Yes, but simply adding them assumes that the contribution to valve temperature of cylinder head cooling and exhaust gas heating are equal. There is no a priori reason that assumption is true, but of course it might be.
They do not need to be equal, they just need to contribute in the same direction. Lower CHT is better and lower EGT is better. One of them may of course have more effect than the other and the EGT measurement itself has errors. There will probably exist sums that are “better” than the other even if the value of the sum is the same, but that effect would be small I should think (The engine still has to be operated within it’s limits and the ignition is constant when using mags, so there is not much you can do. With EI this becomes more problematic perhaps?).
They do not need to be equal, they just need to contribute in the same direction.
Yes, I agree. The “rule” is likely to be not far off so long as one is hanging around between plausible boundary conditions.
There is a funny backdrop to this, because EI got sued by JPI (of EDM700 fame) for alleged patent or copyright infringement. The action was widely regarded on the US GA scene as an abuse of corporate spending power and abuse of the patent system (the latter is pretty standard in about 99% of cases 
) and JPI did not win any popularity points for it. Obviously the two firms don’t like each other very much. But the technology (not a word I would use because it is 1970s electronics combined with a century old prior art) is so old now that I can’t see any valid (or enforceable) patents still existing.
At the end of the day JPI seem to be way ahead in marketing terms, although the EDM700 user interface makes a 1983 VCR look intuitive.
Next time, I would be tempted by at least using the EI probes (which are JPI compatible, generally – they are all just type J or K thermocouples) because they are thicker. I have replaced all my EGT probes over the 13 years, because the erosion just eats them, and then you get a bit of worry because you see crazy stuff happening on the EGT but if the CHT is steady you know it is just the EGT probe.
One of the greatest benefits of running lean, or better yet, LOP, is the diminution of lead content in the mixture.
This, by itself, helps to get significantly better engine reliability and life.
It also enhances safety since a very lean mixture reduces the CO to nil thus in the event of a exhaust leak in the cabin, CO poisoning is virtually eliminated.
I find it strange that this aspect was completely ignored by the article.
My question was whether a GAMI spread of 50°F would be enough to put the richest cylinder at 50°F (or more) ROP.
Obviously the question was rather whether a GAMI spread of 0,5 USG would be enough to put the richest cylinder at 50°F (or more) ROP.
They do not need to be equal, they just need to contribute in the same direction.
The direction doesn’t matter. Basically, the summing means that you are taking the average of the two figures, but instead of dividing by two (as in a proper average), you compare to a value which is twice as high.
You should really use a weighted average, but as long as the weights are close to 1:1, a simple sum will do.
Of course it matters. The idea is to reduce the temp of the exhaust valve. The temp of the valve can in simple, inaccurate but correct terms be described as:
T_v = CHT + k(EGT – CHT)
where k is some variable or function describing the added temp due to the temp difference. So,
T_v = (1-k)CHT + kEGT
We have no idea what k is, and it can also vary with T, CHT, EGT and other stuff, but it must be larger than zero and less than 1 (K = 0 then T_v = CHT which is too little, K = 1 then T_v = EGT which is too much). Let’s say k is 0.5, the “average” between 0 and 1 
then
T_v = 0.5*CHT + 0.5*EGT = 1/2 * (CHT + EGT)
It looks like an average or weighted average because of the simple function, but it is a weighted sum where k is limited by physics. No matter what k actually is (between 0 and 1), as long as the sum is reduced, then T_v is reduced. In reality T_v could be any function F(CHT, EGT, other stuff), but in some way or another the temperature difference (EGT – CHT) has to be the dominant one because EGT heats it and CHT cools it.
No matter what k actually is (between 0 and 1), as long as the sum is reduced, then T_v is reduced.
Why, of course… But the point was not that T_v is reduced if EGT or CHT is reduced. The point was that the valve temperature is proportional to EGT+CHT so that you can limit the valve temperature by limiting this sum. And that is only true if k≈0.5.
Yes but you still cannot give any absolute limit for EGT+CHT because
So then only thing you can conclude is that EGT does contribute to heating the valves.
And then we have Mike Bush and his flying laboratory in which he conducts live experiments and if I understood correctly his engines have about 3000 SMOH, who says that EGT is irrelevant to engine life.
The EGT value is no more than a distant proxy for the varying temperature of the combustion chamber contents. It is a very averaged value over the complete cycle, and is itself reduced by the expansion cooling of the gas as it escapes into the exhaust pipe.
It’s funny when you think that the top of the piston is getting the same gas up against it, yet aluminium would melt well before 1500F… there is a lot of “averaging” going on.
This discussion backs into the key issue with four stroke reciprocating engines, their trick is ‘averaging’… and thereby running fairly high gas temperatures while maintaining fairly low material temperatures – the economic way to produce a thermally efficient engine. No wonder that it’s hard to figure out what to measure. They weight more because they ‘average’ over time and space but it’s either that or super alloys. Neat stuff.
