For those not familiar with rich vs lean and engine load allow me to explain. No, wait there's too much. Allow me to sum up. Lean provides a more complete combustion and generates more heat as a result. The recipient of the extra heat is the exhaust valves that evacuate the spent gases. As you heat (then over heat) the exhaust valves they will expand, reducing the surface area of the valve seal by increasing the diameter of the valve seat. This leads to seat leakage and steam cutting of the valve seats. Results in a head rebuild over time.
To rich is incomplete combustion and unspent fuel going down the exhaust pipe (black soot). Because unspent fuel is expended the exhaust temperatures are lower than too lean a mix. Therefore the heat damage associated with too lean does not occur. New engine designs are incorporating direct injection to take advantage of the cooling provided by blowing fuel into the combustion chamber directly (vice throttle body injection). This provides a large delta T (differential temperature) inside the combustion chamber and uses the 1st law of thermodynamics to greatest effect, (heat always goes from a higher source to a lower source and never in the reverse) to cool the exhaust valves.
Exhaust pipes do two things, direct the spent gases away from the engine intake and provide an expansion volume for the spent gases. This second purpose is important to engine "breathing." The volume expansion allows the exhaust gases to achieve super-sonic velocity and thus create a low pressure point in the system. The exhaust pipes actually educt spent gases out of the combustion chamber when the intake valve overlap occurs (intake and exhaust valves open at the same time). This provides a more complete evacuation of spent gases from the combustion chamber and a better fresh charge of air for the next combustion cycle. Changing the diameter and length of the exhaust pipes changes the point at which the super-sonic flow occurs and changes the timing required for the overlap. (Side note: 2-stroke expansion chambers achieve the optimum eductor effect for a given bore and stroke using the mathematics involved). So changing from stock pipes to TORs (which are shorter) changes where the eduction occurs. If you put a restiction in the stock pipes, the engine would run rich with stock settings. Increasing the flow rate through the pipe (TOR) means more fresh air is available in the combustion chamber. With a stock tune the engine is already running on the leanest possible settings. Increasing exhaust flow leans it even more and you get the Orville Redenbacher exhaust note (pop-pop-pop) as expending gases hit super-sonic velocity early in the overlap.
To fix the problem, buy a Tuneboy and get to a dyno tuner.
Class dismissed.
