Tech Archives - Carb Swap

I have been thinking that maybe my lame gas mileage has something to do with my carb being a bit on the large size for a stock 273 with a .425" lift, 260 duration cam... Right now I average around 10-11 mpg, and now that my Kickdown is working, I drive a little mellower... I have a 600cfm 4 bbl Eddie on there now, which I have even jetted and rodded down to a "Stage 10" from the tuning guide, which is as lean as you can make this model...
Chuck

While we are on this subject, I too am in a quandry about carbs. Using the formula of 2CFM per horsepower, (is this a good formula?) a stock 273 should be happy with a 450 CFM, 4bbl. This should be adequate for up to 225 HP, which would be fine for a mildly tuned, low-compresssion 273. So, does anyone have a recomendation for such a carb? Also, can someone recomend a duel-plane manifold for this carb and engine? I'm told the duel-plane will work best with a mild engine and automatic. It supposedly gives much better low-end torque. What say you?
Steve Leek

Years ago I had a 65 Formula S that I swapped the Carter out for a Holley 450 4bbl and it was on the stock single plane manifold it gave me everything I wanted. I ran it allot on the 1/8 mile and it held it's own with the Mustangs & Nova's this was a 4 speed car but I think that a single or dual plane manifold will give you all you want. I don't think at the weight of are cars you will really notice the difference between the two manifolds unless you are planning on building it up and racing it. There is my 2 cents worth and good luck.
George Walling

Last car we built was a 55 chevy with a well built 355. It originally had a victor jr. single plane. From about 3200 rpms and up, hold on. But below that, it wasn't too enjoyable; wouldn't pull from down low, shuddered, etc... We switched to a edelbrock RPM air gap dual plane intake (rpm range 1500 - 6500) and couldn't have been happier with it. It ran just as good for being a street car and was much more enjoyable stopsign to stopsign too.
Arron DeRousse

There is no reason a properly setup 600 cfm carb won't be fine. The engine only uses what the air flows to it when it is correctly carbureted. I ran a Carter 770 cfm carb from my 68 Charger R/T on my 65 Formula S 273 4-speed 3.55 gear, using a carb to manifold adapter for over 50,000 happy street and strip miles. No problems. Of course, I was not searching for optimum mileage. The plugs showed it was fine. If you want optimum mileage, get a 2bbl intake and carb from a 318, put 2.76 gears in the rear and drive like you have an egg under your foot that you would rather not crack. You will get 20 to 23 mpg on the highway like my automatic 318 Scamp does. First order of business should be to tune what you have with carb, plugs, and timing.
Tom Stroup

I run nothing but Holleys. They are easy to tune and work on. The GoldFish got 18-19 MPG as long as you kept it under 70 on the highway (317 RwHp, 318, Holley 680 4150(vac 2ndry), Edlebrock Performer (a really good choice for a 273 too), Hot Hughes mech cam, A904 Auto, 3.23:1 8.75", 225 50 15 tires). The 680 Holley (650 with the choke horn cut off), worked really well on the old 273 too.
Dave Mapes