Topic: New Carb....
in Forum: C3 Fuel, Emission Control, and Exhaust Systems
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Hey everyone. I'm back from another 4 month deployment...got a little extra jingle in my pocket, so it's time to throw some more $$ at the vette.
I'm looking into a new carb. My Q-jet is just too much of a pain for me...sick of tuning and retuning. I see that most guys with a stock 350 are sticking with the 650 CFM. Any particular reason for that?? Whats everyone's opinion on a larger CFM carb?
I drive pretty aggressive from time to time...the more power the better in my opinion. My engine is a stock chevy crate motor...no cam, nothing fancy (yet
) Just lookin for some opinions...
I'm looking into a new carb. My Q-jet is just too much of a pain for me...sick of tuning and retuning. I see that most guys with a stock 350 are sticking with the 650 CFM. Any particular reason for that?? Whats everyone's opinion on a larger CFM carb?
I drive pretty aggressive from time to time...the more power the better in my opinion. My engine is a stock chevy crate motor...no cam, nothing fancy (yet
) Just lookin for some opinions...SPONSOR AD:: (Our Sponsors help support C3VR)
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Ken(and a few others here) can better explain the intricacies of matching CFM with engine efficiency, but the fact is, bigger ain't always better, when it comes to carbs. Most stock 350s just aren't able to use the extra flow capabilities of a larger carb, and it also actually hurts efficiency in most cases. A 600/650 is pretty much the best all around choice for a stock, or mildly modified 350...
Joel Adams
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Lifetime MemberManteca, CA - USA
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Yep, what Joel said is curiously true. despite the fact that most of the Quads we are running are 750 and 850 cfm, don't use that as a gauge to buy an equal carb as thier air flow design is unique (most of the flow is through the secondaries). The "Quadrajet" is really a great carb and normally doesn't need adjustments. Have you seriously considered having it professionally rebuilt ?
Jimmy B.
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If you go with too big of a carb, you reduce the flow rate through the carb. To move the same amount of air through a hole, the air flows faster through a smaller hole, and slower through a larger hole.
For a carb to work, it needs volocity of air flow. With low engine speeds you get less efficiency,but the throttle plates are also closed causing a quicker air flow around the plates and the idle jets then a larger bore carb would provide.
Any carb is most efficient at wide open. The problem is at wide open it also supplies extra fuel for max performance. That extra fuel has to blend and atomize well. With too big of a carb, the flow is not enough to atomize the fuel well, and you just dump a lot of liquid gas in the engine. Liquid does not burn. It must turn into a vapor. As a result performance suffers, and the engine bogs.
Without the enrichment a carb delivers the best efficency at about 3/4 throttle. Again a larger carb is not as far open as a smaller one, so it's efficency suffers, and mileage drops as well as throttle response.
The small carb is always better, until it gets too small to supply all of the engine needs. At that point efficency is way up, but performance suffers.
This is the advantage of the q-jet. The small primaries create a very good efficency of a small carb, and provides better tip in throttle response as well as better mileage in a street driven car. You in effect have a tiny carb working well for most driving. BUT when you want the power the moster secondaries open up and provide the volume needed. Needed, not excess.
On the other hand the uneven arrangement does cause a slight distribution varation of fuel to the cylinders. The rear gets slightly more fuel than the front. On a track this can slow you down a tiny bit. A Holley type carb with the same size openings at all 4 spots gives a slightly better fuel distribution at wide open throttle. So on a track the Holley design can perform better. Granted we are talking hundrends of a second. On a track that matters, on the street you can't tell the difference, but you will lose a bit of mileage and tip in, just what you need on the street.
But either way, going to big drops the car efficency, and everything suffers. Common mistake.
Ditto for me! After I had my Quadrajet professionally rebuilt and set up on a flow bench, the result has been one of the best driving cars I have ever owned ( and I've owned quite a few ). The car starts right up, idles great, creeps when you want it to, and jumps when you step on it. Couldn't be happier!
Barry
Thanks for the advise fella's. I just ordered headers, but still am undecided about the Carb. Seems an overwhelming support to keep the q-jet and just have it re-worked...but i still would like to have the ability to do work on the carb myself...which is why I'm leaning toward a new product all together. However, I don't want to take a hit in performance. Anyone running the Holley 0-80555C Carb? Its the q-jet replacement.
in Forum: C3 Fuel, Emission Control, and Exhaust Systems
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