Having your cake and eating it to. Isn't that the ultimate goal for every hot rodder? For a hot street car, this means having lots of power without turning it into an undrivable monster. What we are looking for is a car that idles clean, cruises down the road with decent fuel economy and yet has the beans to click off an 11-second trip through the quarter. Not a terribly tricky thing to do with a boosted or EFI car, but it can be a little challenging to pull off with just a simple carburetor. So when Edelbrock introduced their new 800-cfm AVS, we thought there was a chance we could put a combination around it that would give us what we wanted.
These new carbs from Edelbrock are a bolt-on replacement for the units that came on many of the factory Mopar musclecars, but they'll also work just fine on most any manifold that has a square flange bolt pattern. Edelbrock has refined the tooling and manufacturing process over the years so while this carb traces its roots back to the old Carter AVS, it is actually a new design that has been re-tooled and improved where possible. One major difference is the fact that this style of carburetor is now available with an 800-cfm rating. The 800 cfm is a great compromise size for a healthy big-block motor, big enough for 550-plus horsepower, yet small enough that idle quality and part-throttle operation are not heavily compromised.
What makes the AVS a great carburetor for a high-powered street car is the use of a spring-loaded door that controls the secondary air flow, as well as the use of stepped metering rods to control the air/fuel mixture. Since metering rods are available with different step combinations, it is possible to independently alter the air/fuel ratio for both cruise and power modes with a single rod change. For example, a metering rod that is thicker on the big step but thinner on the small step will richen up the power mode air/fuel ratio (A/F), while simultaneously leaning out the cruise mode. Accomplishing that same trick on a double-pumper carb would require disassembly of the carb and drilling the power valve restriction channel.
In order to get a feel for this new AVS carb, we strapped our test car to a Dynojet chassis dyno that was equipped with both a load brake system and a wide-band oxygen sensor. This setup allowed us to see what the air-fuel ratio was doing throughout the entire operating range of the engine. What we were looking for was a tune up that resulted in an A/F ratio of 14.5:1 at idle and cruise, but richened up to 12.5:1 when the engine was in power mode. Power mode for this engine was determined to be anytime the manifold vacuum was less than 6 inches. This power mode point varies for different engines depending on the compression ratio and the size of the camshaft. In our case, the engine idles at 850 rpm with a vacuum reading of 14 inches, so we had plenty of operating range in which to work. The power mode circuitry in the AVS carb is turned on by manifold vacuum working against the step up spring. When the manifold vacuum drops due to load, the step up spring forces the metering rod up and the step down in rod size allows more fuel to flow through the jet. There is a variety of step-up springs available from Edelbrock so that the turn-on point of this power mode enrichment circuit can be varied.
The first dyno pull with the 800-cfm AVS right out of the box netted us a near perfect 12.5 A/F ratio, but the mixture at cruise was too rich since the A/F ratio there was only 12:1. Consequentially, we had to make a jet change and a metering rod change in order to get the cruise A/F ratio leaned out. Leaning out the cruise circuitry then forced us to richen up the secondary jets by one step in order to get the WOT A/F ratio back to the desired 12.5:1. Total tuning time only took a couple of hours and then we were on our way. The engine now idles smooth and steady, gas mileage is significantly better in cruise mode, and the engine puts down more than 450hp to the rear tires at WOT. Sounds like having your cake and eating it too!
Here is why the Edelbrock...
Here is why the Edelbrock AVS makes a great street/strip carburetor. These are the metering rods and they allow you to quickly tune the cruise and WOT air/fuel ratio. The step in the rod changes the effective jet area between high and low manifold vacuum conditions. The small area at the tip of the rod is the power step, the thicker area next to it is the cruise step
Strapped to the dyno and ready...
Strapped to the dyno and ready to grunt! Our '65 Coronet 500 test mule runs big 315/45R17 tires on the back to put the power to the rollers. 6,500 rpm in 5th gear is 145 mph, so those rollers are really whizzing by.
Typical Mopar stroker motor....
Typical Mopar stroker motor. Putting a 440 crankshaft in a 400 block and topping it with Edelbrock heads created 550hp on the engine dyno. Rear wheel torque is over 400 ft.-lbs. from 2,500 rpm to 6,000 rpm, yet it idles at 850 rpm with 14 inches of vacuum. Our camshaft is an ancient Mopar Performance 0.528 solid lifter grind. I suppose we should step up to something modern one of these days!
The AVS carb can be plumbed...
The AVS carb can be plumbed with dual fuel inlets, but we chose to only run the single feed in this setup. This turned out to be fine for our power levels.
Metering rods can be changed...
Metering rods can be changed in seconds by removing the single hold-down screw. A metering rod swap can alter the fuel curve fairly significantly so if the tune-up is close, you do not even need to change the jets.
We used Edelbrock tuning kit...
We used Edelbrock tuning kit #1480. This kit is actually for the AFB model 1407 but we figured it would work. By the time you read this, there may be a specific kit available from Edelbrock for this carb.
A braided steel fuel line...
A braided steel fuel line allows you to take the top off the carb without breaking the connections loose. The tall throttle bracket setup is from the Mopar mavens at Mancini Racing. It works perfectly with the Edelbrock carb on a tall manifold.
Dyno operator Steve Heino...
Dyno operator Steve Heino has the AVS opened up and is swapping the jets. We couldn't get exactly where we needed to go on this engine with just a metering rod change so the top of the carb had to come off. No fuel bowl draining is required though since the gasket surface is above the fuel level.
Our first dyno pull with the...
Our first dyno pull with the carb right out of the box gave us a maximum of 446hp at the tires, but the jetting was a tad rich at part throttle. After a change in jets and rods to fix that rich condition, we picked up about 6hp everywhere on the power curve. All figures are SAE corrected.
|Tune Up menu for 800 CFM AVS Carburetor|
|Primary Jet Size: 0.113|
|Installed metering rod: .068 x 0.047|
|Cruise Mode|| Power Mode ||Rod needed|
|4% richer||No Change|| 0.065 x 0.047|
|4% richer|| 12% leaner|| 0.065 x 0.057|
|8% richer|| 8% richer|| 0.063 x 0.037|
|4% leaner|| 4% leaner|| 0.070 x 0.052|
|4% leaner|| 4% richer|| 0.070 x 0.042|
Mixing different metering rods and jets together allows the carb to be tuned richer or leaner in either cruise or power modes. This chart only shows a few common combinations, there are actually several hundred different possibilities available if all combinations of rods and jets are used.
|Thunder Series AVS carbs currently available|
|Manual Choke||Price||Electric Choke||Price|
|Prices are as of 11/03 from Summit.|
Edelbrock has several different models of the AVS now in their line-up. Prices are very competitive with typical double pumper type carbs.