Carburetor cfm calculator
For a streeter;
Your engine is max airflow limited by; the cam, the heads, and the ability of the exhaust-system to get rid of spent gasses. This adds up to VE aka volumetric efficiency.
If your carb is sized at least adequately for what the engine can thruput then a bigger carb is not gonna give you much; see Bewys post #25 I think it was.
On the street, where rpm may seldom exceed 4000rpm. I see no good reason to be carrying around a whopper-cfm carb for the once a month blast.
This calculator;
(rpm x cid)/3456 times VE = carb size in cfm
actually does work pretty good for a streeter; You just gotta be honest about your VE.
This formula has a basis in reality. The 3456 is not some random number plucked from a hat, but is a "constant" that takes into account your displacement, your rpm, the number of cylinder-fillings per revolution, and converts it to cubic feet, to spit out a number that at any particular rpm represents what your engine should be able to physically ingest at STP (standard pressure and temperature).
The problem is that your engine is rated in CID by it's
swept volume, and not it's
Total volume.
For instance;
say you have a 367 like me, which calculates to a swept volume of 752cc per cylinder. But the total chamber volume is another 75cc in my case, so when the piston is at the bottom of the intake stroke, atmosphere wants to fill it with 827cc .. ok,
but at 4.04 bore, this is 827 is actually equivalent to a stroke of 3.92 with a combustion chamber of zero. So then, doing the math, my 367 at 100% VE would be, for the purposes of this formula be.....404 cubic inches. And so;
(404 x 6400)/3456 time 100% VE=750 cfm.. Whereas at 367 it comes to just
(367 x6400)/3456 times 100% VE=680cfm
That's what the atmosphere sees. Whether or not at 6400 it can do it is depend on the VE of your total engine combo.
Now for Dan, estimating a top rpm not exceeding 4000, and a compression ratio of 8/1; lets plug this in.
A stock 360 has a CID determined to be 737.2cc and
At 8/1 she would have a Total Combustion space of 737.2/7=105.3cc for a grand total of 842.5cc. At 4.00 bore this represents a stroke of 4.09inches. so then the displacement becomes 411. and
(411 x 4000)/3456 times 100% VE= 476cfm NOT
(360 x 4000)/3456 x 1.00= 417
Of course at 4000 with the right induction/combo, the engine is much more likely to be able to operate at 100% VE, so 476 should be pretty accurate. In any case the next closest size is a 500, just right.
Or is it?
Not all carbs use the same rating system. There are about eight ways to screw up.
Carter's system yields higher numbers than Holly's system.
A two barrel is rated differently than a 4bbl. If you install 3 of 320cfm 2bbls onto a 6-pack intake, you will not get a 960. If the engine cannot pull more than 750, then that is what you get. You may get more power from the 6-pack, but it won't be from more airflow.
The mighty 340 Thermoquad is an 800 only by Carter's rating system.
In any case; for Dan, any carb rated at 500 or more will get the job done.
But get this;
(411 x 4000)/3456 times 100% VE= 476cfm as we just calculated. and I suggested that a 500 would be just right. Ok but this assumes the engine can pull the intake vacuum down to the level, that was used to establish the rating of the carb.
If the engine only pulls 70% thru that 500, then it is no longer a 500; but is whatever it is.
This is what makes a spreadbore so great on so many vehicles; the smaller primaries can do the work maybe 80 or 90 per cent of the time, below say 3000 rpm ........
And in my opinion for Dan's application; a lil too small is better than a lil too big.
Some of you guys probably missed the First post. or the subsequent one which said;
Yes it is and it probably won't see much over 4,500 rpm's for the most part. I thought about a 650 eddy and a performer intake or would the rpm performer intake be a better option? This car will not see any track time, probably some stop light to stop light fun once in awhile. My goal is for the car to have good throttle response and decent low to midrange torque. I want the car to be able to get out of it's own way and be able to pass other cars with out difficulty