MotorHead

Just for fun, go to this page and put in the numbers and then make a post of what the formula spits out and what carb size you are actually running

http://www.gtsparkplugs.com/CarbCFMCalc.html

Mine says for a 427ci turning 6000 rpm with .90VE I need a 667cfm carb . I am running a 825 Demom that flows 975cfm and the motor runs perfect

*Just a note, very few engines get 90% VE and the older the motor the less it's going to be to from worn valves and rings so you should be putting in 80-85%. I also believe that the calculator is for a brand new engine so adjust accordingly. A 750 Holley out the box is not a modified carb.

bluedawg

It says I need a 610 on my 400. I'm running a prosystems 780 that according to prsystems flows 820 cfm. It's also a double pumper with an auto tranny.

Shark Racer

I'm running a 750, with cute little primaries and big ole secondaries.

The calculator says I should run 500cfm.

63mako

If you figure 427 6500 rpm, .9% VE and you are running a modified carb which a demon really is it figures 886 CFM. My 383 @ 6500 and .9% needs 795 in a modified carb. My Tripower setup is 816 CFM with the difference between 2 bbl rating and 4 bbl rating calced out. It is really pretty close on a high perf, High VE, modified carb build. They should have an adjustment for vacuum. low vacuum needs bigger carb.

Les

Reminds me of a chick I picked up one night- Of course, in my own defense, it was late and I was just about blind drunk.

Now to answer the question- My 427 is running a Barry Grant modded 750 Holley DP rated at over 1000 cfm. I'm clearly overcarbureted.

TheSkunkWorks

IMCO the calculator in question is hyper-conservative to a fault when using the "Stock" carburetor formula, and the impact of VE% on results is likely over-emphasized in either carb mode. In any event, one can't specify desired max pressure drop at WOT...

BTW, I've successfully run a 1050 Dom on a "mild built" street 427 BBC. Guess I'm swimming upstream too.


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bluedawg

Ah crap. Damn predictive text. At least you didn't high light tranny.

Les

TheSkunkWorks

bluedawg

Could you elaborate a little on why low vacuum engines need larger carbs

DZRick

It tells me 686 CFM while GM calls for a 750 Holley, which is what I have.
ZZ430 clone btw.

Belgian1979vette

Really ? I'm running 6,5 inches of vacuum. Guess the 650 dp is going to be too small.

Matt Gruber

the formula i use is 1.6 cfm per HP.
Most guys have a good idea what HP they might have. But VE? Who knows that?
BUT, that formula is if there is no carb to test. I hook up a vacuum gauge and check it at WOT at max power to redline. If it is over 1.5", there is more hp to be had with a larger carb.

vetteguy75

What if you run a larger carb than the "charts" recommend? Does the engine bog down? Run rich? Waste gas? OR do you get more "punch" at lower RPM's? Higher RPM's? Like the wounded bank robber said to Dirty Harry (in the first Dirty Harry)... "I gots to know."

Matt Gruber

Too BIG?
most common is it loses power on a shift, recovers too slow and bogs off the line.
often it wastes gas at cruise.
has poor manners, cold starts require a warmup without moving at all, or it stalls or backfires. In other words, your fast car can't get out of the driveway as fast as a 4 cyl VW.

johnt365

I am afraid I am the guy who followed the chart All those years of hearing RTFM I thought I was safe.

360 cid 5250 rpm .90ve = 603 cfm and I am using a Holley 600

I can tell you this, there is no low end hesitation or problem between shifts. However, I have left some top end power on the table as a result... I learn a lot on this forum from you guys.

Matt Gruber

600 DP sounds like a good street carb. With a 5250 shift, i don't see how a bigger carb will help. Now, if u start shifting at 6000, put on a vacuum gauge, and see what it pulls.
OTOH, i'd rather use a qjet than a 600 VS.
and i prefer a 650 DP spreadbore over a qjet.

MIKE80

The calculator tells me with 6500 rpm/ .90 VE I need about an 800 carb, I have a Speed Demon 750 w/ mech sec.

I have 385 ci. Also, I only have about 6" of vacuum.

scottyp99

Ahh, thanks! I was having a bad day, and this made me laugh!

Scott

Snoopysvet

This gives similiar results...

http://www.holleycarbs.com.au/chooseCarb.htm