Info on backpressure.

......yeah look anyone that writes articles and precursors it with the "Myth" statement is usually parroting someone else. If what he is saying is true EFI engines wouldn't experience the drop in torque they do.

There's lots of factors involved in so called backpressure and torque, but backpressure is a misnoamer association as it is really describing the system resistance at various gas velocities. The significant factor is the velocity the exhaust escapes (and the scaveging effect it has). If the exhaust piping is sized for high revs then the velocity must be lower at lower rpm. Conversely if the gas velocity is high at low revs it will be much higher at high revs and therefore the "backpressure" will increase as will the choking effect.

If you have low velocity at low revs the exhaust gas will hang around the exhaust port like a stale bottle of piss and mess up the exhausting of the cylinders.

Burnt valves can be the result of bad seat contact or insufficient contact face. If the valve faces are too big there may be insufficient contact pressure and on leaded engines this was a problem because leaded fuel has a low combustion swirl and the valve contact faces tended to build up with carbon and lead deposits.

The valves will burn with lean mixtures, but to say the carby models got double doses of fuel because of a reversion wave going back through the venturi is pushing it a bit. If you think about the valves and how they overlap then more than double dipping comes into play: Imagine a piston starting on the downstoke and both the inlet and exhaust valves are partially open. The exhaust gas is supposed to be be under the influence of a slightly negative pressure from the exhaust piping and the intake mixture under the influence of both the negative pressure the piston is creating and the exhaust. Being heavier the fuel mixture tends to displace the exhaust gas and given a poor cam setup some of the fuel mixture will be drawn out with the exhaust, resulting in lean burn. But if you reduce the negative pressure on the exhaust port you will obviously reduce the scavenging of the combusted gases and the the amount of fuel mixture entering, thus leaning out the mixture in the pots and combined with the residual exhaust gas a slightly hotter one at that.