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4 barrel or 2 barrel carb?

Acgreen3256

New member
Ive got a 77 scout II traveler with the 5.6L V8 2 barrel factory carb, wondering if i should get a 4 barrel carb. what are the pros/cons of more barrels, how will my engine handle a 4 barrel carb, and if i do decide to go for 4 barrels what brand/model of a 4 barrel carb should i get?
 

Scoutboy74

Moderator
A good 2bbl carb is all that engine needs. Aftermarket Holley 2bbls in the 350cfm rating work well on these engines right out of the box. Avoid the 500cfm rating, however. To do a proper 4bbl conversion, you also need a matching 4bbl intake manifold. Further, there are both spreadbore and squarebore versions of 4bbl intake manifolds for the 345 and 392. Same with the 4bbl carbs. Squarebore means all four holes are the same size. Spread has small primary holes and much larger secondary holes. Its not a one size fits all scenario, so definitely not something to do in willy-nilly fashion. Using an adapter to make a 4bbl carb fit a 2bbl manifold is totally pointless. You don't get the use of the secondaries with that adapted setup, which defeats the purpose of having a 4bbl carb in the first place.
 

1975IH200

Member
Holley List 0-80350 for a 350 CFM carb with electric choke.

Holley List 0-7448 for a 350 CFM carb with a manual choke (cable operated choke).

Don't bother with a 4-barrel carb for your SV-345......unless you get a good Carter ThermoQuad carb and spread-bore intake manifold and know what you are doing.
 

mallen

New member
Holley List 0-80350 for a 350 CFM carb with electric choke.

Holley List 0-7448 for a 350 CFM carb with a manual choke (cable operated choke).

Don't bother with a 4-barrel carb for your SV-345......unless you get a good Carter ThermoQuad carb and spread-bore intake manifold and know what you are doing.
My truck came with a Motorcraft 2bbl carb with a broken flange. Since I had to get a new carburetor I decided to go the 4bbl route. Easy as pie. It should take a few hours or so_Or so I thought. So I found a spread bore manifold on ebay for a reasonable price. I got the manifold and tried scrubbing off the 40 years of baked on grime to no avail. I finally gave up and took it to a machine shop and had it hot tanked. While that was the first instance of the cost and effort going up, it had a bright side, because I found a machine shop with guy with a big grey beard that said "I havent seen one of those IH SV engines in years" (So, you HAVE seen one before!) He hot tanked it for 25 bucks and I threw some Case IH Red tractor and implement paint on it (for another 10 dollars for the paint and hardener, which BTW held up perfectly and still looks great) I found a Thermoquad on a 74 1110 at the local wrecking yard a month later for about 50 bucks. So far, it appeared my luck was holding. Then I found out the previous owner had completely destroyed said Thermoquad. (which was probably why the otherwise serviceable looking truck was sitting there to begin with.) On top of that, it didnt occur to me that I could have made ANY similar Thermoquad into the one I need for that truck by using the rods and jets from the ruined one, so the box containing the dissasembled Thermoquad was thrown away when I moved) I then got an Edelbrock 1406 that was taken off a running truck from Craigs List (arent they all) and an adapter plate for the spread bore manifold and an aftermarket throttle cable bracket. After much tuning and many sets of rods and jets and a full rebuild with an Edelbrock rebuild kit,and a Edelbrock phonelic spacer to stop the fuel from boiling in the summer ($+$$+$$$), I finally got it running reasonably "not particularly bad". (I still would not call it good). I also found that Edelbrock does not sell the manual choke bracket for their carburetors, so its far more difficult to convert a automatic choke to a manual one than it is to go the other way. After many hours on the phone to them, I was told that the part was simply not available and that even though they would love to sell one to me that they just could not get one. So I changed tracks and called aftermarket tech support and told them that the bracket on my brand new carburetor had developed a crack, broken and as far as I could tell probably fallen off on the road somewhere. Two days later the bracket arrived in the mail. So here I am some years down the road, and I still don't like the way it runs so I finally broke down and picked up a "Superquad" aftermarket Thermoquad (which the previous owner, bless his soul, told me absolutely does not work and at the very least needs a rebuild kit if its even fixable at all, which as it turns out it is) and found three of the original International Harvester master rebuild kits that contain the jets and rods and plan on putting that on this spring, so finally I will have an fuel system that SHOULD work right. Remember, this was all supposed to take an afternoon to get running.

