2 bbl to 4 bbl pros/cons?

[Larsen74Dart]

Hi. I have a stock '74 Dart 318 2bbl. I was offered a 340 intake with a 4bbl carb. off another mopar. Not sure if it was from a Dart or other model.
Question is:
1. Will this exchange fit without any modifications.
2. Will there be any noticeable improvement in performance/HP?
3. Will it be worth the conversion?

Pros/cons?

This is just an occasional driver. Not daily, but more than just weekends.

 

[toolmanmike]

Nothing but positive improvements. 340? What year and what carb? Stock or aftermarket parts? It will fit with very few modifications. You may have to mod the throttle and kickdown linkages to make it work properly but that has been don a million times and is really not a big issue. There's really no downside to a 4 barrel 318 other than you might see a reduction in fuel mileage because your foot will be planted to the floor more often because of all the fun you will be having when you do. . That's a beauty of a 74. Love that color. It glows in the sun. Here's a link to some good information.

 

[Professor Fate]

I'll get crucified for this, but if it's just an occasional cruiser you won't notice a lot of difference- unless you've got your foot in it all the time.
When you say "a 340 manifold and carburetor", I'll assume it's a factory iron manifold and not aftermarket. This means it could be an early unit with a Carter AVS, or the different later unit with a Carter ThermoQuad.
Either one will require different throttle linkages and transmission kickdown linkages from what you have on your 2 bbl. setup. There may be other considerations depending on your options, like cruise control or AC.
There is also a small difference in port sizes, but it isn't much of a factor on an otherwise stock engine.
And if you increase your intake's ability to breathe, you'll need to consider some exhaust work too, at least duals if not headers. Welcome to the rabbit hole.
IMO, if you've got a nice-running 2 bbl. with no issues, I'd keep it.
Not to say I wouldn't get the manifold anyway in case you get the hankering for a little more power later on...

 

[Tims 66 Cuda]

I did it to a ,73 Satellite years ago. There will need to be some mods on the throttle and kickdown linkage. Some of those manifolds had a mid rise profile so watch hood clearance. It may add additional HP but you wouldnt want an oversized carb. it will be cool, fun its all in what you want. Just my .02 cents worth.

Tim

 

[67Dart273]

With single exhaust it will give you little or nothing. Used to be common, back in the day, to get a muffler shop to hack off the Y pipe and add a second pipe out the back

 

[YY1]

Dual exhaust will add just as much HP as the 4 barrel, but...

The 4 barrel will not add the full potential unless the exhaust is done first.

Cool factor will go way up with a 4 barrel, though.

 

[jeff alder]

That is an incredibly nice looking and very clean car. You aren't going to set the world on fire with a 4 BBL. If the engine compartment looks as nice and clean as the exterior of the car and it were mine. I would leave it be.

 

[Larsen74Dart]

Hi. I have a stock '74 Dart 318 2bbl. I was offered a 340 intake with a 4bbl carb. off another mopar. Not sure if it was from a Dart or other model.
Question is:
1. Will this exchange fit without any modifications.
2. Will there be any noticeable improvement in performance/HP?
3. Will it be worth the conversion?

Pros/cons?

This is just an occasional driver. Not daily, but more than just weekends.

View attachment 1716377825

View attachment 1716377826

Wow. Thanks for all the input. The intake is factory, not aftermarket. Not sure what the carb brand is yet. Already have dual exhaust. I think I may just get the intake and carb, and hold on to it for the time being. I just got the car in the fall and have only driven it a few times. Been at the shop getting a few tweaks done over the winter. All lights working, brakes, carb, timing tuned up, washer resv., alignment, few other items. Got itchy and had to take it out today for a 2nd test drive after the work.

 

[413]

Swap out the 2.7 or 2.4 rear end gears for some 3.2 or 3.5 gears, that's a big improvement.

 

[CRUZE 418]

I would leave it alone. If you want performance, go in another direction. Just because you install a 4- barrel, doesn't make it cool. In these days, it's nice every once in awhile to see an unmolested hacked engine bay.

