I currently have my intake manifold off of my '68 350 4bbl and was thinking of plugging up the exhaust crossover's with some freeze plugs. What I was wondering is the size of the plugs. I know the big blocks use a 18mm or 18.05mm but I haven't found out what size the 350's use. Also I live in Western Washington and the winters are for the most part mild (usually wet and in the high 30's to high 40's) Would this be considered to cool of a climate to block off the exhaust passages? I drive this engine daily to and from work and driveability is a huge factor, but I don't like my intake getting so hot either as well as the gas in the carb during the summers. Looking for opinions and anyone else that has blocked off the exhaust crossover passages and how they liked/dislike it. Thank you,
Pros and cons eh? Well.....My Regal has them completely blocked and my LeSabre has one per side blocked. The reason I did one per side was strictly experimenting with freeze plugs. But I forgot the size. I have no winter trouble with it at all. A well tuned quadrajet helps me I'm sure. The Regal is completely blocked with homemade plates between the intake and the heads. I've started the car in winter, but seldom used it in extreme cold. I tend to let it warm up a bit. So I can't really say how it would do in the real world. But what little use its had seems trouble free. But like I said, I tend to let it warm up.
Todd, The up side of blocking the crossover is the intake stays much cooler resulting in a denser cooler intake charge. It also provides insurance against vapor lock in the summer. The downside, you can't run an automatic choke because there will not be enough heat, quickly enough to cause the choke to come off. In the winter, the car will take longer to really run well because the intake will warm more slowly. Intake heat helps with fuel vaporization. Your intake will eventually get warm but it takes much longer. An aluminum manifold will help with this because aluminum conducts heat faster. The only other thing to be concerned with is a phenomenom called "carburetor icing" Because of the pressure drop inside a carburetor, and lack of intake heat, ice can actually form around the throttle blades under conditions of high humidity, and temps around 35-40*(just the conditions you describe for your winters). When this happens, the engine will want to stall, and not idle. I've had this happen to me a few times, you have to keep the engine above idle at a traffic light, and it's a pain. Buick and other manufacturers dealt with this by using exhaust crossover heat, and heating of the air entering the air cleaner. For a daily driver, I'd leave your crossover unblocked, or do as Dan did and block one side, or block it with the plugs and drill a small (1/4") hole in one of the plugs on each side. That and a working thermostatic air cleaner should prevent the icing. Hope that helps.
To add to what Larry has posted - your carburetor was calibrated with a heated intake manifold in mind. Removing the heat will increase the density of the intake air... which will result in leaner operation, possibly too lean. Increasing the main jets will only help at WOT. The off-idle and part-throttle conditions (where a daily-driver spends most of its life) will still be too lean. You'll need to change the metering rods as well. What values to use? Ahh, that's where the fun (or frustration) begins. In fact, here's a quote from the book, "How to Hotrod Small Block Chevys" (hey, it's a still a gas-powered V8, the same principles apply): Blocking the heat riser for the intake manifold can never be recommended for the street. This modification creates numerous ills for the street-driven machine. It creates poor vaporization and this worsens mixture distribution to the cylinders. It also makes the exhaust system louder because the heat riser acts as a balancer between the two sides of the engine. Note: the balancing effect on the exhaust only affects dual-exhaust setups.
I have the heat crossover blocked in my 71 GS. They're 69 heads, I had the machine shope thread and install pipe plugs in the heads (2 per side) I tried using the metal from and old bathtub style intake gasket, but that burnt out in no time at all. In colder weather if I have to drive the car its a little boggy for the first few minutes. But I have a Holley carb with no choke, T/A intake, And T/A 310 Cam. I cant really say if blocking the exhaust crossover or lack of a choke cause this, but like I said its only boggy for the first few minutes, which I really dont care. Mark
I wouldn't do it for your situation. It does affect cold driveability, and the small HP increase doesn't make up for it. Mine's blocked off, plus I have a big cam, etc. When it's cold out, I have to really baby it, OR let the engine warm up for MINUTES before driving anywhere. My $.02
Thank you for all the help everyone, I appreciate it! :TU: I have John Osborne working my carb over and we talked awhile about my setup and climate and we agreed to try putting in freeze plugs but drilling a 1/4" hole in them. It will slow down the choke which is good since it already opens way too fast. Cheers,
