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2bbl vs 4bbl
https://slantsix.org/forum/viewtopic.php?t=57270		 Page 1 of 2
Author:  camiking23 [ Thu Mar 12, 2015 7:28 am ]
Post subject:  2bbl vs 4bbl
Someone want to help me out with the hp rating and mpg between the two? Gains and drops Etc
Author:  Nicademas [ Thu Mar 12, 2015 9:13 am ]
Post subject: 
All I say it what I have experienced. I had my 225 rebuilt over a year ago. They put a 265 cam in it and shaved the head to bring up the compression to 8 to 1. I ran the stock intake/exhaust and 1bbl carb. I got 28 mpg with the A-833 trans and 3.55 rear gears. This is in a Dodge D-100. Latter I got the clifford intake/headers and the Edelbrock 1801 500 cfm 4bbl. Its a little to much carb but it hauls ass at WOT. I tow a lot too and I can hold 60 MPH in forth gear climbing a hill while towing my 74 powerwagon on a trailer. But just cruising I get 22 mpg at best 18 at worst. You may do better with a Holly carb, different cam, intake, gears....... Its just all trial and error.
Author:  wjajr [ Thu Mar 12, 2015 9:23 am ]
Post subject: 
Take a look see at http://www.slantsix.org/articles/Josh_Carb_Article.htm by Josh Skinner that explains what carburetor is suitable with certain engine builds and related powertrain parameters.

Once one part is changed such as a carburetor from stock like equipment the whole powertrain performance picture is changed most often in the wrong direction resulting in less performance. Generally several upgrades have to be made to get the new parts to play well with the existing set-up and realize a performance gain.
Author:  sandy in BC [ Thu Mar 12, 2015 9:39 am ]
Post subject: 
You can do a lot of performance upgrades before you need anything bigger than a 2 barrell.

Milling for compression and a suitable cam will make a way bigger difference than an Offy and Holley on a stock motor.........

For street torque and driveability ....Super6 and BBD.
Author:  '67 Dart 270 [ Thu Mar 12, 2015 9:39 am ]
Post subject:  I'll start...
HP and mileage are dependent on many factors, the carb is only one. With the appropriate engine build, you can run a 2bbl and have way more HP and better mileage than a car with a 4bbl if the engine isn't built appropriately for it (i.e. the 4 bbl can be overkill, simply a waste of fuel).

If you have a stock engine or even a mild build, I would suggest a 2bbl is the way to go, especially if you are interested in gas mileage.

Carbs have to be matched to an engine, e.g. there are 2bbl's matched to smaller engines (by CFM/gas supply) that would be inappropriate, inadequate, for a 3.7L slant six, and there are likewise 2bbl's matched to V8's that would be too much carb for a slant six. Keep in mind that many carbs have jets that can be changed to increase and decrease the ability to regulate how much fuel is supplied. So, you can take a carb that was designed for a smaller or a bigger engine and adapt it for use in a SL6 by changing out the jets - if the total CFM flow is appropriate. In other words some carbs simply don't have enough flow for 6 cylinders even if you could get more gas through them.

There are folks on this board that know way way more than I do on this (and many other) subjects. I'm sure through using the "Search" function you can learn a ton, and also people must certainly have some suggestions on good carb choices for a slant six. The first thing you need to supply to this thread is what your engine is or will be as to build, stock build, cam choice, exhaust system, ignition type (points, electronic, HEI), etc.

Brian
Author:  70valiant [ Thu Mar 12, 2015 1:37 pm ]
Post subject: 
I never went faster in the 1/4 with my 465cfm Holley than I did with the V8BBD, though I'm convinced the secondarys never opened on the 4bbl. The bbd got me a best of 21.1mpg while the Holley never did better than 20mpg
Author:  Reed [ Thu Mar 12, 2015 1:54 pm ]
Post subject: 
You can do a great deal with the stock one barrel before it really becomes necessary to switch to a two barrel, much less a four barrel. On a car old enough to have points ignition, I would upgrade the ignition first, the exhaust second, the head (compression and flow) third, the brakes fourth, and THEN start worrying about carburetion.
Author:  '67 Dart 270 [ Thu Mar 12, 2015 2:47 pm ]
Post subject:  ignition
Amen on getting rid of those points, Reed. Amazing what electronic does for the slant; I had a terrible time with my points, went quickly to electronic.

