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Old 04-29-2003, 02:23 PM
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Question How big of a carb do I need?

I am building a chevy 350 4 bolt for my 83 olds cutlass 4-door. We are making the car a sleeper. We currently have ported and polished heads, a .302 duration .500 lift cam, torker II intake, roller rockers, bored .30 over, 2500 stall, and headers. we are wondering how big of a carb do we need for this engine? we want to put an edelbrock on it to match the intake. thanks.
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Old 04-29-2003, 03:47 PM
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the formula for finding the correct size carb is CID times max rpm, divided by 3456. for example: 350 x 6000rpm = 2,100,000/3456 = 608 cfm carb. this assumes a 100% efficient engine; keep in mind however that a normal stock engine is 80% efficient, an engine worked over like yours would be about 85% efficient and a fully race preped engine could go 95%, maybe higher. it is always better to be too small than too big, so even though you might think your carb is small you'll be happier with it. I once tested this formula on a 400 cid pontiac with a 600, 750 and 800 cfm carb. the 600 was the fastest; 800 the slowest. not sure if you're thinking of going this way, but, multible carbs are not as efficient as single carbs. while a 600 cfm four barrel might work great on your engine, three two's might need to be 325 cfm each, two fours may need to be 500 cfm each. I have a 348 cid chevy that sports four two barrels at 500 cfm each. sound excessive? I thought so too until I ran it. hope this helps.
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Old 04-29-2003, 07:48 PM
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a 650 to 700 should suit your needs, especially with a open plane intake like a torker. you have to keep in mind that each cylinder draws off the entire mass of the carb, so fewer total cfm are acceptable and will help throttle response so i`d say 650. What gears are you running? if your running stock gears it`s not gonna do much with a upper RPM package as you have. the 82-87 firebird t/a`s and camaro Z28`s, some had posi rear`s with gear ratio`s ranging from 2.73 to 3.73, the guts will bolt right in your rear, this is what i did to my cutlass, i used a 3.23 unit out of a firebird T/A. if your interested in doing this swap let me know and I`ll send you the codes to the F-body rears to look for. good luck.
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Old 04-29-2003, 09:06 PM
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I honestly dont think that formula is very accurate. I have a 600cfm edelbrock carb on my 302 and I have a feeling that its too small. Its pretty modded up, it has aftermarket cam, heads, headers, and intake. It has stock internals so i dont rev it past 6000. My point is this, that formula is the same for a completely stock 302 and my 302, but they will use different carb sizes. At the same rpm, my 302 will draw more air into the cylinder than a stock 302. See my point?
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Old 04-29-2003, 10:38 PM
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that's where VE takes effect....the answer to that you times by the VE

so if you got a 770 out of that eqn and you have a 90% VE.....770 * .9 = 693

simple

more modded engines will have higher VE than stock, as it was mentioned
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Old 04-30-2003, 04:58 AM
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302 X 6000 = 1812000/3456 = 524 X 85% = 445.4cfm. ???????? We have figured the total flow into a 302 engine. Now to convert that number into carb flow numbers. A 2bbl carb is tested at 3" drop and a 4bbl at 1.5". The type of manifold makes a big diference. A single plane can use a smaller carb than a dual plane, on the same motor. The cfms required for an individual runner intake goes way up. If you want good street performance and fuel mileage, use a small carb. If you want all the horsepower you can get, something a little bigger is in order. A 650 is going to be real close from what I can see of the combo. Dan
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Old 04-30-2003, 05:20 AM
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An old mechanic once told me to figure out what size carbureter you need for a street engine was a simple formula. Take your cubic inches and multiply by 2. That will put you in the ball park. You can go smaller but not bigger. It has always worked for me.
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Old 04-30-2003, 11:40 AM
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I'm impressed with all the answers. I anticipated there would be at least a couple of the "dual 850 double pumpers", or "1080cfm Dominator" answers but all the above are very practical. Give yourselves all a big hug!

[ April 30, 2003: Message edited by: willys36@aol.com ]</p>
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Old 04-30-2003, 12:55 PM
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A 750 Holley with Vacuum Secondaries will work fine, many have been set up that way and run strong with the proper jetting.

