1956 1x4 3837109 intake design

[Alexander C.]

When I installed my correctly dated 1x4 3837109 intake manifold, I noted some difference between it and the later '56 1x4 manifold I was running.

As I describe these things, forgive me as I don't know the proper names for things...

The crossover holes under the 1x4 carb base at the rear. On the older 109 manifold, the 2 holes are plugged in the casting, the later intake had these open. What does this effect & why was it changed?

Also I have noticed the paint is burned off my intake now at the crossover runner on either side after only about 200 miles. My late '56 manifold did not do this. Does this have to do with the passages under the carb that I described above?

If it matters, I am not running my heat riser right now on my new engine, would that effect any of this?

Thanks!

[John H.]

Re: 1956 1x4 3837109 intake design

Alex -

What was the casting number and date on your "other" 1x4 manifold?

[Alexander C.]

Re: 1956 1x4 3837109 intake design

Alex -

What was the casting number and date on your "other" 1x4 manifold?

It was a 3735448 dated G56.

[John H.]

Re: 1956 1x4 3837109 intake design

Alex -

Those two holes allow hot exhaust gases from the crossover passage below the manifold plenum floor to warm the carburetor baseplate adjacent to the two primary throttle bores at cold-start to improve cold driveability; when the heat riser valve opens, flow is reduced. With today's gas, it's a good idea to plug them to avoid heating the carburetor body, which causes fuel percolation in the float bowls.

It's normal to have the paint burn off the top of the crossover passages - they're HOT.

[Alexander C.]

Re: 1956 1x4 3837109 intake design

Ok perfect, thanks! Sounds like I'm better off with the correct earlier intake anyway then!

Alex

[Alexander C.]

Re: 1956 1x4 3837109 intake design

So it just occured to me....

If these passages under the rear base of the carb are blocked off in these 109 castings, then why were heat risers even installed at the factory? Why not just a spacer?

[John H.]

Re: 1956 1x4 3837109 intake design

So it just occured to me....

If these passages under the rear base of the carb are blocked off in these 109 castings, then why were heat risers even installed at the factory? Why not just a spacer?

Alex -

The two holes in the carb pad are a secondary function, primarily for heat to avoid carburetor icing on the throttle blades. The primary function of the heat riser system is to heat the floor of the intake manifold plenum, to improve fuel vaporization during cold-weather cold-start and warmup so the fuel remains in suspension with the incoming air on the way to the intake ports instead of "wetting out" on the cold walls of the manifold runners, causing a lean condition. That exhaust gas crossover passage is directly below the floor of the manifold plenum you see when you look into the intake without the carb installed.

[Alexander C.]

Re: 1956 1x4 3837109 intake design

Ok, now I gotcha. Thanks!