Should I buy a different Carburator?
Thread Starter
Racer
Joined: Jan 2005
Posts: 499
Likes: 0
From: Enfield CT
Should I buy a different Carburator?
Welcome fellow forum members with Carburetor opinions.I have a small block 350 ,auto tranny ,pushing about HP at the crank.Its got a mild cam 210@50.On the engine now is a Eldebrock AVs Carb 650 cfm.The car runs well with no starting,stalling or driveabilty problems.I have read on the forum that some guys have gone to Holley style carburetors and noticed a huge performance gain!!What do you guys think?I was thing about a Barry Grant.I assume there quality control has gone up.
Last edited by bozzman3; Aug 8, 2007 at 04:36 PM.
Burning Brakes
Joined: Feb 2006
Posts: 1,024
Likes: 1
From: AL
i don't have a recommendation for the carburator, but was looking at your photos, in particular the dyno graph; i thought that the hp and torque curves were supposed to cross at 5250 rpm. was wondering if you or anyone else knew why these don't??
Safety Car
Joined: Aug 2000
Posts: 3,711
Likes: 43
From: San Antonio TX
I think it greatly depends on the combination. If you were under-carbed, then you could pick up some top end. But from what little you have revealed about your combination, I'd say to let things be as they are.
Thread Starter
Racer
Joined: Jan 2005
Posts: 499
Likes: 0
From: Enfield CT
S489 I dont know the reason why they dont meet.Gerry72 the rear gears are 373s,turbo 350c,2500 stall converter 64cc heads Trickflow,4 bolt L82 lower,Davis HEI with a Mallory cd type digital ignition.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
My opinion is that those who feel replacing the Q-Jet on their SB C3 was a major improvement...should have had their Q-Jet rebuilt by a reputable source. The standard issue Corvette Q-Jet has all the capacity one would need [unless major mods to heads, cam, and/or stroked] and it will be one heckuva lot more efficient than a Holley, etc.....if it is set up right. But, as I say, that's only IMO....
Burning Brakes
Joined: Feb 2006
Posts: 1,024
Likes: 1
From: AL
i think it should as long as the engine can take it (not bog) - it should with those gears and stall. i only have experience running mech sec with a straight shift . . .
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Le Mans Master
Joined: Jun 2004
Posts: 6,231
Likes: 65
From: Seattle WA
St. Jude Donor '14
In a nutshell, no - a carb swap isn't going to make more power. People that note a dramatic improvement in performance from a carb swap just got a carb that was closer tuned to their engine out of the box. Spend the money on a good dyno tune and you'll get all the performance your engine can deliver.
Engines are an air pump - they can only use the amount of air they're built to regardless of the carb you put on. Too-big carbs make for poor low-speed driveability and poor fuel economy. The formula for CFM is below; as a rule of thumb engines that make 1.2HP/CID or less have around 80% VE, 1.2 or more, 90% VE.
CFM = ((CI x RPM)/3456) x VE
Mechanical secondaries are racing carbs, pure and simple. They're made with the idea the full-throttle operation is the primary use for the engine. A properly tuned vacuum secondary carb will perform within a couple of percent of a mechanical secondary carb if the engine is making less than 1.4HP/CID...and provide FAR better driveability and fuel economy.
Engines are an air pump - they can only use the amount of air they're built to regardless of the carb you put on. Too-big carbs make for poor low-speed driveability and poor fuel economy. The formula for CFM is below; as a rule of thumb engines that make 1.2HP/CID or less have around 80% VE, 1.2 or more, 90% VE.
CFM = ((CI x RPM)/3456) x VE
Mechanical secondaries are racing carbs, pure and simple. They're made with the idea the full-throttle operation is the primary use for the engine. A properly tuned vacuum secondary carb will perform within a couple of percent of a mechanical secondary carb if the engine is making less than 1.4HP/CID...and provide FAR better driveability and fuel economy.
Drifting
Joined: May 2005
Posts: 1,629
Likes: 4
From: Somers CT
The secondaries on a Q-Jet are mechanical. Have your carb rebuilt or rebuild it yourself if you're able, it's the best thing going. Seems like there's no expense spared when rebuilding rear ends, trailing arms, waterpumps, front ends, etc but when it comes time to rebuild or refirbish a 30 year old Q-Jet most folks want to dump it and spend $600 or so on a different beast then spend the next year trying to tune it to the motor they have (if they're lucky). I don't believe you'll see any gain over a properly tuned and set up Q-Jet.
