Would running an intake valley pan with the heat cross over not knocked out be better?
Better for what, I guess, is the best question. The short answer is yes. The long answer involves whether you've got an automatic choke you'd like to keep, and how long you want the car to be grumpy on cold starts.
Blocking the heat crossover makes vapor lock less likely and improves performance due to the cooler, denser air getting to the cylinders. Denser air means more molecules per cubic foot, so more oxygen molcules are available to burn. In extreme cases, say going from a factory heat-riser air cleaner to a full-bore Pro Stock hood scoop, you might even need to re-jet to avoid a lean condition. The B/RB Mopar benefits greatly from this, since the bottom of the intake has no contact with hot oil splash, nor is there a coolant passage in the intake. Eliminating the heat crossover means the intake can stay quite cooler compared to something like an LA.
The downside, of course, is cold-start and cold-weather operation. Right off the bat, a factory-style Chrysler automatic choke is out because it will stay on for an eternity before it's warm enough to open, especially on a big block. If you've got an electric choke, of course, that's not really an issue. However, the cold intake manifold and the air passing through it keep the cylinder head ports and intake valves cooler for a longer time, which decreases the evaporation of the fuel in the incoming A/F charge and from the thin film of gasoline that forms in your intake manifold and ports (wall film
*).
Even with a perfectly-functioning choke you might find yourself feathering it for a little while on brisk or downright cold mornings, but to me it was not a big deal to sit in the car for a minute or two until the idle levelled out and it would run on its own. Blocking the heat crossover is something I've done since I was a teenager, and in most of my cars I never even ran a
choke.
Still, they'd run on their own after a minute or so, and I'd go back in the house to warm my bones.
End of answer most people would expect. Keep reading for the Jass over-explanation.
CAUTION: INTERNAL COMBUSTION FUELING LESSON AHEAD!
Engines don't run on liquid fuel; in fact liquid gasoline is relatively hard to light off (hence extended cold-weather starting). Engines run on the gaseous form of gasoline, not the atomized liquid that comes out of a carburetor or fuel injector; it changes state when it hits the hot intake valve. It also evaporates from the hot intake runners and port walls
*. This is why your car originally had a heat riser and choke stove on the exhaust manifold--to get heat into the intake tract more quickly and get those port walls and intake valves warm to accelerate the evaporation of the fuel. It's also why a warm engine will start so much more quickly. The whole point of the choke mechanism on a carburetor is to essentially dump fuel so that there's more wall film available to evaporate, which will allow the engine to run. On fuel-injected engines, the "choke" function is effected by increasing the injector duty cycle. The reasons that fuel injection works so much better in cold weather, particularly direct-port injection, are because well-atomized fuel vaporizes so much more easily, but also because the fuel is run at high pressure. Increasing pressure increases heat; that's why your garage air compressor head gets so bloody hot. Even the 10-15PSI of pressure in a throttle-body system warms the fuel significantly. That warmer fuel not only is more easily vaporizes, it also works like a radiator in reverse and helps increase the temperature of the intake ports and valves. Kooky, right? Atomization and vaporization is so poor with a carburetor, heat crossovers and risers, hot-air intake tubes, etc. were virtually required to make the thing run worth a damn for the average non-gearhead living in a cold climate.
* The study of the behavior of fuel in a film on an intake manifold or intake port wall is fascinating stuff. The accelerator pump in a carburetor, or the acceleration enrichment algorithm in an EFI application, are
not in place to supply the combustion chambers with additional fuel. The extra fuel required actually comes from the increased evaporation of the wall film due to the inrush of air. The additional fuel is actually pumped or injected to keep the wall film at a level
consistent with steady-state operation. On your better EFI systems, acceleration enrichments are calculated using something called X-Tau, which attempts to model the evaporation of wall film at various loads using the instantaneous changes in throttle opening, manifold pressure, and RPM as its variables against a time constant as well as a calculated amount of wall film. It's kind of tweaky shit, but it makes all the difference in drivability.
* Sequential multi-port EFI engines, contrary to popular belief, do
not fire the injector when the valve is open. Firing an injector while the valve is open would allow raw liquid fuel to come into contact with the cylinder wall, which would wash away the oil needed to lubricate the pistons and rings. Injectors are timed so that the fuel hits a hot, very much closed, intake valve, as mentioned above. Timing the injection pulses correctly for sequential injection is fussy work, requiring timing the pulse for each cylinder in degrees of crankshaft rotation to hit the back of the intake valve milliseconds
prior to it opening to allow it to not only hit a closed valve, but have time to fully evaporate before it's sucked into the combustion chamber. The amount of time needs to change with both engine load and RPM, it's not a "set it and forget it at one RPM and it works everywhere" process. This is why true sequential injection, for the hobbyist, really is not worth the effort. Improvements from properly-tuned SMPI are usually in the 2-5 horsepower at peak on a max-effort engine, and
maybe .5MPG at peak economy (expect half that), over a well-tuned batch-fire system (firing multiple injectors in groups, such as left and right banks on a V8). It matters when you're fighting both the EPA and CAFE requirements, which is why production models have
had to have it for several years. At ~$100/hour for dyno time, I'm not
that concerned about either one. I'm not going to work that hard to extend my range by 4-8 miles on a 16-gallon tank, and I'll stand by that statement right up until I run out of gas 3 miles from a station.
Oh, and if you're wondering how direct injection doesn't damage the internals with fuel wash since it fires fuel directly into the cylinders, look into the fuel pressures they're using. No, those ultra-high pressures aren't there to keep the injectors clear of carbon deposits. At literally tens of thousands of pounds per square inch, gasoline undergoes a state change from a liquid to a gas due to the enormous pressure differential across the nozzle of the injector. Hey, that's how diesels have worked for decades!
Got a headache yet?
Unless I can find an electric choke set-up for my Thermoquad. Chevy motors that used the TQ had them for a couple years.
