I received a call last summer from a long time member of POCI, Jon Hardgrove. Jon operates The Carburetor Shop at Eldon, Missouri, which has been in the carb repair/service business for a large part of this century. Jon had noted our consistent and impressive performance with the Q-Jet carbs, and wondered if we had tried a Carter Thermo-Quad. l had briefly tried several in the late 70’s or early 80’s, but was not very successful. After visiting with Jon, and reviewing some literature on the T-Quad, I realized there was much more to this carb than I had previously known.

The Thermo-Quad is a four barrel spread bore unit (small primaries and large secondaries) with an adjustable secondary air valve, manufactured by Carter and it was used on many of Chrysler’s production vehicles in the 1970’s – 80’s. (Carter also produced many of the Q-Jets for Rochester.) It has the same base mounting pattern as the Quadra-Jet, but differs significantly in construction. The center section of the T-Quad is made from a phenolic resin which does not conduct heat as efficiently as metal. The metering circuits are suspended from the aluminum cover, and with the phenolic bowl, this combination results in very good fuel isolation from the heat in the manifold. Of course, less heat in the fuel provides better and more stable performance. The T-Quad was first placed in service in 1971 on the MOPAR 340 engines. It later was used on the complete range of Chrysler engines ranging from the 318’s to the 440’s. However, this was the beginning of the serious attempts by the industry to gain control of the exhaust emissions, and as a result, all of the production T-Quads were calibrated relatively lean. As many of the interior passages in the carbs were designed smaller for the leaner mixtures, richening the actual metering components does not have the expected effect. This explained why my early attempts in using these carbs was not very satisfactory.

Jon pointed out that Carter also produced two additional series of the Thermo-Quads for aftermarket performance use. The 9000 Series, known as “Super-Quads”, are for street and strip use, and are rated at 800 CFM. The “Competition” series is for strip only, and is available in both 850 and 1000 CFM sizes. The 9000 units incorporate in all electric choke, and are available with either Chevrolet or Chrysler throttle linkage. These two performance oriented series have overall fuel metering more suitable to better acceleration as compared to the original equipment production models. The factory settings for the production T-Quad fuel/air ratios ranged from 15.7 to 1 on the early carbs to 18.7 to 1 on the late “Lean Burn” applications. By contrast, the 9000 series is calibrated at 14.7 to 1, and the Competition series is set at 14.2 to 1. The 14.7 ratio provides a combination of good acceleration and reasonable fuel mileage, and the proven combination of small primary throttle openings combined with very large secondary openings, provide the same good overall performance from idle to top RPM as the Q-Jets.

Jon offered to furnish one of the factory re-conditioned 9000 Series 800 CFM for trial on my 455 powered LeMans wagon. We are always interested in such tests, and arrangements were made. In addition to the carb, Jon also sent a Carter “Strip Kit”, consisting of a range of different primary and secondary metering components, and several sizes of inlet seats. The “Chevrolet” linkage style carb was selected for our test. After installation of the carb exactly as received in the sealed box, (more on the installation later), we noted excellent street drivability and throttle response. Transition from idle to cruise was very smooth, and both the primary and secondary power circuits operated predictably and without hesitation. The idle adjustments had adequate range to obtain optimum idle characteristics. The carb height is within .1 inches of the QJet, and the air cleaner base mount is identical. Accordingly, my air cleaner and cold air system fit perfectly. At the drag strip, we obtained 1/4 mile performance within .1 second and 1 MPH of my highly tuned 800 Q-Jet. A later trip with a slightly richer secondary mixture provided performance within several hundreds of a second on my Q-Jet. (In both cases, the carbs were switched at the track for best accuracy of testing.) I suspect that further careful tuning of metering and fuel pressure would have obtained equal or even better 1/4 mile performance. At Jon’s suggestion, the regulated fuel pressure was set to 6# for all T-Quad testing. a have found that 3# maximum provides the best performance with my Q-Jets). Remember that my Q-Jet has been tuned/optimized over the years and hundreds of drag strip runs, and the engine already has excellent heat isolation in the form of cold air into the carte, and welded closed exhaust heat crossover openings that isolate exhaust heat from the carte. Bracket racers now using the T-Quad report that it is very consistent under different weather conditions, and requires far less adjustments during a typical race in order to compensate for changes in temperature and humidity. The unique non-metallic fuel bowl and suspended metering components undoubtedly are responsible for the improved consistency! We were impressed with the “out of the box” performance of the Thermo-Quad (12.40’s to 12.50’s at 109 to 110 in the heat of summer).

Concerning the installation, the Thermo-Quad throttle linkage connecting point did not provide the correct throw (travel from idle to full throttle) to match my throttle cable, so a small aluminum bracket was installed on the throttle linkage (using existing holes), to obtain the correct travel. Additionally, the linkage sets outboard further from the left side of the carb, necessitating a slight readjustment of my throttle cable bracket. The throttle return spring required custom fitting. Lastly, the fuel inlet is located on the right side toward the rear of the carb. We bent a length of 3/8″ aluminum tubing (copper could be used) to extend from the existing carb connection to the T-Quad inlet, and used short lengths of rubber fuel hose to connect both ends. The 9000 T-Quad is equipped with a PCV inlet, and features both full and ported vacuum. In summary, the Thermo-Quad will fit well in place of a Q-Jet, but is not an exact replacement. However, in comparison to some of the after market carbs, it was quite easy to install, and no permanent changes that affected the Q-Jet reinstallation were required.

For more information on the application and availability of these performance carbs, or service on your carb, give Jon Hardgrove a call at 573392-7378 in Eldon, Missouri. Be prepared for some interesting information on these units as well as Pontiac history. Jon has been an ardent Pontiac enthusiast and collector for years and is presently collecting parts and planning for a very fast Pontiac 350 powered street/strip machine.

Some readers will note the absence of any comments about the Holley spread-bore carbs in this, or any of my writings. In my opinion and testing, the Holley spread-bore is a basic unit that does not have the sophisticated metering circuits that both the Q-Jet and T-Quad incorporate. While it will start and run decently, it will not provide the combination of excellent drivability, fuel mileage, and drag strip performance that the Rochester/Carter spread-bores provide. Similar views and reports have been received from many of our readers.