Hey Guys, how do you know what size carb to select for your combos. The big motor is almost back together. I bumped the cubes up to 572" and I have my pro systems 1050 to get me started but, I'm not sure if it's the right size. Here's a little bit about what the combo will be

572" Alum Mitchell block
440-1 heads flow @ 800 395 int / 300 exh (2.25 int valves)
15:1 comp
comp roller w/ 1.6 rocker .800 in/.76 ex; 286/300 @ .050
440-3 indy intake
727 auto w/ brake - 5000 sth
cuda - 3300 w/ me in it

Thanks

Your shift point or safe redline on your engine is the best way to figure out a drag race only carb size....

Guessing your redline is 7500, says you need 1297cfm....I have a 1250 Thumper Carb n my 605...seems about right as I have no intention of going to 7500..

http://www.wallaceracing.com/intakecfm.php

I tried out a big bore 1.720 Dommy with .950 skirted banjos that flows 1230+ cfm and my little 470 loved it and went a 1.34 60 ft. with lots of air under the ft. tires and that's footin to 2250 from a stomp. A 1050 will support good power but the bored main body's and the skirt creat great velocity, throttle response and power................

Carburetors do not have fixed flow capacity, it's based on engine demand. For a single plane manifold with 100% VE Wallace shows 1,241 CFM @ 1.5 Hg" WOT vacuum (4 bbl. flow rates are presumed at 1.5 Hg", don't use this to compare 1 or 2 bbls.). If you use a dual plane manifold, the carburetor can be much bigger.
If you use a smaller carburetor, the WOT vacuum will be higher. Large manifold volume (TR "shoebox") allows smaller carburetors, it's a compromise.
If you use a larger carburetor, the WOT vacuum will be lower. If 1 Hg" is acceptable (makes low throttle openings shaky, and delays main system operation) you can use 1,520 CFM, etc. To correct the calculation, multiply the CFM by the square root of the old ÷ new vacuum. To use 1.25", it's 1.5 ÷ 1.25 = 1.2, ^.5 = 109.5%.

EFI will function as low as .25 Hg" since the signal doesn't actually move fuel out of the bowl, it just provides information for the ECM.

Thanks Guys, I was thinking the shift point will be between 7000 - 7500. Never really thought about the vacuum factor being WOT, Good info.

Dom, I was thinking I'd be calling you later in the season

Many A > B comparos of larger/smaller carburetor show better or worse results than predicted. Sometimes this is "what does a larger carburetor do to the BFSC?".
Mixture automatically becomes richer as engine speed increases, unless air is added (HSAB + leakage). A larger carburetor on the same engine may begin to lean out (13.0 > 13.2, etc.), but the cure is probably a combination of different main jet and smaller HSAB.
If the math supports a bigger carb but it doesn't add power, check the WOT mixture.
The reverse may also hold: if going smaller does not reduce power (and the sizes work out near the Wallace #s) you may have gotten the richer mixture it always needed by accident.

The vacuum test is a good way to see it there's a restriction at WFO or not but not the end all imo and call me anytime if I can help ya out...........

An engine like that would pick up power with a tunnel ram and dual carbs. If you stay with a single carb then you can use something pretty big. I was running a 1250 Dominator on my 514 at 900 hp. That Dominator worked really well on the dyno from 4000 to 7000. I sold the Dominator and used the money to put EFI on that engine. The throttle body is over 2000 cfm so that allows it to make even more power than it could with the Dominator.

I wouldn't use any less than a 1050, and probably a 1250 will run quickest. And I've run with WOT vacuum down to .3" Hg using a carb, signal isn't an issue. It's about getting the least amount of restriction to lower pumping losses and still enough to sufficiently vaporize fuel for relatively equal distribution. Every engine is different and each is a juggling act to find the balance.

I had my Indy tunnel ram listed for sale on Facebook and a guy that worked at APD carbs messaged me to see if I wanted to do some horse trading. I had a couple of 1150's sitting here but kinda thought I needed a bigger carb for my B1 headed 572. They recommended a 1250 so that's what I got.



Untitled by John Cadamore, on Flickr

Only 1?

Carb Porn.... lol

It's all about the distribution!!!

AIUI the final conversion from small droplets (from the booster) to almost complete vapor occurs as the charge passes the valve seat, which is why current head theory says not to use a radius anywhere on the intake valve seats - the sharp edges between the 15° angles break up droplets.
Different question: is homogeneous (easily combustible, consistent quality) charge what you want? Or is maximum charge mass preferred? According to Vizard, it depends on whether the engine can get enough air. If not (from his huge work developing the BMC Mini), droplets will produce more power.

I guess that depends on the volume of fuel in the intake. When you consider the amount of gasoline the engine uses on each cylinder firing is about .10 cc, you are not displacing much air. Everything I've dyno'ed made better power with better atomization from the carb, and allows for a larger carb to be used. Maintaining sufficient atomization with a larger carb lowers pumping losses, more available power. There is always a point where you lose too much atomization, vaporization and distribution when going bigger. Each engine has that point. Fuel type affects it, engine temps affect it, you have to take it all in consideration.

Hey Jack when do you plan to get the car out with the ne eng ? Ron

Hey Ron, I hoping to be out by late June early July. Everything is one order (pistons, rods, crank) Heads are ready. I expect parts mid May. She should run pretty good.

1250's are available in both a 4150 style or standard, both billet. Oval blade 4150 style is also available.

Can I stop now???

No, I probably can't...

Works of art for sure and I'll be testing a twin blade on my junk here soon and will report back w/the results...........