how much vacuum should a 289 v8 with 2v carb have at cruise reason i am asking is that i am trying to determine what size carb to use on my rebuild (very mild rebuild)
Vacuum
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cruise should be pretty high, about 20 in-hg.
A lot of it depends on the engines condition, cam overlap and elevation. I have never seen over 17in of Vacuum @ my 3100ft elevation even with a fresh engine.
Originally Posted By: brandontyler65
how much vacuum should a 289 v8 with 2v carb have at cruise reason i am asking is that i am trying to determine what size carb to use on my rebuild (very mild rebuild)
You kinda need to rephrase your question as it doesn't make a lot of sense. I could put a little 1bbl, or a 650 cfm Holley on it, and cruise vac would be the same.
displacement (CID) X (RPM/2) X VE/1728 = MAX CFM
how much vacuum should a 289 v8 with 2v carb have at cruise reason i am asking is that i am trying to determine what size carb to use on my rebuild (very mild rebuild)
You kinda need to rephrase your question as it doesn't make a lot of sense. I could put a little 1bbl, or a 650 cfm Holley on it, and cruise vac would be the same.
displacement (CID) X (RPM/2) X VE/1728 = MAX CFM
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are you sure punisher cause a 1V 2V 3V and 4V will all have diff vacuum cause of the restrictions in flow created by the different barrels and sizes. i used the same formula basically CFM = CID * RPM * VE * 1/3456 but thats just for how many cfm your motor will pull at the max RPM. in order to determine your actual carburetor size you need you use the formula CARB = CFM / squareroot( desired vacuum / 1.5(4V) or 3.0(2V) but i dunno what my desired vacuum should be i am planning on using a 2V carb. i found this site Go Fast For Less which is stating what i am trying to say
Originally Posted By: punisher
You kinda need to rephrase your question as it doesn't make a lot of sense.
Quote:
How much vacuum should a 289 V8 with 2V carb have at cruise? The reason why I'm asking is that I'm trying to determine the right size carb to use on a very mild engine rebuild.
How's that?
You kinda need to rephrase your question as it doesn't make a lot of sense.
Quote:
How much vacuum should a 289 V8 with 2V carb have at cruise? The reason why I'm asking is that I'm trying to determine the right size carb to use on a very mild engine rebuild.
How's that?
Oh and by-the-way, a 500 CFM 2V Holley works real well on a 289 Ford.
Originally Posted By: Merkava_4
Oh and by-the-way, a 500 CFM 2V Holley works real well on a 289 Ford.
I was thinking the same thing. He may need to grind the throttle bores out in the intake manifold to let the ones on that carb fully open.
Another option would be an Edelbrock 500cfm carb. It's a Carter AFB copy. Holley has the old stand by 1850 which is 600cfm, could change the spring in the vacuum diaphram to limit the air flow some.
Oh and by-the-way, a 500 CFM 2V Holley works real well on a 289 Ford.
I was thinking the same thing. He may need to grind the throttle bores out in the intake manifold to let the ones on that carb fully open.
Another option would be an Edelbrock 500cfm carb. It's a Carter AFB copy. Holley has the old stand by 1850 which is 600cfm, could change the spring in the vacuum diaphram to limit the air flow some.
Originally Posted By: brandontyler65
are you sure punisher cause a 1V 2V 3V and 4V will all have diff vacuum cause of the restrictions in flow created by the different barrels and sizes.
I am pretty sure on this one. Whether you have a 2bbl 500cfm (at 3" of depression) Holley, a little 1bbl, or a 4bbl XXX cfm (at 1.5" of depression) the determining factor for vac at cruise will be RPM, displacement, cam specs, and engine load.
Eventhough the carbs may have a different restriction due to different number/sizes of barrels, you will be opening the throttle more on a smaller carb to acheive the same airflow (hence manifold vac) for a given operating condition. Whether you have a high CFM carb open 1/4 throttle, vs a smaller carb open 1/2 throttle, the airflow will be the same for X amount of depression, RPM, displacement, cam spec.
I do understand your original post now. You aren't so much concerned about total airflow, but airflow at X amount of depression. I am slow sometimes.
are you sure punisher cause a 1V 2V 3V and 4V will all have diff vacuum cause of the restrictions in flow created by the different barrels and sizes.
I am pretty sure on this one. Whether you have a 2bbl 500cfm (at 3" of depression) Holley, a little 1bbl, or a 4bbl XXX cfm (at 1.5" of depression) the determining factor for vac at cruise will be RPM, displacement, cam specs, and engine load.
Eventhough the carbs may have a different restriction due to different number/sizes of barrels, you will be opening the throttle more on a smaller carb to acheive the same airflow (hence manifold vac) for a given operating condition. Whether you have a high CFM carb open 1/4 throttle, vs a smaller carb open 1/2 throttle, the airflow will be the same for X amount of depression, RPM, displacement, cam spec.
I do understand your original post now. You aren't so much concerned about total airflow, but airflow at X amount of depression. I am slow sometimes.
ya basically i didn't know how to word it. and i got a 465 holley and and 2 625 carters, and 3 600 holley but i am going to use one of my autolites and keep it original looking
Originally Posted By: tom slick
cruise should be pretty high, about 20 in-hg.
Dunno what sort of rig you drive that CRUISES at 20" hg but that's a decent amount of vacuum at IDLE. None of my vehicles cruise at anywhere near that level; 13"-15" is more normal. Doesn't take much of a hill or headwind/tailwind to change that by several inches.
Any given engine at "cruise" (whatever that is, pretty loose statement really) will require a given amount of air. It matters VERY little what size the carb is; as long as it's big enough the the vacuum will be the same at the same power loading.
As power/speed increases, the throttle has to open more to allow additional air/fuel mixture in; vacuum drops accordingly until it gets to zero.
If the carb is too small the engine will eventually need more air than the carb can flow at WOT. Vacuum will then start to climb from zero, indicating a need for a bigger carb.
Joe
cruise should be pretty high, about 20 in-hg.
Dunno what sort of rig you drive that CRUISES at 20" hg but that's a decent amount of vacuum at IDLE. None of my vehicles cruise at anywhere near that level; 13"-15" is more normal. Doesn't take much of a hill or headwind/tailwind to change that by several inches.
Any given engine at "cruise" (whatever that is, pretty loose statement really) will require a given amount of air. It matters VERY little what size the carb is; as long as it's big enough the the vacuum will be the same at the same power loading.
As power/speed increases, the throttle has to open more to allow additional air/fuel mixture in; vacuum drops accordingly until it gets to zero.
If the carb is too small the engine will eventually need more air than the carb can flow at WOT. Vacuum will then start to climb from zero, indicating a need for a bigger carb.
Joe
A small 4 barrel carb is best.
The primaries are much smaller than a 2 barrel's bore.
It will meter and respond better at part throttle.
4 barrel carbs are rated at 1.5" pressure, and 2 barrel carbs at 3". BTW.
They don't directly correspond with size rating [a 600 CFM 2 barrel vs. a 600 CFM 4 barrel].
A vacuum reading won't tell you anything useful about sizing a carb.
The primaries are much smaller than a 2 barrel's bore.
It will meter and respond better at part throttle.
4 barrel carbs are rated at 1.5" pressure, and 2 barrel carbs at 3". BTW.
They don't directly correspond with size rating [a 600 CFM 2 barrel vs. a 600 CFM 4 barrel].
A vacuum reading won't tell you anything useful about sizing a carb.
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