2bbl to 4bbl conversion?

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glhx

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ive got a Holley 600 cfm 4 barrel on my engine. The 2 barrel that originally came on the engine was a Carter BBD 285 cfm.

The conversion formula is cfm x 1.414
Does this mean that my 285 is actually a 403 cfm carburetor?
 
No. Four barrels are rated at a 1.5” of mercury vacuum standard during the rating process and two and single barrel carburetors are rated at 3” of mercury. A two or single barrel will only flow 70.7% of what a four barrel will at 1.5 inches of mercury.
 
Or the two barrel flow rating divided by the 1.414.
 
Well; the carb will flow whatever the atmosphere is able to jam thru it.
A slanty at 1000rpm and WOT might pull 65cfm. by 3000 it might be up to 196cfm @100% VE. By 4500 shes wanting 293cfm at 100%VE. But in stock form;more likely is 60% VE so 176cfm.
A 318 of course will be 318/225=1.41 better or 249cfm@4500@60%VE.
In reverse math; 285cfm is good for a 318 at up to 5150 at 60%VE That doesn't mean anything special will happen at 5200.lol It's just done making power...... which is Ok cuz the stock cam power-peaked a long time ago anyway,lol.
Ok so but say you put a 600 on the stock 318, and increased the VE to 65%. And with a cam that peaks at 4500, you might shift at 4800, so the 318 might pull 286 cfm@4800........ thru that 600, so the only change is that the air is moving more slowly thru the 4bbls 4 throttle valves than when it was hustling thru the 2bbl.
But will it by faster? IDK, math ain't everything.
If nothing else, it sure sounds faster.
 
96.2% of everything I know about automobiles is that it sounds faster.. And my fastest vehicle only has 3 cylinders so I probably don't know squat!
I had a 3-cylinder once.
A 1975 GT750 Suzuki 2-stroke motorcycle; affectionately called a WaterBuffalo, because it was liquid cooled and weighed about 7000 pounds. Great motorcycle, unkilleable. In it's time, for 70hp, it was pretty quick..... in a straight line .......... at sealevel. Or at least on level ground. Climbing mountains, I had to downshift twice getting up the Crowsnest Pass as the CV carbs couldn't keep the needles up. But man did it fly on the Prairies.
Two years later it became obsolete when Suzuki introduced the 750 4 cylinder 4-stroke. And a year after that the GS1000cc at just 6 pounds heavier than the 750. After I got that 1000, I quit buying new motorcycles. What a machine. It wasn't particularly great at any one thing; but it was really great at everything.
 
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The reason I’m asking.....is if you look at my picture. I ran double 2 barrel Carter BBD carbs back to back. Both are the 285 cfm carbs.

Now I’m running a 600. I took them off due to a stripped fuel inlet that was leaking.

When I saw the conversion I was trying to figure out if I was running 800cfm progressive

Or 570cfm

I want to make sure I’m not over carbureted in cfm.
It did run fine before I took it off. I had it set at 60% for the primary and then the secondary kicked in....Was hard to tune but I did get it tuned...and the fact that I was working with a worn points distributor was a contributing factor I think....as well as other reasons.

I was also dealing with two master carbs. They were run identical with factory jets. The second carb wasn’t sealed at all....lol. It should have been a slave carb and was basically a vacuum leak. I don’t know how it even ran like that but it did and it ran ok. Kind of hard to start.

know a little bit more about timing and jetting now and want to fine tune them. Seal up the second carb a lot better and rejet them both. Total timed to 36° at 3k

Just making sure it isn’t too big....and belongs on something that flows more cfm like a built 360 instead of a stock or slightly built 318.

The 318 stock 1976 compression and will eventually have a 262 cam with ported heads and matching parts....streetmaster intake with headers and such

These carbs did run that engine for two years and they seemed ok but it doesn’t hurt to get second opinions.
 
