General Fit TalkGeneral Discussion on the Honda Fit/Jazz.
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Can someone dumb down the explanation of an i-VTEC engine? I have found several sources on the internet that give 60 paragraph explanations of it, and none of it makes sense to me
I'm just trying to figure out exactly what it does different than a typical engine, and how it does it.
Thanks!
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__________________ 2007 NBP Fit Sport MT Honda Floor Mats || Honda Cargo Net || 35% tint || custom CAI || Blox shift knob || LED dome + cargo lights || Megan axle-back || Zeta dead pedal || "Dirty White Boy" air freshener :-D
Well unless you're looking into the 2009 Fits, then you'll learn nothing on your current Fit since it does not have i-vtec technology. However, vtec and i-vtec engines vary the cam profile to either better performance of the engine or better economy. i-vtec in the Civic Si changes the cam profile for more lift once it hits a certain rpm giving it more power. i-vtec in the r18 civic and on the 2009 Fit will "intelligently" retard the cam profiles to improve fuel economy and let it run at regular settings when more performance is needed. That's vtec in a nutshell, there are quite a few more factors that you could research.
The Fit has i-VTEC? Basically it's variable valve timing and lift. Some newer cars have this and VVT (variable valve timing) is becoming almost standard now days.
A dumb down answer would be it allows way more control over the engine so it can be more efficient and powerful.
One of the key factors in engine performance is valve timing. That means how long the valves stay open, as well as how much they open, during the intake and exhaust strokes. This is controlled by the shape, or profile, of the lobes on the engine's camshaft. Before the advent of variable valve timing, any given engine had a single cam profile to activate the valves. You had to choose between economy and performance, or make a compromise between the two. With variable valve timing, you have two specific cam profiles; one for performance, and one for economy. The engine's electronic control activates and deactivates the two different cams depending on input from the driver's right foot.
[quote=KCHoosier;351984]Can someone dumb down the explanation of an i-VTEC engine? I have found several sources on the internet that give 60 paragraph explanations of it, and none of it makes sense to me
I'm just trying to figure out exactly what it does different than a typical engine, and how it does it.
Vtec is indeed a very complicated process but I'll give it a try.
Vtec is one of several solutions to having two - or more - cam profiles to operate a valve. The valve lets the fuel air mixture into the combustion chamber or lets the combusted gases out of the combustion chamber. The combustion itself is what gives the engine power to move the car and controlling that valve is the job of the camshaft which has one or more cams called lobes for each valve.
A cam lobe is an oval shaped surface that pushes a valve down and controls the return as it rotates and the length of time the valve is 'open', or the time from starting to open til it closes. The ovality of the cam 'lobe', as its called, changes for better efficiency as the rpm increases. Vtec is Hondas way of changing the 'ovality' of the lobe by having two different profiles side by side and use one or two followers to 'follow' the profile of one or the other of the two profiles. That follower imparts the up and down motion from the cam lobe to the valve. At low speed the lobe moves the valve only a short distance and at high rpm not only 'opens' the valve a greater distance but also keeps it open longer. That is it starts to open sooner and closes later the higher the rpm. Which follower is operated is chosen by the compurter. By this process efficiency of an engine is increased greatly so both power and mpg can be improved. hondas vtec is only two stages so there is only low rpm mode and a high rpm mode which is certainly an advantage.
The Fit has i-VTEC? Basically it's variable valve timing and lift. Some newer cars have this and VVT (variable valve timing) is becoming almost standard now days.
VTEC is a Honda thing. VVT, I believe, is Toyota. Different means to the same end.
__________________ 2007 NBP Fit Sport MT Honda Floor Mats || Honda Cargo Net || 35% tint || custom CAI || Blox shift knob || LED dome + cargo lights || Megan axle-back || Zeta dead pedal || "Dirty White Boy" air freshener :-D
Ive seen some people on the forum use the phrase "keep it out of VTEC" when referring to improving fuel economy. Is there a certain point, say RPM, or something when that happens? And would it make a difference?
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08 A/T Sport NBP | 20% Tint All Around | K&N Short Ram Intake | Floor Mats
On the Fit vtec is always on >= ~3400 rpms. Below that it may or may not be depending on throttle input and other factors.
BTW, unless I'm mistaken, the current gen Fits do NOT have variable valve timing at all (only 1 cam profile). This will be available in the the 09's. VTEC, as used in the 07 and 08 fits, refers to how many valves are opening - the valve timing/cam profile is not affected at all:
At low RPMS, only 1 intake valve/cylinder opens.
In VTEC/high rpms, both intake valves/cylinder open.
So at any given time, the engine is running with either 12 or 16 valves opening. I'm pretty sure that's it - no cam profile changes in the current engine.
I'm sure someone will let me know if I'm missing someting.
On the Fit vtec is always on >= ~3400 rpms. Below that it may or may not be depending on throttle input and other factors.
BTW, unless I'm mistaken, the current gen Fits do NOT have variable valve timing at all (only 1 cam profile). This will be available in the the 09's. VTEC, as used in the 07 and 08 fits, refers to how many valves are opening - the valve timing/cam profile is not affected at all:
At low RPMS, only 1 intake valve/cylinder opens.
In VTEC/high rpms, both intake valves/cylinder open.
So at any given time, the engine is running with either 12 or 16 valves opening. I'm pretty sure that's it - no cam profile changes in the current engine.
I'm sure someone will let me know if I'm missing someting.
"If we look at this principle in relation to whether a VTEC mechanism is full 16valve 3-rocker arms or a 12v/16v 2 rocker system, then we can see that there is no physical law that dictates that one must be for power or the other must be for fuel economy. The 16v 3-rocker system adjusts valve timing whereas the 12v/16v system actually adjusts valve sizing."
__________________ 2007 NBP Fit Sport MT Honda Floor Mats || Honda Cargo Net || 35% tint || custom CAI || Blox shift knob || LED dome + cargo lights || Megan axle-back || Zeta dead pedal || "Dirty White Boy" air freshener :-D
"If we look at this principle in relation to whether a VTEC mechanism is full 16valve 3-rocker arms or a 12v/16v 2 rocker system, then we can see that there is no physical law that dictates that one must be for power or the other must be for fuel economy. The 16v 3-rocker system adjusts valve timing whereas the 12v/16v system actually adjusts valve sizing."
Yep - thanks for the pointer.
Performance wise, what the valve sizing does is:
At lower RPMS, a small valve size increases the intake velocity, makes for more turbulence, which in turn gives better a better air/fuel mixture and better combustion.
At high rpms the piston is already sucking in the air very quickly. A bigger size makes sure enough air can get in in the much smaller time that the valves are open.
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