[jetallman]

Putting my 455 together with some extra parts I have around as a budget build, I have an edelbrock 1406 600 cfm sitting on the shelf I'm thinking it's to small, I have two of them and would like to do a duel setup later but just want it going for now

[A.W.Dille]

The 600 Edelbrock is way too small for a 455, I wouldn't even run one on a 350 Pontiac. Find a good Q-jet and you'll be alot happier.

[Bruce Meyer]

To just get it going I don't see a problem. When you have more funds then step up to a larger carb. I have run a 600 DP Holley on a mild 455 before and it ran strong.

[jetallman]

Originally Posted by Bruce Meyer

To just get it going I don't see a problem. When you have more funds then step up to a larger carb. I have run a 600 DP Holley on a mild 455 before and it ran strong.

I have an older 750 holly DP but never had any luck getting it to work right since new, never could get fuel level to set right so I threw it on the shelf, I may think about refurbing it also

[MescaBug]

I had a 1406 on the stock 400 in my 72 GTO. Switched to a 72 Q-Jet, no noticeable difference in performance.

I used the same 1406 on a 455 with 7K3, no porting, RAIV cam, running strong but couldnt get an acceptable air-fuel ratio. Switched to Holley 750 with mech sec., what a difference. The 1406 was a little too small for that combo.

On a stock 455, I dont think the 600cfm carb is too small. When do you use that much airflow anyway except on the strip?

[jetallman]

My bad, after double checking, one turned out to be a 1407 which is a 750 cfm. I'm thinking I will do a regasketing on it since it's been sitting for years and give it a shot.

[gtofreek]

One thing about carbs is you don't want to run a carb that barely supplies the amount of air the engine requires. In other words, if you have an engine that has an air requirement of 600 cfm, you don't want to run a 600 CFM carb because your working the carb at 100% of it's capacity. This makes the engine strain trying to suck the air through it, and power will suffer dramatically. If you have a well prepared carb that can maintain good air fuel ratio at any air flow, and the engine now only works the carb at 705 of it's rated capacity, the engine will breath much easier and make more power. It's like sucking all your breath through a drinking straw compared to a 1" diameter straw.

A good example is when we used to build engines for the NASCAR SW tour. The engine was 353 CID. They had to run a 390 Holley carb. Jeff, here at this shop built one that flowed about 540 CFM. On the dyno the engine made 580 HP. He swapped on a 950 HP Holley that he built for it[for the open carb tracks] that now flowed 1045 CFM and the engine made 673 HP. the 390 was choking off almost 100 HP. Obviously this is a race engine with good flowing heads, better than what we are talking about here, but it still gets the point across.

[jetallman]

I'm gonna try the 1407- 750cfm, I also have a 1405 and a 700 Holley DP all sitting around to compare, I'm gonna regasket all of them since they have been off a car and on a shelf for several years. Surely one of them will work

[paul s.]

ABSOLUTELY use it. They are nice carbs. Especially since you just want to get it going for now. Easy carbs to tune too since all you have are metering rods, jets, accelerator pump setting and step up springs for the basic tuning.

[jetallman]

Originally Posted by paul s.

ABSOLUTELY use it. They are nice carbs. Especially since you just want to get it going for now. Easy carbs to tune too since all you have are metering rods, jets, accelerator pump setting and step up springs for the basic tuning.

I noticed the one on you homepage engine. good looking engines you build there
Think I'll attach mine to a Torker 2 I have on the shelf and see what happens

[MG12692]

The edelbrock is a reliable carb however if you have the factory dual plane manifold I would run the Q-jet. Nothing sounds like a Q-jet when the secondaries open up!

[paul s.]

Originally Posted by jetallman

I noticed the one on you homepage engine. good looking engines you build there
Think I'll attach mine to a Torker 2 I have on the shelf and see what happens

Thanks for the compliment, I try.

I believe that will be a good match for you. A small(ish) carb on a well built 455 will pull HARD on the idle circuit causing a rich condition and a single plane dampens that yet supplies the engine pretty well up to 5,500. Assuming a mild combination anyway. I use that 750/Torker II combination on my Street Performance engines destined to go in 2ng Gen F-bodies.

I am finding even the 400's equipped with the RPM intakes, even with their short stroke, are pulling hard on the 750's as a few customers of mine noticed they couldn't lean them down enough at idle. So I went to the 800's and so far, no complaints.

[Cliff R]

I don't agree anyplace that the Edelbrock AFB clones are good units. It is true that they have good parts support, and relatively easy to tune. They down side of them is that the secondary air door is NOT adjustable. Direct testing here, dyno, street, and at the track has shown that they can and will stumble/hesitate/bog going quickly to full throttle on many applications, and this can NOT be tuned out.

