Re: Another carb CFM question
[Re: LowDeck451]
#2546812
09/06/18 06:07 PM
09/06/18 06:07 PM
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Joined: Oct 2007
Posts: 4,213 New York
polyspheric
master
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master
Joined: Oct 2007
Posts: 4,213
New York
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For a street-only car, the only serious mistake is making the primaries too big. Factory engines as small as 230" have used QuadraJets.
Boffin Emeritus
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Re: Another carb CFM question
[Re: LowDeck451]
#2546935
09/06/18 10:17 PM
09/06/18 10:17 PM
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Joined: Oct 2007
Posts: 4,213 New York
polyspheric
master
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master
Joined: Oct 2007
Posts: 4,213
New York
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The place where air speed matters is inside the primary booster.
Boffin Emeritus
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Re: Another carb CFM question
[Re: lewtot184]
#2546974
09/06/18 11:29 PM
09/06/18 11:29 PM
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Joined: Jul 2004
Posts: 19,318 State of confusion
Thumperdart
I Live Here
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I Live Here
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Too add to this, a too small of a carb on a hot motor can be real sensitive and a bear to tune sometimes because of the high velocity however I have Dommy's on cars/trucks that the "pros" said would never work for the reasons you mentioned above as in not enuff velocity..........I've had 1250 Dommy's on my dart for street testing with good results and why is that............ i'd say your keeping the rpm up. That's one of the criteria and I have a fairly loose 5200+ vert, 12.1 comp, 4.11 gears, fairly light etc........Like Mr.PBody said, he had a 1050 on a 300 something cubic in. small block BUT super light, good heads and combo plus high rpms. I've learned a couple hard humbling lessons lately about high winding SB Mopars. They love CFM and Shiloh and Tory will attest, they have HUGH carbs that some big blocks cant utilize and they run stellar #'s for their combinations.............Fun stuff, so the whole "theory" CFM calculators don't seem to take these things into account at least the ones some speak about on here........
72 Dart 470 n/a BB stroker street car `THUMPER`...Check me out on FB Dominic Thumper for videos and lots of carb pics......760-900-3895.....
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Re: Another carb CFM question
[Re: polyspheric]
#2546975
09/06/18 11:31 PM
09/06/18 11:31 PM
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Joined: Jul 2004
Posts: 19,318 State of confusion
Thumperdart
I Live Here
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I Live Here
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The place where air speed matters is inside the primary booster. Important for sure and that sets up the rest of the tune...........
72 Dart 470 n/a BB stroker street car `THUMPER`...Check me out on FB Dominic Thumper for videos and lots of carb pics......760-900-3895.....
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Re: Another carb CFM question
[Re: LowDeck451]
#2547276
09/07/18 05:38 PM
09/07/18 05:38 PM
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Joined: Oct 2015
Posts: 570 UK
rb446
mopar
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mopar
Joined: Oct 2015
Posts: 570
UK
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Not only do SB mopars like cfm as my stock CR 340 did (850DP+2/10ths over a 750DP).. we had a well known racer with a Mk1 Ford Cortina running a Rover 3500c (215ci buick V8) stick car, lots of rpm etc. He amongst all the other racers kept on about my 850 carb being too big, so I persuaded him to try my 850 straight on his little 215ci motor,.....result.. 2/10ths faster, he couldn't believe it, throttle response wasn't as good as his 600 carb of course but I proved a point to them all back then in 1984.
1969 'Cuda 446ci, best 9.96@133.9 in 1990 1971 340 'Cuda, best 11.01@122.8 in 1987
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Re: Another carb CFM question
[Re: rb446]
#2547456
09/08/18 01:39 AM
09/08/18 01:39 AM
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Joined: Jul 2004
Posts: 19,318 State of confusion
Thumperdart
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I Live Here
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Not only do SB mopars like cfm as my stock CR 340 did (850DP+2/10ths over a 750DP).. we had a well known racer with a Mk1 Ford Cortina running a Rover 3500c (215ci buick V8) stick car, lots of rpm etc. He amongst all the other racers kept on about my 850 carb being too big, so I persuaded him to try my 850 straight on his little 215ci motor,.....result.. 2/10ths faster, he couldn't believe it, throttle response wasn't as good as his 600 carb of course but I proved a point to them all back then in 1984. Yes sir, see it a lot where a "calculated" computer program spits out cfm recommendations but there are too many variables to nail YOUR specific cfm needs........
Last edited by Thumperdart; 09/08/18 01:40 AM.
72 Dart 470 n/a BB stroker street car `THUMPER`...Check me out on FB Dominic Thumper for videos and lots of carb pics......760-900-3895.....
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Re: Another carb CFM question
[Re: crackedback]
#2547664
09/08/18 06:30 PM
09/08/18 06:30 PM
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Joined: Jul 2004
Posts: 19,318 State of confusion
Thumperdart
I Live Here
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I Live Here
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Yep........And monkey see monkey doer's follow not realizing it....
Last edited by Thumperdart; 09/08/18 06:31 PM.
72 Dart 470 n/a BB stroker street car `THUMPER`...Check me out on FB Dominic Thumper for videos and lots of carb pics......760-900-3895.....
