fcx/

ok try that

 

Nope.

 

DEVO, I mean you are incorrect when you say "the lobes get phased in after the vtec rpm crossover point is reached". When you use the term "phased in" it sounds like one at a time, and this what the oneonline was saying and is definately not the case. But if thats not what you meant then I misunderstood you.


Bass, both your links are the same

There are already fuel cell powered vehicles on the roads today. In BC there is a fleet of buses running on this technology (supplied by Ballard Power). The hard part is to make refueling easy...and SAFE. This is going to take a while.

 

here is somthing i found on camless engines for your reading pleasure
http://www.ipd.anl.gov/carat/1998pro...la_results.htm