Originally Posted By: rationull
It seems like most econo-car engines, even those in the B-segment (Yaris, Accent, Versa for instance) tend to be DOHC these days. When manufacturers like Honda and Mitsubishi can get by with SOHC designs with 4 valves per cylinder (and still make comparable or better power than the competition), why is this the case? Given SOHC heads are smaller, lighter and (I assume) cheaper, why go DOHC for smaller, relatively low power engines?
Well, the assumption that SOHC is necessarily smaller/lighter/cheaper is flawed. Given that there are a multitude of ways to actually actuate the valves from the cam (bucket followers with shims, roller fingers with or without hydraulic lash adjusters, rocker arms with or without hydro lash adjusters, single rocker driving two valves from one cam, single finger driving 2 valves from 1 cam, individual finger or rocker per valve, etc.) Clearly there are a LOT of variables to play with, all of which affect cost to assemble, Noise-Vibration-Harshness (NVH), engine package size, and performance.
A good case study for you would be the original Chrysler 2.0 engine from the first Neon, as it came in both SOHC and DOHC flavors. When you peel away the packaging, a fundamental difference boiled down to the included angle between the intake and exhaust valves, and the effect that had on scavenging and flow. But then, that could have been altered by a different valve actuation method, too!
It really is too complicated to just say "SOHC is cheaper, DOHC is better." In fact, I suspect that the little throw-away super-eco cars tend to go to DOHC because it can actually be made very compact- often more compact than SOHC if you're willing to compromise on valve angle (and econo cars aren't going for ultimate performance anyway). It can also be made very cheap to produce if the very inexpensive method of having the cams act directly in-line with the valves is used, although that costs a little more overall engine height.
