We had Edelbrock's latest carb, the new Thunder Series AVS,on our 340 Dart. To get the bes
A few months ago, we brought you an introduction to the new Thunder Series AVS carb from Edelbrock. We installed the carb on our 340-equipped '69 Dart and were more than happy with the performance. We had previously been running a race-calibrated, mechanical-secondary carb. It worked great for wide-open throttle response and power, but at cruise it sucked down fuel like booze going down in a Girls Gone Wild video. In fact, when making the high-speed, 200-mile trek from the shop to Westech's dyno facility, a fuel stop was required little more than halfway there. With the old carb, we estimated the fuel economy at an appalling 7-8 mpg. With the swap to the AVS carb, we noticed the fuel economy was up, while the WOT performance was still good. The balance of performance and economy seemed good with Edelbrock's factory calibration. However, we were intrigued with the idea of seeing just how well we could dial in the combination with a sharp dyno-tune.
As we made our way to Westech, it was clear the AVS had improved the fuel efficiency of the Dart. With the out-of-the-box calibration, mileage had improved to 12.5 mpg-not exactly thrifty, but it was an increase of 66 percent. It was a gain that made us take notice with our racy 340 engine. Our plan was to tune the carb for maximum performance and economy for our 340 engine's combination.
The Dart's engine was our dyno-test-mule 340, which had been detuned from its peak dyno co
Longtime readers will remember the solid-lifter 340 in our Dart as one of our dyno mules. Ultimately, we had seen 477 hp on the engine dyno with this mill. However, it was detuned from the hottest test configuration as installed in the Dart. Without the race headers and carb spacer, we had dynoed it at 458 hp at the flywheel. We also removed the 1.6:1-ratio rockers, which were worth 15 hp in a similar solid-cammed 360 and were running through a full 2.5-inch exhaust with quiet, reverse-flow mufflers. All in all, the engine was detuned a good 50-60 hp from its top engine-dyno configuration. Still, the 340 was strong and responsive, and we expected it to turn in a healthy performance on the chassis dyno.
We tuned and tweaked on the dyno and found 10 rear-wheel horsepower, a decent gain for only jetting changes. More importantly, the carb was dialed in to our engine combination, gaining more than a full ratio point in air/fuel calibration before we left. This means more power from less fuel, from a carb that was already well balanced in this regard. We tuned the full-throttle performance, but we also took the time to dial in the cruise mixture for maximum efficiency and driveability-after all, this is a street car. We were happy with the improved mileage we found over our old carb, but we were downright thrilled when our return run from Westech netted us 16.6 mpg. And we didn't baby it. The accompanying photos show what we accomplished on Westech's dyno. The calibration and jetting numbers are always going to be unique to the specific engine combo in question, so don't be tempted to just drop in the same jets, rods, and springs as we did. The lesson here is a good dyno tune can work wonders in terms of getting the most out of a combination.
One of our main objectives was to dial in the jetting of our new AVS carb. With 800 cfm at
Our first dyno pulls were made to obtain a baseline output level, and then to check the ai
Crunching the numbers, it was clear the engine was rich at WOT, indicating a leaner mixtur
A lot of guys find changing jets in this style of carburetor a little intimidating, but ge
The various linkages, which run between the main body and the top air horn, need to be dis
The metering rods should also come out before removing the top air horn. loosen or remove
Torx-drive screws bolt the upper air horn casting to the main body.
The air horn should lift cleanly off the carb body. If it sticks, a few light taps with a
The jets are in the bottom of the main casting. The fuel bowl baffles can be pulled up and
Rear Wheel Horsepower
Superflow Chassis Dyno
Tested At Westech
On the secondary side, we replaced the original 407 jets with a pair of 392s. On the prima
Satisfied with the WOT mixture, we turned our attention to the part-throttle and cruise re
To fatten up the cruise mixture, while keeping the WOT setting unchanged, we needed a mete
The rod change brought the cruise mixture to where it needed to be, showing a 13.5:1 air/f
We figured that getting the rods to switch from the cruise to power position sooner would
With the power and part-throttle tuning done, the icing on the cake was setting the idle.