carb sizing help
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that is not a step up imo. i'd sell that ASAP. But,that's me; others love VS.
the 113's are ported to what? they are only 163 cc stock. about the worst head for a 383. flowing 600cfm at 1.5 is unlikely, more like 550.
a 600 DP will flow 780 cfm at 3" of vacuum.
i'd rather have the 600 DP any day on this combo
the 113's are ported to what? they are only 163 cc stock. about the worst head for a 383. flowing 600cfm at 1.5 is unlikely, more like 550.
a 600 DP will flow 780 cfm at 3" of vacuum.
i'd rather have the 600 DP any day on this combo
"600 DP will flow 780 cfm at 3" of vacuum" Why didn't you say that before?
There is a lot of contradicting info going back and forth here
113's can make 350 hp in stock form, saying that they are the worst choice for a 383 is going over board. My builder has looked over my work and thinks it will make 400hp no problem. I realize they aren't the best head but it's what I've got for the moment
Last edited by ekess744; Sep 28, 2009 at 10:09 AM.
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you are assuming 100% efficiency from this forumla.
no NA engine will give that. maybe close but not quite.
if it is a well tuned, efficient, large valve engine that say was giving 90% of what it was capable of the 600 would be perfect
660*.9=594cfm
if you already have the carb and it is a manual. go for it.
no NA engine will give that. maybe close but not quite.
if it is a well tuned, efficient, large valve engine that say was giving 90% of what it was capable of the 600 would be perfect
660*.9=594cfm
if you already have the carb and it is a manual. go for it.
I do think that most, but not all, people should use the older formula, though. The metering circuits will come much closer to the application and there will be fewer, if any, tweeks needed to get the calibration right for the engine.
But if you are up to speed on the fuel and air circuts and how the emulsion and the rest of the separate circuits really work, then sure, you can have a good outcome running a 4779 Holley 750 DP on a relatively mild 350 with an automatic. Not much to it really, but you will never just stumble on a proper calibration to match your engine.
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"then sure, you can have a good outcome running a 4779 Holley 750 DP on a relatively mild 350 with an automatic."
??? it's a 600 dp or a 750 vac.... its a 383.... and once again it's not an auto, it's a 3000lbs manual trans with a 3.07
??? it's a 600 dp or a 750 vac.... its a 383.... and once again it's not an auto, it's a 3000lbs manual trans with a 3.07
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From: San Antonio TX
A 4776 (600cfm DP) will work fine on your 383 with a single plane manifold. It would run a bit better at the top end with a 700-750 (either vacuum secondary or double pumper) but if you're not going to be buzzing the engine over 5,000rpm regularly, then the 600 is easier to live with in every day life. Just my opinion.
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i see the heads might be lots better than i thought.
Here is the test, where u prove it.
Have a friend read a vacuum gauge while u wind it out to your shift point; if it is 6000, call out NOW when u hit 5500 in 2nd gear. By the time your friend reacts it will be 6000 and he calls off the vacuum. I expect it will be maybe 1 or 1.5", my carb is barely off zero, but if your putting out, say, 490 hp it will be at 3 or even higher w the 600. Then u have proved it maybe could use a bigger carb. i say maybe as i don't know if it is a street car. a drag strip car would go bigger, street, maybe, maybe not. it is confusing, if the 600 looks clean, i'd try it.
Nascar restrictor engines use only a 390 cfm!
Here is the test, where u prove it.
Have a friend read a vacuum gauge while u wind it out to your shift point; if it is 6000, call out NOW when u hit 5500 in 2nd gear. By the time your friend reacts it will be 6000 and he calls off the vacuum. I expect it will be maybe 1 or 1.5", my carb is barely off zero, but if your putting out, say, 490 hp it will be at 3 or even higher w the 600. Then u have proved it maybe could use a bigger carb. i say maybe as i don't know if it is a street car. a drag strip car would go bigger, street, maybe, maybe not. it is confusing, if the 600 looks clean, i'd try it.
Nascar restrictor engines use only a 390 cfm!
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A 4776 (600cfm DP) will work fine on your 383 with a single plane manifold. It would run a bit better at the top end with a 700-750 (either vacuum secondary or double pumper) but if you're not going to be buzzing the engine over 5,000rpm regularly, then the 600 is easier to live with in every day life. Just my opinion.
Last edited by ekess744; Sep 28, 2009 at 10:31 AM.
