best size 2bbl carb or small 4 bbl for my 318?

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63VALIANT*KISS

63VALIANT*KISS

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i want to get better gas millage so i figure take the 650 carter 4 off it and get a small 4bbl or a 22bbl with an adapter on the eldy performance manifold

but whats the best size where i wouldnt suffacate the engine or it run like a ford (slow)? please help me
 
650 AVS carb? Or is it a AFB? AFB's are 600 unless there the 625 cfm Emmissions carb, but I can't see where the extra 25 cfm comes from.

Anyway, that carb is just fine for mileage and should simply be jetted lean. An 02 sensor would help dial it in best. Next would be rear gear ratio followed by an overdrive transmission. Then taller tires.
That would end the mechanical part of mileage getters.

Non moving mileage getters would be;

Free flowing air flirter and if the houseing was open, even better.
You allready have a good carb. A 500 wouldn't really help any and even if you found mileage with a 500 Edel. Is it really worth the expense?
Perhaps a 4 hole spacer under the carb.
The intake is good.
Heads leave stock. A valve job would help, but it's also a bang for the buck issue not really worth doing.
Exhaust, dual 2-1/4 for a stock or near stock 318.
Headers do well as well.

Electrical;

A Multi spark box is excellent at adding mileage.
Also, new wires, I like MSD wires, with the lowest ohms per foot and new plugs.
Adjust spark delivery for max. mileage. (Advance it, but be aware this may require a higher octane to run. The cost of gas is a marginal increase on the wallet vs. a possibley slightly better margin of mileage. But it is very possible.

Liquids;

Synth. liquids help. It's a small gain, but just keep adding small gains up and you'll find another mpg in there. Also, cool fluids do the job better and longer. Deep trans pan/oil pan and external cooling lines do the job well. (OK, a bit silly & hard for the rear end, but it's doable.

Parasidic drag;

Can you get rid of some? Clutch fan, P/S ???? can you live without it. This is another bang for the buck issue on the P/S, but not so much for the electric fan.
Taller tires I mentioned, but also keep them thin. Over inflated tires have been proven to get better mileage.
Lower the car, less drag.
Lower the weight, less energy spent moving a heavier car.
 
There are alot of fans of thermoquads for better gas milage if you can keep your foot off the gas. The primaries are pretty small on that carb but the secondaries are there when you need them.
 
Vacuum secondaries only open under load. So it is only there when you need it. I would would look at tuning what you have.
 
63VALIANT*KISS said:
Evreyone Has Told Me A 2bbl Would Get Way Better Gas Milage Then A 4bbl
Click to expand...

That's simply not true. I get better gas mileage now with my 625 Carter AFB than I ever did with my old Carter BBD. If you only drive with the primaries you're pretty much just using a 2-bbl. anyway. Even if you did gain any MPG going to a 2-bbl. it's going to be much, MUCH slower. What are your current rear gears and transmission?
 
dont put the 2bbl on it .youll regret it in the long run.tune the carb you have. rumble had some good suggestions.i had an afb on a 318 in a 73 valiant with 276 gears and a 904.it got 20 mpg at 65 on the highway,its actually the same carb refered to in the previous post.ive had a few 3/4 ton truchs ive tried to tune for max economy over the years.when i acheived the best mpg they were misrable to drive.id rather have a good balance of mpg and power and enjoy driving .jmo.
 
leave the 4bbl and disconnect the secondaries

I've always heard this but always wondered why doing this doesn't limit, in a big way, your top speed or why doing so wouldn't cause your engine to run too lean at high speed?
Could someone give me a not to technical explanation?
Thanks,
C
 
IMO, disconnecting the secondary side of a 4 bbl. carb is a dumb move. If such a move was done, then I'd say install a 2bbl. carb.

SBKNUT is on it!

Heres the deal with 2 vs 4 bbl. carbs for starters. There rated differently. Different pressure drops. They flow differently at the sme drop. You want the 4bbl. and you want facts not some *** saying run the 2bbl. because it is smaller etc....
Garbage! Pure garbage.

For starters, sbknut is right. Your right foot will control 90% of your mileage at all times no matter what freakin carb is on top! BAR NONE!

1967dartgt's idea of a T-Q is an excellent idea but I left it out on purpose since you allready have a carb that is absoultley capable of getting everything you need. The bang vs the buck in swapping to a T-Q is not really there. The advantage of the smaller primaries for a greater velocity of air and fuel is a good thought and if tuned right, would show a marginal improvement.

Badart is right! Tune your carb and stop listening to people who have absoultely no idea what there talking about.
MOPEkiD-3 has said something very important and I have also been there and done that with exactly the same move he did. I moved from the 2bbl. BBD to the Carter 625 and found better milage. After I did that, I made many of the changes I mentioned above. These changes done to my car not just did the trick, but changed the car in everyway one would seek to do except consume more fuel.

The car was my 1979 Dodge Magnum with a 360. The car only lacked a moon roof or T tops , but had everything otherwise and a working coooold A/C system.

I went from a perfect emissions passing 10 MPG's Hwy. to 20 mpg's and still passed emissions and I feraking passed it better than the stock leanburn or the "FIX" Chrysler did for that disaterous computer, the "Orange box" did.

The following is ALL i did to the car.

