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I have a 1969 corvette big block. It had an aftermarket intake with a double pumper carb. I was replacing it back to the original 3x2 when I noticed some thing funny on the intake holes. The middle holes on both sides were completely blocked off by the gaskets (As seen In picture below). Then with the new gaskets it has openings for the middle holes on the heads. Is this something I should be worried about? Thanks for any responses.
This is the original gasket next to the heads. [size=13px]original[/size] intake that was blocked off.
From: At my Bar drinking and wrenching in Lafayette Colorado
A carbureted engine is what is called a "wet flow" system. In order to properly atomize the fuel coming out of the carb and going into the intake, there must be some heat in the intake manifold. The crossover provides this pre-heat for the fuel, in addition to providing heat for the divorced choke system. A street-driven car with a wet-flow carburetor on it tends to perform poorly under normal driving conditions if the heat crossover is blocked. It is a common "performance" mod, and misconception, to block off the crossover to make "more power." Data I have from GM testing and from the SAE disputes this common "speed trick" for street-driven vehicles.
Lars
I’m about to replace an intake also (but a small block LT-1). The gasket set came with some small metal plates that slot into the crossover port and reduce the size of the crossover from a large rectangle to a small circle, probably one quarter of the original area. Lars, any thoughts on using those?
A carbureted engine is what is called a "wet flow" system. In order to properly atomize the fuel coming out of the carb and going into the intake, there must be some heat in the intake manifold. The crossover provides this pre-heat for the fuel, in addition to providing heat for the divorced choke system. A street-driven car with a wet-flow carburetor on it tends to perform poorly under normal driving conditions if the heat crossover is blocked. It is a common "performance" mod, and misconception, to block off the crossover to make "more power." Data I have from GM testing and from the SAE disputes this common "speed trick" for street-driven vehicles.
Lars
Hey Lars,
Nice to see you’re still spending time here.
Back in the day, it was common for many people who lived in the western and desert states to block off the heat riser ports going to the intake to aid in problems they were having regarding percolation of the fuel and heat soak issues.
Have you seen any data that supports this?
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
You do want to block that cross over. Not doing it is inviting heat related issues by heating the carb which will lead to fuel evaporation, heat soak and possible vapor lock. ITs akin to leaving your timing at 6 or 8 degrees because GM says its good for the car. All of the performance intakes block it for a reason. The most populart is the Air Gap styles that promote under carb cooling.
You are asking for issues with todays ethanol corn fuel. I have a tripower and had zero heat issues last year and this summer until last week when the ambient temp was in the 90's and the humidity was too. I run it blocked and that was the first time I had a heat soak issue. Once it cooled the issue was gone.
Thanks a lot for the advice. I was already leaning toward blocking the crossover as I’ve had heat soak issues this summer, but I’m replacing a no-choke double pumper and generic intake with the correct OEM intake and carb and wondered if they would fare better.
FWIW we have 93 octane non-ethanol premium just up the road and that seems to make a positive difference too.
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
Lars says the choke wont function correctly without the crossover. You can either leave the choke so its always open, open the crossover on that side and use the stock choke, or get an aftermarket electric divorced choke and modify it to work with the tripower.
69 vette weird intake question.