I don't think Mopar ever made a 2bbl so big you couldn't run two of them at WOT on any 318 to 360
Trying to get the car moving is another story,lol. Ima thinking lotsa rear gear, a bit of a loose TC, and lazy low-rpm timing
I would not "seal" one carb. The transfers have to be operational or no matter what, you will have a tip-in sag, until the dried up transfers get moving.

I've never run two of those So I can't help except to suggest lazy idle timing. The lazy timing will require a larger throttle opening. With a small cam, you may have to restrict the PCV to a smaller idle feed. This too will require more throttle opening to compensate.
If it was mine I would set all the transfer slots to square to a little wider than tall, and reduce the fuel level at least a sixteenth. Then I would set the idle-speed with timing and restricted idle-bypass air. but I wouldn't touch the Curb-idle screws again after the sync was set.
I would start the timing curve about 150 rpm higher than the idle speed. The rest of the timing curve is compression and rear-gear related. I would not be in a hurry to get to max timing but I would try a two-stage curve with about 28* at 2800, then slowing down.
 
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There’s really no reason to run two carters.

Rochester’s are much better.
But I had the adapter and the carbs sitting around and decided why not see what happens.

So I hit McMaster and bought some hardware and it was done

What do you think about sealing up most of it and leaving the transfers exposed? Right now it’s got a pretty big ring of light around it. Maybe run them square and slightly exposed. There is a paste made out of graphite, jb weld, and acetone that hardens up and helps seal where you need it. The Rochester guys use this quite a bit when running primary carbs on the back and with tri power carbs.


Even my Holley has some light around the secondary but it isn’t much. It made a huge difference in tuning when I made that as little as possible as someone had messed with it before I owned it.

It was open like 1/16” on the secondary and jetted down 2 with the wrong pump cam ....too small.

I put the 600 back to spec and it was a lot better but it took a lot of understanding to figure that out....you helped a lot with that.

which led me to looking at the problems with the double 2 and why it was so hard to tune.
 
Thank you aj
All that makes a lot of sense.

Definitely not in a hurry o get it back on. Working on it slowly

I want to get the engine finely tuned and then start messing with the dual carb set up so I'm not chasing my tail

I'm running 2.76 gears and a stock converter so it might be pretty slow off the line.
 
If you get the transfers synced up and running with a reduced idle-timing, and the engine has no tip-in sag, then you can reduce the fuel from the idle mixture screws.
You gotta keep in mind that the transfers are your primary low-speed fuel suppliers, and the mixture screws are just idle trimmers. So if you double up the transfer fuel, and the AFR goes really fat, the you're gonna have to figure out how to burn all that fuel,lol... or how to lean that circuit out. It's not easily adjustable so you are gonna have to re-engineer it. You're gonna have to find the fuel jets, or just holes in this case which if I recall, are at the bottom of the removable clusters,and make 'em smaller. And you may have to fudge the air-bleeds at the top too. And perhaps trim the whole she-bang with fuel level.
I suppose on a single-plain intake you could stage the one carb to come in later, like you did before, which might make it easier to take off with, but I think there will still be some cylinders going lean. Maybe with a larger plenum spacer, but I'm just guessing.
With a dual plain I would think non-progressive would be just too jumpy no matter how lazy the timing was. I think I would just run a 2800 TC, and let the low speed go fat and let it get lost in the fluid coupling.
Get the idle right with all 4 transfers contributing first, and see where it takes you.
I think I would change the throttle ratio so it takes more pedal for a given throttle opening. That will make it easier to drive.
A long time ago, I wanted to do just what you are now doing, so I'm real interested to see how you make it work,
As for the two-stage timing curve, most smogger-teens, with electronic ignitions,that I have worked on had those from the factory. If you pop the timing plate up, you will see the two advance weights and their springs. You will see one spring with a long loop. Until the flyweight s are spun up to the limit of that loop, it does nothing. So both weights are working on the other spring until that point. Don't put a chevvy spring on there, cuz the timing will come in, in a real big hurry. Find a spring that is a lil lazy to start advancing, but hits in the neighborhood of 24 to 28 degrees by about 2800 to 3000. Then let that long looper slow the curve down to all in around 3400 to 3600. You may have to start with an initial timing of 8 or 10 to to get a slow enough idle speed with all 4 transfers flowing. I wouldn't worry about it cuz it's an automatic, and if it's idling it ain't pulling. And with a 2800TC it won't be pulling for a long time, and when it gets to 2800, well, hopefully you will have figured out how to get the 28* up there,lol. How the timing gets to TC stall would not be a big deal for me. Cuz when the teener hits 2800 with 28* , it will be a happy camper.
If you have a factory 1750 type stall convertor, your tune is gonna be a whole lot harder to hit.
If it's fat below stall, I would put some hotter plugs in there until you get it bugged out, or else it may start eating plugs.
Anyway, like I said, I've never done what you are doing, so this is all just Idle chit-chat.
Someone else may be along shortly and have a whole nuther story.
No matter what, Happy HotRodding.
 
