VTEC discussion (from iVTEC sticker thread)

[MrX]

Originally posted by Jeff TYPE R
actually, the reason ppl remove vtec under high boost applications is because the aggressive lobe is TOO aggresive...theres so much overlap that the boost basically forces the uncombusted mixture out the exhaust valves before the exhaust valves have a chance to close.

So with a less aggressive cam profile or a custom grind where overlap is minimized, you may be able to take advantage of the vtec technology on boosted honda/acuras.

[350hp_or_Bust]

Originally posted by Jeff TYPE R
actually, the reason ppl remove vtec under high boost applications is because the aggressive lobe is TOO aggresive...theres so much overlap that the boost basically forces the uncombusted mixture out the exhaust valves before the exhaust valves have a chance to close.

Oh yeah... didn't think of that.

Originally posted by MrX


So with a less aggressive cam profile or a custom grind where overlap is minimized, you may be able to take advantage of the vtec technology on boosted honda/acuras.

Thats what I was just thinking too .... except a less aggressive profile would negate the purpose of having VTEC ... but a custom grind with high lift on the secondary lobes with either a shorter duration than the stock ones or advanced just enough to prevent much valve overlap would be where its at.

[THREE40SEVEN]

Valve overlap is mainly determined by lobe seperation angle. The higher the #, the less overlap-not duration.

[350hp_or_Bust]

not for DOHC

[THREE40SEVEN]

Originally posted by 350hp_or_Bust
not for DOHC

Explain to me why that would make a difference- 2 cams vs 1?

[350hp_or_Bust]

there is no such thing as lobe separation in DOHC ... not in the conventional sense anyhow.

for DOHC, there's typically one cam dedicated to the intake valvetrain, and one dedicated to the exhaust ... valve overlap is mostly a function of cam timing/synchronizing/degreeing ... i.e. you can decrease overlap if you degree the exhaust cam a few degrees retarded ... or the intake cam a few degrees advanced, or both (valve overlap will be reduced the same amout of degrees as the degrees you advance/retard the respective cams) ... the problem then lies in the duration cause if you advance the intake cam too much for example ... and your duration is too long, it will open the valve too early .. same story for the exhaust cam.

In the case of SOHC, or conventional push-rod valve trains (OHV) .. the exhaust AND intake profiles are cut on the same shaft, and (as I'm sure you know) the advance/retard of the intake and exhaust events relative to each other are determined by lobe separation.

hows that? sometimes I'm ass at explaining things