Replacement carb?

[dab67]

What carb if any, would you replace the Rochester 750 cfm on a stock 350 V8?  Just about everyone I know that has replaced the 750 Rochester on a 350 have stepped down to a carb somewhere between 550 and 650 cfm. Biggest question is why did the first gen 350's have such a large carb compared to what is suggested to replace it with?

Thanks!!!!!!!!

dab67

[CNorton]

If the original casting were too out-of-spec to salvage, my personal choice for a replacement would be another Quadrajet.  I'm a bit biased but I believe that one will have a difficult time improving on it.  The ratio of primary-to-secondary throttle bores plus the vacuum-controlled secondary opening rate provides the Q-jet a level of flexibility that is hard to match in terms of versatility.  A few years ago, Edlebrock produced a run of new carburetors based on the original tooling.  Those of us who compete in NHRA Stock Eliminator have had excellent results in using those units, tweaked for maximum acceleration.  They work well.  I've put the Edelbrock version on several big and small block street performers and had good success.

[joesauer]

I run a 350 with around 330 HP.  Changed my quadrajet to an Edelbrock 1406, electric choke.  It's 600 cfm, more than enough to handle the HP.  Stay away from the endurashine finish....it can flake off internally & clog up the carb.  Like the 1406 because I can easily change the metering rods "on the road" to compensate for high altitudes (ie going from sea level to 5000 ft.)

[dab67]

thanks for the info guys but does anyone know why the carb was so big back in 67-68 and 69 on the 350 V8?

dab67

[JohnZ]

thanks for the info guys but does anyone know why the carb was so big back in 67-68 and 69 on the 350 V8?

dab67

All Q-Jets were rated at 750 CFM (a few were larger); that rating is at wide-open throttle with the secondaries fully open - not an issue, as the primaries were quite small for good throttle response and a strong metering signal.

[dab67]

Thanks John:

Never really understood why it was so much larger than todays carbs. so the size was determined at wide open throttle? Can you tell me why most recommend a smaller rated carb now if you replace it?

Is there any difference in performance if the carb is repalced with smaller cfm carb? My engine (350) is basically stock other than headers.

Thanks

dab67

[Stingr69]

Generally speaking - Carb sizing is about determining the size that is big enough to adequately feed the engine but not so big it will reduce the response and metering ability. Most sizing formulas use engine size, rpm and volumetric efficency to determine the theoretical volume of air an engine can pump if it were an air pump. Very simple calculation. The Cubic Feet Per Minute (CFM) number you get from that calculation (or chart based off that calculation) will get the job done. A smaller rated CFM carb will still run, but it will hold you back from a performance standpoint. A somewhat larger carb will allow you a little more HP but it will be less responsive. If you go much larger in size above what is on the chart you will loose response and metering ability.

Carbs are rated at the CFM they will flow at a given pressure drop. 2-barrell carbs are rated a different pressure drop as compared to 4-barrell carbs.

Vacuum secondary type carbs are more flexible so they work better than a full mechanical secondary carb when they are both equally "oversized" based on the calculations. Vacuum secondary carbs are generally preferred for automatic transmission cars but will also work well with manual tranny especially for a street car. Quadrajets are very flexible due to the mechanical secondary with a vacuum operated air door on top. The advantage here is very good drivability over a wide range of conditions regardless of the size rating. Quadrajets are more sophisticated than some other carbs but tuning parts are harder to find for a quadrajet in my opinion. Holley parts have allways been easier to find and more people seem to know how to modify Holleys as compared to a Quadrajet.

Manual secondary carbs are good for racing and some higher HP stick cars but they are less forgiving when it comes to oversizing. When you stomp the gas, the carb will open all the way and the accellerator pumps will empty regardless of the true needs of the application at that particular operating point. Fun, but not allways the best choice.

-Mark.

[dab67]

stingray69

Thank you for the detailed explanation
dab67