Who is running the same carb size "the formula" spits out for their engine ?
#1
Race Director
Thread Starter
Who is running the same carb size "the formula" spits out for their engine ?
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.
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.
Last edited by MotorHead; 02-28-2013 at 10:46 AM.
#2
Safety Car
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.
Last edited by bluedawg; 02-27-2013 at 01:01 AM.
#4
Race Director
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
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
Last edited by 63mako; 02-27-2013 at 12:51 AM.
#5
Race Director
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.
#6
Le Mans Master
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.
.
BTW, I've successfully run a 1050 Dom on a "mild built" street 427 BBC. Guess I'm swimming upstream too.
.
Last edited by TheSkunkWorks; 02-27-2013 at 01:05 AM.
#10
Safety Car
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.
#12
Melting Slicks
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.
#13
Race Director
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.
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.
Last edited by Matt Gruber; 02-27-2013 at 07:29 AM.
#14
Melting Slicks
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."
#15
Race Director
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.
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.
#16
Drifting
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.
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.
#17
Race Director
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.
OTOH, i'd rather use a qjet than a 600 VS.
and i prefer a 650 DP spreadbore over a qjet.
#18
Drifting
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.
I have 385 ci. Also, I only have about 6" of vacuum.