Today I am looking at the head combustion chamber, valve, and valve cage. I start by pulling in a model of a valve I have used before and tweak it to fit. This is used in the Wallaby, a 1″ stroke and 1″ bore engine.
The first thing I find is that there is not much room for the lip on the valve guide in the Offy cone shaped combustion chamber. In the past I have used a light press fit with Loctite to retain the valve cage in the cylinder head. Terry used a bit looser fit and then relied on a steel pin to retain the valve cage in his Offy.
The second thing I see is that due to the large angle of the where the manifolds mount, and the angle of the combustion chamber, there is not a straight shot for the intake charge or exhaust gasses in and out of the combustion chamber through the valve cage.
The way the side hole is drilled in the valve cage, the edge disrupts the gas flow in and out of the combustion chamber. OK, I thought, I will install the valve cage in the head without the side hole drilled, then drill the hole through the head and the valve cage at the same time. This approach is shown below:
And then the resulting hole through the side of the valve cage would look like the following:
I don’t like the shape of the resulting valve cage above. The lip is too thin and could deform over time. Also it is a risky machining operation where the valve seat could be damaged. I rejected this idea.
Another idea I considered for a moment was eliminating the valve cage and fabricating the seats directly in the cylinder head. I rejected this idea as it would be too risky, I would hate to reject a head because I messed up one of the valve seats. I like to test the sealing of my valves and valve cages before I install them in the head.
I also looked at altering the angle of the hole in the head that interfaces to the hole in the side of the valve cage, I tipped it up, tipped it down, but did not find an orientation that significantly improved the air flow.
As can be seen above highlighted by the circle, the valve will hit the top of the piston. I can rectify this by raising up the combustion chamber or by having a more complex shape at the top of my piston. The original had a tent top shaped piston. Whatever I do, my compression ratio will be affected and I need to take this into account.
The head is quite riddled with holes and there is very little area to run coolant. The two red circles above highlight the only area that is really available.
I might be able to get away with a smaller valve and valve cage. The one I started with is from an engine with a 1″ bore and stroke, where as the Midget Offy model has a cylinder volume about 56% of that. The diameter of the cylinder is 75% (.75″/.1″) so theoretically I could get by with a smaller valve and valve cage. Below is a valve cage from Westbury’s seagull that has a .75″ bore. This is from the Model Engineer Magazine dated September 14,1950.
I love Westbury’s drawings, everything is done in fractions, nothing in thousandths.
I know bigger throat and valves are better, but it is good to have another data point.
If I increase the size of the combustion chamber I need to understand the ramifications on my compression ratio. So I repeat my modeling of the space inside the cylinder at top and bottom of the piston stroke.
When using a flat top piston I get a compression ratio of 4:1, a bit too low. So I model a piston with a tented top. I also add the space at the top where the spark plug is and the area the valves extend into the combustion chamber.
By adding material to the top of the piston I can increase the compression ratio from 4:1 to 9.4:1. What this tells me is that I can increase the size of the combustion chamber to give additional clearance to the valves and adjust the compression ratio within a reasonable range just by changing the shape of the piston top.
So, where did I end up today?
The above cut away sums up where I am with the combustion chamber, valve and valve guide.