You’re right, in general terms. That’s why four valve (typically DOHC) engines have become so common. The small block Chevy engine has avoided it, but not many others.
One can have a SOHC with four valves and a central spark plug, it takes some interesting rocker arms:
First, some examples. DOHC with 2 valves per cylinder, Jaguar XKE, Fiat 124 Spyder (60’s-70’s). Examples of SOHC with 4 valves per cylinder, Honda Civics and Accords (many), Chrysler 3.5L.
Honda has not had any issues with the optimum placement of spark plugs in its SOHC 4 valve engines.
As for higher intake velocities with a single intake vs. dual intake valves, that is true. The early V-tec engines, and maybe the current ones too. only open one of the intake valves fully at low RPM. It just cracks the other one. This causes much greater swirl in the combustion chamber during the intake a combustion strokes yielding better efficiency. At high RPM, both valves open fully.
Having 4 valves per cylinder yields a larger curtain area. The curtain area is the circumference X lift. The 4 valve arrangement uses smaller valves, but the total circumference of the two valves is about 50% greater than the two valve design.
The larger curtain area gives better breathing at high RPM for a give cam profile. A high performance 2 valve design needs to have a high lift and a long duration to breath adequately at high RPM, that sacrifices low end torque and a smooth idle. A 4 valve design can use a milder cam profile for good low end torque and still breath at high the high end. Best of both worlds.
The cam angle between the intake and exhaust valves can be continously varied with a DOHC, not doable with a SOHC although a V-tec SOHC can have two valve angles. Varying the cam angles widens the torque curve.
The cam angle is the angle between the peak of the exhaust lobe to the peak of the intake lobe.
You mean for more power?
More power and reduced emissions. DOHC allows independent cam timing adjustment, improving both power and emissions.