Edelbrock knows this, and why they introduced the Thunder Series (AVS clones). They have a spring loaded adjustable air door, and a much better choice for these engines if you choose the square bore intakes and those type of carburetors.

As far as cfm requirements, you will see no difference from 600cfm to 800cfm below about 4500rpm's in a well built 455. On the dyno going larger with the carburetor will start to show significant power gains at higher rpm's. Even though all the formulas for cfm to cid requirements show 650-700cfm is capable of supporting 455cid to past 5000rpm's, in actual use they will not. Those engines are underquare and by design not overly efficient at high rpm's with the conservative cross section the heads we typically use on them have. This doesn't mean they woln't make great power, but it takes a lot of camshaft, and they will see small carburetors on them as a restriction when they really start sucking on the intake manifold (high rpm's). I wouldn't use any carburetor smaller than 750cfm on any 455 build, 800-850 would be better, unless you've put a tiny cam in one and it's not going to pull past 4000rpm's anyhow...FWIW.....Cliff

[CROCIE JR]

If a 600 on a 455 doesnt pull vacuum at WOT at peak rpm. I would run it. I try to run the smallest carb I can get away with. 1.5" (or more) of vac. in the manifold, at WOT at peak, carb is too small. Keeping the primary throttle bores small for good response, snappy-ness and proper idle mix and control are important to me. But on square bore carbs the pri. and sec. usually grow at the same rate as rated CFM increases. This is why Qjets are awsome. My problem with AFBs is the metering rod tips change in almost all sizes. They are not standardized like a Qjet. You have to also change the jet to keep either WOT mix or cruise mix the same. You cant just change the rod to lean down just the cruise, because it has a different "power" tip and now WOT is affected. Goofy.

Bottom line, tape a vac. gauge to your windshield. This is normally taped next to the fuel press. gauge!

[paul s.]

If an engine will not accept full wide open throttle from a moderately sized AFB style carburetor, that tells me more about the engine than it does about the carb.

Many times a poorly tuned ignition curve will ACT like a poorly tuned or misapplied carburetor because combustion efficiency is lacking. If you were to have, let's say, a 9:1 scr 461 with a Ram Air IV clone cam and a 2.11" intake valve with a 30 deg. seat with the cam advanced 4 degrees and 12 degrees ignition timing on an "outdated" intake---take your pick of any older intake manifold, invert it; Do the intake runners "T" as they leave the plenum, or do they distinctly branch out their own directions? Like the old tripower intakes, if you can look into one port on the passenger side, and see the port exit on the driver side--- what do you think that does to the charge entering a given cylinder @ 90 ATDC intake cycle, or on overlap?) Now imagine a long, lazy intake lobe and a lot of overlap...

You are probably going to need every crutch imaginable to get that reversion happy combination to run well down low as your metering signal has turned to... well, you know and idle vacuum will necessitate the use of fast bleed lifters to "trick" the engine. What will the use of the fast bleed down lifters do to this combination's dynamic compression? Get ready to use premium.

Now, Why do I say "reversion happy"?

1. Early opening intake valves are known to introduce exhaust pollutants into the intake tract due to residual combustion pressure in the cylinder on overlap.

2. Outdated intake manifolds like previously stated.

3. Thirty degree seats have been known to invite reversion during overlap along with the low lift flow we all know about. Makes sense as the intake seat shape adjacent to the exhaust valve has a nice "lead" to it.

4. Big overlap numbers... that speaks for itself.

5. ALL of these factors lead to a low vacuum condition where fuel droplet size is erratic to say the least, and THAT, leads to detonation. As much or more than a 1/2 a point of compression will.

I do agree that if you have an engine like this, where multiple carburetor modifications are necessary to get the engine to idle and transition properly, then the out of the box AFB probably isn't for you.

I believe the real culprit here is the decision-making process when the components are chosen for the build from the very beginning. There are a number of airflow restrictions in an intake tract. Assuming the camshaft and the carburetor are the only ones that matter are a BIG mistake.

I am all for using stock parts whenever possible. You just have to consider HOW IT ALL WORKS TOGETHER.





The metering rods on the AFB carbs are stepped so you can absolutely change only the cruise mixture if you are so inclined. As an example, a popular rod is the 71/47, which is the "base" rod in a number of carbs. There are also 75/47, 73/47, and 68/47 rods--- the "47" being the "power" tip designation, and as one can see, it stays the same. Of course, there are other rods that use different power tip sizes.

