I guess that I don't need to explain why flathead ie. side-valve engines are so rare nowadays. So, I'll leave the history for historians.
I see potentials in flathead engines. Some of them are very unusual.
- Pop-up pistons. These may be used to increase compression ratio without damaging breathing.
- Roller cams and lifters. These reduce friction losses.
- The turbulence grooves on combustion chamber's squish area, by Somender Singh. These are used for improved fuel mixture.
- Variable valve timing (VVT). Due to concentric cam shafts (by Mahle and Mechadyne) variable timing is possible also in cam-in-block engines such as OHV and side-valve engines.
- OmniValves special intake valves to improve low rpm torque.
- Cup valves, especially exhaust valves, could be literally cool exhaust valves. The cup valve design enables higher compression ratio and valve cooling with lubricating oil. See the valve patent by John Hutchinson and later patents by Michael L. Clements.
- Combining VVT and OmniValves could lead to a throttleless design, because adjusting intake valve timing radically in a flathead engine does not cause a collision between valve and piston. Why use a throttle if you could do that with the intake valve timing?
- Using all those possibilities could lead to incredibly high compression rate. Sounds like a case for laser ignition.
- US 6,237,579
- US 6,598,577
- US 1,250,463
- US 6,328,012
- US 6,601,558