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2 Barrel to 4 Barrel Manifold and Carb
#1
I have a 302 motor, if I changed from a 2 Barrel to 4 Barrel Manifold and Carb would I need to use premium gas instead of regular?

Whats the difference on MPG between the above?
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#2
Machman;82022 Wrote:I have a 302 motor, if I changed from a 2 Barrel to 4 Barrel Manifold and Carb would I need to use premium gas instead of regular?

Whats the difference on MPG between the above?

Just changing from 2 to 4 barrel will not require use of premium gas.
Changing the heads to get a higher compression ratio or advancing the timing will require premium gas.

A 4 barrel carb increases the volume of gas and air that can be combusted by the engine during wide open throttle. If you're a lead foot
then your MPGs will be much worse with a 4 barrel. If you stay off the gas and drive like a grampa, your MPGs will be about the same as a 2 barrel.

1971 Mach1
351C-4v
C6 is history-->>now TKO-500
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#3
Pretty much what he said..

'Mike'
73 Convertible - 351C/4V CC heads/4bolt/forged flat tops/comp 270/rhodes/mallory unilite/tri-power/hookers/glasspacks/c6/3.50 limited slip/Gear Vendors/Global West sub frames, strut rods and shelby style traction bars/ Rear sway bar/tilt steering (not original)

Pics of modifications included in:
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#4
I don't think you have to drive like a grandpa to get good mileage with a vacuum secondary 4 barrel-with a double pumper with mechanical secondaries you have to be very conservative.

The primaries on most any 4 barrel are smaller than most any 2 barrel. Keep in mind cfm ratings between the two are measured differently and therefore are not accurate indicators of anything.

If you are driving mostly on the primaries you can even see a slight increase in mileage.

But here is something to consider that many people choose to ignore. A carb must be matched to your vehicle by changing jets, adjusting floats, adjusting fuel pressure, setting accelrator pump clearances, changing accelerator pump cams, changing squirter sizes and sometimes (very rarely) modifying air bleeds and transition slots.

Sounds complicated huh? Well it can be and most of us don't want to accept the challenge to learn to tune our carbs properly so we buy one that is what we think is good for our application, slap it on, adjust idle mixture and idle maybe and then end up unhappy that it doesn't meet our expectations.

Most engines need an air fuel ratio within a fairly narrow range. Carbs work by allowing air to be draw through them which in turn draws the fuel into the venturi. Jet size and air bleed size are coordinated to get good atomization. So while most carbs that bolt on your manifold will let your engine run, and even give you a fair air fuel ratio, an under or oversized carb will mess with fuel atomization, vacuum etc.

Tuning a carb is neither difficult nor expensive, but it takes some time and if you swap carbs, build in a few days of testing and tuning time to really enjoy the fruits of your labor.

[Image: 1_01_07_15_8_53_18.png]

"I love my Hookers!" and "Get some Strange" probably have a different connotation to non automotive enthusiasts!
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#5
Thank you all for the replys and info , it really helps.

As a guideline could someone post the following info for me so I have a ball park on what to choose.

For a 302 motor what should the following specs be;

Jet size or number

venturi size ( as I noticed some carb seliers list 1.21 , 1.09 , etc)

appx fuel pressure
( as mentioned in an earlier post I may have an incorrect new mechanical fuel pump and not too sure what the correct one should be so I am thinking about exchanging it, putting in a block off plate and using an electric one to solve it)

Any other specs that may be useful
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#6
Machman;82264 Wrote:Thank you all for the replys and info , it really helps.

As a guideline could someone post the following info for me so I have a ball park on what to choose.

For a 302 motor what should the following specs be;

Jet size or number

venturi size ( as I noticed some carb seliers list 1.21 , 1.09 , etc)

appx fuel pressure
( as mentioned in an earlier post I may have an incorrect new mechanical fuel pump and not too sure what the correct one should be so I am thinking about exchanging it, putting in a block off plate and using an electric one to solve it)

Any other specs that may be useful
You didn't say what brand or model carb you're using. Jet sizes would also depend on how your engine and exhaust system is set up.
The stock mechanical fuel pump should work fine unless your engine is setup for very high performance.
Alot of folks have different preferences on brand of carbs. I would suggest an Edelbrock, they seem to require the least fussing. Mine worked fine right out of the box, jets were fine. It has run great for 4 years with very minor tweeking. And the best part, no leaks!

1971 Mach1
351C-4v
C6 is history-->>now TKO-500
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#7
Jeff73 had some very valid points,but I am going to read between the lines and guess you are going to hit the key, go drive to the local burger joint or drive your car as a daily driver with out working on your car all the time. I am also jumping to conclusions about that it is going to be a very cost effective change out, less spent the better! So if this is the case like when I started out so many years ago the tried and true Holley 600 cfm vacuum secondary carb with either an aftermarket intake aluminum 100-125 bucks everywhere or stock 4v 50-100 everywhere will work for most small blocks, stock to mildly modified or even a much larger say 429-460 that is what the factory used originally carb wise. If you are concerned about gas milage you can insert a 8-32 screw in the plate where the rod actuates the secondaries. When you get a 4v vacuum carb watch how the secondary butterflies open up ,you can adjust how much you want it to open with a simple little screw. For about 250.00 or less depending how good your shopping skills are it will most definetly put a smile on your face when you mash on the loud pedal!Wink
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#8
1971mach1;82286 Wrote:
Machman;82264 Wrote:Thank you all for the replys and info , it really helps.

As a guideline could someone post the following info for me so I have a ball park on what to choose.

For a 302 motor what should the following specs be;

Jet size or number

venturi size ( as I noticed some carb seliers list 1.21 , 1.09 , etc)

appx fuel pressure
( as mentioned in an earlier post I may have an incorrect new mechanical fuel pump and not too sure what the correct one should be so I am thinking about exchanging it, putting in a block off plate and using an electric one to solve it)

Any other specs that may be useful
You didn't say what brand or model carb you're using. Jet sizes would also depend on how your engine and exhaust system is set up.
The stock mechanical fuel pump should work fine unless your engine is setup for very high performance.
Alot of folks have different preferences on brand of carbs. I would suggest an Edelbrock, they seem to require the least fussing. Mine worked fine right out of the box, jets were fine. It has run great for 4 years with very minor tweeking. And the best part, no leaks!

Thank you for your reply, the carb is some type of motocraft 2100 (no number plate so I do not know specifics) and the car still has the orig dual exhausts.

The heads and intake manifold were replaced on the orig motor w/ ones from a 77 so it can run on unleaded gas.

Since it was farted around w/ in the past and I do not know what should be the orig specs of things I wanted to get some general guidlines on venturi, jet size, and fuel pump pressure for whats there to better help me choose what to get if I do a change over

AJ
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#9
my quick 2 cents: if you have a 500cfm 2 barrel and you decide to run a 750cfm 4 barrel then if you use only your primaries then you will only being using 375cfm.

now since you have a 302, you most likely have a 350ish cfm 2 barrel. now most people who have a stockish 289-302 will run a edelbrock 500cfm. therefore you will only have a 250cfm on your primaries and you mileage will increase some. hell, if you put a 600cfm on there on a more warmed up 302 then you're primaries will still be at 300 which is less than a 350cfm two barrel.

u dig?
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