4bbl. vs. 2bbl. HP

I have a '72 Nova (4-door), 44,000 miles, with a 307 2bbl., no a/c and single exhaust. I understand this motor has about 200hp at 4600rpm. It’s nearly all original and I’d like to keep it that way as much as possible, but I’d also like to up the HP a bit, without changing motors. How much more HP would a 4bbl. provide? (P.S. Car is also for sale at $7900 :slight_smile:

The big question is if the rest of the engine set-up can take advantage. Some engines simply cannot draw enough volume of air through a bigger carb to take advantage of the larger air intake, and the engine will bog down, and drive worse. A '72 small block V8 should be able to handle a small 4bbl (like a 550 cfm), but, unless you replace the intake manifold with a 4-bbl manifold, a 2bbl-to-4bbl adapter will kill most of the advantage of adding a 4bbl. Plus, the factory-style single exhaust is pretty restrictive. If air cannot get out efficiently, It cannot get in efficiently.

For that engine, if I were to change to a 4bbl, I’d also change the intake manifold to a better flowing Edlebrock, put in a better breathing camshaft, like a Crane street cam, better flowing headers, and the take it to a muffler shop for dual exhaust. Otherwise, your not getting the best bang for the buck.

Locate and install a stock Chevy 4V intake manifold and Quadrajet carb. Also, dual exhausts, but the stock manifolds will be OK. Headers not worth the trouble on a stock engine. Now you have maybe 225hp, enough to feel but no hot rod.

If you want SERIOUS power, select stock parts from the 67,68, 69 model cars. Heads, cams, Corvette stuff…you might want to drop in an HEI ignition from 75-80 model Chevy’s and get rid of the points…You may have to "dimple’ the firewall a little to get it in…

The least expensive way to increase power is to install dual exhaust pipes. Try that first and then consider the more expensive changes. You might consider increasing the exhaust pipe diameter to increase flow, too. I agree that the OP must change the intake manifold if the carburetor change is made. Edelbrock has performance 2-bbl and 4-bbl intake manifolds that would provide a boost.

The HP number is about impossible to say, but it would not be earth shattering or worth the trouble. 10 HP, maybe???
The four barrel is only going to help at wide open throttle and you have to ask yourself how much of that do you do. Not much probably, and especially with a 307.

Getting serious, noticeable HP is going to require a cam, opened up exhaust, intake manifold, with some cylinder head work and a 307 is really not worth the effort. The 350 is an easier option.
Going to a four barrel may be impressive on the looks but you’ll seldom use those back barrels unless the pedal is to the metal.

Depending on the carb you use, sometimes going to a 4 barrel will actually make the car more anemic during in-town driving and you may wish you had the 2 barrel back on it.

If the car is really a low mileage original, don’t mess it up looking for a couple more HP. Anyone willing to give you $8K for it will probably want it as original as possible.

The stock 2bbl will actually produce more low end torque, which those old cars needed a lot of, especially with the exeedingly steep-geard automatic transmissions. In terms of raw horsepower, a stock 307 probably won’t notice a bigger carb in terms of airflow, although having secondaries can deliever better gas mileage and a somewhat peppier engine.

1 Like

Agreed! If you restrict the intake and exhaust a little, you will keep the torque curve farther down the rev range, giving you more useable power. A 2 1/4" dual exhaust system will not hurt, but IMHO, opening the exhaust more than that, or moving to a 4 barrel on a stock motor, it’s just not worth it. You can get 390 CFM 4 barrel carbs, but it’s really not worth it in this application.

A good example of carburetion changes affecting peformance could be an old Triumph Bonneville motorcycle I had many years ago. The Bonneville was the “hot rod” of the line with dual carbs, hotter cams, etc.
Since this bike was used primarily around town I swapped a TR6 (bike, not the car) cylinder head with a single carb onto it and the difference was day and night.

The single carb made that bike 200% more pleasant to ride and it was actually far quicker from a dead stop up to about the 2 or 3 block mark. The dual carbs only helped on the long runs; say a 1/4 mile or better.

Not enough to matter. You will get enough to notice, but the next owner can spend the money and time on it willingly, or won’t bother because it’s a four door and doesn’t look like a hot car.

I must agree with the leave it alone option except to mention that if the car seems to lack power check the spark advance, both centrifugal and vacuum and check the valve in the passenger side exhaust manifold for proper operation (better stuck open than closed).

That 200hp is the old SAE (optimystic) number; it is more like 145hp measured by the current SAE method. You need the get more flow through the engine, and I would also get dual exhausts. The engine itself is robust enough to not need additional reinforcing.

I think your 10 horsepower figure may be correct. I remember the ads for the 1955 Plymouth V-8. The engine displaced 260 cubic inches. The horsepower was 167 with the regular 2 barrel carburetor and 177 if the four barrel was ordered. I don’t know if the four barrel came with dual exhausts or not. Chevrolet offered a package of a four barrel carburetor and dual exhausts on its 265 cubic inch engine for 1955. This may have boosted the horsepower from 160 to 180, but I’m not certain of these figures.

A 1972 Chevy II with a 307 does not seem like a collector’s car. But I would not make any changes if you are going to sell it. FWIW, eBay has cars like this for around $8000 with a similar drive train and mileage.

Don’t touch this car in any major way unless it needs a major repair. However, I agree with some of the responses to tweak it. An HEI would certainly help. Alternatively, there are kits that convert the points to electronic ignition using the same cap and rotor for original look. Thinking outside the box, how about getting rid of the stock cooling fan and going electric. You would be amazed at how much the stock unit can drag an engine. How about re-jeting the carburetor? Think about a K&N air filter too. I’m not against duel exhaust either as this was an original option. (very cool.) Remember to save all the original parts.

Thanks so much for all the replies and comments! Very much appreciated. Made my decision much easier. I’ll stick with the 2bbl. to maintain its original/stock equipment. More important (to me) than the additional 10-25 HP, work and cost. No, it’s not a hot rod, but it still gets a lot of comments. I will do the dual exhaust, though. Again, thanks to all, and hope your Christmas stockings are filled with all the parts you’ve been looking for (car parts, that is)!

Thanks for the pics. I think jtsanders is wrong. That looks like classic muscle car stock to me. And, judging by some Nova owners I know, they are very collectable. Just not rare enough yet for the high-dollar returns, like the Hemi Challeger or Yenko Camaro.

That’s a great looking car. I’ve always been partial to most of the greens used back then. Very nice survivor Nova! Most of them have long since been beaten into the ground or become 1/4 drag cars.
Recently met a little old man (about 80 or so) and he was still driving a 71 Buick Skylark that he bought brand new way back when. The car only had 31k+ miles on it and was the same shade of green as yours, or very close to it.

Not necessarily. Most four barrel carbs are a progressive setup, with only the front two barrels opening at part throttle. If the two front barrels are smaller than the original 2 barrel, then the 4 barrel will produce better lower end torque, better throttle response, and also flow more at higher throttle positions when the rear barrels open.

Driving through Virginia, I saw a little old lady, barely able to see over the steering wheel of her pristine silver '70/'71 Chevelle - complete with black stripes. Wanted to follow and ask if she’d sell. Sad when you’re envious of a little old lady!