The much more rare 840cfm...
The much more rare 840cfm Quadrajet carb.
Until throttle body fuel injection started taking over in the 1980s, if you had a General Motors product with a V-8 engine, you could just about guarantee a Quadrajet manufactured by Rochester would be sitting on top of the intake manifold.
And while Quadrajet-style carburetors aren't nearly as popular these days-especially when it comes to stock car racing-as the iconic Holley-style four-barrels, you'll still find them on both the street and the racetrack. This is particularly true in the Pure Stock and some IMCA Modified classes that require racers run the same carburetor that originally came installed on the engine. Of course, racing with a Rochester Quadrajet isn't exactly a death sentence to poor performance and on-track frustration. When properly prepared and tuned correctly, these carburetors actually are very good performers. The trouble is working with a Quadrajet is becoming a bit of a lost art.
Sean Murphy of SMI (Sean Murphy Induction) is one of the few carburetor builders and tuners around who is still trying to advance the art of racing with a Quadrajet. We recently spent some time with him and he graciously shared several tips for getting the most out of these carburetors. What follows isn't your typical rebuild story-if you want that, there are plenty of books out there-instead, Murphy gave us the real lowdown on what it takes to turn these carbs, which can be rather pedestrian in stock form, into real screamers. The tips were so good we decided to pass them right along to you in his own words.
A stock 750cfm Rochester Quadrajet...
A stock 750cfm Rochester Quadrajet carb.
An 800cfm Q-Jet.
How Big is It?
There are basically three different sizes Rochester produced for the Quadrajets. Image 1 is a stock 750cfm carb. If you look at Image 2, that's an 800cfm. The main difference with these two is the bump on the venturi wall is significantly larger than the others. That bump was to enhance the venturi effect, which increases airspeed across the booster and, in turn, increases the signal at the booster. But it comes at the cost of decreased overall flow.
Image 3 is the 840cfm carburetor. It's easy to identify because it doesn't have the secondary booster ring. That's how it gets such a larger airflow. These carbs are more rare because the only place you'll find them is on '71 and '72 high-output applications on Pontiacs outfitted with 455 engines.
By removing that bump on the venturi wall, you can really increase the amount of cfms these carburetors will flow, but you have to be careful. You don't want to get too happy with the Dremel and mess up the shape of the venturi. As an example, you can look at Image 4 to see a before and after. On the left is a venturi that we've modified for maximum flow, while on the right the venturi is still the way it came from the factory.
When we go in and modify these carburetors, we can get as much as 850 cfm out of the 750. And on the 800 we can get it all the way up to 890 cfm, but it really depends on what your engine needs. You can over-do it with the carburetor size. Remember that many of these engines are limited by the rules, so if you get too large you will lose responsiveness off the corner. In circle track applications the most popular carburetor to start with is the 750.
We also look at the quality of the airflow through the carburetor. In the stock carburetor (Image 5), you can see casting flash in the secondary venturis. To get a little more airflow and also decrease the turbulence as the air moves through that area, we will polish the secondaries (Image 6). It doesn't look like much, but by decreasing the turbulence you can increase the overall airflow.
On the left is a venturi that...
On the left is a venturi that we've modified for maximum flow, while on the right the venturi is still the way it came from the factory.
In the stock carburetor you...
In the stock carburetor you can see casting flash in the secondary venturis.
To get a little more airflow...
To get a little more airflow and also decrease the turbulence as the air moves through that area, we will polish the secondaries.
The fuel comes through the...
The fuel comes through the secondary main wells then comes back up and through the jet. You can see how restrictive this is.
We will work the secondary...
We will work the secondary main wells in order to improve fuel flow through them.
Here, you can see the different...
Here, you can see the different springs that are available for you to tune with.