O yea Ignore all that is posted below here, These people will confuse the hell out of you with there long winded answers!! Except for 4 Jaw

[ May 01, 2003: Message edited by: roys63 ]</p>
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Old 04-30-2003, 01:27 PM
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I am running a bit of a beefed up 350 in my camaro and I have a double pump 650 Holley which seems to be doing just fine. You can always rejet it if necessary if you find you need a bit more 'squirt'
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Old 04-30-2003, 06:57 PM
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Ignore all of the above.Here is the easiest way to pick the right size 4 barrel carb.For an automatic trans street strip or bracket race engine,multiply the anticipated flywheel horsepower by 1.7 and that is the cfm rating of the carb you need to buy.
Here is why you dont want to get all mixed up with the formulas above.
The cfm rating of a carb is a nominal rating that doesnt really have anything to do with how much air a particular engine will need or use.4 barrel carbs are rated at 1.5" of pressure drop,while 1 and 2 barrels are rated at a 3" drop.So 1 and 2 barels arent less efficient,they are just being rated at a different level.
In a well set up street car with an automatic trans{having the right convertor to match the cam is the most important factor in this respect}you want to hav about 1" of manifold vacum in the manifold at wide open throttle at peak horsepower rpm.With an unlimited class race type engine you can go as low as .75" of vacum and still get a useable carb signal.So if a 4 barrel carb is able to flow 750 cfm at 1.5" of drop,it is only flowing about 600 cfm at 1" of drop,and that is where you want to be.
The formula I gave you above really has a very specific logic to it.Dyno operators know that it takes a specific amount of air to make 1 horsepower,this is independent of engine size or speed.They also know that if you want to maintain only 1" of manifold vacum at peak airflow,you dont want to use a carb that needs 1.5" of vacum to acheive this level of flow.You can put a little 750 cfm carb on top of a prostock engine and make 1000 horsepower with it with 3" of vacum in the manifold,{comp eliminator racers do this all the time because of the rules that limit them to a single 750 cfm carb},but common sense tells you that if you put 2400 cfm of domintors on a tunnel ram on top of the same engine it will make 1400 horsepower and only have .75" of vacum in the manifold.The same holds true for any engine.Be realistic about your horsepower expectations or judge you numbers off of a similar combination to yours,and use the formula.Good luck.
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Old 04-30-2003, 07:35 PM
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I think there's lots of great advice in here. One more thing as stated before(sorta). Run as big sa a carb as you want to just nothing over 1 1/4 venturies. Any more than that and you will be begine to sacrifice idle and part throttle response. You won't like it. Your car will seem slow untill high RPM and by that time you have moved out of the RPM range of the cam so what the point. A 750 Holley is about as goof proof as yuo can get.
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Old 04-30-2003, 07:50 PM
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Not that I want to start an argument Super Streeter :p but I think you are confusing vacuum signal with airflow, the actual vacuum drop across the carb only indicates the restriction presented by the carb, airflow creating the vacuum is what pulls the fuel out of the bowls. More airflow=more fuel.

For example you can whack the throttle open at idle and cause the manifold pressure to go to zero but that doesn't mean any fuel is going to flow (hence why you need an accelerator pump). Since your carb bowls are at atmospheric pressure how would fuel flow if the manifold and the outside air is at the same pressure? Air velocity causing the vacuum drop is required to make fuel move in a passage...no air flowing...no fuel.

As for your comparison of 2bbl to 4bbl ratings your correct in the manner that they are measured, 1.5 inches of mercury drop for 4 bbls and 3.0 inches of drop for 2bbls. The reason for the difference is because each type of carb presents a different restriction to your air pump. For actual vacuum at WOT at full load the ideal vacuum level in the intake would be zero...no restriction. That is how you make the best power/torque. Strangely it is also how you get the best fuel efficiency also. These recommendations for cfm sizing are just guidelines not hard fact, I have seen 1150 cfm Dominators on 302 cubic inch engines run just fine with no bog or hesitation from being too big. Did it need that big a carb? Probably not but it made the best power with the combination on the dyno, so....

My opinion for your carb Dan is I would run a Holley 750 cfm double pumper carb if it is a standard trans or a 780 cfm vacuum secondary carb for an automatic. These carbs have larger midrange fuel passages as they are designed as high perf carbs to begin with and restricting fuel flow is a lot easier than trying to get more when you have reached the limit of what the carb was designed for. For proof put it on the dyno and compare, choose your carb by what the engine requires to produce the best power if that is your goal. Balancing throttle response to power production is what you are accomplishing by choosing a carb size, don't forget that.

Of course I am just a guy on the internet giving free advice, what the heck do I know?
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Old 05-01-2003, 04:48 AM
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Rich, You state .75" and 1". I've always used .5" as a max for the street, and .75" as being the point where power starts to be lost. For a race car, where drivability and mileage aren't an issue, I shoot for 0".

My question is, What changed? Cam profile? Head flow? 1" sounds like a bunch to me. Dan
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Old 05-01-2003, 06:49 PM
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I gotta agree with ya 4-jaw. Most circle trackers are restricted to two barrel carb and yet they find as much as 500 horses with them. Great throttle response as well. Although my racing buddy has a two barrel carb you can drop a Silver dollar striaght through it. It's legal by the way.

Iv'e seen coutless cars running dominator's on near stock engines at the races in the stock chassis class. 350 or smaller CI and 1050 carbs making 400 horsepower. Size is just a guideline. B
ooster signal is what's most important.
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