Lynn or Bill,
Having a quick look at the web sites, I see the choke does restrict flow and
increase venturi effect, which is good for sucking up fuel from the float
bowl. However without maintaining that restricted size, through to the
combustion chamber, the flow returns to a slower flow. So I don't see a
benefit for increased VE without the smaller sized manifold inlet tube (
matching the choke size) OR a reducing diameter manifold inlet tube (a style
Lynn mentioned in an earlier post).
Or am I wrong?
George ( down under)
Any time we change anything in a dynamic system, we remove energy. So we need to avoid changing anything. The only source of energy available for us to push air into the engine is the difference between the low pressure generated by the engine spinning and the local (Ambient) air pressure.
As you see the carburettor has to have that venturi in order to develop a dramatic difference in pressure in order to function. The rush to fuel injection is the result of folks noticing that with no venturi, or choke in the inlet runner higher velocities could be maintained. In other words we avoid the energy removed by the velocity changes induced by the choke. The flow speeds up through the choke, (Dropping some energy) and slows again when past the choke (dropping more energy).
Here is the answer to part of your question. The velocity past the choke returns to nearly the same speed as before the choke. Energy is lost for sure but not much. I also point out that the amount of flow you can get through a small well designed choke (Weber wrote the book) is enormous. So wide open throttle performance is very close to fuel injected engines. About 6 HP on a 250 HP base.
A 36MM choke can deliver engough airflow for 120 HP (one housing) a 44MM choke can deliver enough airflow for 155 HP (one housing)
And here we see the change over to fuel injection is because we can build a slightly smaller engine with the same power as the early carbed engine. We can also do many electronic tricks like shut off fuel flow on spool down. We can skip injections and run engine's with dead cylinders to save fuel. We can run engines out of coolant, and use fuel to cool cylinders not being fired. So the carb was dead in short order for street cars just because fuel injection is cheaper and works better for nearly anything, so long as there is electrical power.
At any rate the throttle body can be slightly oversized, so as to not change the velocity of the incoming flow, and so, not remove any energy. And if that is the case, the flow at whatever velocity can continue into the port face. So on identical engines the injected engine will have higher velocity and make more power.
You can imagine that higher velocity right where the rotor shears of a slug of incoming air as though it was a near solid, like sausage. So, the higher velocity gives you a longer piece of sausage during each open port session. At 6,000 RPM, 100 RPS, how long is the port open? If the port were open for some great portion of that time? about 10 milliseconds per revolution and the port is open less than half of that?
Check the math for me I'm old.........
So time is of the essance. Our sausage links are too short.
So if you work only on the intake system. And we have fuel injection with a throttle body slightly bigger than the runner diameter, or a carb with about a 36MM to 40MM choke size, we want to maintain the diameter from air cleaner to within about 8" of the port, then taper smoothly to the port shape. Leaving the shape in the irons stock. Remember curves remove energy, the taper will be removing energy, and the choke in the carb will be removing energy. The very slightly over size of the throttle body is only to account for the butterfly and shaft in that unit.
By increasing the velocity we overcome some of our short links. We could increase port timing to allow for more crankshaft degrees of port open time, but that adds bowl volume and volume reduces velocity.
This works wonders but removes mid range performance. In cars. Planes don't use the mid range, so this might work out very nicely. And if all else fails to generate a Viking departure, we overpressure the intake system with a turbo or mechanical air pump of some sort like a Roots blower. Now our sausage links can be real long.
Why is the Periphery ported engine so much better at making power over the side ported engine?
Even though the port timing is recorded in crankshaft degrees, this is misleading. Neither the intake or the exhaist port ever closes. That little apex seal going by does not close off anything. It may change the direction of the flow but does nearly nothing to stop it. It is turbine like in its air consumption. Just add lots of fuel and hang on. The sausage just gets little dents in it. One long link till you shut it off.
Lynn E. Hanover
My bridgeport opens at 110 degrees BTDC and closes at 85 ABDC
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