2 bbl or 4 bbl non GM - Chevelle Tech
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post #1 of 20 (permalink) Old Mar 19th, 20, 8:32 AM Thread Starter
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Dave
 
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2 bbl or 4 bbl non GM

I am building a 390 Ford, small cam and headers. This is for a 71 F-250 4x4 with 4:10 gears and a 4 speed to be used as a work truck with some hauling and trailer pulling. My question is should I keep the factory 2bbl carb or go to a small 4bbl carb. Which would be best for fuel mileage or would it make any difference.
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post #2 of 20 (permalink) Old Mar 19th, 20, 8:42 AM
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Re: 2 bbl or 4 bbl non GM

Quote:
Originally Posted by 25C 70 View Post
I am building a 390 Ford, small cam and headers. This is for a 71 F-250 4x4 with 4:10 gears and a 4 speed to be used as a work truck with some hauling and trailer pulling. My question is should I keep the factory 2bbl carb or go to a small 4bbl carb. Which would be best for fuel mileage or would it make any difference.
Obviously the 2bbl will have better gas mileage. Have you driven it with the 2bbl? Are you able to pass someone or does it feel under powered when towing? You can always try the 2bbl and then put an intake and carb on later after it is in the truck.
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post #3 of 20 (permalink) Old Mar 19th, 20, 8:55 AM
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Re: 2 bbl or 4 bbl non GM

It used to be popular to swap a Holley 390 cfm 2-barrel in place of the factory 2V. If you are keeping your foot in it and use it as a truck, perhaps the 2-barrel will afford some fuel economy (ha, with 4.10s in an F250 4x4).

Those FE manifolds are heavy. I would've gone with an Edelbrock Streetmaster 390 intake but are they even still available? Maybe an Edelbrock Victor then & a probably experiment with a 4-barrel around 650 - 800 cfm. Are you averse to EFI?

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post #4 of 20 (permalink) Old Mar 19th, 20, 9:17 AM Thread Starter
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Re: 2 bbl or 4 bbl non GM

Truck is a low mile 16,000 truck formerly owned by the local fire department and used as a brush truck. The original 360 has a dead cylinder, low compression. I purchased a new built unfired 390 +.030 for $300. Thought this would be better for towing. I guess what I am asking is a 4bbl running on the front primaries more efficient than a 2bbl being the venturis are larger on the 2bbl.
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post #5 of 20 (permalink) Old Mar 19th, 20, 9:53 AM
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Re: 2 bbl or 4 bbl non GM

Q-jet!
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post #6 of 20 (permalink) Old Mar 19th, 20, 12:34 PM
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Re: 2 bbl or 4 bbl non GM

I have tested many carbs as I almost always have over 50 to choose from.
Absolutely the best MPG I have received was with a Q jet on a Q jet iron intake.

When I had my 6000 lb ford van I used it almost daily for 13 years I built the engine and suspension and ported the stock 2V intake and ported the iron manifolds and stuck a 3" single exhaust on it.

I was shooting for 20 MPG I got 18 many times and usually 17.6 MPG.
Quick timing curve 22 initial and 35 total.
All in by 2100 rpm on it.

I tried many 2 barrels and an econ master Holley 2 barrel.
465 cfm AFB 600 edelbrock 750 edelbrock and 2 different Q jets.

Once all of them were dialed in there was not 1/2 MPG difference in any of them.
For off road and just plain reliability and simplicity I kept the Motorcraft 2 barrel on it.
Many thousands of miles driven with other carbs so not a simple day test.

I ran the stock Ford cam advanced 4 degrees and the 260H comp cam gave me 2 MPG better than that.

The shape of the 1984 Ford van was a basic brick.
You may not believe this but the spare tire mounted on a tube grill guard on the front gave me another 2 MPG.

I vote to keep the 2V Motorcraft and dial in some much needed initial timing.
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post #7 of 20 (permalink) Old Mar 19th, 20, 4:27 PM
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Re: 2 bbl or 4 bbl non GM

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Originally Posted by jeff swisher View Post
I vote to keep the 2V Motorcraft and dial in some much needed initial timing.
Agree

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post #8 of 20 (permalink) Old Mar 19th, 20, 8:42 PM
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Re: 2 bbl or 4 bbl non GM

run a 600 CFM 4-bbl. Like the tried and true LIST #1850. what Jeff said about the tining.
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post #9 of 20 (permalink) Old Mar 19th, 20, 11:39 PM
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Re: 2 bbl or 4 bbl non GM

My parts chaser is a 1970 F100 with 360, 4 speed and 3.50 gears. I swapped out the original 2bbl for an earlier iron intake (those suckers are HEAVY!) with an 1850, 600 cfm Holley. Much more adjustable, and I'm not sure I've ever opened the secondaries. Mine's completely stock other than the intake swap, and really not happy above 3500 rpm.

