what would happen if the cam shift point was moved from 6k to 3k or lower, besides more gas being used? would the car just be really, really powerful all the time? is this easier accomplished with a vafc?

 

Honestly this is a shot in the dark because i dont know much
but what i would assume is since the lower engagement of vtec it would lose top end power or you would need ridiculous secondary cams or something to make that an advantage

 

Actually, that wouldn't really help. People would have done it or even Honda would have done it if will achieve superior performance.

In any case, the purpose of VTEC is so that you two sets of intake/exhaust valve timing profiles at different engine speeds. I'm not a mechanical engineer myself, but to make it simple, and correct me if I'm wrong, the reason of having a higher lift/longer duration profile @ higher engine speeds for your valves is because of, more of less, air velocity. You will need to open your valves for a longer period of time (relative to the piston's cycle) only at high speeds, since your combustion cycle has shortened in time, but air doesn't necessarily flow much faster when you do that. To compensate for that, they have increased the overlap in the opening "moments" of the intake valves on the more aggressive cam profile (which is not needed when the combustion cycle is longer at lower engine speeds. Sufficient air will flow through when the valves are open).

I apologize for not being able to articulate this very well, but te conclusion is, having VTEC kicking in at 2000rpm wouldn't exactly help, since it's not needed, and would defeat purpose of VTEC. VTEC isn't anything magical, it's a simple concept for internal combustion engines and their valving timing related issues, that lets the car idle well (if your engine uses VTEC at idle, well, that's not ur original question, but just more of any fyi, your engine would have a really rough idle) and to have good fuel economy at low (engine) speeds and allow it to achieve more efficiency and higher engine speeds. It's not quantum physics, it's just that Honda has done a such good job in making it right and tight. Honda doesn't really create any ground breaking technology IMO (well, besides the attempt of ATTS, 4WS, VGS?) they don't create generations of turbos, 4WD, or any super hi-tech traction system for their vehicles. Rather, they spend their time making sure everything's where it's supposed to be at its cost, like having a fun manual transmission that doesn't feel like rubber or rotary engines that break even after 30 years of research and production. They just use basic engineering ideas ...

 

There are basically TWO cam profiles on a VTEC motor. The primary cam has a smaller grind (less valve lift and duration) and is made for more torque in the lower RPM range...1000-6000 rpm. The secondary cam or VTEC cam has a larger grind (more lift and duration) to make more HP in the upper RPM range...6000+. Each cam has and RPM range that it makes the most power in. Altering the VTEC to come in earlier would not be benificial because VTEC just does not make much power below 6000 RPM. If it did Honda would have designed it to engage earlier.

 

Dam KS320...you must have been typing the same time I was.

 

S2KSpaYellow:

Haha yup! Well, I made the message in 1 minute before you did But then, looks like you made it into S2ki one day before I did

 

I would think it would stall the engine....too much fuel at that RPM. The air flow would not be greatly enhanced, because the engine should suck the same amount of air at 3k RPM no matter what the cam setting, because the engine will not need anymore, as it is not going faster. The VTEC, as I understand it, is just to let the engine breathe better at high RPM. The car does not need to breate that much at lower RPM's, and the air will not be "forced" into the engine if the valves are open longer.

I would think, based on my logic, that the engine would just run rich, and the ECU would cut the fuel flow to compensate, thus basically eliminating the aggressive cam timing. I think this is the point of the O2 sensor. Cant force more air in by just opening the valve.

John

ps- Please let me know if this logic is off base. I am just starting to learn about the internals of the engine, and am looking to learn above all else.

 

Honda set VTEC engagement at a specific rpm for a reason. Even with heavily modified engines you will find the ideal VTEC engagement point is very close to where Honda set it (i.e. my 320hp turbo Civic ideal point was still around 4200rpms). I would suspect that modded S2K's are also finding stock VTEC engagement at 6K to be the best. Some aftermarket cams may prove to be different though.

 

Vtec alters the valve opening overlap, ie the time that both the intake and exhaust valves are open at the same time.

The reason for it is that the exhaust gasses rushing out actually help to pull in the fresh charge of fuel and air, and allow it to mix better. But the downside is that some of the fresh intake charge can also be expelled which would cause pollution and and general inefficiencies. There is a lot more to this but i am simplifiying the little that i do know.

The benefits of greater overlap are best realized at higher rpm's, and if the extended overlap were present during idle or lower than ideal rpm range it would be more detrimental than beneficial.

The stock vtec engagment point can be lowered, but 2k or 3k is way too low.

 

There is a scavaging effect when the valves open and close. Opening them sooner/later with more/less duration CAN effect the amount of air going into a cylinder at the same RPM.

I am by no means an expert on the subject either.

For anyone that wants to learn about how an engine works and how cam profiles, heads, etc...can affect engine performance you should pick up a copy of DESKTOP DYNOS. This is a cool program that allows you to change all kinds of specs to see how the dyno chart changes.