Originally Posted by rugsr
I LOVE IT!!!!!!
I knew how to spell Vtecks....or vtek...or VTEC sorry(lol) and I knew what it stood for and what it did. BUT UNTIL now i couldnt picture it. DUDE NICE LINK. VTEC is truely nice
QUESTION HERE IF I MAY? Buy getting bigger cams you increase the length that the valve is open correct, which allows more air in for a bigger bang. SO does this increase the compression then, making bigger cams bad for turbo. lets say on a GSR?
Umm... let's back up a step.
Cam lobe profiles are defined by two separate factors - LIFT and ENDURANCE.
LIFT is measured in varying ways, but the basic principle is that a taller lobe will open the valve further, allowing more air (or exhaust) to flow into or out of the engine in
a fixed amount of time.
ENDURANCE is determined by the ramps on either side of the lobe's wear surface. A more gradual slope to each face increases the amount of time the valve spends opening and closing. This also increases flow by providing more time for air (or exhaust) to flow into or out of the engine over the course of a given 4-cycle combustion cycle.
Now that we've got those terms estabilshed, consider the difference in engine dynamics between a naturally aspirated buld and a turbocharged build.
With an N/A motor, the exhaust charge is shoved out by the upstroke of the piston and drawn through the header by virtue of tuned pipe diameters within the exhaust header and so on. On the intake side, fresh air waits at near atmospheric pressure, to be drawn in as the piston begins its intake stroke.
N/A cams feature high endurance and high-lift profiles to provide additional power by exploiting the benefit of valve overlap. The cam profiles intentionally increase the period of time when the exhaust valve is open at the end of the exhaust stroke and the beginning of the next intake stroke. By using refraction tuning within the exhaust header, a scavenging effect at high rpm helps draw more fresh air into the cylinder. The end result is more oxygen to be utilized in the combustion cycle, producing more power.
With a turbo motor, the dynamics are entirely different at high-rpm. Within the exhaust manifold, there is a significant amount of pressure due to all the exhaust gas waiting to drive the exhaust impeller on the turbo. Likewise, there's plenty of fresh air waiting to flood into the combustion chamber when the intake valve opens.
Given these circumstances, turbo cams take an entirely different approach than N/A cams. Valve overlap is eliminated and valve lift is increased to better expel the exhaust charge and receive the incoming pressurized intake charge.
So, it's isn't a matter of bigger versus smaller cams when turbocharging a motor. The issue is to match the cam profile to the method of tuning.
Class dismissed. :run: