The Stovebolt Site Forums The Shop Area Engine Shop SB 327 with a 4 barrel carburetor

Do you carburetor experts know how I can identify the carburetor needed for my SB 327? I have a motor that came out of what they think was a 67 station wagon. I have the engine pretty much rebuilt but have been puzzled about what carburetor this thing should have on it. Of course it was missing when I was given the engine.
It appears to need some wide base quadrajet. I have the 4 round holes, 2 small, 2 large. I am guessing that means round bore?

The intake has the oil spout in the front. That makes me think it is older than 67...? I got a 65, 238 that look almost the same except it has a 2 barrel carburetor and the little "power pack" heads. This 327 has the camel humped heads with no bold holes for an alternator bracket. I understand the heads to be more HP than stock. I have the numbers off the block if someone knows where I can look that up I would really appreciate it.

if your looking to keep it original, i'm not sure..if not you can put a quadrajet, holley, edelbrock , etc. you can get the plate to put whatever 4 barrell ya want.

The q-jet uses what is called a spread bore, which is what you have described. This was the stock carb for the sbc for ages. It's a good carb for milage and operating medium performance engines. The other carbs use what is called a square bore pattern, all 4 holes are the same size.

For carb info go to www.carburetion.com

For V8 casting numbers go to www.mortec.com

It must have had a Q-jet on it. They came out in 1966. You have to watch on those early manifolds. They had heat riser holes going up to the base of the carb. If it has those you will have to put a plate over them. If they are not sealed you can get vacuum leaks & exhaust leaks. I like the late 70,s early 80,s Q-jets. Get one without the electronics. Put an electric choke coil in it with a shield in front of the choke to keep cold air from blowing on it.

The "camel-hump" heads without the accessory bolt holes were used on the "300 HP" 327 engines as early as 1964, or possibly a little earlier, with a Rochester 4GC carburetor, or a Carter AFB for some applications. Those heads have 64CC combustion chambers, and make about 10:1 compression on a 327 with flattop pistons, way too much compression for any pump gas except maybe 93 octane. If you can establish the year of manufacture and the HP rating by using the 2-or 3-letter suffix to the block number, I've got Mitchell service manuals from that era which should list the proper carb application, right down to the carb part number. Drop me an IM or an email if you need to.
Jerry

Having "too small, too large" does not necessarily denote Q-Jet, depending on the poster's definition of small and large.

Many of the "square bore" intake manifolds have "2 small, 2 large" bores, where there is about 1/4 inch diameter difference from small to large.

With the Q-Jet manifold, there will be between 3/4 inch and 1 inch diameter difference from small to large.

Once you have identified the engine, this page from our website will tell you the correct carburetor:

http://www.thecarburetorshop.com/Kchevrolet3.htm

Jon.

the Q jet was the big thing back then,,came on small and big blocks not on the Hipros....doc

Like Jerry mentions the double hump heads would lead me to believe it is at least the 300 horse engine which was introduced in 1962. The 300 horse engine had the Carter AFB whereas the 250 horse engine utilized the Rochester carb and the smaller, single hump/point heads. There was a higher horse version, I'm thinking 340, with an aluminum intake and Carter AFB used mostly with the Corvettes.

My 327 came from a 66 impala with the carter afb. An older fellow tells me it is really an avs.I don't know the difference. The holes look the same size in the manifold, but there is a slight difference.

When I bought my truck, it had a 327 Camel Hump motor, with an older style ( Oil filler spout up front ) intake manifold. It has an Edelbrock 650 cfm Performer carb which worked fine. When I rebuilt the engine, I bored it .060 over, with a new Edelbrock intake - carb still handles the load.

It was a Rochester Quadrajet on it originally. My '68 Corvette had this set up.

They're good carburetors until they wear out. If you want to stay with that, Summit Racing sells rebuilt Q-jets with 3 levels of performance. They range from about $330 to $390 bucks.

Holley used to carry a spreadbore carburetor, which my 327 was switched to. Much easier to tune, but the secondary bores are not nearly as huge as the Rochesters. I remember Q-jets. You really felt them when they went into the secondaries. Kind of felt like an afterburner.

Another thing you could do is go to a modern intake manifold. An excellent choice is the Edelbrock RPM Air Gap. It is so good, I switched to that on both my 68 Firebird and my 68 Corvette. The difference in power is very noticeable, and will allow you to use a standard square bore carb.

The formula for carb sizing is:

CFM= Displacement x redline RPM x Volumetric efficiency / 3456

Volumetric efficiency is about 0.90 for a good street motor. Race motors approach 1.00. Turbo/supercharged motors are over 1.00.

So for a stock 300 horse 327 with about a 5500 rpm redline, here's your goal:

CFM = 327 x 5500 x 0.9/3456 = 468 CFM

So you really don't want a real huge carb. Since you can't get a 468 cfm carb, you can step up to a 600 cfm maybe. The closer you are to your ideal cfm range, the easier it will be to tune and the more responsive the engine will be. This is important for a double pumper especially.

Vacuum secondary carbs can be bigger, because the engine will determine how far the secondaries open. So you can go larger with a vacuum secondary carb. A 750 cfm in this case would not be a problem. Vacuum secondary carbs can be used in manual tranny cars, but are usually used with automatic transmissions.