Edelbrock's Thunder Series AVS Carburetor - Tweaking An Edelbrock AVS
Tuning For Horsepower And Fuel Economy- It Can Be Done
From the February, 2004 issue of Mopar Muscle
By Steve Dulcich
Photography by Steve Dulcich
We had Edelbrock's latest...
We had Edelbrock's latest carb, the new Thunder Series AVS,on our 340 Dart. To get the best combination of power and economy, we headed to Westech to dial it in with a razor-sharp dyno tune.
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...
The Dart's engine was our dyno-test-mule 340, which had been detuned from its peak dyno configuration with smaller headers, no carb spacer, lower ratio rockers, and a full exhaust system. We figured that even in its milder state, there was well over 400 hp at the crank from our solid-cammed 340.
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...
One of our main objectives was to dial in the jetting of our new AVS carb. With 800 cfm at hand, it offers significant airflow for our 340. Even with the factory calibration, the output seemed strong, and fuel economy was up significantly over our previous race-prepped carb. For a dual-purpose balance of performance and economy, it seems to be a very good carb.
Our first dyno pulls were...
Our first dyno pulls were made to obtain a baseline output level, and then to check the air/fuel ratio as recorded on the dyno. Power stood at 315 rear-wheel horsepower at 5,800 rpm.
Crunching the numbers, it...
Crunching the numbers, it was clear the engine was rich at WOT, indicating a leaner mixture could improve WOT output. We had an average recorded A/F ratio of 11.04:1 at WOT. It was time to go into the carb for a jet change. The plan was to tune the WOT mixture with jetting changes, and then check and dial in the part-throttle cruise-mixture, which we hoped could be accomplished with only metering-rod changes.
A lot of guys find changing...
A lot of guys find changing jets in this style of carburetor a little intimidating, but getting into the AVS for a jet change isn't all that complicated. The top air horn needs to be removed to access the jets. Begin by unhooking the fuel line.
The various linkages, which...
The various linkages, which run between the main body and the top air horn, need to be disconnected. This includes the accelerator-pump linkage, shown here. Just remove the small clip and the rod is disconnected.
The metering rods should also...
The metering rods should also come out before removing the top air horn. loosen or remove the screws to get the cover plate out of the way, and out the rods come.
Torx-drive screws bolt the...
Torx-drive screws bolt the upper air horn casting to the main body.
The air horn should lift cleanly...
The air horn should lift cleanly off the carb body. If it sticks, a few light taps with a plastic mallet will usually free it. The gaskets can typically be reused many times, though we recommend a good drenching with WD-40 before reassembly.
The jets are in the bottom...
The jets are in the bottom of the main casting. The fuel bowl baffles can be pulled up and out to make access to the secondary jets easier.
Rear Wheel Horsepower
Superflow Chassis Dyno
Tested At Westech
On the secondary side, we...
On the secondary side, we replaced the original 407 jets with a pair of 392s. On the primary side, the factory 413 jets were replaced with leaner 404s. With the jetting change, power went up to 325 at the wheel, while the average air/fuel ratio leaned out to 12.21:1. More power and less fuel-we like that.
Satisfied with the WOT mixture,...
Satisfied with the WOT mixture, we turned our attention to the part-throttle and cruise response. With the leaner jets, we now developed a lean condition at part-throttle.
To fatten up the cruise mixture,...
To fatten up the cruise mixture, while keeping the WOT setting unchanged, we needed a metering rod with the same power step diameter at the tip, while having a thinner part-throttle step higher up the rod. We settled on a 6547 rod, which measures .065 inch on the cruise step and .047 inch on the power step. The stock rods were 6847s, which measure .068 and .047 inch on the cruise and power steps, respectively.
The rod change brought the...
The rod change brought the cruise mixture to where it needed to be, showing a 13.5:1 air/fuel ratio. While it cruised cleanly, there was a lean hole when applying throttle from cruise to power, or tip-in.
We figured that getting the...
We figured that getting the rods to switch from the cruise to power position sooner would allow the mixture to fatten up a little sooner as tip-in throttle is applied. Changing the metering-rod springs will affect how early the rods will move from cruise to the power position. Metering-rod springs are rated by the vacuum level at which they will move the metering rods to the power step. Stiffer springs will lift the metering rods to the power position sooner. We used the silver springs, which move the rods to power mode at 8 inches of vacuum.
With the power and part-throttle...
With the power and part-throttle tuning done, the icing on the cake was setting the idle. With some careful adjustments using a vacuum gauge, we finally ended up with 11 inches of vacuum at a 750 rpm idle-not bad for a cam with 239/243-degrees duration at .050 and .537/.549-inch lift.