# 400 carberator



## liljohn442 (Sep 9, 2010)

In the process of rebuilding my 400 I needed a new carberator. I recently aquired a Holley 600cfm spread bore which will match my intake, but i am concerned if that will be enough. All of the GTO catalogs have either a 650 or 750 cfm as far as carberators. Any help would be apreciated.


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## freethinker (Nov 14, 2009)

liljohn442 said:


> In the process of rebuilding my 400 I needed a new carberator. I recently aquired a Holley 600cfm spread bore which will match my intake, but i am concerned if that will be enough. All of the GTO catalogs have either a 650 or 750 cfm as far as carberators. Any help would be apreciated.


that will be fine for street use if jetted correctly.


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## BearGFR (Aug 25, 2008)

liljohn442 said:


> In the process of rebuilding my 400 I needed a new carberator. I recently aquired a Holley 600cfm spread bore which will match my intake, but i am concerned if that will be enough. All of the GTO catalogs have either a 650 or 750 cfm as far as carberators. Any help would be apreciated.


Hi LilJohn,
600 is going to be "close" depending on how stock that 400 is. Even the base QJets that came with all 400's flowed 750 cfm. If it's a stock motor and not a Ram Air III, IV, HO, doesn't have an aggressive cam, and has D-port heads, you'll probably be ok. A factory QJet, properly set up, would probably run better though.

Bear


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## Eric Animal (Oct 28, 2007)

:agree find a proper Q jet!


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## dimitri (Feb 13, 2009)

BearGFR said:


> Hi LilJohn,
> 600 is going to be "close" depending on how stock that 400 is. Even the base QJets that came with all 400's flowed 750 cfm. If it's a stock motor and not a Ram Air III, IV, HO, doesn't have an aggressive cam, and has D-port heads, you'll probably be ok. A factory QJet, properly set up, would probably run better though.
> 
> Bear



:agree:agree:agree:agree:agree:agree:agree:agree


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## liljohn442 (Sep 9, 2010)

If all goes right it will be factory bore. This cam 51-223-4 - Xtreme Energy? Hydraulic Flat Tappet Camshafts. And a Torker II intake from Edelbrock.


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## Mr. P-Body (Jan 20, 2011)

Bear's correct. 600-650 is simply not enough for a performance 400. 750 MINIMUM. We use 800 CFM Holleys on performance 400s. A well-done Q-Jet is "the ticket" for most street Pontiacs. Pre-74 are 730 CFM, post '73 are 800. ALL are "vacuum secondary". 
The XE268H is a decent cam with a manual trans. It won't "like" the stock converter (been there, done that...) if an auto trans. Works best in 9.3:1 or lower compression. 

If you're really rebuilding the engine, it will need to be "bored". Otherwise, it's an "overhaul" (legal disctinction). We recommend boring when wear exceeds .003", either "taper" or "out of round". Fresh cylinders are the purpose, not increasing displacement. A "new" engine will always outperform a a "used' one if done right.

FWIW

Jim


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## liljohn442 (Sep 9, 2010)

This is really just a mild street performer, not many racing intentions for this car.


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## Mr. P-Body (Jan 20, 2011)

Wear knows no performance "level". It's either "in spec" or it's not. Best advice is ALWAYS "Do it right, do it once". Inevitably, if one avoids necessary operations, they WILL be "revisited". 

Jim


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## liljohn442 (Sep 9, 2010)

Engine is in the process of a rebuild, but still the question is will this carb perform on a daily driver basis.


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## BearGFR (Aug 25, 2008)

liljohn442 said:


> Engine is in the process of a rebuild, but still the question is will this carb perform on a daily driver basis.


If it was my car, I wouldn't run it. A lot depends on what you expect out of the car and how you're going to drive it though, and those are questions only you can answer. Personally, I'm concerned that 600 cfm won't be enough to feed the motor "everything it wants" at wide open throttle. If you can honestly say that you're "never" going to drive it like that, then it won't matter.

Bear


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## jetstang (Nov 5, 2008)

liljohn442 said:


> Engine is in the process of a rebuild, but still the question is will this carb perform on a daily driver basis.


Carb will work just fine for a daily driver, and get better MPG than other larger carbs. The optimal carb is larger, do it as your time and money permit.


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## freethinker (Nov 14, 2009)

liljohn442 said:


> Engine is in the process of a rebuild, but still the question is will this carb perform on a daily driver basis.


yes it will be fine. realize that if you wanted you could put a stock 2 barrel rochester carb that only flows 350 cfm and it would work fine as a daily driver.


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## BearGFR (Aug 25, 2008)

jetstang said:


> ... and get better MPG than other larger carbs.


Not sure I'd buy off on that, because...

At any specific rpm and load, and engine needs a precise amount of fuel to run correctly. It doesn't care what kind of carburetor it's squirting though, or what the maximum flow capacity of the carb is (or even whether the fuel is being provided by a small gnome perched on the intake with a very precise eye-dropper ) --- as long as the proper amount of fuel is being mixed with the volume of air the engine is pulling, it's going to be happy and the exact same amount of fuel is going to be needed. To get "worse" mileage, a larger carb would have to be providing more fuel than the engine needed, either because it was jetted/metered "too fat" or had some sort of malfunction (like a ruptured power valve diaphragm due to a previous backfire that Holley's are -notorious- for...). CFM rating is a measure of a carb's maximum airflow capacity, and as such does not necessarily mean it's going to use more fuel.

Here's the exception: If you have a carb that is WAY too big for the engine (think putting a 1050 Dominator on a stock VW 4-banger), then the motor is never going to be able to pull enough air through the carb to activate the main metering circuits. It's going to be running on the idle and transition circuits all the time. Those circuits are usually a tad on the rich side because at very low flow velocity the fuel has a tendency to "drop out" of the air stream and puddle in the intake manifold, so to compensate those circuits are often a little fat to try to make sure the proper amount of fuel stays in the air stream and gets to the cylinders.

Bear


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## Jeff's Classics (Feb 24, 2010)

That cam is too big for an auto-trans daily driver, IMHO. Instead of dropping compression and going with a Comp XE series cam why not keep the stock setup and use an "old school" cam with more overlap that bleeds off the cylinder pressure of a higher compression engine? I ran a XE256 in a big block Chevy and the idle was rough (for a small cam, relatively speaking), even with the lower compression.
My GTO has a Ram Air II cam with stock compression and heads, has a better idle than the XE cams I've seen, and makes great power through the RPM range. I prefer roller cams for use with the newer lubricants, but I have to say I love this RAII cam! Sounds great, too, with the high compression.
My $.02. 
Jeff
P.S. I think the 600 is too small for your setup but as others have said it will run and drive.


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## liljohn442 (Sep 9, 2010)

The cam is still under debate, but i am at least going to give the carb a shot. Hell i got it for nothing and it already fits the intake.


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## liljohn442 (Sep 9, 2010)

Oh and the car is just going to get me to and from high school, and occasionally be hammered down on. This car has the possibility to kill me, I don't want to make it have the probablility to.


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