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I would throw that 40 year old design TQII manifold in the garbage though!
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Single planes can use smaller carbs because of the plenum volume. If you read articles on carb sizing one of them will say to hook up a vacuum gauge to the intake and go out and drive it at WOT right to redline in 2nd or 3 gear. If the vacuum start to rise above some number like 4 inches then you might think about increasing the CFM. But people that say that they ran out of carb and had to put on a 750 have problems other than their carb size. Because with everything right 500 CFM restricted circle track racing is alive and well making @550 hp motors
I would throw that 40 year old design TQII manifold in the garbage though!
I would throw that 40 year old design TQII manifold in the garbage though!
at this point I think I will try both carbs!
Last edited by ekess744; Sep 28, 2009 at 10:42 AM.
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From: San Antonio TX
...If the vacuum start to rise above some number like 4 inches then you might think about increasing the CFM. But people that say that they ran out of carb and had to put on a 750 have problems other than their carb size. Because with everything right 500 CFM restricted circle track racing is alive and well making @550 hp motors
I would throw that 40 year old design TQII manifold in the garbage though!
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If you're pulling more than 1"hg at peak horsepower, you're suffering an intake restriction. And the engine builders running these mills in the 2bbl restricted classes make nearly everyone here look like pikers -myself included.
I would too. But then it's easy to get the right combination when you're spending other people's money.
I would too. But then it's easy to get the right combination when you're spending other people's money.
But I would run a 600 mech secondary in a heart beat.
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Im just afraid it won't be free in the end, he gave a little wink like he might want cash later on and I don't like running deals like that.
That and he was kind of an an arshole when I told him it was only 600cfm and not 650 like he previously said
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From: Carlsbad CA
I started out with "as a general rule of thumb." Engines are nothing more than air pumps. Most people put to large a carb on their engine thinking bigger is better. After all the back and forth in this the thread, the general opinion is that the 600 cfm is to small which is what I originally stated.
I was at a local carb shop (he builds carbs for over 50 professional race teams) the other day having some work done and was talking to the owner about this forum. He said he posted on it to try and help but all the "experts" would rip him so he left. I can see why and it is no wonder Lars left.
I was at a local carb shop (he builds carbs for over 50 professional race teams) the other day having some work done and was talking to the owner about this forum. He said he posted on it to try and help but all the "experts" would rip him so he left. I can see why and it is no wonder Lars left.
Burning Brakes
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From: Europe, France
I'm using a Holley 600 cfm Double Pumper with a Holley single plane manifold on my 350. Redline is @ 6000 RPM.
Choke tower is milled and I have a stubstack too, so probably the carb is making between 625 and 640 cfm now. It works great, very crispy throttle response.
The Innovate LM-2 showed me that it is still pushing hard at redline too...
If you intend to use the same max RPM, a single plane manifold and a double pumper, I think you should use a 650 or 700 cfm with your 383 CI, not higher.
But if you want to use your engine over 6500 RPM, you can go up to 750 cfm.
With a dual plane and a double pumper, you can use a slightly higher cfm carb ( maybe 50 cfm more ), so between 700 and 750 cfm.
With a vacuum secondary carb, it's possible to go even higher and still have a good off idle response.
The bigger the carb, the bigger the venturis. Too large venturis weakens the carb signals and can make a too big carb very hard or impossible to tune.
If you choose the right carb, the jetting should be pretty close to perfection or slightly rich right out of the box...
Choke tower is milled and I have a stubstack too, so probably the carb is making between 625 and 640 cfm now. It works great, very crispy throttle response.
The Innovate LM-2 showed me that it is still pushing hard at redline too...
If you intend to use the same max RPM, a single plane manifold and a double pumper, I think you should use a 650 or 700 cfm with your 383 CI, not higher.
But if you want to use your engine over 6500 RPM, you can go up to 750 cfm.
With a dual plane and a double pumper, you can use a slightly higher cfm carb ( maybe 50 cfm more ), so between 700 and 750 cfm.
With a vacuum secondary carb, it's possible to go even higher and still have a good off idle response.
The bigger the carb, the bigger the venturis. Too large venturis weakens the carb signals and can make a too big carb very hard or impossible to tune.
If you choose the right carb, the jetting should be pretty close to perfection or slightly rich right out of the box...
Last edited by 73StreetRace; Sep 28, 2009 at 11:45 AM.