 

[RustyRatRod]

I'll get crucified for this, but if it's just an occasional cruiser you won't notice a lot of difference- unless you've got your foot in it all the time.
When you say "a 340 manifold and carburetor", I'll assume it's a factory iron manifold and not aftermarket. This means it could be an early unit with a Carter AVS, or the different later unit with a Carter ThermoQuad.
Either one will require different throttle linkages and transmission kickdown linkages from what you have on your 2 bbl. setup. There may be other considerations depending on your options, like cruise control or AC.
There is also a small difference in port sizes, but it isn't much of a factor on an otherwise stock engine.
And if you increase your intake's ability to breathe, you'll need to consider some exhaust work too, at least duals if not headers. Welcome to the rabbit hole.
IMO, if you've got a nice-running 2 bbl. with no issues, I'd keep it.
Not to say I wouldn't get the manifold anyway in case you get the hankering for a little more power later on...

I agree. I'd leave it as is.

 

[Dan the man]

Put a curve kit in. 3.23's would be a nice addition.

 

[YY1]

Changing ratios in a 7.25 axle is WAY more of an intensive and costly job than a 4 barrel swap.

 

[toolmanmike]

Wow. Thanks for all the input. The intake is factory, not aftermarket. Not sure what the carb brand is yet. Already have dual exhaust. I think I may just get the intake and carb, and hold on to it for the time being. I just got the car in the fall and have only driven it a few times. Been at the shop getting a few tweaks done over the winter. All lights working, brakes, carb, timing tuned up, washer resv., alignment, few other items. Got itchy and had to take it out today for a 2nd test drive after the work.

Without going too far into it for modifications, a 4 barrel, dual exhaust, and a recurved distributor are good things to do. The next steps like cam, compression, headers, torque converter, rear gears and the like get more involved and spendy.

 

[toolmanmike]

Swap out the 2.7 or 2.4 rear end gears for some 3.2 or 3.5 gears, that's a big improvement.

Put a curve kit in. 3.23's would be a nice addition.

I went from a 2.76 to a 3.23 and then to a 255-60/15 tire and didn't notice much difference.

 

[Dan the man]

I went from a 2.76 to a 3.23 and then to a 255-60/15 tire and didn't notice much difference.

3.55's?

 

[Professor Fate]

Changing ratios in a 7.25 axle is WAY more of an intensive and costly job than a 4 barrel swap.

FWIW, a 'teener in '74 would most likely have an 8 1/4- my '75 does, but I have seen an occasional 7.25 too. Ya never know with Ma Mopar!

 

[413]

I went from a 2.76 to a 3.23 and then to a 255-60/15 tire and didn't notice much difference.

That's because the end result was you didn't change the ratio with the taller tires, and if the wheel/tires weight was more you killed it that way also.


"Biggest tire that will fit" is not the answer for performance.

 

[413]

Changing ratios in a 7.25 axle is WAY more of an intensive and costly job than a 4 barrel swap.

Never said the keep using the 7.25, or that it was cheap, only that different ratio is very effective in getting the car moving.

Remember riding you 10 speed? Hard to start out on 10th gear! Change gear, much more performance, same power on the pedals, same total weight, mechanical improvements win the day.

 

[CRUZE 418]

This is a question about going from a 2-barrel to a 4-barrel. How did we get off into the weeds with rear ends. The car appears to be a very nice driver. Maybe he doesn't want to start hacking it.

 

[67Dart273]

On a bone stock 318 with duals, stock cam, a 2bb performing UP TO PAR aka jetted for performance without being "black," and as mentioned, the dist. curve redone, you can get a surprising amount of performance out of them. You also might gain a LITTLE if you built or bought an adapter and put a 360 size (bigger bores) 2bbl on the 318 manifold. The air filter base, however will not fit.

