Better fuel economy from a small 4bbl or a stock 2bbl?

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Robj

Robj

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I see discussions from time to time on whether or not you can actually pull better fuel economy from a small 4bbl if you stay off it and keep the secondaries closed.

What are the opinions here?
 
Think thermoquad. Very modern fuel metering in the primaries. Three step metering rods. If you stay out of the secondaries it is a very efficient and thrifty carb.
 
X2 on the 4 barrel. a lot of it depends on the driving style, and a solid fuel carburetor (for me) has worked more efficient than an air bleed caruretor. most thermoquads, afb's, avs', quadrajunks, are solid fuel metering systems. holleys are air bleed setups. kind of hyppocritical since i run holleys....but i loved my TQ, it actually ran harder than my 4175 spreadbore holley, but i couldnt seem to get the damn crossovers that are glued to the bottom of the main body to seal up so it would always bypass fuel. It definately put my BBD's and holley 2210's to shame, for being user friendly, driveability, and mileage (i gained 3 mpg) i never looked back
 
A correct 4 barrel can and will be made to get better MPG then a 2.

JMO and it worked for me
 
I recently switched from a stock 2 barrel to a factory 4 barrel setup on my 65 273. It's a 500 CFM Carter and I rebuilt it before putting it on. My mpg around town, driving conservatively, went down about 1/2 mpg.

The 4 bbl was the correct part number for my car but I have no way of knowing whether anyone ever changed the rods, jets, or springs.

The car also has trouble starting hot after it's set for a little while. Maybe that's affecting my fuel economy.
 
If a Carter or Edelbrock start hard when hot. It's probabvly because you have more than 5psi fuel pressure. I fought it for years before a pressure regulator fixed the problem.
 
Ted Carlson said:
If a Carter or Edelbrock start hard when hot. It's probabvly because you have more than 5psi fuel pressure. I fought it for years before a pressure regulator fixed the problem.
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I have 8 or 9 psi. If the pressure is too high after I shut it off, do you think it could be too high when running as well? Could that be affecting the mileage? Other's have recommened an insulator between the carb and intake.
 
8 psi is way too much pressure on an edelbrock carb. Yes it will have a negative effect on fuel metering.

Without know what the A/F ratios are, you have no way to tell if one setup is better than the other. ex: 2bbl running at 14:1 cruise and the 4 bbl running at 12.5:1 or worse, yep the 2 bbl is going to be better on fuel economy. you have to get the fuel curve right and equivalent to get any type of comparison.
 
65DartGTConv said:
I have 8 or 9 psi. If the pressure is too high after I shut it off, do you think it could be too high when running as well? Could that be affecting the mileage? Other's have recommened an insulator between the carb and intake.
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that is pretty excessive. you may be pushing fuel past the needles and seats and dumping gas. those carbs only need 5 -5.5 pounds of fuel pressure. i would get it down to that.
 
4bbl has the advantage to be able of having smaller primarys but if both set up for optimal fuel curves there really should get similar gas mileage but its kind of hard
compare 2bbl and 4bbl cfm because 2bbl are flowed at 3@inHg and 4bbl are flowed at 1.5@inHg so a 500cfm 4 bbl would be equal to a 700ishcfm 2 bbl.
 
273, do you know where a calc for this is? I was wondering about the 6 pack carbs flow to a 4bbl. flow for a compare.

Robj. For certain! I know! I have done this swap before and will always swap a 2 for a 4. Every time without fail. However, replacing a 2bbl. with a 750 4 bbl. is about a dead even swap due to the primary size of the 750. A better size would be a 600/650 in place of a 2bbl on a small block.

For mileage purposes, I also do a MSD or equal type of ignition and a dual exhaust over the tiny single exhaust pipe. In total, these 3 things will improve power, mileage and driveabilty. You need to only take the time to tune it well. Really get in there and keep tweaking it for best running conditions. Emissions will remain low if that is an issue.
 
rumblefish360 said:
273, do you know where a calc for this is? I was wondering about the 6 pack carbs flow to a 4bbl. flow for a compare.
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i don't have a formula i just use my dyno program and keep uping the 2bbl flow intil it equals 4bbl power numbers the six pack is probaby 900ish cfm.
 
The carb with a 3.0" Hg rating will flow 1.414 times more air than if you had rated it at 1.5" Hg.

A 500cfm 2bbl flows 353cfm at 1.5" pressure drop
 
Ok, thanks man. Just found this on the web; " If you want to compare the CFM rating of a 2 BBL to that of a 4 BBL multiply the 2 BBL rating by 0.707."

You can not directly compare the CFM rating between 2BBL's and 4BBL's as the test pressure that is used to measure the CFM is different between the two. The 4BBL is measured at a test pressure of 1.5 inches of Mercury (Hg) of pressure drop, while the 2BBL is rated at 3 inches HG. To convert the 2BBL into 4BBL use the following formula:
4BBL Flow = 2BBL Flow / 1.414.
So that 500 CFM 2BBL Carb actually flows 353.606 CFM on the 4BBL scale
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http://speedtalk.com/forum/viewtopic.php?f=1&t=24093

and the list goes on here; I put this into the google search box. "2bbl. vs. 4bbl carb flow"
 
So cracked, a MoPar Holey 6 pack flows like???? on a 4bbl. scale.......
 
Isn't a six pack 2 500cfm and 1 350cfm= 1350cfm x .707 = 950cfm
 
crackedback said:
the carb with a 3.0" hg rating will flow 1.414 times more air than if you had rated it at 1.5" hg.

A 500cfm 2bbl flows 353cfm at 1.5" pressure drop
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Ops! Edit, 500 divided by 1.414 = 353.
OK, what is the center 6 pack carb at? 350 sez 273, never mind..... 243 cfm when converted.
Isn't a six pack 2 500cfm and 1 350cfm= 1350cfm x .707 = 950cfm
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That is how I understood it. Just checkin with the crew.

Does anybody know what the various 2bbl carbs flowed that came on the MoPars?
 
waaay back in the day, my dad had a '67 catalina with a 400 2 barrel. pap had a '69 bonneville with a 428 4 barrel. yep, bonneville got better mileage. i can't help it that they didn't have enough sense to run mopars. ( i will admit i loved that bonneville)
 
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