1 hi am putting on a rpm edelbrock intake on a 350 cu. in. engine non egror or mech. choke. should i block off the exaust crossover or leave it open

 

My heat crossovers on the heads of my new engine are lead filled and I did some research on it when I pulled the intake and found it. There are a couple posts on CO I think that talk about it and it seems that there are varying opinions on its benefit.

If someone could educate me/us on the purpose of the heat crossover maybe we could understand the theory a bit more.

 

I used the steel plugs to seal them up. Intake runs way cooler now.

 

The crossover allows hot exhaust to warm the coil in the choke pot. When the engine is cold the exhaust bypass valve (the flap with the coil ) is closed and forces some exhaust into the crossover which heats the choke mechanism. As the bypass warms it opens to allow full flow through the exhaust.

 

The intake manifold exhaust crossover is designed to quickly heat the carburetor under cold conditions, improving engine running and warm-up, and reducing the possibility of carburetor icing. It is designed to work in conjunction with a thermostatic heat riser valve, which blocks exhaust flow out of one bank of cylinders (unless "blown open" by the force of exhaust gases at high revs), forcing them instead through the heat riser passage in the intake manifold, past the choke spring or tube, under the carburetor, and out the exhaust pipe on the opposite side.

The crossover is a compromise design that improves some things while making others worse. It heats up the carburetor, increasing evaporation of gasoline out into the atmosphere (where it can't make your car go faster), and reducing the density of the air-fuel mixture.

Eliminating the crossover will improve exhaust flow at high RPMs (RPM levels your engine would almost never have seen under normal design conditions as a "regular car"), will improve air-fuel charge density, thus slightly improving performance, will significantly impact warmup times and low-temperature driveability, and will render your automatic choke nearly useless.

Whether you decide to block your crossover will depend on how your motor is built, what sort of use you intend for it, and what sorts of temperatures you intend to operate it under.

- Eric

 

Probably zinc/Al mixture or Al
MIKE

 

The heat crossover is designed to heat the manifold under the carb. This helps to vaporize the fuel rather then let it puddle on the manifold floor which happens when you have a temperature difference much like condensation. It's purpose was to help driveability initial and secondly to help emissions. The fact that it is also used to heat the divorced chokes as well as the heat stove tube assemblies is just an added bonus that was utilized by GM. (Read cheap but functional, rather then using an electric choke)
There are pros and cons (as with most things in life ) with using blocked crossovers. Pros: You'll get a denser fuel mixture for power. Cons: It will take longer to warm up and may give driveability issues along with poor fuel economy early on when cold but that clears up as the intake warms a bit.
If you were going to drive it all year around it may be a pita. If just a summer car I'd go for it.

 

thanks for the input. i think i go with closing them .i found some exaust crossover plugs from summitt for 10 bucks , there for a big block hope i can make them work

 

How did you get them in?

 

A grinder and a big hammer. I just ground them down till they almost fit. And then pounded the heck out of them. Then used the grinder on whatever still stuck up above the surface of the head. When using the grinder on the plugs just take a bit off of each side and occassionally test fit.

 

Good writeup!

 

Yeah, not sure why I said lead...

 

redman dont do it like stickman grind them on your workbench and blow them off before each try you dont want to get metal shaving in your engine plus hammering them in could crack the head his advice is scary

 

If you grind them so they simply slide it they won't be tight.