Presently I have two carbs, and EDL-1405 (currently on a 350 engine) and an older Holley, that I have no idea what it is. Only numbers I can find are L-7455 and 1676. (It came with the 402 I purchased)

Would either of these be ok on a stock (other than cam and intake) 402? The Holley for sure would need rebuilt. It's been sitting for years. The Edelbrock seems just fine, as my 350 runs nice and strong. I haven't decided on a cam and intake yet. Would that affect the carb decision?

Either carb is not suitable for a 402. The Holley crosses to a 450 cfm quadrajet replacement. I would either go with a 750 Vacuum secondaries List# 3310 or even better a 770 Street Avenger by Holley.

Dave

Ok, thanks, Guess I'll be carb shopping now also

Off the top of my head I think the 1405 is the 600 CFM manual choke Edelbrock Performer carb? If so that would work quite well on a 402. You may, depending on other parameters, leave a very little top end on the table but at the gain of much more tuning capability.

I have run both the 600 and 750 Edelbrock carbs on my 402 with a relatively big hydraulic roller cam (236/242 @ 050). With the 750 the vacuum changes too much too fast and it is very difficult to tune the stumbles out. When the vacuum drops below 7" or so at low RPMs the idle circuit stops providing any fuel. And the mains have not started yet so the engine starves lean.

With the 600 the vacuum changes are much more progressive so you will have much less trouble with the idle circuit dropping out before the mains come on. The 600 is actually a bit of a spreadbore with smaller primaries where the 750 is totally square (they both have the same size secondaries). The higher venturi air speed (and thus venturi vacuum) also assists fuel atomization.

My motor will spin the tires about the same with either carb, but I can stomp on it from just about anywhere with the 600. Running the 750 I had to be at or above 2000 RPM before I could stomp on it.

Some of this will depend on your cam. I might not have the same issues with the 750 with a milder cam and more idle vacuum (I get 11"). Some of this is that a 402 is really not that much bigger than a SBC. I would probably have better luck with the 750 on a 454 or larger. So consider that you will be happier with a bit too little carb than you will with a bit too much in your planning.

Not to slam Edelbrock carbs but they design the bog into their own carburetors. They have an air door that slams open with no speed adjustment. Worse of all there is no like squirter to overcome the bog. Even Quadrajets use little fuel bleed holes for their air doors, actually there is a tube and a well that draws fuel up for the air door opens.

Dave

Not to slam Edelbrock carbs but they design the bog into their own carburetors. They have an air door that slams open with no speed adjustment. Worse of all there is no like squirter to overcome the bog. Even Quadrajets use little fuel bleed holes for their air doors, actually there is a tube and a well that draws fuel up for the air door opens.

Dave
"Not to slam..."?? Then you go on a total slam. But even worse you make a claim and then contradict your own point. I rather doubt that the air door slams open. I will conceed that it likely overshoots a bit on initial opening, but that is far from slamming open. And it will quickly retract back to stable state, likely within a pump shot's duration.

It is possible to have a bog with any carb and in function, if not actual design, the Edelbrock carbs are really not any different. They do have a secondary enrichment device just as the Quads do, and it even works under the same principle. It is a demand-driven design as opposed to a pump that provides enrichment whether it is needed or not.

It is probably closer to the truth to say that Edelbrock carbs are fairly basic carbs with more fuel tuning capabilities than an equivalently basic Holley (the Holley power valve is a bit limited compared to the variety of rods for Edelbrocks). As such they work fairly well within the range of applications for which they were designed/calibrated. Outside of that range they start to have problems just as any other basic carb would. What is useful is to understand where the limits are so you can compare them to your own setup and get appropriate expectations of how well they will/might work for you.

You will not find adjustable air bleeds, secondary idle circuits, or annular boosters in the Edelbrock carbs. So if you want/need to go outside the basic design box covered by Edelbrock... there are a lot more Holley (or derivative) models designed specifically to cover those different requirements.

My motor will spin the tires about the same with either carb, but I can stomp on it from just about anywhere with the 600. Running the 750 I had to be at or above 2000 RPM before I could stomp on it.?????????????????