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I'm using a Holley 600 cfm Double Pumper with a Holley single plane manifold on my 350. Redline is @ 6000 RPM.
Choke tower is milled and I have a stubstack too, so probably the carb is making between 625 and 640 cfm now. It works great, very crispy throttle response.
The Innovate LM-2 showed me that it is still pushing hard at redline too...
If you intend to use the same max RPM, a single plane manifold and a double pumper, I think you should use a 650 or 700 cfm with your 383 CI, not higher.
But if you want to use your engine over 6500 RPM, you can go up to 750 cfm.
With a dual plane and a double pumper, you can use a slightly higher cfm carb ( maybe 50 cfm more ), so between 700 and 750 cfm.
With a vacuum secondary carb, it's possible to go even higher and still have a good off idle response.
The bigger the carb, the bigger the venturis. Too large venturis weakens the carb signals and can make a too big carb very hard or impossible to tune.
If you choose the right carb, the jetting should be pretty close to perfection or slightly rich right out of the box...
Choke tower is milled and I have a stubstack too, so probably the carb is making between 625 and 640 cfm now. It works great, very crispy throttle response.
The Innovate LM-2 showed me that it is still pushing hard at redline too...
If you intend to use the same max RPM, a single plane manifold and a double pumper, I think you should use a 650 or 700 cfm with your 383 CI, not higher.
But if you want to use your engine over 6500 RPM, you can go up to 750 cfm.
With a dual plane and a double pumper, you can use a slightly higher cfm carb ( maybe 50 cfm more ), so between 700 and 750 cfm.
With a vacuum secondary carb, it's possible to go even higher and still have a good off idle response.
The bigger the carb, the bigger the venturis. Too large venturis weakens the carb signals and can make a too big carb very hard or impossible to tune.
If you choose the right carb, the jetting should be pretty close to perfection or slightly rich right out of the box...
Good advise.
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Burning Brakes
Joined: Aug 2005
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From: Europe, France
Of course he can use it and try. A carb change is easy. If he was given a 600dp, why not try it !
It will at least give him a good starting base and help him determine if he really needs a slightly bigger one, or not...
But with a decent cam, heads, single plane, 383CI and 6000+ RPM, it's probably too small...
He will certainly loose some high end power and torque with this one. However, a 600DP will make the car very streetable.
Last edited by 73StreetRace; Sep 29, 2009 at 01:40 AM.
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St. Jude Donor '14
Hoo boy.
If you want to use the carb for break-in, the 600 isn't going to make your engine explode
The rule of thumb - it ain't a "formula" as it's not always correct - is CFM = ((CI x RPM)/3456) x VE, with VE at 100% around 1.4 HP/CID. Generally a smaller carb will give better off-idle and low-speed throttle response at the expense of a couple % HP up top for a street engine because of the venturi size impact as noted above. If you're one of the "racing engine on the street" guys it's a different deal completely.
Without more specs it's hard to say, but I think you'd find a combination of a good dual-plane with a 700-750 vacuum-secondary carb to be a better overall combo unless you're going drag racing.
Otherwise, you can't really over-carb with a vacuum-secondary carb, and given the choice of a single-plane manifold off-idle and low-speed driveability probably is already out the window to some extent.
So, if you're buying a carb a 750 CFM vacuum-secondary would be my suggestion. But you can make the 600DP work with what you've got. Invest in a little dyno time and let us know if it's too small
If you want to use the carb for break-in, the 600 isn't going to make your engine explode
The rule of thumb - it ain't a "formula" as it's not always correct - is CFM = ((CI x RPM)/3456) x VE, with VE at 100% around 1.4 HP/CID. Generally a smaller carb will give better off-idle and low-speed throttle response at the expense of a couple % HP up top for a street engine because of the venturi size impact as noted above. If you're one of the "racing engine on the street" guys it's a different deal completely.
Without more specs it's hard to say, but I think you'd find a combination of a good dual-plane with a 700-750 vacuum-secondary carb to be a better overall combo unless you're going drag racing.
Otherwise, you can't really over-carb with a vacuum-secondary carb, and given the choice of a single-plane manifold off-idle and low-speed driveability probably is already out the window to some extent.
So, if you're buying a carb a 750 CFM vacuum-secondary would be my suggestion. But you can make the 600DP work with what you've got. Invest in a little dyno time and let us know if it's too small