[downest]

I tore down a D16Z6 today with a friend of mine. It was the first SOHC VTEC I've opened up. Why are there extra lobes for only the intake valves? On the DOHC motors, they are on both, right?

 

[crxgator]

on d16z6/y8 , it is only vtec for the intake valves...

 

[jfrolang]

On the DOHC motors, they are on both, right?

Most of the time. I think the only exception is the K20A3, found in the 2002-2005 Civic Si and the 2002-2006 RSX base model.

 

[bobski]

Why are there extra lobes for only the intake valves?

Well, think about the function of the intake vs. the exhaust valves. The exhaust valves allow hot, pressurized gas out of the cylinder. The intake valves allow air/fuel mix in at atmospheric pressure. Since the air mix is under less pressure, it's going to be more difficult to get it past the valves quickly. As such, a greater performance gain can be had by changing the cam's intake profile than the exhaust. Intake valves are larger than the exhaust valves for the same reason.
I guess they left the exhaust gear out on SOHC engines to save space and mechanical complexity. They are supposed to be economy motors after all.