 

[413]

This is a question about going from a 2-barrel to a 4-barrel. How did we get off into the weeds with rear ends. The car appears to be a very nice driver. Maybe he doesn't want to start hacking it.

Because I wanted to that how. Any other ?? Why aren't you bitching about the exhaust posts here?

Because it is about the combination. And the gears and exhaust are a part of that.

Just adding more air/fuel to the top won't necessarily make it any better performing. Exhaust and mechanical advantage both have place. Can you think of anything else to help this guys car?

 

[toolmanmike]

That's because the end result was you didn't change the ratio with the taller tires, and if the wheel/tires weight was more you killed it that way also.


"Biggest tire that will fit" is not the answer for performance.

Ya, no kidding. I knew that. If I would have left the 2.76 in there with the tall tires it would be a bigger pooch. A gear change won't get you much until you go to a 3.55 or higher. Jumping one ratio is a waste.

 

[toolmanmike]

Because I wanted to that how. Any other ?? Why aren't you bitching about the exhaust posts here?

Because it is about the combination. And the gears and exhaust are a part of that.

Just adding more air/fuel to the top won't necessarily make it any better performing. Exhaust and mechanical advantage both have place. Can you think of anything else to help this guys car?

See the 318 recipe in post #14. Go that far and no further for a stock Teen unless you want to go off the deep end with modifications.

 

[rumblefish360]

Hi. I have a stock '74 Dart 318 2bbl. I was offered a 340 intake with a 4bbl carb. off another mopar. Not sure if it was from a Dart or other model.
Question is:
1. Will this exchange fit without any modifications.

Yes on the direct part swap question but
NO on the transmission linkage. This MUST be correctly readjusted or use the 340’s parts as the 2 & 4 barrel linkages and brackets are different. A 2bbl linkage can be made to work easy enough.

2. Will there be any noticeable improvement in performance/HP?

Yes but only in the mid range and top end.

3. Will it be worth the conversion?

Depends on the goal or reason. If it is more power, then yes.

Pros/cons?

Pros, more power for getting on or passing on the Hwy. Cool wide open throttle sound, depending on the two barrel size, better mileage as long as you drive it responsibly.

Con: Some see the conversion as a detractor, a loss of the cars value, so retain all the OEM parts. Added expense for parts you get don’t realize. Carb to transmission linkage as stated earlier, fuel line adjustment or if the correct OEM look, new fuel line. (All cheap but added expense.)

New air cleaner. An OEM 4bbl air cleaner can be pricey compared to an aftermarket open unit.

Temporary loss of mileage. Consistently stepping on the GO pedal really hard. Perhaps even more tire wear? LOL!

I’ll add in recurving the distributor for more power and mileage as a minor expense.

This is just an occasional driver. Not daily, but more than just weekends.

IF you decide on doing this swap and the only occasional weekend use character usage of the car, IMO, it’s not a big deal to do and a fun project. Again, just retain all of the parts you swap. Someone may actually want the 2bbl back.

I am in the process of changing back from a 4bbl. conversion back to the OEM 2bbl. On a ‘66 Charger. I have everything except the intake when I got the car. So I had to search for an intake. It’s not a date matched intake but it will do just fine.

If also agree with Toolmanmike. Those are excellent suggestions.
I’d call them a stage one hot rod step.

A dual exhaust is a very good idea for power and mileage. I also mention a distributor recurving. These two in addition will really help wake up the basic 318. After that, it gets pricey with cam and lifters and/or headers. Going a step further gets more pricey to the point of you might say “Ouch!” when you start seeing g the prices. The next parts are an upgraded torque converter and rear gears. On the high end, a converter is somewhere between (generally speaking) $600 - $800.
The rear gears themselves aren’t too expensive but the process can be a pain and pricey. A lot depends on the rear end size. 8-1/4, 8-3/4?

For now, just do the 4bbl swap, dual exhaust and recurve the distributor for a nice, easy & cheap upgrade that will yield nice results you can notice.

Enjoy