Gage

My 72 that I just bought came with an Edelbrock 600 on top of an edelbrock intake. Being that I prefer original stuff, I put the newly rebuilt original Rochester back on. Ran nice and smooth, but I think it had more power with the Edelbrock...Am I just imagining this, or does this this jive with others have experieced. It is just the base 350 motor with a mild cam upgrade. Thanks for the help in advance

lars

The Q-Jet, if correctly set up, will outperform the Edelbrock (a reproduction of the old Carter AFB) by quite a margin. The Q-Jet may not be set up right for your application if the AFB is running better. GM stopped using AFBs in performance applications in 1966 in favor of the better-running Q-Jet.

marshrat99

The Rochester is also probably 750cfm vs the 600cfm Edelbrock.

lars

That's correct. All Q-Jets except 455SD Pontiacs and 455Stage1 Buicks are 750 cfm. The SD and Stage 1 carbs are 800.

dgruenke

I could have sworn that some of the Q-jets found on the 350's were 795 cfm.

Boofers

According to "Rochester Carburetors" by Doug Roe there are two different flowing Quadrajets. One has 3/32" primary venturis and flows 750CFM, the other has 7/32" primary venturis and flows 800CFM. The book goes on to say that a 350CID engine running at 6000RPM will only require 600CFM.

page62

The contemporary Edelbrock QJets claim to be 795 cfm. Go figure...

In any case, comparing cfm ratings of a QJet and a square-bore carb like an Edelbrock is an apples-and-oranges type thing. For starters, the QJet is a spread-bore carb: small primaries, BIG secondaries. But the secondaries on a QJet are controlled by the air valves (the big flaps on top of the secondaries) in addition to the mechanically actuated throttle plates on the bottom. The theory behind this (correct me if I'm wrong here) is that the air valves allow the carb to adapt to the engine's needs, providing the correct amount of flow as needed. Of course, proper jetting is critical, but the unique design of the QJet allows it to be used in a variety of applications.

Gage

Well I hate to admit it but I put the Edelbrock back on this weekend and the car performs noticeably better than with the Rochester at all speeds. I loved the Rochester on my 69 427 ,but my impression is that the 350 can't take advantage of the huge back barrels on the Roch. On my 427 vette it felt like someone hit the nitrous button when back barrels opened. The fact that it had 3.55 gears vs 3.08 on new one probably helped as well. I thought others who are thinking about switching might like to know the results of a head to head test on an otherwise stock 350.

page62

This is not an accurate comparison, unless the QJet is not properly tuned, and comparable intakes are used. If all this happened, I'll bet the QJet would give better overall performance. The beauty of the Edelbrock is that it will provide good performance out of the box. The QJet must be set up to do the same.

Do a search, there was a very detailed discussion of this recently. Or get a copy of Lars QJet tuning paper at http://www.corvettefaq.com

Larry B.

Well... That's the beauty of a Q jet. The Air valve adjustments and it's related parts like the choke pull off (also functions as a dampener for the secondaries) only allow the engine to use what it needs. Kinda like using the maximum for the conditions. Theoretically you sould be able to use a Q jet on almost any engine and it would adust itself to the engine.. Fuel on demand. That's in theory... It still needs the correct jet sizes and other parameters tailored to each application. That's whey there are so many different Q-jets for all of the vehicle/ engine combinations. Also the small primaries give the great low end throttle response while the big secondaries begin to open before the primaries a fullt open. Now... If you go to Lars tuning paper you will find the calbration info need to attain maximum performance.


[Modified by bluevetteman, 10:09 AM 3/30/2004]