Originally Posted by ludeboom
he was right about the 2 cams per engine, there are 3 lobes per 2 valves, but the theory behind VTEC is that you can have "2 different types" of camshafts in the head at one time. A normal profile cam, and a racing profile cam. The engine would idle erractically and under-perform overall if the high cam was used all the time
the point is to add performance when needed, and allow for driveability and emissions at the same time.
as far as the VTEC controller goes...you will want to get one that also allows for minor adjustments of the fuel/ignition curves. otherwise it could do more harm then good, by creating an excessively lean situation at a lower cross-over point. remember that honda uses MAP to determine air entering the engine, not MAF, so by opening the VTEC earlier the computer cannot enrich fuel enough on its own, since its measuring pressure and not actual airflow like a MAF sensor.
anyway, whoever said that opening VTEC sooner (creating a leaner condition) will cause flames at the muffler is wrong. flames out the tailpipe are categorized by an excessively rich running engine.
ie. when a high boost turbocharged motor spits flame on a shift, the blowoff valve is releasing a ton of air (boost) into the atmosphere, that air was already metered by the MAF sensor (a good amount of turbo cars have a MAF instead of a MAP), so the engine thinks its getting more air then it really is.
the result: an excessively rich situation, because the engine still thinks that X amount of air needs X amount of fuel to prevent detonation and make good power.
the popular example is the WRX which uses a MAP (like honda). this allows boost to be vented to the atmosphere since its sampling the intake air charge in the plenum or throttle body, after the BOV.
i would personally recommend the VAFC-II, its more functional in a small way over the VAFC-I, plus it changes color when VTEC engages, and of course its newer.
either way MAP or MAF, both have their shortcomings and strongs.
~boom
Er.... seems like there is widespread confusion about how VTEC actually works..... shame.
For a start, the engine does have 2 *camshafts*, as it is like many jap engines, a DOHC. One of these *camshafts*is the intake cam responsible for the two intake valves per cylinder, and one is the exhaust cam responsible only for the two exhaust valves per cylinder.
Each 2 valve pair is operated by a common set of *cam lobes*. There are effectively 3 *cam lobe profiles* per valve (or per two valves, but each pair run off the same set of three lobes so its the same thing). These are used in three stages across the rev range, not two stages!!! These cam lobes are called Primary, Secondary and Mid. The Mid lobe (commonly known as the *high* cam lobe etc) is the lobe which comes into play at the top stage of the VTEC operation. The VTEC system operates by changing which cam lobe is operating the rocker arm driving the valve, not by changing which camshaft is operating as you suggested.
Next, my point about the exhaust flames (which was just a passing comment btw).... Flames in the exhaust are most commonly caused by a phenomenon called "valve overlap" which racing cams and the high cam on the VTEC engines have. If you use these cams at low revs then you will most likely have flames from your tail pipe. Your point about the rich fuel mixture is true, but the most common cause is valve overlap.
I do plan to get the VAFC-II, for this exact reason of tweaking the fuel curve like you say...