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How big of a carb do I need?

12K views 30 replies 16 participants last post by  Dans83Olds 
#1 ·
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.
 
#2 ·
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.
 
#3 ·
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.
 
#4 ·
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?
 
#5 ·
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
 
#6 ·
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
 
#7 ·
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.
 
#9 ·
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>
 
#11 ·
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.
 
#12 ·
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.
 
#13 ·
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? :D
 
#14 ·
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
 
#15 ·
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.
 
#16 ·
My referances are for a typical downleg booster Holley carb in a single 4 barrel tune.Of course we would like no vacum in the manifold,this is the biggest bonus of fuel injection,you dont need vacum since you dont need boosters restricting the airflow in order to move fuel.If you selected a throttle bore size that offered no restriction to airflow at wide open throttle and therfore had no vacum at peak load,and then installed a set of downleg boosters inside the venturies you would have at least an inch of vacum at the same load.So we select larger throttle bores to reduce this loss in flow,but in reality,as long as those boosters are inside those bores,you have some sort of restiction.The engine is moving air and that air is being used to create a pressure change that will draw fuel out of the boosters.Whenever airflow is used to create a pressure differential across a surface,some of that flow is lost.
Since we arent talking about changine the boosters or air bleeds here,we have to stay focused on carbs that will deliever thier rated flow at 1.5" of pressure drop,being used on an engine that will tolerate a bit less of a drop.Stock engines are teamed up with torque convertors that have too low of a stall speed for even a stock engine,so soo much throttle area can easily leave you with an engine that will never move enough air to create a good signal at low engine speeds under a high load,so regardless of how much power the engine makes,if the carb is within my guideline,it will cause the car to be soft at low speed/high load.This isnt the carbs fault,the combination is mismatched.Even a bone stock 350 smog motor would perform better with a 2500 stall convertor,so it isnt the carbs fault if the engine performs poorly.In a street car,convertor selection is still a bit conservitive.In reality an engine that really needs 3500 stall will be teamed with at least 1000 rpm less then that,and you need to keep this in mind when selecting a carb,but even a mild race motor could stand less then that if it doenst have to operated outside of it's operating range.
The long and the short of it is that no matter how you slice it,most people dont know of cant figure their volumetric effiency when building a street car,so simply using the rpm and engine speed formulas leads then to believe they need a smaller carb they they can really use,and most would never have a clue about how cfm is rated and where and engine wants to be rated at.Look at most good running engines that have the optimum size carb on them,and see where they fall in the hp per cfm scale.Look at most common bracket race and street cars that run well for their respective combination,and see how their hp to cfm ratio scales up.No rocket science just a quick way to put the right size off the shelf carb on a well matched street car.
 
#17 ·
As for big power from little carbs,just keep this in mind.Nobody says you cant make big power with a little carb,but you are still throwing power out the window.Winston cup engines should easily make 2 hp per inch,but they dont because they are limited to a smaller then optimum carb.They spend thousands of hours tweaking rod ratios,cam designs and intake runner shape to overcome this shortoming,but they still dont make 2 hp per inch.If you gave them a tunnel ram and 2,1050 carbs,they could take the same basic parts[Of course without the bandaids that masked the small carb},and make 2.25 hp per inch or more.The same goes for stock class 2 barrels carbs.500 hp out of a 350 with a 2 barrel sounds inpressive until you figure out that if you slapped an 850 on that engine and tuned it to match the big carb you would make another 200 horsepower.

[ May 01, 2003: Message edited by: Super Streeter ]</p>
 
#18 ·
Originally posted by Super Streeter:
<strong>As for big power from little carbs,just keep this in mind.Nobody says you cant make big power with a little carb,but you are still throwing power out the window.Winston cup engines should easily make 2 hp per inch,but they dont because they are limited to a smaller then optimum carb.They spend thousands of hours tweaking rod ratios,cam designs and intake runner shape to overcome this shortoming,but they still dont make 2 hp per inch.If you gave them a tunnel ram and 2,1050 carbs,they could take the same basic parts[Of course without the bandaids that masked the small carb},and make 2.25 hp per inch or more.The same goes for stock class 2 barrels carbs.500 hp out of a 350 with a 2 barrel sounds inpressive until you figure out that if you slapped an 850 on that engine and tuned it to match the big carb you would make another 200 horsepower.

[ May 01, 2003: Message edited by: Super Streeter ]</strong><hr></blockquote>

All true if you want to run your engine at 7500RPM for 3 hours at 200mph before it self destructs. However, if you need to get to work and back in city traffic 5 days a week at 2000rpm or less and occasionally make a run down the quarter mile @ 5000 rpm, stick with the smaller carb. You will be MUCH happier.
 