For a bolt-on-and-go type carb, I've known a lot of people to be happy with Edelbrocks. Personally I've been tempted to try a Qjet on mine. They are more difficult to tune, but infinitely adjustable.

Like Jeff said, with proper timing and a good carb tune, any carb should bring very similar mileage. It all depends on how much time you want to spend dialing it in.

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post #10 of 20 (permalink) Old Mar 20th, 20, 1:59 AM
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Re: 2 bbl or 4 bbl non GM

Quote:
Originally Posted by 25C 70 View Post
Truck is a low mile 16,000 truck formerly owned by the local fire department and used as a brush truck. The original 360 has a dead cylinder, low compression. I purchased a new built unfired 390 +.030 for $300. Thought this would be better for towing. I guess what I am asking is a 4bbl running on the front primaries more efficient than a 2bbl being the venturis are larger on the 2bbl.
Depends on the 4bbl but generally the 2bbl will out MPG a 4bbl.
Does the 390 come with a manifold ? I had a 400CI Pontiac with a 2bbl carb on a stock factory manifold and that thing did not lack any power even when that car was loaded down with stuff.

John
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post #11 of 20 (permalink) Old Mar 20th, 20, 9:44 AM Thread Starter
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Re: 2 bbl or 4 bbl non GM

I have the 2bbl intake and motorcraft carb that is on the original 360. I would like to thank the members of this sit for their advice. I will be reinstalling the original 2bbl intake and carb, thanks again.
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post #12 of 20 (permalink) Old Mar 20th, 20, 9:51 AM
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Re: 2 bbl or 4 bbl non GM

the tuneable accel pump system on the Holley style 2 and 4 bbl carbs is a huge factor in the acceleration of a loaded truck if for instance you want to merge onto the freeway
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post #13 of 20 (permalink) Old Mar 20th, 20, 3:52 PM
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Re: 2 bbl or 4 bbl non GM

There is no need to do anything but a stock engine if you are going with the 2 barrel carb. A 390 will run of of air very quick with a 2 barrel carb. Don't need a small cam if you are out of air at 3500.

Ford put a 4180 series Holley carb on some of th mid 80's trucks. I have used it and it works very well for both power and economy. I really doubt you will find one but it is a possibility.

I would just do the 1850 600 Holley. It is tried and true. Economy will be just as good with a 4 brl as a 2 brl if you don't open the secondaries. You would have power if you need it. There is nothing worst than a weak tow or work truck. If you have to buy all this I would just use what you have and try it. If you have the 4 brl and intake I would use it.
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post #14 of 20 (permalink) Old Mar 20th, 20, 7:20 PM
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Re: 2 bbl or 4 bbl non GM

I had a 400" Ford engine, 2 barrel with 260H cam it went 5000 rpm, That was plenty.

I won a bet one time with a buddy he had a 1977 TA with 400" [email protected] .050 cam and headers.
I bet him my little 200" inline 6 with 1 barrel would rev higher RPM than he could before valve float or just nose over.

He could muster 6500 rpm I went 6700 and took his 20 bucks.
I was not making any power but it was not nosing over either just kept slowly revving and gaining speed.
Poor thing.


I had a 1969 2 ton box truck 26 foot box cab over deal 2 speed rearend.
It had a 350" chevy in it.
It had a 2 barrel carb and intake.


A 4 barrel on a heavy deal like that would not make any more power. it would just make noise and use more fuel.
If you ever pulled 8000+ lbs behind your vehicle and opened the secondaries on the carb you will notice little if any gain.. sure not enough to justify keeping the secondaries open.
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post #15 of 20 (permalink) Old Mar 20th, 20, 8:47 PM
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Re: 2 bbl or 4 bbl non GM

Jeff, you are onto something. Torque of course is what you need in a truck, not peak h.p. Ford used to do a 477 Super Duty topped with a 2-barrel on h.d. trucks. Also operating economy is very important to operators, plus these motors are often governed down to 3400 rpm maximum to promote engine life & relative fuel economy. Chevy also did a tall-deck BBC 366 version with a 2-barrel, replacing the 348 2-barrel.


That is funny story about revving your old 200 six. I remember beating a '75 Comet on my bicycle back in the mid-70s, and if I were a betting man you would've had my $20..
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