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

thanks Daniel for the reply...pitty, nobody seems to run that carb on the 351C....so i'm on my own perhaps not on this forum here , as i have seen Panteras "for sale" with the Edelbrock carb..... yes, i studied the charts for jet/rod calibration and ordered a set to play with...first remark was that the charts propose a combination jet/rod which #'s are not in a calibration "set" ??? so i've ordered a set plus additional some rods....i've got already a better performance by taking the secondary jets a step greater as out of the box....( drilled them a bit larger ) and now waiting for a calibration set i was thinking of grinding a flat side at the "rich " rod part....as it's all about square inches "jets" openings obstructed by square inches "rods" material .....to calibrate the gas flow.

Is there a explication of why a Holley should make better performance compared to the Edelbrock ? Myself had the choke tower grinded off on my Holley and didn't needed it : just a few pedal pumps , start the Corvette and only needed a few seconds to help reving before i had good idle....perhaps i don't need all that choke equipment on the 351C either ... should experiment with it ....even open that butterflie/tunnel stays there as a air "restrictor"... ???
Ok, no big deal ...but i like to play with it to see if i can gain some HP's.... ..and yes, i should instal a wideband A/F gauge to get it on the right spot.

We tune quite a few of the Edelbrock carbs at the shop. I have to say that they are dead simple to tune as long as you have a wideband air fuel meter. You dont have to spill any fuel like you do with the Holleys. Remember when tuning that the chart provided by Edelbrock is your friend. If you follow this chart you can typically have the carb tuned in one disassembly. Also remember that the chart is only for cruise and part throttle so dont look to it for full throttle fueling. This is done by the secondary jets only and has its own small table for jet selection. These are in my opinoin one of the best street carbs out there even though there are those who dont like them.

thanks Blaine for the info.... jets and rods are arrived so i can play with them now good to hear full throttle is more about the secondary jets...what i already did by drilling them a bit larger... now i can replace them with #'s jets....would like to have the engine on the "rich" side , rather have once a foulded plug then a blown piston ? see my plugs are still white to light brown...got my '70 Corvette great running at the time but with rather "dark "plugs
Strange ,Edelbrock info gives only a chart for cruise/power mode about the primary jets/rods and just some remark that one step higher secondary jets as out of the box can "in some cases" give better performance ... ??? Think our Cleveland wants/needs some numbers higher could be because we have headers and i also a performance camshaft.....
keep you posted what i found running best....

Hi there.

Might I suggest my home page to maybe help out, it's not too long:
http://www.tuningmadeeasy.com/...a-carburetor-part-1/
and
http://www.tuningmadeeasy.com/...a-carburetor-part-2/

And by the way, never drill jets, they're finely machined and flow tested, and drilling them destroys that, you risk flowing less if you drill them. Buy a set (as you have done)

You're welcome to contact me directly if you want.

just to give a "final" answer to my Edelbrock tuning ....after playing with jets & metering rods i came back to primarys the stock setup but secondarys 0.104 jets instead of the 0.098 stocks ( Blaine had it right )... had today a 100 miles trip and never got a flat "spot".... as experienced with all other combos , also felt a rich running with richer primarys.Remarkeble is the ( spring loaded,adjustable ) air valve in the secondarys, with the out-of-the-box setting it was , once i had the jets right , as a turbo-lag going WOT ( a hesitation and then woooop ) .... now i have the "perfect" results with that spring on the softest setting, which means almost no resistance : if i push the valve open by hand , it almost doesn't return closed due the very little spring tension.
I must add here that the electric choke wasn't working as there is no voltage on that wire...so i rotated the choke housing to full open for the moment as to be certain that valve isn't in a partial closed position... could have influenced some of my trials...next job is to follow that wire ( not easy in our mid-engine cars ) and see where it is supposed to get it's voltage, i assume original there was no eletric choke ... so someone has ever got placed that extra wire in the car... so far , i seem to have no problems starting the engine without that choke valve working....
so.. do i need it ?

greetz from a happy man with a great running engine now....