#19 ·
The big tradeoff is always driveability and power, I have always wanted to try out a Predator carb mainly because with a variable venturi you should be able to overcome these limitations. Anyone who has driven the latest generation of Japanese motorcycles will tell you "have your cake and eat it too" with slide valve carbs and needles. I never did understand why this never became popular on large V8's when it works so well on everything else. I would like to try a set of 8 bored out 44mm modern flat slide Mikuni's on an engine, that would look cool and should really haul on the top end. It may not be fuel injection but the look alone would be worth it.

Anyone here ever try a Predator carb? I would like to know what tuning that carb is like. Seemed like the answer when this carb first came out, hardly see them anymore.

<a href="http://www.mikuni.com/carburator/hsr_group_l.jpg" target="_blank">Imagine a eight pack of these on top your engine?</a>

[ May 02, 2003: Message edited by: 4 Jaw Chuck ]</p>
 
#20 ·
According to super streeters calculation I should be running 544cfm carb,
according to the most widely touted carb calc formula I should be running 547.2cfm.

I'm actually running a 600cfm carb which works fine so I'd say both calcs are just about right.

The guy also stated he wants to use edelbrock (so bye bye all you holley fans).

To answer the original question I'd go with an Edelbrock Thunder Series AVS Carburetor - 650cfm

Why do you guys always have to over complicate things? You start hugely technical arguments that end up having the same answer anyway!
And then there's always someone who says "holley 750" as if it's stamped on their forehead.

Sorry Chuck you usually give good advice but I went through the same process picking my carb and everyone said "holley 750, holley double pumper, holley holley holley", well I tried one and it ran like ****e, went through all the tuning instructions and still ran like a bag of poo. Bought a 600cfm edelbrock performer and it bolted straight on and ran fine, some say they run a little rich out of the box but my engine likes it just the way it is.

Although got to agree with you on the slide valve carbs, they rock on bikes so no reason why not on cars too. If I had to choose another carb I may well give the predator a try.
 
#21 ·
Holleys are an acquired taste, I like em. Never had a problem tuning them unless the carb was crap to begin with. The Carter/Edelbrocks are somewhat easier to tune and cheaper but I have found them less forgiving with wild cams due to the metering circuit design. They seem to be more apt to lean sneeze with long overlap cams because the accelerator pump isn't big enough, you need 50cc's for most big cam engines. I once tried a Carter carb on my 12A rotary engine and it just couldn't cut the mustard because the accelerator pump can't deliver the volume required, it is commonly known that modified rotary engines need the same amount of fuel as a modified BBC. 4 miles to the gallon is the best I could muster tuned right with the power in the 270 range.

The reason why the term "Holley 750/780" comes up so often is because this is their premier performance carb, the 3310 series carbs are different internally than say a Holley 600 vacuum secondary. It's all where you can't see it.

The big problem with Holleys is they never seem to be a good buy used because they are usually shot, thats why people are selling them. I learned my lesson long ago to just buy a new one and forget buying used Holleys.

I have found the 3310 series Holleys to be quite universal carbs. Can't say the same for their lesser 600 cfm vacuum secondary side hung float cousins, got three in the drawer for parts because I don't use them. The metering plate idea sucks if you ask me.

To each his own, the world would be an awfully boring place if all everbody ran was a Carter carb. Same goes for Holley's. ;)
 
#22 ·
Originally posted by 4 Jaw Chuck:
<strong>....I have always wanted to try out a Predator carb mainly because with a variable venturi you should be able to overcome these limitations. </strong><hr></blockquote>

One American Big Three idea that should have been the end-all, be-all to having your cake and eating it too is the Rochester QuadraJet. Tiny primaries for great low-end efficiency and HUGE secondaries for unlimited top end that are called on by engine demand should have been a killer combo but it never really panned out. They just never addressed the hi-po market so the thing has been generally relegated to the grocery-getter crowd. Too bad.
 
#24 ·
I agree Willy, the Quad is a good carb just never got the development it needed to be truly universal. Metering circuit signal in a Quad is nearly twice what a regular single downleg booster can muster so the mixture can be so finely adjusted that you would swear you almost got fuel injection control. Nearly every single Chevy equipped with a Quad from the early sixties to the late seventies got better mileage than say a Ford with an Autolite 2 bbl on it.

I still want to try the eight Mikunis one day, I think it would sound fantastic at WOT.
 
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