Bill
Le Mans Master
Joined: Jun 2004
Posts: 6,231
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From: Seattle WA
St. Jude Donor '14
Somewhat; the butterflies are mechanical, but the top throttle plates are vacuum activated by the adjustable air valve. Unless the top plates open, there's no flow through the butterflies. So it's both...but I qualify it as a vacuum secondary carb as there's no flow through the secondaries without vacuum.
that the right carb in good shape and properly matched and tuned is better than anything that's going to come "out of the box".
I've bought 4 nearly-new Edelbrock 1406's on Craigslist for $100 or so that were "complete pieces of crap" (previous owner)...because they changed the secondary springs, jets/rods. Taking it back to stock config and tuning from there has made a great carb for 4 rebuilds I've done. This isn't a Holley vs. Bg vs...thing; it's just about tuning the ignition and carb properly for the engine.
that the right carb in good shape and properly matched and tuned is better than anything that's going to come "out of the box". I've bought 4 nearly-new Edelbrock 1406's on Craigslist for $100 or so that were "complete pieces of crap" (previous owner)...because they changed the secondary springs, jets/rods. Taking it back to stock config and tuning from there has made a great carb for 4 rebuilds I've done. This isn't a Holley vs. Bg vs...thing; it's just about tuning the ignition and carb properly for the engine.
Melting Slicks
Joined: Sep 2002
Posts: 2,783
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From: Engineers do it better.
By Holley themselves ...
QUESTION How do I know if a vacuum or mechanical carburetor is best for me?
ANSWER For street cars the vacuum secondary carburetor works best on midweight or heavyweight cars with an automatic transmission. They are more forgiving than a Double Pumper is because they work by sensing engine load. The mechanical secondary carburetor is best on a lighter car with radical camshaft and a lower gear and manual transmission or on a car that is going to be used for racing purposes.
More info available here... http://holley.com/TechService/FAQ.asp
Le Mans Master
Joined: Jun 2004
Posts: 6,231
Likes: 65
From: Seattle WA
St. Jude Donor '14
By Holley themselves ...
QUESTION How do I know if a vacuum or mechanical carburetor is best for me?
ANSWER For street cars the vacuum secondary carburetor works best on midweight or heavyweight cars with an automatic transmission. They are more forgiving than a Double Pumper is because they work by sensing engine load. The mechanical secondary carburetor is best on a lighter car with radical camshaft and a lower gear and manual transmission or on a car that is going to be used for racing purposes.
More info available here... http://holley.com/TechService/FAQ.asp
QUESTION How do I know if a vacuum or mechanical carburetor is best for me?
ANSWER For street cars the vacuum secondary carburetor works best on midweight or heavyweight cars with an automatic transmission. They are more forgiving than a Double Pumper is because they work by sensing engine load. The mechanical secondary carburetor is best on a lighter car with radical camshaft and a lower gear and manual transmission or on a car that is going to be used for racing purposes.
More info available here... http://holley.com/TechService/FAQ.asp
Drifting
Joined: Jul 2006
Posts: 1,871
Likes: 6
From: Sault Ste. Marie Ontario
I had a 780 Holley on my small block 56 Chevy. It did nothing but bog and drink fuel. Changed ot out for a 650 and the car came to life.
Melting Slicks
Joined: Sep 2002
Posts: 2,783
Likes: 2
From: Engineers do it better.
Just yanking your chain
Drifting
Joined: May 2005
Posts: 1,629
Likes: 4
From: Somers CT
Somewhat; the butterflies are mechanical, but the top throttle plates are vacuum activated by the adjustable air valve. Unless the top plates open, there's no flow through the butterflies. So it's both...but I qualify it as a vacuum secondary carb as there's no flow through the secondaries without vacuum.
Le Mans Master
Joined: Jun 2004
Posts: 6,231
Likes: 65
From: Seattle WA
St. Jude Donor '14
Le Mans Master
Joined: Jun 2004
Posts: 6,231
Likes: 65
From: Seattle WA
St. Jude Donor '14
The secondary butterflies are opened mechanically, what is vacuum operated are the secondary metering rods which allow only the amount of fuel into the engine that it requires. The metering rods are pulled up by the action of the top plates which react to the vacuum now present by the butterflies opening. That's what makes a Q-Jet so versatle, it only delivers the amount of fuel that the engine can handle.
I oversimplified Regardless, I think we're in agreement that the Q-jet functions as a vacuum secondary carb.