Baffled

[Bill Braun]

Well, I'm still going through the '67 convertible I bought a few weeks ago. I noticed that the baffle (according to my AIM: part 3884575) that is usually sandwiched between the carb and intake manifold is missing. I assume (usually incorrectly...) the baffle is intended as a barrier to heat transfer. Could its absence cause problems in hot weather? Would I be well-advised to order and install one?

Bill Braun 33186

[Joe L.]

Re: Baffled

Bill-----

I've forgotten which engine that you have. But, if you have one with a cast iron intake manifold, the baffle is ABSOLUTELY CRITICAL unless the carburetor heat "groove" at the front of the primary bore holes is "decommissioned". I recommend this be done with all manifolds so-equipped. This can be done by drilling and tapping the round holes at the either end of the groove for 1/4" allen-head pipe plugs. Aluminum manifolds didn't have the heat "groove" or require the baffle.

With this carburetor-destroying heat passage suitably eliminated, the carb heat shield is superfluous. I'd probably still use it, though, to insure exact correct geometry for the divorced style choke.

[Bill Braun]

Re: Baffled

Joe: Thanks for the quick response. My '67 is a big block (replacement from 1968) and has an aluminum intake manifold. The Holley 4150 (not stock) presently on the car is a manual choke variation, and I wasn't keen on running a choke cable into the driver's compartment. I have taken the advice of several people on the DB and ordered an electric choke. Since the absence of the baffle will not affect anything, I'll put it on the list of low-priority things. 'Preciate the help!

Bill Braun 33186

[John M.]

Re: Baffled still

Joe

Does the shield breakdown or is a matter of insufficient protection which damages the carb over time? I was also wondering why the groove even exists ? John M NCRS #28796

[Joe L.]

Re: Baffled still

John-----

No, the shield does not deteriorate. It is made of stainless steel and it's pretty much a "forever" type item.

This system is designed to "pre-heat" the primary side of the carburetor for better cold engine warm-up performance. With this system, exhaust gases which normally pass through the intake manifold exhaust heat crossover are partially diverted through passages which take the gases to the hole on the passenger side of the groove. The exhaust gases then pass under the carburetor through the groove to the hole on the drivers side. From here they re-enter passages which take them back into the primary exhaust heat crossover passage and out the left side exhaust pipe.

The insulating carburetor base gasket has a groove in it which corresponds to the groove in the manifold. The heat shield is mounted between the carburetor base gasket and the carburetor. In this configuration, the hot exhaust gases pass directly beneath the carburetor, seperated from it by only the thin stainless steel shield.

In my experience, this system causes the carburetor to be exposed to excessive heat which wreaks havoc on the carburetors. Warped throttle bodies and float bowls are some of the problems created. The benefits are minimal and, in my opinion, are FAR outweighed by the disadvantages. Why was this system ever used? Remember, good cold running performance is particularly important to selling cars; longevity of carburetors is not.

This system was used on many small block Chevrolet engines with cast iron manifolds as far back as 1957. It was used on big block engines with cast iron manifolds, too, from 1965(passenger car only)thru 1969. The system was totally abandoned for all engines in 1970, never to be seen of again. The only aluminum intake manifolds that I am aware of that used this sytsem were the 64-65 350 and 365 hp manifold and 68-69 390 hp 427 Corvette manifolds.

[Bill Braun]

Re: Baffled still

Joe,

Now you've got me worried again, because the motor in the car is a 390 HP replacement with a block casting code of H27 8. If they kept the intake manifold with the engine, then it may indeed be better off with the baffle. I guess its best if I pull the darn thing off and have a look to be sure.

Bill Braun 33186

[Joe L.]

Re: Baffled still

Bill----

In my original post, I should have qualified my statement regarding aluminum manifolds not having the heat groove to provide for the few exceptions. However, in your post you mentioned that you have a Holley 4150 on your car. The 68-69 aluminum big block manifolds are the only GM aluminum big block manifolds that have the heat groove and they are designed for use with a spread bore carburetor, not a Holley 4150. So, unless you have some sort of adapter plate, your aluminum manifold couldn't be the 68-69 style. NO aftermarket aluminum manifolds that I am aware of have or ever had the heat groove.

GM never made an aluminum manifold designed for oval port, big block heads which used a Holley "square bore" flanged manifold.

In any event, the 1968-69 big block, spreadbore, aluminum manifolds are GM casting numbers 3919849 and 3947801. There was also another late 68 manifold which was used for a very short time, but I don't have the number handy.

[Bill Braun]

Re: Baffled no longer

Joe,

Thanks. I think I'll let sleeping dogs lie.

Bill Braun 33186