What I am saying if you have a bog with a Edelbrock Performer series carb how do you cure it? On a Quadrajet there is a spring on the secondary air valve you can tighten or loosen. Actually I have even drilled a bigger hole in the choke pull off/ air valve vacuum diaphram to even speed them up more. At least their new thunder series carbs they have stepped up and added some adjustment feature to their air valve.

My motor will spin the tires about the same with either carb, but I can stomp on it from just about anywhere with the 600. Running the 750 I had to be at or above 2000 RPM before I could stomp on it.?????????????????

What I am saying if you have a bog with a Edelbrock Performer series carb how do you cure it? On a Quadrajet there is a spring on the secondary air valve you can tighten or loosen. Actually I have even drilled a bigger hole in the choke pull off/ air valve vacuum diaphram to even speed them up more. At least their new thunder series carbs they have stepped up and added some adjustment feature to their air valve.
A fair question.

Since we are talking about a squarebore carb, what if you bog before you even get to the secondaries?

Based on jet sizes, there is a fuel flow area for the main/secondary of about 0.01802 sq in. The idle jet is about 0.00096 sq in. or about 5% of the mains. If the proportions hold, that corresponds to about 37.5 CFM of airflow that can be fed by the idle circuit. But note that this will only feed if there is sufficient manifold vacuum to overcome the idle air bleeds, which is about 5-7" depending. Once vacuum drops below this point no fuel will flow from the idle circuit. This point comes up quick with the larger primary bores of the squarebore carb.

A 402 CI engine displaces about .23 CF. If we presume 80% volumetric efficiency (a common working optimistic assumption) at max torque of 6000 RPM, we get around 552 CFM to feed the engine. Hmmm... that sounds like a 750 carb is overkill. VE has to go above 87% to need more than a 600 CFM carb on our little BBC.

On a 750 CFM squarebore carb you have 375 CFM on the primaries presuming equal flows between primary/secondary. And continuing the presumptions, the smaller 600 with identical secondaries runs 225 CFM through the primaries.

What is the volumetric efficiency (VE) under 2000 RPM? Maybe 40%, maybe less? It will be maximized at peak torque and less everywhere else. The following table crudely estimates airflow and shows the percent of rated flow for the primary bores on the 750 squarebore and smaller 600 carbs:

RPM CFM 750 % 600 %
2000 92 25 41
1500 69 18 31
1000 46 12 21

Now this is really a rough/simplified analysis which misses a lot of details. But on the low end 40% is probably a bit optimistic for VE. The real point is at some low end number the main fuel circuit will not generate enough venturi vacuum to pull fuel because there is insufficient air flow. Further, low vacuum will reduce fuel atomization. And these issues are independent of secondary operation.

Note that the spreadbore carbs like the Quadrajets are specifically optimized around this problem. The smaller 600 CFM Edelbrock has a similiar benefit from different bore sizes.

Normally you get overlap between idle circuit and main circuit. But if the vacuum drops too fast and flows are too low, this overlap dissappears. That is when you get the stumble.

Very good analysis. I agree the 600 will be a snappier carb on your 402, but given the rpm you are ringing your engine at, it seems the 750 would benefit. Your cam being 236/242 @ 050 its large enough to be meant for 6000 rpm shifts. Like you said earlier it seems like you are saying the carb kinda stalls, the engine doesn't pull enough vacuum to turn on the main metering. My own car I hit it right at idle once in a while , but with the converter in it my rpm's flash right 4500 rpm's, with your stick its hard to get your rpms to climb like that.

Dave

Actually I rarely ever see 6000 RPM in my driving around on the street :D I just use the wheel hop affects as my butt dyno :p

The cam is another interesting, long, and unrelated story in itself. Suffice to say that you may or may not get a deal off of Ebay but you will always get an education.

The whole carb thing was just supposed to be a temporary thing until I could do the EFI conversion anyways. But after running carbs with a vacuum gauge along with a wide band O2 you could say I have learned a fair amount about how carbs work. And having both CFM ratings has allowed me to compare and contrast a bit as well. For example the 600 is expected/tuned to pull harder on the secondaries than the 750 based on the much different secondary air bleed configuration.

Bringing it back to the original topic, I still think that for most little BBC's the 600 CFM carb will be a fine choice. And what it may give up in absolute top end it more than makes up for by being a fairly forgiving tuner with good driving characteristics.