Erland Cox wrote: ↑Tue Sep 08, 2020 5:25 pm
The area in the exhaust port is usually smaller than the area in the header.
And a valve in the middle if the port blocking the port and forcing the wave to turn around it in all directions will make
the wave see the the valve as a restriction and reflect with the same sign.
See it like this, if you flow the header backwards without the the head it will flow more than if the head is mounted with the exhaust at full lift.
I basically disagree with you.
Yes, the exhaust port may be smaller than the header primary in many cases, but especially in a single cylinder turbo engine I see no reason why it should be. For example, in my car, I have two 34mm exhaust valves and 40mm head exhaust port matched to a 40mm cast exhaust manifold. Working it out with throat sizes, stem diameters, etc, the cross sectional area with the valve open is about constant.
Contrast this with the 100mm bore and about 500cc cylinder and combustion chamber volume at the relevant crank angle degree. That’s a big increase in the cross-sectional area and a geometry that works pretty close to an ideal helmholtz resonator. I think that the open exhaust valve is going to send out, at least initially, the opposite wave.
I’m more interested in pressures and less interested in the flows that these pressures cause at this point. I think the wave is going to mostly just propagate around the open poppet valve.
Now, there are people more experienced with modeling these things and they might be willing to chime in and tell us who is right.