Thank you aj
All that makes a lot of sense.
Definitely not in a hurry o get it back on. Working on it slowly
I want to get the engine finely tuned and then start messing with the dual carb set up so I'm not chasing my tail
I'm running 2.76 gears and a stock converter so it might be pretty slow off the line.
IMHO; 2.76s will never fly with dual 2bbls, it's gonna be hard enough to tune with the stock TC.
IMHO, even a 4 bbl is not gonna fly... because
I'm guessing your 318 at a 4000 power-peak and 75% efficiency, will mathematically want 286cfm
So with a 600,Firstly; if you start to bring the secondaries in any sooner than ~3000, you're likely to have a bog. And 3000 will be 33mph, so the reason for the 4bbl is what again? Ok but say you are a magician, and get the secondaries to start coming in at 2800. Hang on a sec; the 318 at 2800 and 100% efficiency, will need (318 x2800)/3450 =258 cfm... but at say 50% efficiency more like 129cfm....and you have a theoretical 600cfm available. So hang on; lets do the math backwards; and I get 600cfm is good to 8680 rpm at 75%efficiency. Hmmmmmm. And 75%, IMO, is very generous.
I'm guessing your 318 at a 4000 power-peak and 75% efficiency, will mathematically want 286cfm
Soooo the point is; what are the primaries good for. Well firstly your 318 is never gonna drag the primaries down to 1.5 inches so lets just re-rate the 300cfm primaries to 3 inches and call them a 212. So 212 is good to 3300. Ok so 3300 is where the primaries may top out at 3" mercury. So 2800 is a reasonable start point.
But hang on, peak power in a stock 318 is at about 4000 rpm and the secondaries let's say in your combo are good for 14hp... at 4000, and you are running out at 3300 so we can throw this out there; from 3300 to 4000 we are expecting 14 hp, so that is about
2hp per 100 rpm, beginning at 3300.
I think this is pretty generous. And 3300 with 2.76s is 36mph in 2.45 low gear with 26.1" tires.
The point of this exercise is to see that the secondaries even if the are wide open at take off, the engine will not pull 1 cfm more thru them, than it could thru the primaries alone, until after 3000 rpm@33mph, probably closer to 3300/36mph. Course if it spins the one tire, you could get to 4000 in a hurry. But the rear end is not gonna take that for very long.
But it's worse; What if the 318 never pulled the vacuum down to even 3" Oh my.

You say those 2bbls are 285s@3" mercury............... each. Ok I'm just messing with you now.
If the math is right, then your engine will be pulling ~128cfm thru each of those 2bbls, and I bet it could do that with the butterflies somewhere around half closed. So what jets are you gonna start with? IDK either,lol.
What about the jets in the 600? IDK that either.

