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  1. #1

    Default C.I.D math for a carb conversion

    Help me on this math... For a 350 Chevy c.I.d. 4" (bore) x 3.14 (pi R squared) x 3.48 (stroke) x 8 (number of cylinders) = 349.67 c.I.d. Same math with a 4.3 Chevy but with 6 instead of the 8 and I get a 262.4 c.I.d. but when I do that same math with a 235 Chevy inline six I get a bigger c.I.d. 3.562 X 3.14 X 3.94 X 6= 264.4 Am I doing something WRONG? Every other engine I've tryed it on comes out what the mfg says. I also looked up the 235s big brother engine which is a 261 inline 6, its identical other than the bore is 3.75 as opposed to 3.562. And it comes out to 278 c.I.d. multiple sites have the specs and they are the same... Anyone have a clue what's wrong with my math? What I want to do is put the 235 carb on a 4.3 Chevy v6 on the TBI manifold with an adaptor, change the dizzy to an hei and make it run, but I don't want it to run too rich or too lean. Some one told me the carb would probably work if it within 3.5% of the cylinder air volume from the original engine. Hence my math homework. :-)

  2. #2


    So, your saying Chevy lied about the cu. in, of the 235 all these years?

    Put the 235 carb on the 4.3. it will be fine. Both motors are about the same cu in. The worst that could happen is you might have to rejet the carb a bit, but I'm betting you won't tell the difference.

    What I find funny was that you were OK with putting a carb off of what you thought was a smaller motor on to the bigger one, but now both are the same size, you are concerned? Gene

  3. #3


    No, not saying they lied.... Just bad at math! :-) as for the latter part, I'm just on a really tight budget. The original engine runs and runs well for a 1954 235 i6, but the driving I do is mostly highway, and I find the power quite lacking. When I bought the truck, it was just to be driven on occasion, and life has changed. I now need to drive it every day, and oil leaks and low power won't work. I have changed the 3 speed to a T5 and a rear end from a 4x4 s10. I had a 350 but the t5 is rated for 300 HP and 330 lbs of torque. A mild 350 would probably break that tranny. So, I know that in the 85-86 astro vans they put a t5 behind a 4.3 v6. Hence the v6... Plus I basicly have an s10 with a 53 Chevy truck frame and sheet metal. I have bought 2 running engines for $100, freshened one with new, rings and bearings, but my wife got transferred and now we need to move, before I got chance to buy the TBI harness. Thats when I came up with making an adaptor for the one barrel carb and buy a hei dizzy. Much cheaper that way. Plus its a little more unique vs a SBC. moving from wichita Kansas to Tampa Florida is a bit of a culture shock. I have $11k in the truck and other than the engine it done... Have 200 miles on it, I don't want to sell it. It would of worked great the way it is in Kansas but not Florida. From what some people say is the vaccum pull air over the venturi it pulls a certain amount of air and fuel, if the engine is bigger most people think the engine will run lean, but its the opposite, its rich. The air speeds up and pulls more fuel and the oxygen is the same. Rejetting may or may not be needed, but if it does I'm not sure I can get jets for a carb that old. I do thank you for your opinion Gene.... And any cheap ideas will be appreciated! Dave

  4. #4


    I understand that you want to use as much of what you already have as possible, but I believe the investment of a 2bbl carb will be a worth while investment. The old 1 bbl worked on an inline motor OK, but the same or slightly larger size cfm 2 bbl should be more efficient on the V6 and would likely provide more power at a usable level. Your making an adapter anyway, a 2bbl adapter would probably be easier to make.

    The reality is I would want to invest in the TBI harness as soon as possible. The humidity in FL is much higher then it was in KS, and a carb will give you more issues in high humidity.

    As for the T5 behind a a 350 holding up will depend on how you drive it. If you drive it like the commuter ride the vehicle has now become, it will likely hold up fine. If you want to play race car driver, expect it to be short lived, unless you don't have any traction. Spinning tires don't have much negative effect on the life of a trans, but good traction has different results.

    Back in my younger days, I put a pretty potent Mopar 340 in front of a stock 904 (Chrysler's small automatic trans) in a 69 Dodge Charger and beat the living crap out of it everyday for 2 years without issues (I sold the car, the the new owner wrecked the car without hurting the trans). Play out the numbers and the trans didn't stand a chance, but it did not fail.

    Sometimes, you just have to refrain from driving it like you stole it, and actually drive it like you need it to keep running.

    As I understand, you have a rebuilt 350, you have the clutch & bell for the T5 and you have the T5. I'd put it together and take it easy until you can line up a tougher trans. Although I wouldn't be opposed to the 4.3 & the T 5. My 48 Plymouth coupe has a TBI 3.9, 5 speed. Its fun to drive and pulls down 17 mpg in town. Gene

  5. #5


    All the details... I still can't decide... Hotrodding it supposed to be fun, and I'm beating my brain to get the ride the most economical ride. Over the last few days I sold a few things to get some extra money. I now have enough for the harness, but all the little things are nickel ands diming me to death.. . a carb would definitely fix my money situation, but the reliability would be worse... My desired wish is efi TBI system, just not enough money. Ahhhhh... Not even sure I have time to even do it. I'm not a racer any more. I have 4 kids and a wife... But I don't really want to do an SBC. Love the 4.3s what do you guys think of putting a edelbrock 500 or 600 on it?

  6. #6


    You will find that the nickels and dimes add up faster then the big $$ things do. Most guys only add up the big ticket items when they estimate the cost of a project, and they end up way short of the actual cost. The big ticket items are about 1/3 of what the project is going to cost, the rest will depart your wallet $5 at a time, very often.

    Past experience tells me that if your desire to to have EFI, your never going to be happy with any carb. That said, if a carb will get you on the road sooner, there are a butt load of carbs that will work. GM used that 2bbl Rochester on small blocks for years. Even a Quadrajet would probably work, if its set up right, the vacuum secondaries will only flow as much fuel as the motor can use. I think I would trend away from a Holley. An Edelbrock 500 might work. See what is available in your area. Gene

  7. Default

    the formula you are looking for is:

    ( CID X RPM X VE ) 3456 = CFM













    DO NOT shy away from Holley carbs, tuned properly they will out perform anything short of FI.

  8. #8


    So by your math I need a 238.8 cfm carb. That would be (262x4500x.7)3456=238.8 does anyone have an idea on what the 2gc carb cfm is from a 283/327/350? I bought a new one from eBay for less than $100. Let's hope its 238ish...

  9. #9
    Join Date
    Aug 2015


    A stock Rochester 2GC is about 280 cfm. In years of building motors and tuning them - I personally hate the formula. Its a guideline, but all too often I find that its more of a "bare minimum". As a rule of thumb - you can always over compensate for CFM (air - its not bad for the motor) with proper jetting. If you want to run the 4.3 with the 2GC it should work (of course you need the adapter which will raise the plenum height) and not be too thirsty on the fuel. At WOT you may experience some lack of potential power but that is about it. If you go with a Holley 4160 (500 or 600 cfm) you would benefit from the additional airflow, ease of tuning the jets and the benefit of the aluminum intake (Edelbrock 2114 or 2111 depending on the head)

  10. #10


    Thanks for the replies... I only went with a 2 bbl because its $100 bucks new on eBay. Chances are its a Chinese knockoff but if it works... Yay. Ultimately I will be going efi.... That will fix all the issues... Lol... Just need money and time. Think a lot of people he that same problem. Now, if the 2 bbl works well, I'll leave it alone... Just because of simplicity. Other than I suck beyond suck at tuning carbs... Any suggestions? I got a vac gauge I that helps.


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