Reply to "Edelbrock Thunder carb"

> anybody using a Edelbrock (Thunder series) carb on his 351C ?

I didn't see anyone respond to your post so I thought I would comment.
I don't have one on a 351C but I do run an AFB on a 3.5L Rover in my
Triumph TR8. The AVS is essentially an AFB with a spring-loaded secondary
(versus the AFB's counter-weighted approach). AFB and AVS carbs use
stepped rods which slide up and down in the jets (actuated by a
vacuum-referenced spring and piston). When cruising, the rod is on
the lean step. Under high load, low vacuum, conditions the rod moves
to the rich step. The rods and springs are easily accessed from the
top without disassembling the carb. To get to the jets you have to
remove the lid. Carter AVS carbs used longer 3 step rods under a raised
cover on the primary side. Most AFB's used 2 step rods under a flat cover.
I'm not sure if the Edelbrock AVS versions uses the AFB 2 step rods or
the 3 step but, from a quick glance at their site, it appears that 2 step
rods are used. 3 step Carter rods could likely be substituted if you used
the raised covers.

On a Holley, fixed jets and power valves serve the same purpose as the
rod/spring/jets. During cruise, you run lean on the jet only and during
high load and/or WOT the power valve introduces additional fuel. To tune
a Holley correctly, you have to install the proper power valve and quite
often you have to adjust the size of the power valve channel restrictors.
The PVCR's are fixed orifices that are located behind the power valve.
On most Holleys, to adjust them they must either be drilled out or epoxied
shut and re-drilled to a smaller size. Some aftermarket Holleys and clones
do have screw in PVCR's now. The bottom line is that it is easier to
tune an AFB for fuel economy but it can certainly be done to a Holley.
Holleys will generally make better power, though.

The secondary butterflies on an AFB are opened mechanically in response
to accelerator pedal position. However, a counter-weighted flap, the air
valve, is positioned over the secondaries. The opening rate and amount
the flap opens is controlled by the airflow demand and the counterweight
resistance the airflow must overcome. In theory, it shouldn't open the
secondaries until there is sufficient air flow. One of the drawbacks of
the AFB is that to adjust its secondary opening rate you must add or
subtract material to/from the counter weights. The AVS, like the later
Thermo Quads and Quadrajets, uses a spring-loaded air valve that can be
easily adjusted. AFB's use a conventional booster venturi to deliver the
secondary fuel flow. The AVS (Air Valve Secondary) uses an adjustable
spring-loaded air door that is placed above the fuel-discharge point.
The placement creates a low-pressure area at the fuel-discharge point so
conventional booster venturis aren't needed. Instead AVS carbs used fuel
spray bars. Originally, the AVS was only offered as an OEM carb, primarily
on Chrysler products but also on some Chevrolets but Edelbrock offers it
as an aftermarket carb now. It's also available is a version with
spring-loaded needles and seats (for off-roading).

If it is of any use, I went through a tuning exercise using a wideband O2
on my TR8 and documented it. Edelbrock also has a tuning guide that you
can download with metering rod and jet charts. Some combinations are
not on those charts and Carter offers different jet and rod sizes so I
wrote a program to calculate the effective cruise and power jet areas of
all the possible combinations. I also have a copy of the Federal Mogul
Carter carb guide. Let me know if any of that is of interest.

Dan Jones