[Cliff R]

Paul, my comments do NOT pertain to an engine accepting the AFB style carburetor at full throttle. Any one I've every tested with correct tuning will tear your head slam off to the shift point once the secondaries are on line.

My comments ONLY pertain to transition onto the secondaries, which is mostly controlled by the weighted secondary airflap. Since it is NOT adjustable, they will often times be a lean condition, especially on really high HP to weight ratio set-ups when you bring in the secondaries really quick, from idle, or just off idle, etc.

This is a momentary lean condition caused by the flap opening BEFORE any fuel is flowing, and it can NOT be tuned out with jets, metering rods, and pump shot from the primary side accl pump.

Your comments would be correct otherwise, if we were discussing full throttle tuning past transition. Hope this clears things up some?.....Cliff

[Cliff R]

Just a comment on Rhoad's lifters.

Back to back testing with Rhoad's lifters vs standard 951 lifters show no difference anyplace from 3000rpms and up. What we did see was 2" more vacuum with the Rhoad's lifters, and noticeably quicker throttle response.

Rhoad's lifters should only be used with longer duration (advertised) camshafts. Adding them allows for the use of a slightly larger camshaft than would normally be ideal for good idle characteristics in a particular build. These engines benefit further by adding high ratio rocker arms, so when the lifters are fully pumped up the cam acts even bigger than what it is at higher rpm's.

The dyno numbers we achieved with an "old" design flat cam, Rhoad's lifters, and high ratio (1.73) rocker arms rivaled a very well chosen "modern" hydraulic roller cam. During the same dyno testing we also tested a solid lifter cam, but chose it with 10 degrees more duration at .050", and it made less power than the other two cams did.

Pretty good learning experience, although a bit costly. Consider the cost of the flat solid cam/lifters, then a complete HR set-up, plus two weekends on the dyno.....YIKES!......Cliff

[paul s.]

I believe I do understand your comment. The ONLY time I have ever seen a momentary lean condition was when I was attempting to tune a low compression Buick 350 in a stock weight '72 skylark saddled with a TIGHT convertor. Under cruise conditions and applying throttle to "tip in" the secondaries, that did exhibit a lean condition like you state. Otherwise, secondary actuation lean conditions have always been cured with a different jet size.

In my experience.

EDIT: They did make secondary air valves of different weights for guys that wanted to tune them. I believe the weights are different for a 600 vs a 750 and so on.

[paul s.]

Originally Posted by Cliff R

Just a comment on Rhoad's lifters.

Back to back testing with Rhoad's lifters vs standard 951 lifters show no difference anyplace from 3000rpms and up. What we did see was 2" more vacuum with the Rhoad's lifters, and noticeably quicker throttle response.

Rhoad's lifters should only be used with longer duration (advertised) camshafts. Adding them allows for the use of a slightly larger camshaft than would normally be ideal for good idle characteristics in a particular build. These engines benefit further by adding high ratio rocker arms, so when the lifters are fully pumped up the cam acts even bigger than what it is at higher rpm's.

The dyno numbers we achieved with an "old" design flat cam, Rhoad's lifters, and high ratio (1.73) rocker arms rivaled a very well chosen "modern" hydraulic roller cam. During the same dyno testing we also tested a solid lifter cam, but chose it with 10 degrees more duration at .050", and it made less power than the other two cams did.

Pretty good learning experience, although a bit costly. Consider the cost of the flat solid cam/lifters, then a complete HR set-up, plus two weekends on the dyno.....YIKES!......Cliff

I'M SURE THAT WAS EXPENSIVE!

I am not questioning your numbers, or the outcome of your testing, but I have a hard time believing a roller could not keep up with a flat tappet. I guess what I am saying is: was the combination optimized for the flat tappet? What was the peak rpm for this combo? What springs were chosen? Open and closed pressures? Installed height? Total valve lift for each? What rockers were used with the rollers?

See my point?

[carbking]

No comments about the clones, as they are permanently banned from our shop; however:

Cliff - some (not all) of the genuine Carter AFB's can definitely be tuned on the secondary transition by modifying the secondary starter circuit. And while I understand your comment concerning the auxiliary air valve, personally I have always liked the weighted air valve better than the spring-loaded ones; as they are more reliable for street engines. Springs fatigue. Of course, not everyone has my supply of Carter parts.

And just a tip if/when changing air valves: many enthusiasts add/subtract weight from the air valve weights when, in fact, better timing adjustment may be realized by changing to valves with different angles.

And just for the record, my own Pontiac 350 has an 850 CFM Carter sitting on top. It runs great. Just wish I had more time to enjoy the car!

Jon.