The moral of this story is, while changing things around CAN end up with a vehicle that runs correctly (for various values of "correctly") its usually a lot more effort than you expect,and a lot more money than you expect, and the farther you change it from stock the more effort it takes to get it all tuned in and running right.
 
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FDChappie

Active member
I put a Holley 390cfm square bore carb on my warmed over 345. Got about $1k into it and can get 14mpg on a tank. It also has better throttle response at low rpm's. There is a write up on the tuning process on this forum.
 

47 dodge

New member
A good 2bbl carb is all that engine needs. Aftermarket Holley 2bbls in the 350cfm rating work well on these engines right out of the box. Avoid the 500cfm rating, however. To do a proper 4bbl conversion, you also need a matching 4bbl intake manifold. Further, there are both spreadbore and squarebore versions of 4bbl intake manifolds for the 345 and 392. Same with the 4bbl carbs. Squarebore means all four holes are the same size. Spread has small primary holes and much larger secondary holes. Its not a one size fits all scenario, so definitely not something to do in willy-nilly fashion. Using an adapter to make a 4bbl carb fit a 2bbl manifold is totally pointless. You don't get the use of the secondaries with that adapted setup, which defeats the purpose of having a 4bbl carb in the first place.
May I ask why not the 500cfm two barrel?
 

Scoutboy74

Moderator
Yes you may. For one thing, these engines can never utilize that much delivery. They rev too slow and the valves are floating well before you get to the point of needing that much cfm. For another thing, that particular list number carb is designed for engines that are running at or near wide open throttle all the time ala circle track racing. Kinda taking me right back to my first point. Again, these engines are rarely seeing more than 3k rpms and even when they do go above that it's just momentarily, rather than sustained. It's a lot like using a sledge hammer to drive a thumbtack. They run like pure shit with those carbs.
 

47 dodge

New member
Thanks. Then when would you say there is a need for the 4 barrel? I do have a 4 barrel manifold off a 392, although not sure if that will fit the 345. I have a 345 in a school bus by the way. 65 mph is at 2800rpms, and for me that is all day at that. It is converted to a motorhome. I am currently running with a Holley pro jection unit. The oxygen sensor has gone bad and they are not available anymore, so thinking about going back to a carb.
 

Scoutboy74

Moderator
Yes, the 4bbl intake will fit the 345. It is important that the carb design match the intake design, ie square bore to square and spread bore to spread. The 392 intake should be a square bore...all 4 holes same size. For a heavy vehicle, like you have, a small cfm 4bbl such as the Holley 4150 that came oem on many of the 392 engines can be a help. Vacuum secondaries ensure that the engine is only able to use what it demands in terms of air and fuel, based on engine load. Part throttle cruise on flat or down hill will only be running on the primaries.
 

47 dodge

New member
I do have the carb that was on the intake, and yes it is a square bore carb and intake. I will look into what that might need. It had been sitting for many years .
 

47 dodge

New member
Well just put a 2300 350 cfm carb on for now. My 4 barrel manifold is in rough shape the exhaust crossover is rusted out. I am pleased to say it runs great, right out of the box. No tuning was needed other then setting the idle speed. I do have an air/fuel meter so can tell what is going on. Not sure if it is my imagination or not but power seems slightly down.
 

47 dodge

New member
Yes did check and the throttle is opening fully. I was actually surprised that the throttle linkage matched right without adjustment from the pro jection unit.

Must admit the common idea is bigger is better, more power etc. so going smaller is just contrary, even when it really does make sense.
 
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