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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 :)


  • edited December 2007
    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.
  • edited December 2007
    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 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...
  • edited December 2007
    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.
  • edited December 2007
    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.
  • edited December 2007
    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.
  • edited December 2007
    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.
  • edited December 2007
    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.
  • edited December 2007
    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.
  • edited December 2007
    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.
  • edited December 2007
    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).
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