How do I pick what CFM carb I should be using? I have a 71 olds 350 bored .030 with flattop Pistons ,oversized valves ,and a fair sized cam . Don't know the exact numbers . I have a quadrajet,I belive its a carb from 1977. I don't want to go and buy a carb and find its too small or too big . Any help would be appreciated thanks

 

Not hard to figure out:

350 cubic inches = 0.2025 cubic feet
Since your 350 is a 4-cycle engine, it requires 2 revolutions to cycle all of its cylinders, so
0.2025 cubic feet ÷ 2 = 0.1013 cubic feet per revolution.
0.1013 x 5,000 RPM (probably a bit higher redline than reality, but I'll use it anyway) = 506 CFM
506 CFM x (very roughly) 80% efficiency = 405 CFM the carburetor air flow you need.

Your 1977 QuadraJet is 750 CFM, or possibly 850 (depending on the model).

- Eric

 

remembering that cfm flow across an orifice [like a carb] is rated at a certain pressure drop. Different pressure drops will create differing flow figures. I believe there is a standard pressure drop that pretty much everyone uses to rate a carb, in order to compare apples to apples.

 

A 1977 quadrajet is a 800 plenty for any 350. They come a little lean idle and transition. Get cliff ruggle book on quadrajets . then calibrate to your needs.

 

Somwhere I've read that a carb shoud have less than 1" Hg pressure drop at WOT.

Carb sizing is one place that bigger is NOT better.

Gerry

 

According to Doug Roe's Rochester book, 4BBL carbs are rated at a pressure drop of 1.5" Hg. Single and 2BBL carbs are rated at 3" Hg. The higher the pressure differential, the higher the measured flow - just like if you increase the water pressure in a hose with the same nozzle, the higher pressure will provide more gallons per minute.

Since the pressure drop across a carb changes with throttle opening and engine RPM, the calculation based on engine displacement and RPM is only a very rough sizing tool. Pressure differential across the carb drops as you get closer and closer to W.O.T., so the actual flow at W.O.T. may be less than the carb rating. You almost always want to go larger than the number you get from the calculation.

 

Is that the stock (non W31) redline on a 4-bbl 350? Makes me wonder what the factory redline on a 2-bbl 350 is.

 

Stock, these motors with automatic transmissions would shift at 4,000 to 4,500.
I went up a bit because he listed some minor modifications.

The Olds motor was not designed to be a screamer.

- Eric

 

Holley still builds a 650 Spreadbore that would bolt right on the engine. That would work very well and can be tailored to your cam quite easily.

 

My '70 Supreme (350-4bbl) shifted at 4800 RPM. No mods to engine or trans at that time (late '70s).

 

remember that while qjets are generally 750 or 800cfm castings, depending on model, the secondary air flaps had a stop that would limit their travel and thus the cfm capability of the carb.

thats how they used essentially the same casting for v6 engines up to big block V8s.

My advise would be to go with a quadrajet set up for your combination. A good rebuilder/restorer will be able to custom tailor the calibration for your setup.

 

I stand corrected. I'll say 4,500 to 4,800 RPM, then.

- Eric

 

My engine is specked very similar to yours with Flat tops, and mild cam. I had a 600 cfm Edlebrock with an Edlebrock ep2 intake. Techg8 built a nice performance tuned Q'jet for me. It really woke my engine up. You should talk to him about what he could do for your Q Jet. He knows his stuff.......

 

Thanks for all the help guys, gonna stick with the qjet