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Re: Another carb CFM question
[Re: LowDeck451]
#2547683
09/08/18 08:37 PM
09/08/18 08:37 PM
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Joined: Oct 2007
Posts: 4,213 New York
polyspheric
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master
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New York
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"Smallest carb CFM to possibly run"? Definitely not, MUCH smaller works perfectly well.
Boffin Emeritus
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Re: Another carb CFM question
[Re: Thumperdart]
#2547689
09/08/18 08:48 PM
09/08/18 08:48 PM
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Joined: Jan 2003
Posts: 12,327 Benton, IL.
DaveRS23
Special needs idiot
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Special needs idiot
Joined: Jan 2003
Posts: 12,327
Benton, IL.
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Why does Holley still say to use a power valve half the idle vacuum and recommend CFM be based on that old equation? Shouldn't both be updated? Or better yet, be eliminated all together. I'm not sure either one was ever accurate. But we sure know better than to use them today. Bring the on!
Master, again and still
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Re: Another carb CFM question
[Re: DaveRS23]
#2547702
09/08/18 09:40 PM
09/08/18 09:40 PM
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Joined: Jun 2008
Posts: 3,916 usa
lewtot184
master
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master
Joined: Jun 2008
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usa
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Why does Holley still say to use a power valve half the idle vacuum and recommend CFM be based on that old equation? Shouldn't both be updated? Or better yet, be eliminated all together. I'm not sure either one was ever accurate. But we sure know better than to use them today. Bring the on! they're trying to make their product idiot proof. dumbing people down seems to be the American way anymore.
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Re: Another carb CFM question
[Re: LowDeck451]
#2547753
09/08/18 11:32 PM
09/08/18 11:32 PM
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Joined: Feb 2015
Posts: 2,086 CA
crackedback
top fuel
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top fuel
Joined: Feb 2015
Posts: 2,086
CA
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You can put a 1bbl carb on a 440 and it will run perfectly well... Won't do much for power, but some have to pick everything apart. 390's ran great on the older Busch series cars. Not as well as if they had a worked 750+ carb on them... but perfectly well.
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Re: Another carb CFM question
[Re: LowDeck451]
#2547894
09/09/18 12:34 PM
09/09/18 12:34 PM
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Joined: Oct 2007
Posts: 4,213 New York
polyspheric
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master
Joined: Oct 2007
Posts: 4,213
New York
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some have to pick everything apart
So annoying - someone had the correct answer, and it wasn't you.
Boffin Emeritus
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Re: Another carb CFM question
[Re: LowDeck451]
#2547897
09/09/18 12:37 PM
09/09/18 12:37 PM
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Joined: Oct 2007
Posts: 4,213 New York
polyspheric
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master
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New York
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The equation prevents 2 things: 1. Children asking yet another question 2. Selecting a carb large enough that idle and tip-in are poor (although there is no input for "primary venturi area per inch of engine displacement"). As the total CFM goes up, the primary:secondary balance of all square bore carbs (4150/60, AFB, AVS) changes.
Boffin Emeritus
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Re: Another carb CFM question
[Re: LowDeck451]
#2547905
09/09/18 01:03 PM
09/09/18 01:03 PM
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Joined: Jul 2006
Posts: 4,319 Puyallup, WA
StealthWedge67
master
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Joined: Jul 2006
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Puyallup, WA
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For what it’s worth, The Holley website carb-selector does not just blindly follow the common equation. I actually followed its advice on my car, which was an HP1000 on my 452”, 6300 rpm mill.
LemonWedge - Street heavy / Strip ready - 11.07 @ 120
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Re: Another carb CFM question
[Re: LowDeck451]
#2548198
09/10/18 12:56 AM
09/10/18 12:56 AM
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Joined: Oct 2007
Posts: 4,213 New York
polyspheric
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master
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Holley also assumes that all calculations would be based on a V8 engine with all cylinders drawing from all carb throat simultaneously. This means a single plane manifold, not a dual plane, divided single plane, or "360°".
If using a V8 with individual throttle bodies such as stack injection, the formula will invariably return a size far too small to work. E.g., if a single Holley 4500 supplies 1,000 CFM to a 500" engine at 7,000 RPM, the intuitive assumption is to divide the 1,000 by 8 to get the ITB CFM, or 125 CFM each. This is only slightly more than 1" diameter... There is a correction for 6 cylinders or less supplied by the same carburetor, multiply this number by the usual Holley formula: 7 or more cylinders: 1.0 6 cylinders: 1.1 5 cylinders: 1.3 4 cylinders: 1.6 3 cylinders: 2.2 2 cylinders: 3.3 1 cylinder: 6.6
The CFM for the 500" engine would each be 825 CFM (assuming no manifold volume). At about 125 CFM per square inch, this makes the throat 6.6 square inches, or about 2.9" diameter. How odd, that's the size of the Crower BBC stacks...
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Re: Another carb CFM question
[Re: polyspheric]
#2548216
09/10/18 01:42 AM
09/10/18 01:42 AM
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Joined: Aug 2003
Posts: 43,352 Bend,OR USA
Cab_Burge
I Win
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I Win
Joined: Aug 2003
Posts: 43,352
Bend,OR USA
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some have to pick everything apart
So annoying - someone had the correct answer, and it wasn't you. What is perfect for one may or may not be perfect for someone else Being the racer I am may have some influence on my choices on many things dealing with motors, especially Hi Po motors
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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