The exhaust crossover goes back many years in deetroit iron. Worked in conjunction witha "heat riser" valve (flapperthingee).
Later on became more important with all that deetroit and IH emissions workaround shit. Onna sv, that's a prime location for a vacuum leak and all kindsa issues regarding bringing old motors to life.
Makes no difference in engine warmup if all engine systems are tuned for heat crossover elimination!
I have to call bs the exhaust crossover is a sealed passage through the intake, a leak at the manifold to head interface will result in an exhaust leak from that joint not a vacuum leak. Now on some old school 4 and 6cyls where the top of the exhaust manifold forms the floor of the intake then you can get a vacuum leak when the gasket fails. On the amc 6 cyl used in IH apps the intake forms the top of the exhaust manifold so it will cause an exhaust leak when the gasket between the two fail.
While you are correct that it makes no difference in warm up time it does make a shit load of difference in the driveablity if you don't live some place like the balmy wilds of orygone. Say you live in big sky country, nodak or the great white north, and it's 40 below, then you need that crossover to prevent extreeeme driveabilty problems.
In cold weather the fuel will tend to drop out of suspension where it has to take a turn, the ridged intake bottom helps to hold the fuel there while the exhaust heat e
vaporates it. That is what we are after fuel
vapor, cuz it will burn, while the liquid stuff won't. Even once the engine is up to operating temp the heated intake floor is required. Even if the fuel is atomized when it leaves the venturi, by the time it hits the throttle plate the sub zero air it has condensed and will just drip off the plate to the floor of the intake. Now 6 mos later when its 110 degrees it does hurt performance by heating the intake charge but that's what the oe needs to do design for a car that will run acceptably anywhere, any season.
Ron, the crossover is easy to pic out from the intake and coolant passages, it's the little rusty, pitted one with no signs of paint, since it exhaust travels through it. Even w/o a heat riser the timing of the pulses between the banks means some exhaust will flow back and forth as the pressure in the connected ports varies.