Carb flow ratings
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Registered User
Joined: Oct 2010
Posts: 259
From: Vancouver, BC
Carb flow ratings
This is something not terribly critical to me, but I've wondered about it from time to time.
When suppliers give a carb flow rating, what are they saying exactly? I can say a carb is rated at 850cfm, but it is possible to get 850 cfm running through a carb rated at 650cfm... you just have a higher pressure drop through the lower rated carb.
If I am thinking right about this anyone can give a flow rating, but without any idea of the losses at that flow you can't compare with another carb, even if that carb has the same rating.
On top of that, flows would also depend on your air filter... I'm guessing their rating is based on no filter.
Have you guys seen anything that would clear this up? In the aftermarket world especially, I wouldn't be surprised if there is no standard that manufacturers are using to get their specs.
When suppliers give a carb flow rating, what are they saying exactly? I can say a carb is rated at 850cfm, but it is possible to get 850 cfm running through a carb rated at 650cfm... you just have a higher pressure drop through the lower rated carb.
If I am thinking right about this anyone can give a flow rating, but without any idea of the losses at that flow you can't compare with another carb, even if that carb has the same rating.
On top of that, flows would also depend on your air filter... I'm guessing their rating is based on no filter.
Have you guys seen anything that would clear this up? In the aftermarket world especially, I wouldn't be surprised if there is no standard that manufacturers are using to get their specs.
Registered User
Joined: Jun 2010
Posts: 3,613
From: Rogues Island, USA
Standard test vacuum was 1-1/2 inches of mercury for 4 barrel carburetors and 3 inches of mercury “Vacuum” for 1 barrel and 2 barrel carburetors. This was reasonable as it was about what a passenger car would develop. Of course, for the numbers to mean anything, tests would have to be run at a certain temperature and barometric pressure. Or be corrected to standard temperature and pressure. If not otherwise stated, ratings in cubic feet per minute are at standard temperature and pressure.
The above was taken from a race carb sight.
They are rated at standard inches of mercury to maintain a somewhat level playing field. Could you get 850cfm out of a 650cfm carb? Sure if you force fed air through it but that would disregard the standards set. You'll notice head manufacturers will play this game by measuring cfm at say 26 inches of mercury rather than the standard 28 inches. You can play with the numbers this way but it's hardly accurate. That's why they make standards.
The above was taken from a race carb sight.
They are rated at standard inches of mercury to maintain a somewhat level playing field. Could you get 850cfm out of a 650cfm carb? Sure if you force fed air through it but that would disregard the standards set. You'll notice head manufacturers will play this game by measuring cfm at say 26 inches of mercury rather than the standard 28 inches. You can play with the numbers this way but it's hardly accurate. That's why they make standards.
Last edited by TripDeuces; Jun 10, 2011 at 06:36 PM.
Old(s) Fart
Joined: Mar 2007
Posts: 50,815
From: Northern VA
Standard test vacuum was 1-1/2 inches of mercury for 4 barrel carburetors and 3 inches of mercury “Vacuum” for 1 barrel and 2 barrel carburetors. This was reasonable as it was about what a passenger car would develop. Of course, for the numbers to mean anything, tests would have to be run at a certain temperature and barometric pressure. Or be corrected to standard temperature and pressure. If not otherwise stated, ratings in cubic feet per minute are at standard temperature and pressure.
The above was taken from a race carb sight.
The above was taken from a race carb sight.
Registered User
Joined: Jun 2010
Posts: 3,613
From: Rogues Island, USA
http://www.candsspecialties.com/index.html
I copied it from this sight. Better report it to the plagiarism police.
I copied it from this sight. Better report it to the plagiarism police.
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