too many carbs

I've been going back and forth with my engine choice for my 55. 350 or the 265 I already have. The 265 is rebuilt new cam, roller rockers, gapless rings etc. and I want to run multiple carbs. whether it be 2/4 or 3/2 or any combo that may work. Just worried about how much carb will the 265 take.

 

Three 2's with PROGRESSIVE LINKAGE properly set up should work fine.

 

can anybody post the cfm formula? i think it's like cid x highest rpm / 1234 or 3456, something like that. i think i guestimated 327 cid x 7500 / one of those and got 710 cfm as the proper total carb size. i'm too sick for math now though! some kinda bubonic freakin cold or something.

 

It's c.i.d. x highest r.p.m. divided by 3456.

 

Any idea roughly what cfm a 97 Stromberg will flow?

 

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Any idea roughly what cfm a 97 Stromberg will flow?

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Thought I seen on here 300cfm, or was it 150?

 

Carefull with those CFM formulas, gang...they can be a little misleading!

Carburetors, like engines, are not 100% efficient, and they rarely flow the actual rating that they carry. Given the fact that things like the engine's volumetric efficiency, the air cleaner, choke plate and shaft, and even the throttle plates and shaft all impede the flow of air through the venturis, their bore size alone is not an accurate way to determine their actual flow rate when comparing or shopping for carbs.

On my first car, I ran a 780cfm Holley on a 350 Chevy. Every armchair speed shop expert in the world told me that the carb was too big for the motor...and they wouldn't listen to a 14 year old telling them that it probably only REALLY flowed about 625cfm (as a guess) on my particular engine! The quarter mile ETs backed me up, though...as the car was fastest with the properly tuned 780 carb.

(I had a 600cfm carb dialed in for the car too...ran it on the street for better mileage, but it was never as fast at the track with the smaller carb that everyone thought was "just right" for a 350 engine based on the Magic Formula!)

It's hard to over-carb an engine with a vacuum secondaries four barrel...you still jet it to run at the optimal air/fuel mixture (around 14:1 at sea level if you gotta be scientific...or until the plugs show a nice light tan color if you wanna go that route!) and the secondaries can be tailored to open slower or faster...and they are somewhat sensitive to load.

Now, when throwing a bunch of one or two barrel carbs on an engine, you can have too many throttle plates opening too quickly and kill off your throttle response due to the lower velocity of the incoming air/fuel mixture through a greater area.

(Your engine will only 'suck' in a given amount of air...whether it does it through one, two, or twelve venturis...more throttle plates opening simultaneously just reduces the speed at which that given amount of air/fuel mix will enter your engine...which can hurt off-the-line and low RPM performance.)

That's why progressive linkage (be it mechanical or vacuum) helps make multi-carb set-ups far more streetable...especially on smaller or milder engines.

Can a 265 handle a tri-power? Sure! It will work best if the center carb operates as your primary, with the end carbs 'staged' to open later...as rpm and load demand it. You will still have to have the carbs jetted correctly, and you won't need accelerator pumps on the end carbs because the center carb will be responsible for off-idle operation.

(Full race motors can handle the extra shot of fuel from having non-progressive linkage and accelerator pumps on all the carbs...but it would bog the hell out of a mild street engine!)

 

Pretty sure the 56 265 ci Corvette motor could be ordered with factory 2-4s similar to these. WCFB 4 bbl carbs. I know you could get them in 57 on a 283. It developed 270 HP. in 57. with all the other hi-po parts to complete the package. The rear carb is the primary for normal driving. The front one kicks in when you floor it. The Corvette "270 set up" is very pricey. I made this set up for a 292 Ford from flea market carbs. Don't ask me what they came on I love 3-2s but 2-4s were the top of the food chain when I was coming along.

I wouldn't try to run dual AFB's or Holleys on a 265.

 

If I remember right 97s run about 190 and Holley 94s run just over 200. If you go 3 deuces, run progressive. A 265 SBC isn't far off from a 276 flatty. And it'll rev higher.

 

good information guys.

 

excellent info on the carb set up thanks it will help alot. I did check that carb formula and come up with 337cfm for the 265 , that seemed like too small of a number.I saw holley had a 300/350 cfm carb and thought of running two of those. Sounds like a good idea!!!

 

Fat Hack mentioned this. . . "and you won't need accelerator pumps on the end carbs because the center carb will be responsible for off-idle operation"

Does that mean I should remove the accelerator pumps from my outer carbs- if its progressive it seems that the shot may still be a good thing when those secondary carbs kick in

- or is it just too much fuel when you dont need it?

 

Depends on how big your motor is, how radical it is, and on how you got the end carbs dialed in. For a mild street small block, three working accelerator pumps would probably be too much...but for a stout, high compression motor in a light car running steep gears, it may work out well.

You have to set your carb(s) up to work with your total combination.