There are exactly three or four cures for this; supercharging, a higher stall, plus gears, or a manual trans.
My go-to would be a hi-stall and at least 3.23s. The hi-stall lets your engine wind up to where the power might be. The bigger rear gear, multiplies that power out the back door. Together they are a very fine 1-2 punch. Even if you only find plus 80 ftpounds with the hi-stall; switching from 2.76s to 3.23s multiplies those meager 80 another 3.23/2.85=plus 17%, to 94. But something else happens with gears; the engine spools up quicker between shifts. So now you are accelerating quicker with more power per unit of time. NOW the 4bbl has a chance to work... but with the stock engine, shifting at 4500, the Rs drop to 2655 on the 1-2 shift, and then you wait, and wait, and wait, to get back up on the cam with the secondaries again open.
With a stock LA318, I'd sooner have a hi stall and gears than a 4bbl. and a free-flowing exhaust before the 4bbl too. I'd probably be happy with a big 2bbl. Or a really really, really small 4bbl.
IDK
But I always wanted to try a pair of deuces.
I always wanted to say; I got a 323 with a pair of deuces, and a three on the tree.. with a 3.23............ It's late, I'm off to bed;lol.
 
IMHO; 2.76s will never fly with dual 2bbls, it's gonna be hard enough to tune with the stock TC.
IMHO, even a 4 bbl is not gonna fly... because
I'm guessing your 318 at a 4000 power-peak and 75% efficiency, will mathematically want 286cfm
So with a 600,Firstly; if you start to bring the secondaries in any sooner than ~3000, you're likely to have a bog. And 3000 will be 33mph, so the reason for the 4bbl is what again? Ok but say you are a magician, and get the secondaries to start coming in at 2800. Hang on a sec; the 318 at 2800 and 100% efficiency, will need (318 x2800)/3450 =258 cfm... but at say 50% efficiency more like 129cfm....and you have a theoretical 600cfm available. So hang on; lets do the math backwards; and I get 600cfm is good to 8680 rpm at 75%efficiency. Hmmmmmm. And 75%, IMO, is very generous.
I'm guessing your 318 at a 4000 power-peak and 75% efficiency, will mathematically want 286cfm
Soooo the point is; what are the primaries good for. Well firstly your 318 is never gonna drag the primaries down to 1.5 inches so lets just re-rate the 300cfm primaries to 3 inches and call them a 212. So 212 is good to 3300. Ok so 3300 is where the primaries may top out at 3" mercury. So 2800 is a reasonable start point.
But hang on, peak power in a stock 318 is at about 4000 rpm and the secondaries let's say in your combo are good for 14hp... at 4000, and you are running out at 3300 so we can throw this out there; from 3300 to 4000 we are expecting 14 hp, so that is about
2hp per 100 rpm, beginning at 3300.
I think this is pretty generous. And 3300 with 2.76s is 36mph in 2.45 low gear with 26.1" tires.
The point of this exercise is to see that the secondaries even if the are wide open at take off, the engine will not pull 1 cfm more thru them, than it could thru the primaries alone, until after 3000 rpm@33mph, probably closer to 3300/36mph. Course if it spins the one tire, you could get to 4000 in a hurry. But the rear end is not gonna take that for very long.
But it's worse; What if the 318 never pulled the vacuum down to even 3" Oh my.

You say those 2bbls are 285s@3" mercury............... each. Ok I'm just messing with you now.
If the math is right, then your engine will be pulling ~128cfm thru each of those 2bbls, and I bet it could do that with the butterflies somewhere around half closed. So what jets are you gonna start with? IDK either,lol.
What about the jets in the 600? IDK that either.

