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Carburetors Carb discussion and questions. Upgrading your Third Gen's carburetor, swapping TBI to carburetor, or TPI to carburetor? Need LG4 or H.O. info? Post it here.

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Old 08-24-2005, 12:12 PM   #1
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What does cfm mean? What size should I Get?

I'm in the middle of a getting a new 350 put in my car. In the mean time I'm using the old carb from my 305 but I want to get a new carb in the near future. <p>I know that the carb I have is a 4 barrel but that's it. I was wondering what size carb I should get for my 350 and what does cfm stand for. Thanks!
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Old 08-24-2005, 12:18 PM   #2
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"Cubic Feet per Minute". It's a measure of how much air flow capacity a carb has.

However, the measurement isn't standardized. The vacuum below the carb base can be varied, and that affects how much air will flow through it. Most 4bbls use the same standard, though.

The formula for getting the minimum sized 4bbl carb is to multiply engine displacement by maximum RPMs you will run, divided by 3456 (an easy number to remember). A 350 spinning to 5500 RPMs will need at least a 557 CFM carb. So, go bigger than that. 650 CFM usally will yield good results on a 350.
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Old 08-24-2005, 12:57 PM   #3
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If I was to get a larger one, like a 750 would it cause any damage or just guzzel gas?
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Old 08-24-2005, 01:08 PM   #4
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No damage. It won't necessarily guzzle gas. However, economy and driveability are typically improved when the carb is closer to the minimum size required. Performance improvement with a larger carb is measureable, say 750 vs. 650, but typically not felt (assuming both are properly tuned).
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Old 08-24-2005, 07:05 PM   #5
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650 will be plenty. I've got a 650DP on my 383. Broke it in on the dyno tonight, gonna make some load pulls tomorrow. I'll let you know what it does. Last motor we put together made 470 with a 650 and the one before that made 515. Both had the shafts milled down and the choke horns milled off and a little port work. We stuck the smaller of the two on the flow bench and it flowed like 720. Mines stock though, so it's still around 650.
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Old 08-25-2005, 09:48 AM   #6
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I was reading the article How to Choose the Proper Carburetor and Fuel Pump and it said that if you have a healthy 350 you should run at least a 750 cfm. The article went on to say that if you have manual transmission that you should consider a mechanical carb instead of a vacum. Any thoughts on that?
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Old 08-25-2005, 10:35 AM   #7
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Mechanical secondaries are, in my opinion, superior on all counts in a performance application. The manual transmission factor makes that even more so. Going mechanical secondaries also means you need to avoid over-carbureting, which is simply choosing the carb size based on engine size and lowest RPMs at which you would go wide open throttle. The Holley website has a chart to help with that.

Unless you have a huge cam and high numberical ratio rear end gears, you aren't likely to need a 750 CFM mechanical secondary carb on a 350. The 650 is much more likely to produce satisfaction.
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Old 07-27-2012, 12:04 PM   #8
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Re: What does cfm mean? What size should I Get?

I want to get a Holley carburetor for my 305 Iroc-Z it's 670 CFM is it a good idea and how much performance can I get out of it?
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Old 07-27-2012, 01:03 PM   #9
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Re: What does cfm mean? What size should I Get?

Quote:
Originally Posted by blackgoldcamaro View Post
I want to get a Holley carburetor for my 305 Iroc-Z it's 670 CFM is it a good idea and how much performance can I get out of it?
670 is a tad large for a 305, unless it's seriously warmed up (Big Cam, Heads, Headers, Custom tune, etc.). With a Stock 305, You should be looking at a 500-550cfm unit for peak peformance/gas mileage. If you don't give a #@$^ about gas mileage at all, the 650 or 670 will work, and give you a FEW more HP (not much) at the sacrifice of gas mileage.

You should also already know that you're going to have to replace your intake manifold, or use a baseplate adapter (which is not generally recommended)

If you're stuck on Holley, and don't want to consider an Edelbrock / Carter unit, these two would probably be your best choices (Dependant on whether you want a mechanical or vacuum secondaries):

Mechanical:
http://holley.com/0-4776C.asp

Vacuum:
http://holley.com/0-80570.asp


If you're willing to go with an Edelbrock unit (which I would prefer if it were going on a 305 I owned), you can get a closer match for your engine with either of these in the 500cfm rating:

http://www.edelbrock.com/automotive_...erformer.shtml

http://www.edelbrock.com/automotive_.../thunder.shtml


Also, way to resurrect a LONG dead thread :P
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Old 07-27-2012, 03:32 PM   #10
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Re: What does cfm mean? What size should I Get?

How much horsepower will this engine create?
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Old 07-28-2012, 01:26 AM   #11
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Re: What does cfm mean? What size should I Get?

Thanks for the heads up I had no idea about that. In that case I'll go for either the 500 or 550 cfm carburetor. I'm slowly building up my iroc-z to get more horse power. I would like to get a bigger better engine for it but I'm a bit low on money so I'm sticking with what that 305 can give me. I know those 305 have potential to give out more horse power I just gotta know what to add or what to take out. I've heard a lot about Edlebrock so I'll look more into that for my camaro.
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Old 07-28-2012, 07:53 AM   #12
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Re: What does cfm mean? What size should I Get?

Quote:
Originally Posted by blackgoldcamaro View Post
Thanks for the heads up I had no idea about that. In that case I'll go for either the 500 or 550 cfm carburetor. I'm slowly building up my iroc-z to get more horse power. I would like to get a bigger better engine for it but I'm a bit low on money so I'm sticking with what that 305 can give me. I know those 305 have potential to give out more horse power I just gotta know what to add or what to take out. I've heard a lot about Edlebrock so I'll look more into that for my camaro.
Now, If you're planning to add things to the engine later on, you should go with the 550 to start, and save yourself some cash down the line. 550 isn't quite "too big" for a stock 305, and it gives you a little room to breathe for adding small stuff later.

Your best bet is to read as much as you can, both on this site and the 'net in general about the easiest performance mods for a small block.

Now, if you're a bit confused, or don't know which parts to get, you could always look at getting a "matched kit" from Edelbrock. This gives you all the parts you need to do a serious upgrade all in one box. The price isn't "cheap" but all the parts are matched and there's no real guesswork, and you'll be adding the most horsepower you can for the cash (you could expect 270HP/350ftlb easily out of your 305). http://www.summitracing.com/parts/EDL-2022/

If you get that, you'll definitely still want to find a set of headers and have your exhaust done.

By the time all that's done, you'll have spent around 2 grand, but will have a car that will blow you away compared to the way it sits now.
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Old 07-28-2012, 07:53 AM
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