The Stovebolt Site Forums The Shop Area Engine Shop Are carb spacers (insulators) recommended with Offenhauser intake?

I'm putting the Offenhauser dual carb intake on my 235 with Rochester's and wondering if I should bother with carb spacers. I know from a heat perspective they can only help, but with the raised bases already cast into the intake a stock spacer really gets the carb up pretty high. I'm a little concerned about losing my bottom end power or causing starting/drivability issues, and thinking the aluminum intake is already going to be a bit cooler than the stock cast. I am hooking the manifold heat to exhaust.

If it is the manifold that bolts directly to the stock exhaust manifold, I would think you would want the insulators. Aluminium transfers heat better than cast iron. And I would think that OEM used them for good reason. Try it both ways and see if there is a significant difference in performance and heat soaking. An Infrared thermometer is your friend on this one.

SimS

I have run dual carbs on a couple of different manifolds over the years. I did not use a insulator/spacer. I did, and still do, apply heat to the under side of the intake via hot water from the cooling system (if you are not using the stock exhaust manifold). To do that with the offy, you have to flip it upside down. Where the sheet metal core plugs are located, you have to weld little plates over them. Then make up a bolt on plate or weld a plate to the bottom where the stock exhaust would have usually gone, with two fittings to run hot water from the thermostat housing to the manifold, then the manifold back to the cooling system. Some gmc truck lower thermostat housings, like 302 and 270 gmc motors, have a threaded port on both the driver and passenger side of the lower housing to make hook up easier. You can also make up an aluminum spacer to go in between the lower housing and the head with a threaded port in the spacer (also gives some room for aftermarket aluminum valve cover so it can be removed easier.

The spacers won't be needed, it will run better with more heat as the aluminum dissipates the heat much faster the iron. You will however loose drivability if you try and run two Rochester's. You need two carbs that match and are much smaller then what the engine came with. I run dual W-1 Carters made for a 216 on a 250 inline six. I tried two Monojet's that the 250 came with, it would run, idle, and make decent power, but I could see the fuel gauge move as I was driving, I am talking single digit fuel mileage!

I knew it was only a matter of time before someone started up the ole carb debate! I'm using 216 carbs of the same part number.

Joe H is not wrong. Getting a pair of later and larger R's to run decently in a 2X1 or 3X1 configuration is many times an exercise in futility. The best running 2X1 street sets ups I ever ran were set ups using early style Carter YF's for a 216 (manual choke and I want to say 964 but its been a while since I looked so I may have the number wrong- I still have a complete YF set up on a Weiand intake in the basement shop). I switched to a 2X2 configuration later on at the suggestion of Tom Langdon, which has to be like 25 years ago. So the best 2X2 on another weiand manifold I have (using adapters) was using a pair of 1974 ford pinto holly/weber 5200 carbs. Small primaries, larger progressive secondaries.

The bigger carburetors do not get the air volume needed to work correctly, I won't say it won't run, it just won't run like you want it to. I have run different manifolds with and without heat, and here in Kansas City, you need heat under the carburetors, the more the better. I always had iron or steel manifolds so they retained some heat, the aluminum is going to give it up much faster, so apply as much heat to the carbs and manifold as you can if you live anywhere it humid or gets cool. It really helps on restarts after 10 or 15 minute shut downs.

I guess I didn't know the 216's used Rochester carburetors, I thought they all were Carter W-1's.

Carburetion 101:

There are basically 3 major methods to atomize fuel:

(1) Add heat
(2) Increase velocity of the fuel/air mixture
(3) Add additional fuel

Several have mentioned the necessity of adding heat to aftermarket setups. Others have mentioned the heat conductivity of aluminum. While the aluminum WILL conduct more heat from the cylinder head to the manifold, it will dissipate this heat much faster to the air, thus the aluminum manifold will run cooler, almost always requiring heat.

But the OP asked about spacers.

For a trailered race vehicle WITH a 235 that is modified for full-blown racing, the spacers will PROBABLY help at wide open throttle, as the spacers in effect increase the plenum volume of the intake manifold. This tends to raise the RPM of the horsepower peak.

For a street engine, the spacers will PROBABLY require ADDITIONAL heat to be supplied to the manifold; and will PROBABLY hurt low end torque and horsepower.

Please note the use of the "probablys". There is no exact answer to the question. The answer will depend on the actual circumstances.

As to the "ole carb debate" mentioned by the OP: one of the great things about our hobby is the freedom to make our own choices. Some make better choices than others. If we were all to drive the perfect car, we would all be driving white with blue interior 1962 Jaguar E-types, and I would be walking, because I can not/could not afford one

Jon

Joe - not certain exactly when Chevrolet discontinued the 216, but at least in 1951 there was a 216 that used Rochester type B carbs.

However, GM instructed Chevrolet dealers to replace earlier superior (opinion) Carter W-1's with the inferior (opinion) Rochester B as a service replacement.

Jon.

Carter sold a million direct-replacement carbs for Rochesters after Chevy started using Rochesters as OEM parts, and the CEO of Carter had the one-millionth one of those carbs pulled off the production line, gold plated it, and gave it to GM! "The ABC's of carburetion- - - -Always Buy Carter!"
Jerry

But then again back with Rochester into the sixties with the monojet.