There are exactly three or four cures for this; supercharging, a higher stall, plus gears, or a manual trans.
My go-to would be a hi-stall and at least 3.23s. The hi-stall lets your engine wind up to where the power might be. The bigger rear gear, multiplies that power out the back door. Together they are a very fine 1-2 punch. Even if you only find plus 80 ftpounds with the hi-stall; switching from 2.76s to 3.23s multiplies those meager 80 another 3.23/2.85=plus 17%, to 94. But something else happens with gears; the engine spools up quicker between shifts. So now you are accelerating quicker with more power per unit of time. NOW the 4bbl has a chance to work... but with the stock engine, shifting at 4500, the Rs drop to 2655 on the 1-2 shift, and then you wait, and wait, and wait, to get back up on the cam with the secondaries again open.
With a stock LA318, I'd sooner have a hi stall and gears than a 4bbl. and a free-flowing exhaust before the 4bbl too. I'd probably be happy with a big 2bbl. Or a really really, really small 4bbl.
IDK
But I always wanted to try a pair of deuces.
I always wanted to say; I got a 323 with a pair of deuces, and a three on the tree.. with a 3.23............ It's late, I'm off to bed;lol.

Gonna look at this a lot closer and study the answer

To add to this
These carter Bbd 285s

The high tops are different than the low tops. The high top have edelbrock jets that can go in there. So I have a large choice on jetting. Don't get the low tops.

The distributor on there now is a gm style hei. The cheap ones from eBay. The procomp version is the higher quality

Not sure if it will work with the dual carb as it was really long and close to the older points distributor. The gm cap is much larger. We put that points in there just to get the car on the road but it was really shot.

The gm hei is so easy to total time and recurve.

Different fuel fittings Should allow to run it. If not I'll recurve an electronic 76 distributor i have here.
The smog era curves are complete garbage for performance.

With the gm style I can put bolts on the advance plate that are ground down. It makes total timing simple.

When I get back into it I'll let you know here and see where it goes

THe adapter came from vintage speed.

Also ......that street master is a weird intake. It's a single plane
With small runners. Basically made for torque
 
Different fuel fittings Should allow to run it. If not I'll recurve an electronic 76 distributor i have here.
The smog era curves are complete garbage for performance.
IMO, you can't paint them all with the same brush;but I agree; for those Ds that were curved with 30 or more degrees in the mechanical advance, that NUMBER is garbage, but the fix is pretty simple.
I have a 73Mopar D in mine, with the timing limited to 20* and slightly delayed to all in by 3400, and my combo ran a 93 Eighth at 3457 pounds and 930 ft elevation.
Every engine wants it's own low-speed curve; and the lower the rear gear and stall, the closer to the smogger curves she's gonna want. You just cannot run a race curve in a streeter, unless the entire combo is more race than street,.... cuz the vast majority of time, the street engine is gonna be at sub max-timing rpms. You might have to recurve your D 20 times to get it right; Or perhaps 3 times in 8 miles just to get it close, as I once did. The Mopar magnetic pick-ups have been working fine for me since 1971; in 48 years I have seen just one failure.
If you have a stock compression, stock cam engine, then,IMO, you'll probably want a stock Mopar distributor with a minimum amount of fudging, to run those two duals. Maybe just limiting the mechanical timing.
 
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This adapter? ...But with Carter BBDs and progressive linkage? Oh, I thought you had something else.
This one should run hardly different than a 4bbl,almost like a double-pumper.
But I would still leave the back carb open some to keep the transfer slots alive, to prevent a tip-in sag.
That looks pretty adjustable...
and pretty too.
This might sound crazy but;
I might try idling it on the secondary transfers, and dry up the primary transfers a little, but use the primary mixture screws as trimmers; then use pump-shot to eliminate the tip-in sag of the primary carb, if any appears with that funky intake.. As soon as the car is moving it should be fine. Or maybe you already tried that.
With the back carb having a pump on it, you might be able to close the throttles further than I imagine.
In any case, idle-timing to just off-idle timing is gonna play a big role in the success of this combo.You might have to back the idle-timing way down, But then you are gonna want to get the curve started asap. That will require a two-stage curve for sure.Don't know if the Mopar long-loop spring will stay on the HEI pins.
It looks to be a challenge for sure.

2x2withcarbs1.jpg
 
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