........The Q-jet carb on my basically stock 305 needs rebuilt but now I'm thinking of replacing it. I'm thinking edelbrock. My question is should I go with a 600 or a 650? Also will I notice any performance gains over the rochester? Thanks.......Zapr.

I was about to ask the same question

The correct answer is "computer controlled q-jet".

You will not see any performance improvement with an Edelbrock. If you don't have something set right on the q-jet, you might get a driveability improvement by going to the Edelbrock. However, a properly adjusted and operating CC q-jet will drive as well as, get better fuel economy than, and have as much or more power potential than an Edelbrock.

A properly tuned edelbrock is a great thing. The unfortunate side effect of installation is the loss of all CC features. You will need to replace the distributor too. I would look at installing a new or rebuilt CC QJet, that is, unless you are planning some serious mods.

.......My carb is not computer controlled. 85 lg4 Canadian car. So if an edelbrock will not give any more performance, what about a holly. Again, 600 or 650. Thanks........Zapr.

If you were to tune the carb properly, yes, you could see an improvement; out of the box, maybe. The point is really, unless you have other mods (heads, cam, intake) you wont see any improvement leaving the qjet behind.

FYI, i have seen many 12 second cars running QJets. A great carb when it is tuned and calibrated properly. Read up on it, and you may just decide that it is what you want.

as stated above without other modifications your not going to see any improvements over your quadrajet except the fact that the quad is out of tune and needing rebuilt where the new one will not ....

.......I think maybe I'll just rebuild the Q- jet. Thanks for the help guys............Zapr.

I might do the same.. The only thing wrong with my Qjet carb is that there's no more choke on it.. And it takes forever to warm up. And when I drag race and I reach the 5000 Rpm before shifting something the carb chokes.. And it sucks!

Yeah and save yourself the added purchase of a new intake or carb adapter for the Edelbrock.

I know alot of guys here dont like edlebrock carbs, but I just replaced my stock manifold and q-jet with an edlebrock 650 carb and a performer rpm intake. Im thrilled with the results, definitely a good move, I noticed a nice performance gain, and I still have fuel efficiency, but if you are looking for the most performance gain possible, Id say go with a demon.

apples to apples...

No carb will out perform another brand carb

Many test have been done useing the same engine, and same CFM carb but diff brands.. Q-jet, edelbrock, holley, demon, etc

The max diff was 5 HP apart, most was within 1-2 hp of each other.


The main thing with carbs is to dail them in right for your engine, and don't pick one too big for your needs

I run mid to high 12's on a edelbrock 600 cfm

You need to tune the carbs idle, crusie, and power to your engine

Jet it right (metering rods in Q-jets and edelbrocks), use the right power valve (step up spring the the edelbrock), ect


The big gain with the edelbrock carbs are how easdy they are to change adjustments on

Hear, hear. Exactly what edelbrock guys have been saying for a long time.

i see guys asking all of the time which size carburetor is right for their motor, and most always they never get the right answer. while a CC q-jet might be the right answer for some, what size is still outstanding.

this formula may help you determine what size carb belongs on your car: (cubic inches divided by 2) times (RPM divided by 1728) times volumetric efficiency == cfm

cubic inches is easy to substitute a value for. RPM is the maximum rpm the motor will see. volumetric efficiency should be 100, unless you are planning on your combination being less than optimal.

so, plugging some real numbers into the equation: 350/2 X 6000/1728 X 100 == 607.6 cfm.

the only reason i would vary from this equation is if the carb was pulling 1.5-inch-Hg (or more) vacuum at WOT, then it has become a restriction and should be upgraded until that large a pull is gone, which may just take 700 cfm. no one will actually tell you that you only need a 600 cfm carb - because BIGGER is BETTER or some other such nonsense.

make sure the carb is dialed in for your application. this means the proper size jets, metering rods, etc., and that it is tuned properly.

#1 - He has a "basically stock" 305.

#2 - No particularly good reason for replacing it was given.

#3 - The originator has already said he's going to rebuild the q-jet.

#4 - "Size matters" - Not! The q-jet with adjust itself to the demands of the engine. So why are we bothering with theoretical "CFM formulas"? (BTW, why the extra terms? CID times RPM times VE divided by 3456 gets you there. "3456" is a lot easier to remember than "1728".)

#5 - He will not notice any performance gains over the Rochester, especially with a Performer. See #1. So why are we bothering even discussing it? If this was a drag racing dedicated, non-stock class car, there might be some benefits realized - but not in this case.

Of course, the fact that it is a Canadian-delivered car was not revealed in the beginning. However, CC or non-CC q-jet, doesn't make a big difference. See #1...

Actually Five7, #1 sums it up real well, I think that was what i pointed to in the beginning of the thread also.

no one was challenging your answer(s), least of all me. i was merely adding in what i knew to anyone that might be able to use the information. just because this topic was answered doesn't mean someone else won't wander in here weeks or months from now and be able to use the info i posted. or not.

A little too much juice for me today, I guess...

That's why we so often say

Since when does Edelbrock make a 650cfm carb? Ive seen 500, 600, 750 and 800. But no 650. Carter makes a 625, but thats it.

Details, details.

Doesn't make any difference as far as the originator's situation, but it would be good to keep the information contained within accurate.