"Trap Door" 1973 air cleaner with a K&N on top of a 625 Carter AFB on a stock iron intake with working EGR valve, dual exhaust of the manifolds into twin cats, H pipe, into $17 turbo mufflers to the tail pipe. A new viscous fan from MoPar and a MSD being triggered by a junk yard distributor that cost me a whopping $10.
Drive train was a 360/904/8-1/4 w/2.76 gears and 235/60/15's on all 4 corners.
The Magnum weighed in at 3780 lbs before gasoline and me or a spare tire.

IF I can get 20 mpg;s with a 360 in a car totaling over 4,000 lbs, why can't you or anyone do the same/similar or close with a 318 in an A body car?

Tune tune tune!
Do not use a 2bbl or disconnect the secondaries of a 4bbl. carb!:angry7:
 
clhyer said:
leave the 4bbl and disconnect the secondaries

I've always heard this but always wondered why doing this doesn't limit, in a big way, your top speed or why doing so wouldn't cause your engine to run too lean at high speed?
Could someone give me a not to technical explanation?
Thanks,
C
Click to expand...

It will limit your top speed and power a great deal. BUT the engine doesn't run any different A/F mixture at high speeds than low speeds; if the engine is spinning faster (requiring more air), the carb meters more fuel into the venturis accordingly. What you do when you disconnect the secondaries is pretty much turn the carb into a 2-bbl. Really that doesn't make sense to me because the only difference it will make is the secondaries won't open when you floor it, which you shouldn't be doing anyway if you're trying to get good gas mileage! When you drive around town at part throttle the secondaries don't do anything at all on a 4-bbl. carb.

What is your ignition and exhaust setup? If you don't already have them an electronic ignition and free-flowing dual exhaust will help gas mileage. Of course they'll also give you more power, so you'll have to try harder to drive with a light right foot. All power-adding mods done to an engine (aside from a longer-duration cam or big-port heads) will also increase gas mileage because the engine is running more efficiently.
 
63VALIANT*KISS said:
Evreyone Has Told Me A 2bbl Would Get Way Better Gas Milage Then A 4bbl
Click to expand...


This is simply not true, with a 2bbl you are running off of all the CFM of the carb all the time but with a 4bbl you are only running off the primaries unless you have your foot in it all the time, the primaries are usually less than half the total CFM of the carb depending on the carb you have. The thermoquad has the biggest difference between the primaries and secondaries and has the potential to get the best fuel economy but you need to find a good one which is not always easy.

That being said, you don't want a mechanical secondary carb.

Like others have mentioned above. The carb is only a small part in getting better fuel economy, your gears, tranny, tire size, exhaust, timming and a lot of other items need to be addressed.
 
A decent spreadbore carb will do it. Mechanical or vacuum secondaries. 2bbl carb bores are generally bigger than 4bbl primaries to handle power needs through the RPM range of the engine. When I had to drive my '68 GTX during the first "gas crisis" in the early 1970s, I put a stiffer spring in the vacuum can for the secondaries. Since they had dropped the national speed limit down to 55mph, I planned to drive at 60mph on the interstate to better mark time/progress. The stiffer spring held the secondaries closed to about 65mph. It worked great. It is a Holley 750.

I noticed some 6 cylinder 2bbl carbs with bigger bores than 4bbl carbs on many 1970s V8 engines.
 
you can either go to a thermo quad or a holly spread bore as said,, or if your running a 318 mildly built a 500 cfm edelbrock will be plenty,,,and if you keep your foot out of it you will see mileage gains,,,,your factory 2 barrell is not more then 350 cfm,,unless its a big holly 2 barrell economy master they are 500 cfm,,,

i run a 500 edle brock on a 241 ci dodge hemi in my street rod,1940 dodge wit hthe A/C on and cruising at 75 mph i got 18 mpg with 3;23 gears and a 904 with a lock up converter,,

241 hemi pics 011.jpg
 
fstfish66 said:
you can either go to a thermo quad or a holly spread bore as said,, or if your running a 318 mildly built a 500 cfm edelbrock will be plenty,,,and if you keep your foot out of it you will see mileage gains,,,,your factory 2 barrell is not more then 350 cfm,,unless its a big holly 2 barrell economy master they are 500 cfm,,,

i run a 500 edle brock on a 241 ci dodge hemi in my street rod,1940 dodge wit hthe A/C on and cruising at 75 mph i got 18 mpg with 3;23 gears and a 904 with a lock up converter,,

View attachment 102607
Click to expand...
I'd love to find a cast iron 4 barrel intake for my 241....so far I have E-brock's DO410 and the old Offy four barrel intakes.

out-XcbINC6g.jpeg


Offy intake with thier adapter.jpg
 
63VALIANT*KISS said:
Evreyone Has Told Me A 2bbl Would Get Way Better Gas Milage Then A 4bbl
Click to expand...
Well "everyone" is wrong.
Consider that the front half of a 4 barrel is in fact a 2 barrel.
Furthermore;
4bbls are rated differently than are 2bbls. So half a 650 4bbl is NOT the same as a 325 2bbl..

IIRC my formulas, you have to divide that 325 by 1.67, to get 195, when rated as a 2bbl. Well........ what size is a 2bbl from a 318?
Ima thinking it was a 250cfm, but I admit I'm guessing. In any case, the only time a 2bbl is better, is if you can't learn to defeat/ or not use, the secondaries.
and I can give you a recipe for a 360-4bbl that once got me fuel-mileage that a 318 can only dream of.
By itself, the 4bbl is not your fuel-economy problem. But just so you know, the Thermoquad has Triple boosters, and size for size, is a hard to beat combination.

I highly recommend a compression test, and mapping out your timing curves and testing your VA
 
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