For a solid 15-year run from 1971 until 1984, the Carter ThermoQuad (TQ) was ubiquitous on Mopar four-barrel V-8 engines. Really an evolution of the long line of Carter four-barrels, the TQ was a break in tradition, moving from the square-bore configuration of the earlier AFB and AVS carbs to a spread bore layout similar to the Rochester Quadrajet. The original OEM ThermoQuad, debuting on the 1971 340, was a unique unit with different castings, metering circuits, and tuning parts. These 1971 carbs are quite rare, and pretty much orphans today outside the world of authentic restorations. Subsequent carbs were produced in huge numbers, and saw only minor changes through rest of the production run. Though the TQ was an all-new carb, the distinguishing characteristics of earlier Carter four barrels were still there. This was an era where fuel efficiency and emissions were major considerations, and the highly-refined ThermoQuad carb, with its small primary barrels, dual booster venturi, and three-stage metering rods was perhaps the most advanced four-barrel carburetor on the market.
1. TQ’s come in two sizes, big and bigger. The small version features 13⁄8-inch primary ba
Configured with either 13⁄8 or 11⁄2-inch primaries and massive 21⁄4-inch secondaries, the TQ delivered enormous airflow. Wide-open, the "small" TQ was rated at 800 cfm, while the larger unit spec'd at 850 cfm, blowing away the airflow capacity of any previous OEM Mopar four-barrel. While the ThermoQuad certainly had the flow to support big power, the timing of events curbed its chance at glory as a high performance carb. Though originally employed by Mopar on the stout 1971 340 engine, most TQ carbs were on duty during the dark ages of Chrysler's emissions era. This period of declining horsepower, band-aid smog control systems, and compromised engine calibrations was not going to inspire any legends.
The signature feature of the ThermoQuad was its unique phenolic center body, providing an insulating effect to the fuel, significantly reducing fuel temperature (a 20-degree average reduction, according to Chrysler). Although the concept was sound, the "plastic" main body immediately raised the suspicions of old-time mechanics. As the years rolled on, the basic TQ was equipped with any number of combinations of add-on devices, including a crude throttle position sensor for the ESC system, idle stop solenoids, blow vent solenoids, vacuum-controlled auxiliary air bleeds, a high-altitude compensator device, and in the latest versions, a feedback primary metering solenoid. Given the generally terrible factory specs and drivability of those "smog" engines, and all of those extra gadgets on the carb, it was enough to turn most gear heads away from the mighty ThermoQuad. Even to this day, the ThermoQuad remains one of the most misunderstood and even feared carburetors ever produced.
2. The upper airhorn casting carries the majority of the internal parts, including the nee
While the OEM TQ received more than its fair share of disrespect, those in the know marveled at its engineering features, recognized its potential for efficiency, and embraced its tremendous airflow capacity. Nothing else off an OEM engine could deliver the wide-open airflow of a ThermoQuad. A well-tuned unit pulled atmosphere so forcibly it would rattle the fenders, and while ingesting air it delivered a bellowing moan that could be heard for blocks. Due to the TQ's huge production volume, along with a general lack of recognition in the performance world, a used ThermoQuad represents about the cheapest carb per cfm of airflow imaginable.
When considering a carb for the 326-cube small-block freshly built for our 1972 D-100 project, we wanted a low-buck unit that would maximize efficiency, and yet be able to support close to 400 horsepower. For this application, the ThermoQuad was the obvious choice. We had plenty of OEM cores on hand, though these carbs can still be readily found at rock-bottom prices at the boneyards and swap meets. We selected a Carter number 9004, an 850-cfm unit originally from a 1975 360 application. The plan was to give the carb a basic rebuild, and kits for these carburetors are still readily available. Although the ThermoQuad has a reputation for complexity, in reality the carb is fairly simple to rebuild. We'll go through what it takes to get an old TQ ready for service, pointing out some of the common mistakes and trouble spots.