750 cfm carb - tuning - smaller jets or 600cfm? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 01-18-2004, 08:34 PM
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750 cfm carb - tuning - smaller jets or 600cfm?

*************A little info on the truck******

using:

---stock 350 gm goodwrench crate motor, 260 HP @ 5,000 RPM RPM (brand new)

aftermarket parts:

---edelbrock 750 cfm manual-choke
---edelbrock RPM air gap intake

other crap:

---stock headers to 2" individual Flows

*********************************8

here's the dilemma

right now i put around town (San Francisco), mainly on the highway doing around 2-3 thousand rpms

after reading a milion different carb/intake setups here on the forum, i'm thinking 750cfm is slightly overkill for an around-town truck on a stock motor. it performs OK but i really dont step on it too much, i want to keep this engine healthy but reliable and long lasting. starts up on a charm, has ~slight~ bog when i step on it on lower RPMs. bog gets less and less the higher the RPMs

i'm thinking about swapping out the .113 primary and .107 secondary jets for the 600 cfm-rated jets, .100 and .95

this is the question:

** is swapping out the jets going to be acceptable, or do i really need to get a 600cfm replacement carb? **

i'm thinking that the airflow restriction of the 600 cfm carb won't make any difference in performance or economy compared to the jet swap, because the stock engine (i think) sucks as much air as is necessary thru either a 600 or 750 cfm, regardless. either setup i believe won't be restrictive in terms of air. so if i modify just the amount of fuel going in, it should be ok.

i think i'll be fine, but if anyone has some specific info on this move, any help is appreciated

---t

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Old 01-18-2004, 09:11 PM
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if its a stock 350 & will never see other side of 5000 RPMs a 600 cfm is way more than addequete. Infact youll have better thorratle response & low speed torque..
a buddy of mine has the same engine in his truck with a performer intake & a 570 CFM Holley, works great.
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Old 01-19-2004, 12:44 AM
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It is a very common misconception that you can jet down a carb that is too big and make it work on a smaller engine. In reality you end up with a carb that is still too big, and now running too lean.

I think you should have a carb with around 650 CFMs vacuum secondaries. I prefer Holleys, and they are pretty cheap on Ebay.
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Old 01-19-2004, 05:00 AM
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750cfm way too much for a stock 350. Much better to get a 600cfm carb, it will be lots happier. You cannot jet down an oversized carb.

Vince
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Old 01-19-2004, 06:01 AM
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when in doubt always choose a smaller carb. if you went with a 500 cfm carb you would get much more bottom end pull and perhaps loose a little high end horsepower. but try and take your car for a spin and see in what rpm range you drive. a 600 cfm carb is the absolute max you can slap on a 250 hp engine, regardless of displacement. holley or edelbrock doesnt make much difference, i like edelbrocks myself.

(but the only ticket to drivabillity out there if fuelinjection, a megasquirt system costs arround 100 bucks, and a ford or buick throttlebody from the junkyard should fit fine on top of your airgap manifold with a homemade adapter plate.... you should be able to build it for arround the price of a new edelbrock...)

infact when in doubt always choose a smaller carb, taller rear end gears, smaller cam, higher stall converter, higher boost and more nitrous....
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Old 01-19-2004, 07:57 AM
Technical Support Barry Grant
 
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Carburetor Sizing

Most people do not understand how a carburetor works.

Over simplified....

An engine is an air pump it is going to move (suck) the same amount of air regardless of what size carburetor you put on the engine (unless you extremely restricted it)

Based on the difference in the air pressure outside, and the vacuum being pulled through the carburetor by your engine the fuel is pulled out of the carburetor.

If the carburetor is too big for the application the air velocity will be low, not allowing the engine to break the fuel down at low speeds (rich) and not be able to pull enough fuel at higher RPM (lean).

A properly sized carburetor will atomize the fuel more effectively giving you better idle quality, throttle response, drivability, and overall performance.

Depending on the rest of your vehicle specifications you'll want to go somewhere between a 575 and 650 carburetor. Using an Air Gap style intake on a low HP engine like this I'd lean towards a smaller carburetor to keep up the air velocity.
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Old 01-19-2004, 09:32 AM
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I have to agree with all of these responses. I run a mild 350 in my Nova- mild cam, air gap intake, headers-much like your engine. I recently switched out my Q-jet for an edelbrock 1406 600 cfm carb and couldn't be happier with its' performance. go smaller and I think you'll be VERY happy with the results. Good Luck!
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Old 01-19-2004, 02:39 PM
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600 holleys are nice, the edelbrocks are more user-friendly, both will require tuning. A 600 will keep your air velocity up, and that's what you need for this motor-

K
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Old 01-20-2004, 06:13 PM
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hey TECH @ BG, that makes thing more clear about air velocity, thanks

sounds like a 600 is next on the list
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