I'm sure you've heard people say this before. VTEC sucks. VTEC is just for fuel economy. VTEC isn't for performance. blah blah blah.
Well here is a thread you can point them to to show them otherwise. This is my explination of VTEC as I understand it. If anyone wants to add to it or change it, let me know and I'll be glad to check it out. Hope this helps people understand the reason for VTEC and it's existence....
The purpose of VTEC engines is to get more power out of a smaller engine that produces small torque, while keeping fuel efficiency. If you take a typical cam profile in say... a mustang GT and look at it, you will see that as you get to higher rpm, the power starts to drop rapidly and the engine falls on its face. The reason for this is because of the characteristics required to make such great low end power and torque. However, at higher rpm, that same cam cannot fill the engine with air/fuel rapidly enough to keep that power and torque. So there is your low end cam.
Now your high end cam. A high end cam has a greater lift and duration on the valves, allowing more fuel and air to enter the combustion chamber, thus making more power at higher rpm. However, at low rpm, this lift and duration would cause the engine to have extremely weak power as the engine is "saturated" with the fuel/air being forced into the engine. It also creates a really rough idle at lower rpm and is extremely fuel inefficient. There is your high cam.
Now. Take the low cam and throw it in a small displacement engine. (such as a 2.0 liter) Now that engine won't make anywhere near the torque of the 4.6 liter due to it's small displacement and smaller bore and stroke. However, tuned properly it can put out a respectable amount of torque for an engine that size. Example, S2000. It has roughly 76.x tq/liter, which is pretty good when compared to larger engines. So, now our car has respectable low end torque for an engine of its size. Now add the high cam. When the low cam starts to drop and lose its power, the crossover point (known as "hitting VTEC") is when an additional following arm/rocker engages, thus letting the high cam take over. This increases the lift and duration, and creating phenomonal power for such a small displacement engine. (100-120hp/liter)
The advantage to the high cam is two-fold. First, the car can have a high redline. This is because of the fact that the high cam continues to make power at higher rpms. The second part of that is the fact that it allows the car to have extremely aggressive gearing, but still shift at respectable speeds. For example, a car with a 14:1 1st gear overall ratio that only redlines at 6000rpm may have to shift at 32mph. Where a car with an 8000rpm redline and a 14:1 ratio can shift at 44mph because of the fact that the rpms keep going. The reason this is all important is because of the little amount of torque these engines make. Put it all together... Aggressive gearing multiplies the small amount of these engines make, yet the high redline lets them shift at decent speeds. The low cam gives decent torque for a small engine, and the high rpm cam makes great power at higher rpms.
So it's basically taking the best of both worlds,... a high cam and low cam in order to get performance out of a small displacement engine, yet keeping it fuel effiecient and reliable. Forced induction brings less fuel efficiency, and long term reliability is a factor as well.
By the way, VTEC does actually work to try to get more power out of the SOHC engines as well. It is just not as noticable due to the characteristics of the SOHC engine and the VTEC system it uses.
i have a EX (with VTEC) and i drove my friends LX (Non-VTEC) and i can tell you there deffinetly is a difference. My car when it gets to 5 grand goes a whole lot faster than his to redline. Also ask him (Panda) who's cars faster[IMG]i/expressions/demon.gif[/IMG]
As impressive as Honda's VTEC is, I am currently more intrigued by BMW's Double VANOS which is continuously variable and operates on both intake and exhaust valves. This gives BMW engines a broad torque band and high-end power.
Here's the short version for people to lazy to read his informative posts: Peak power in the higher RPM range out of a small displacement engine, with very little power in the lower RPM range. Tries to make up for a lack of torque.
Quote
[hr]Originally posted by: klam
As impressive as Honda's VTEC is, I am currently more intrigued by BMW's Double VANOS which is continuously variable and operates on both intake and exhaust valves. This gives BMW engines a broad torque band and high-end power.[hr]
well ya... its a BMW... god i love those cars... why cant they be less expensive
U suck![IMG]i/expressions/face-icon-small-mad.gif[/IMG][IMG]i/expressions/face-icon-small-blush.gif[/IMG][IMG]i/expressions/face-icon-small-tongue.gif[/IMG]
i hear all this crap about vtec and vvti and all that. all those systems are worth nothing vs. the porsche system that continually changes valve timing throught the entire rpm range.
actually, VTEC is one of three different lobes on the cam. there is the primary lobe (small), secondary lobe (medium), and the VTEC lobe (large). there are three sets of rocker arms for each cylinder, and they are always in contact with the cam lobes, but tak effect when a locking pin connects them to the valve.
Quote
[hr]Originally posted by: S2000man01
The reason this is all important is because of the little amount of torque these engines make. Put it all together... Aggressive gearing multiplies the small amount of these engines make, yet the high redline lets them shift at decent speeds. The low cam gives decent torque for a small engine, and the high rpm cam makes great power at higher rpms.[hr]
Yeah, sounds reasonable. At higher rpm's we need a cam capable of delivering a larger air/fuel mixture. At lower rpm's we don't. If you have only a high end cam the car will either have a high idle rpm, or will run like crap in the lower rpm's. If you only have a low end cam your car will run poorly in the upper rpm's. The EX's have a lower end cam profile that's lower than an LX's and a higher end cam profile that's higher than an LX's. An LX cam is somewhere inbetwean those two profiles. At least theoretically that's the way it should be. A VTEC engine will accelerate faster than a non-VTEC one. Even if the non-VTEC one has a cam profile that's just like the cam profile of the higher end VTEC one. That's 'cause the engine won't be efficient at lower rpm's and will accelerate slower. At least in theory.
