Re: Carb size question
[Re: 68LAR]
#1567056
01/23/14 08:48 PM
01/23/14 08:48 PM
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Joined: Mar 2009
Posts: 2,475 Sydney,Australia
tex013
top fuel
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top fuel
Joined: Mar 2009
Posts: 2,475
Sydney,Australia
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duh , get my remembering cap on as to your first note . Yes the motor is a pump and will only pass through what the smallest restriction allows , for peak flow . If the carb is the restriction and you increase its size that will help . My example is the heads were the restriction - they had reached max flow and a bigger carb did not help power / flow . Yes a smaller carb can restrict flow and reduce power , as I noted with the hp950 I tried plus the back to back carb test on the dyno with new heads. But remember a smaller carb will often have a better signal and can help the short times ,responsiveness , but give up a bit in the top , this can only be seen on the track not dyno . That is why I start with an 1.40/42 venturi carb for performance use , maximum power . JMO Tex
New best ET 10.259@129.65 . New best MPH 130.32 Finally fitted a solid cam, stepped it up a bit more 3690lbs through the mufflers New World block 3780lbs 10.278@130.80 . Wowser 10.253@130.24 footbraking from 1500rpm Power by Tex's Automotive
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Re: Carb size question
[Re: tex013]
#1567057
01/23/14 10:26 PM
01/23/14 10:26 PM
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Joined: Oct 2003
Posts: 4,013 South Park, Pa.
68LAR
OP
master
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OP
master
Joined: Oct 2003
Posts: 4,013
South Park, Pa.
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Quote:
Yes the motor is a pump and will only pass through what the smallest restriction allows , for peak flow . If the carb is the restriction and you increase its size that will help . My example is the heads were the restriction - they had reached max flow and a bigger carb did not help power / flow . Yes a smaller carb can restrict flow and reduce power , as I noted with the hp950 I tried plus the back to back carb test on the dyno with new heads. But remember a smaller carb will often have a better signal and can help the short times ,responsiveness , but give up a bit in the top , this can only be seen on the track not dyno . That is why I start with an 1.40/42 venturi carb for performance use , maximum power . JMO Tex
4 speed street legal. Best time 10.99 @ 124 mph on 93 octane pump gas @ 3926# total weight
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Re: Carb size question
[Re: 68LAR]
#1567058
01/24/14 03:31 AM
01/24/14 03:31 AM
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Joined: Jan 2003
Posts: 27,421 Balt. Md
383man
Too Many Posts
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Too Many Posts
Joined: Jan 2003
Posts: 27,421
Balt. Md
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You know because I kept thinking you ran a Dominator is why I was wondering how well it runs on the street. Dumb me. I guess because I would like to try a Dominator one day I just assumed you ran one. But I take it you like the carb alot that you are running now ? Ron
Last edited by 383man; 01/24/14 03:32 AM.
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Re: Carb size question
[Re: 68LAR]
#1567063
01/24/14 09:50 PM
01/24/14 09:50 PM
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Joined: Jan 2003
Posts: 27,421 Balt. Md
383man
Too Many Posts
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Too Many Posts
Joined: Jan 2003
Posts: 27,421
Balt. Md
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Re: Carb size question
[Re: 68LAR]
#1567064
01/25/14 02:09 PM
01/25/14 02:09 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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There is no "determined size". The real question is "how much pressure differential do you want at WOT?" Big carb = weak signal = difficult to tune, big power: 1" WOT vacuum. Economy car = small carb = excellent response + good mileage: 3" WOT vacuum. Very complex carbs such as Weber DCOE, Dell'Orto DHLA have good response and signal as low as .5".
Carb CFM is not a fixed number. A 750 only flows 750 is the engine demand produces the Holley test vacuum figure of 1.5 inches of mercury. If the engine produces more vacuum (2" etc.) carb flow increases, and vice versa.
Why does high WOT vacuum reduce power? #1. pumping loss #2. the VE is affected by the vacuum reading. No N/A engine has 0 vacuum, which means the maximum VE (absent wave/sonic tuning) is below 100%. Theoretical calculation is ((ATM - WOT loss) ÷ ATM)^.5
Boffin Emeritus
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