There are many threads on this board for conversion to electronic ignition.

b
Author:  spacecommander [ Fri Mar 13, 2015 3:50 pm ]
Post subject: 
Quote:
You can do a great deal with the stock one barrel before it really becomes necessary to switch to a two barrel, much less a four barrel. On a car old enough to have points ignition, I would upgrade the ignition first, the exhaust second, the head (compression and flow) third, the brakes fourth, and THEN start worrying about carburetion.
I swapped from a 1bbl to a offy 4bbl manifold and a carter 400cfm 4bbl. Huge difference in drivability and power on a stock engine (well, exhaust mods - replaced the mile long tailpipe with one that vented out under driver side in front of the rear wheel) - this is in a 1982 W150 truck.
Author:  sandy in BC [ Fri Mar 13, 2015 4:50 pm ]
Post subject: 
...and you would find the same difference if you went to a Super6 BBD and 2 1/4" exhaust.
Author:  Rick Covalt [ Fri Mar 13, 2015 5:49 pm ]
Post subject: 
The 2 barrel intake and BBD are a fine upgrade and you will notice a nice increase in performance / driveability with it. But don't forget that with a small 4 barrel ( 500 Edelbrock for instance) you are only running off the 2 barrels 99 % of the time. I personally have run 4 barrels on bone stock and slightly modified engines with no troubles at all. The Edelbrock has plenty of adjustability and if you are really worried about the secondaries, unhook the link. For the costs of what Super sixes are bringing today I would keep my eye open for a cheap 4 barrel intake and then by a new 4 barrel Edelbrock and be done with it. Then you are set for future upgrades and more power if you need it! Of course if you run into a Super six reasonably priced I wouldn't hesitate to go that route too. I am pretty much a tight wad! A properly adjusted 4 barrel should not get any worse mileage than a 2 barrel if you keep your foot out of it.
Another man's 2 cents.

Rick
Author:  '67 Dart 270 [ Fri Mar 13, 2015 7:35 pm ]
Post subject:  carbs
I have double 2bbls, and yes, since I'm mostly using the primaries, it's like having the best of both worlds, a 2bbl most of the time and a 4bbl when I want it. Gas mileage of 24 mpg on the highway, and average of about 19 overall with lots of city driving.
Author:  spacecommander [ Fri Mar 13, 2015 7:35 pm ]
Post subject: 
Agreed a 2bbl = a 4bbl for most of the usage out there. An improperly setup 4bbl would be much worse than a stock 2bbl. I just happened to have the offy manifold from a 80 Aspen from waay back when and bought the carter 4bbl which I rebuilt by installing a new accellerator pump ;-) for something ridiculous like $120 on the bay of ease (ebay). The holley 390 had the black spring installed so it would only be a 2bbl, but I discovered the holly did NOT like being non-level and this is for a 4WD Truck . . . . Actually quite amazed at how level-sensitive it was, the carter not so much, but still a worry - if this was a offroad build the only option appears to be the 2 x1 bbl offy manifold with the truck 1bbl carbs designed for use at extreme angles.

Next step is to investigate rods and springs for the carter carb - my mileage still sucks and it appears to be rich during normal running - I'm at 5500 feet, so it will need leaner primaries. Can you swap just the springs in the metering rod setup on these cater AFBs?
Author:  spacecommander [ Fri Mar 13, 2015 7:42 pm ]
Post subject:  Re: carbs
Quote:
I have double 2bbls, and yes, since I'm mostly using the primaries, it's like having the best of both worlds, a 2bbl most of the time and a 4bbl when I want it. Gas mileage of 24 mpg on the highway, and average of about 19 overall with lots of city driving.
I like that setup. How did you get the linkage to work? Those carbs seem to take an incredibly hefty pull to get them to open. That is the setup I have and wanted to install on the truck - but am having a hell of a time working up a non-bending linkage. Got the langdon linkage and even bought and fabricated two more supports & bearings for the rod to keep it from flexing, but it still bends and the clamp-on pivot arms still slide over the shaft. Yes, I have the longer carb attachment bolts needed to properly attach the linkage brackets to the bolts, just haven't taken it apart far enough to install them yet - still working on linkage flex and arm rotation.

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Author:  camiking23 [ Fri Mar 13, 2015 9:05 pm ]
Post subject: 
Great information much needed
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