Carb size after engine mods.
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Instructor
Joined: Jul 2001
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From: kennett square,pa,us
Carb size after engine mods.
How do you determine the right size carburetor for your engine?
I am curious why some guys with very similar mods are using 650s & others are using 750s?
If there a formula to calculate the size needed, trial & error, or experience, or what? :confused:
I am curious why some guys with very similar mods are using 650s & others are using 750s?
If there a formula to calculate the size needed, trial & error, or experience, or what? :confused:
Drifting
Joined: Mar 2001
Posts: 1,395
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From: Indiana
Re: Carb size after engine mods. (bud snyder)
I have found several different formulas as well as simple rule of thumb theories. The most common one is:
(CFM = CID x RPM x VE ÷ 3464).
In my case for example, I have a 355cid that can turn 6000rpm. If I assume that I can reach 1.0 volumetric efficiency, then it would end up like this.
(355 x 6000 x 1.0)/3464= 614cfm.
There is also a formula that says that you should multiply the required CFM by 110%-130% for single plane intake, and multiply by 120%-150% for dual plane intakes. Using this formula(614cfm x 110% or 130%), I would need something between 676-799 cfm. I run a 650DP holley and it will rev to 6000 rpm, so I'm not sure that multiplying my 614 by 130% would do me alot of good. It really depends on your type of driving. If you don't mind loosing a little throttle responce to gain top end power, then the bigger carb would not be a problem. If on the other hand, it is a daily driver that rarely sees redline, then I would try to stay with the smaller of the recommended carbs.
(CFM = CID x RPM x VE ÷ 3464).
In my case for example, I have a 355cid that can turn 6000rpm. If I assume that I can reach 1.0 volumetric efficiency, then it would end up like this.
(355 x 6000 x 1.0)/3464= 614cfm.
There is also a formula that says that you should multiply the required CFM by 110%-130% for single plane intake, and multiply by 120%-150% for dual plane intakes. Using this formula(614cfm x 110% or 130%), I would need something between 676-799 cfm. I run a 650DP holley and it will rev to 6000 rpm, so I'm not sure that multiplying my 614 by 130% would do me alot of good. It really depends on your type of driving. If you don't mind loosing a little throttle responce to gain top end power, then the bigger carb would not be a problem. If on the other hand, it is a daily driver that rarely sees redline, then I would try to stay with the smaller of the recommended carbs.
Team Owner
Joined: Sep 2001
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From: Middletown Connecticut
Cruise-In IV Veteran
Re: Carb size after engine mods. (tsw71)
by your calculations, it says that i need 625 cfm, I'm running a 600, that close enough or should i upgrade to a 650? 327cid 6500rpms
Drifting
Joined: Mar 2001
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From: Indiana
Re: Carb size after engine mods. (Corzvette69)
by your calculations, it says that i need 625 cfm, I'm running a 600, that close enough or should i upgrade to a 650? 327cid 6500rpms
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From: Hoosier
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Re: Carb size after engine mods. (tsw71)
by your calculations, it says that i need 625 cfm, I'm running a 600, that close enough or should i upgrade to a 650? 327cid 6500rpms
If it will rev up easily, then I would be inclined to leave it as is. If it feels like it struggles to reach red line, then that would indicate you may need more carb. The most accurate way to determine this is to hook up a vaccum gauge with a long hose that will reach the passanger compartment. Take it for a ride and make a run at wide open throttle all of the way to redline. If you pull less than 1.5" of vacuum, then that indicates that you need more carb. If you end up with more than 1.5", then you have too much carb. I have done this test myself and had a reading of between 1-1.5". A 700cfm carb would probably be better for me, but I'm close enough and it revs easily, so I'll just leave mine alone.
If it will rev up easily, then I would be inclined to leave it as is. If it feels like it struggles to reach red line, then that would indicate you may need more carb. The most accurate way to determine this is to hook up a vaccum gauge with a long hose that will reach the passanger compartment. Take it for a ride and make a run at wide open throttle all of the way to redline. If you pull less than 1.5" of vacuum, then that indicates that you need more carb. If you end up with more than 1.5", then you have too much carb. I have done this test myself and had a reading of between 1-1.5". A 700cfm carb would probably be better for me, but I'm close enough and it revs easily, so I'll just leave mine alone.
[Modified by Road Fever, 3:34 AM 10/2/2001]
Drifting
Joined: Mar 2001
Posts: 1,395
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From: Indiana
Re: Carb size after engine mods. (Road Fever)
tsw..don't you have that backwards? If you show less than 1.5" then the carb is ok...if you show more than 1.5 the carb is undersize. :confused:
[Modified by Road Fever, 3:34 AM 10/2/2001]
[Modified by Road Fever, 3:34 AM 10/2/2001]
Note to self: Do not attempt to type too quickly while watching favorite team durring Monday Night Football. Go Niners :cheers: :cheers: :cheers:
