With force induction you can't have alot of valve overlap right?
If you do you loose alot of boost through the exhaust system.
But the fun cam lobes of a V-Tech engine have $h!t load of overlap.

So then how do you turboed guys compensate for that?
Do you dial out the valve overlap with adjustable cams gears?

Please do tell, spread the


[Modified by vwandhondaboy, 5:38 PM 12/26/2001]

 

You are correct.

 

Correct! The stock VTEC lobe overlap is awesome for supercharging, nirous or N/A, but isn't too bad for a turbocharged application. Dialing in the cam timing like you mentioned is the best solution to reducing the overlap. On many turbocharged engines, import or domestic, a near stock cam is most commonly used. Engineers may play with the duration and lift, but the ammount of overlap is usually kept to a minnimum. Beleive it or not, the area in the intake and exhaust system that has the greatest pressure is between the exhaust ports and the turbocharger (exh. manifold), even greater than the pressure that the intake manifold is seeing from the turbo. So if "too" much overlap is introduced, when both the intake and exh. valves are open, the exhaust gasses will literally be pushed back into the cylinder and possibly as far up as into the intake runners.
But like I said that's under extreme overlap.

 

Even adjusting the cams isnt ultimately enough.
I understand Crower has solved the H22a turbo problems with a set of cams for it

 

Does dialing out overlap have any adverse effects?
Like hotter exhaust temps(exhaust cam advanced), or lowered effective compression by later closing intake valves(intake cam retarded)?