It’s just for exhaust crossover, I think it’s to help equalize pressure and/or to help with heat distribution but I’ve ran gaskets with and without the hole and there’s not much of a difference, in my opinion if the originals have the hole than I’d make holes in the new ones
It does help to equalize pressure to a degree, but it's also kind of a product of emissions. The cross over warms the intake up below the carb, tbi, injection system or whatever is on top of the manifold faster in cold weather, and warmer fuel atomizes better and burns more completely and efficiently. So the choke operates more closely to what it should and "pulls off" like it should as the intake warms up. because anyone that's played with a carburetor very much knows that once the key is turned on and the choke is set, you really only have about a minute or so of "Choke time" until the coil starts heating up and pulling the choke plate open. that's part of why those ports are there, to help get the intake warming up quicker. on a stock, or mostly stock application you probably wouldn't notice much difference in the operation of the engine between having the crossover ports open or closed. However, on an application that is more modified or when you start swapping to performance intakes and putting edgier camshafts with higher lifts and longer durations in the engine and start to get more reversion of the intake charge, you now have more unburned fuel mixing with MORE unburned fuel at low rpm until it starts warming up to temp and atomizing correctly. So we just made an engine that's a little temperamental and cold blooded even more temperamental and cold blooded until it warms up by taking those and blocking them off. Not saying that you can't tune the carb out of it but it will take some time to get those "cold starts" just right. That's the beauty of modern fuel injection, it reads the parameters of the engine and what's going down the tailpipe and just throws more fuel or air or less fuel or air to keep it in the optimum A/F range. With a carb, sometimes that can be touchy. Where I'm from in Illinois, it was 58 overnight on Thursday and tonight the overnight low is supposed to be 28. I've gotten mine tuned now just about where it needs to be, but when you can really only tune for "cold starts" for about a minute or two at a time, it took some time! Lol If the intake warmed up just a little bit quicker with the ports open it would be easier and less temperamental , but for a truck that doesn't get driven but a few times here and there when it's cold, I'm not going to sacrifice a few extra ponies. I'm keeping them blocked off.
