Goal: More power and more fuel economy (through the use of an overdrive unit coupled with the higher power).
Done through:
- Higher volumetric efficiency
- Better/more complete combustion
Limiting factors
- Flathead design
- Head material: Cast iron has a thermal conductivity of 55 W/(m*K). This is rather poor, and means that cast iron has a hard time transferring heat from the chamber to the coolant, making for hot spots in the chamber, which cause detonation if compression is too high. This in turn is one reason why compression is traditionally kept low.
- Flow: Air-Fuel mixture and exhaust gasses have to pass through a U shaped channel, and through the choke point of the combustion chamber. This impedes volumetric efficiency.
- Flame propagation: The combustion chamber is long and irregular with the spark plug in one end, which causes the combustion to push air and fuel in front of it as the piston is pushed downward. This causes detonation of the unburned air-fuel mix. To prevent this knocking, compression is traditionally kept low in flatheads.
- Chamber size: The combustion chamber is long and irregular with a huge surface area relative to the chamber volume. This has one of two effects - either the chamber surface leads to a lot of heat dissipation (and thus energy lost to the cooling system), or hot spots. Thus compression is kept low.
- Valve placement: Due to the valves at the edge of the chamber, the chamber can't be made smaller in that end due to the risk of shrouding of the valves.
Proposed solution
- Head material: Aluminium has thermal conductivity of 204-250 W/(m*K). This is, in the worst case, four times as efficient as cast iron.
- Flow: Porting of the exhaust and intake runners should be easy - hogging out the intake isn't productive for making low-end torque, but the exhaust should flow as freely as possible. Larger valved runs the risk of more excessive shrouding - so none of that so far. Relieving of the block between the cylinder and the valves(the transfer area) shows limited results only, whereas raising the roof over the transfer area makes good power in flatheads. This should be compared to the lowered compression caused by a larger chamber.
- Flame propagation: Shortening the combustion chamber overall length could minimize the effect described above - notice the difference between this original head and its chamber design and this "high performance" head - the original head has a chamber that extends up to 40% into the cylinder diameter, whereas the HP head has a far more "square" chamber (extending less into the cylinder diameter), promoting a more uniform and faster combustion. This, in turn, makes for less tendency to detonate, allowing for more compression. Theoretically. The compromise is of course the transfer area, that means a perfect circular chamber cannot be achieved.
- Chamber size: See above:
- Valve placement: Not a darn thing.
- Baked plaster of Paris with 20% talc added in to prevent cracking, which has been baked to prevent exploding of the casting. Ideally the mold should be multiple use.
- Water jackets; three ideas:
1: Solid casting with drilled-out water jackets. Takes time, only allows for round water jackets that can be drilled from the outside of the head somewhere. Risk of cracking the casting or drilling through the casting.
2: Two-piece casting: Allows for very detailed water jackets, but complicates the process since the mating surfaces need to be machined. Possible leaks around the spark plugs.
3: Cast-in water jackets - hard to make in the dimensions needed by the size of the head, a risk that the material is stuck in the casting. A new water jacket negative would have to be constructed for every head casting.
- How big are the stock water jackets in the head? I need to find out whether aluminum benefits from the same size water jackets, or whether it benefits more from thicker material and less water jacket volume.