[nkaminar]

Warning: Technical discussion

When the exhaust valve opens there is a pressure pulse that travels down the exhaust pipe at the speed of sound. When it gets to the end it reverses as a negative pulse. The negative pulse will suck exhaust gasses out of the cylinder. The length of the exhaust pipe determines the timing of the scavenging and thus the rpm where it is most effective. In a race car you will find all the exhaust pipes in a header the same length and joined together with an added length of exhaust pipe (no muffler). The joined pipes reinforce the scavenging effect from each header pipe. A defusing cone at the end can widen the negative pulse and thus increase the effective rpm range.

Usually the cam is ground to take advantage of the scavenging effect. A stock Model A cam is not ground to use this scavenging. There may be some advantage but not like in a racing engine that has been designed for the scavenging effect.

The intake manifold can also be designed to take advantage of the pulses there. In this case there is a negative pulse that travels up the intake manifold and reverses as a positive pulse which helps push more charge into the cylinder.

the speed of sound in the exhaust and intake systems is dependent on the temperature of the gasses. It can be calculated but it ends up being cut and try for the final lengths and designs.

In a stock Model A engine getting the exhaust out is not a major issue, unless there is a restrictive muffler. Having an open exhaust header may help a bit but not much. I would guess maybe 2 horsepower, but that is just a guess.