carbs

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jbtn
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carbs

Post by jbtn »

how much is to much carb for a 350 40 over dd thanks

Bdriller
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Re: carbs

Post by Bdriller »

jbtn wrote:how much is to much carb for a 350 40 over dd thanks
400 cfm

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Re: carbs

Post by loudmouse »

I would put a 650 mech sec 4150 holley on it without knowing engine specs.
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Re: carbs

Post by jbtn »

engine specs are 350 - 40 over roller cam vortec heads air gap intake block huggers headers everything new thanks

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Deano
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Re: carbs

Post by Deano »

There used to be a real good CFM calculator at http://www.4SecondsFlat.com but I see now it is no longer there.
It's simple straight forward math. You need to guesstimate your Volumetric Efficiency, but .85 or .090 is
plenty close enough to get an idea of where to start.

Figuring you have a .85 VE and a wide open throttle of 3300 RPMs,
you only NEED about 350 cfm for a near stock configuration. You are DD right?

This calculator will show you a couple useful graphs if you want to play around and see for yourself.
http://www.gtsparkplugs.com/CarbCFMCalc.html

This chart shows that a 500 cfm will suffice for up to about 5k RPM for your engine.
http://www.4secondsflat.com/Carburetor% ... Charts.htm

You can use the wrong carb and it will work acceptably after it's properly jetted, but any expectation of having the power and the fuel mileage that most folks are after, will require that the carb be close to the correct size.
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Re: carbs

Post by loudmouse »

Another way to look at it. Airspeed from a v8 through a small carb will cause the engine to have vacuum under wot load. This vacuum is a restriction of air and therefore reduces HP. To maximize the power potential of ur engine it needs to breathe. A 650 will make more power at any rpm on a dyno'ed 350 Chevy over a 500 2b.
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skinny99
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Re: carbs

Post by skinny99 »

loudmouse wrote:Another way to look at it. Airspeed from a v8 through a small carb will cause the engine to have vacuum under wot load. This vacuum is a restriction of air and therefore reduces HP. To maximize the power potential of ur engine it needs to breathe. A 650 will make more power at any rpm on a dyno'ed 350 Chevy over a 500 2b.
This is very incorrect. Period. You don't know what you are talking about.
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Re: carbs

Post by loudmouse »

Maybe u might want some dyno time or some accurate info before u post. 2b are rated differently than 4b(1.5 vrs 3). It takes twice the vacuum for the same cfm thru a 2b vrs 4b. Dyno numbers speak for themselves. Vacuum is a restriction.
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skinny99
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Re: carbs

Post by skinny99 »

loudmouse wrote:Maybe u might want some dyno time or some accurate info before u post. 2b are rated differently than 4b(1.5 vrs 3). It takes twice the vacuum for the same cfm thru a 2b vrs 4b. Dyno numbers speak for themselves. Vacuum is a restriction.
I know all about how carbs are rated. The difference is that I understand how an internal combustion engine creates torque. Which is what turns a prop, especially in a DD application. Much more important to make torque than the volume of air an engine can ingest.is how fast it can actually do it. It is the intake charge/port velocity that allows an engine to provide higher torque numbers at low engine speeds. Lot's of dyno queen's have run like crap when hook up that prop to em. When someone that has a clue gets involved they find out that most of them are over carbed and over cammed. But they made a really big number on the dyno, guy could brag to his friends about how much "POWER" he has but couldn't turn the prop hard enough or with enough pitch.

So forget your dyno go do some real world testing and see what turns the prop the hardest. That is what matters. In general the smallest carb that will flow enough air to properly feed the engine will make the most torque and turn the most prop. And will do it with the best fuel mileage and throttle response/snap.

If they weren't so expensive the 390cfm Holley 4 barrel would be the best choice.Flows more than enough air and is very tunable for the times when less than WOT. The 500cfm 2 barrel has some disadvantages but it is very usable and is cheap and easy to find.
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Re: carbs

Post by loudmouse »

Skinny, it is widely known that a 4b holley properly tuned will give better mpg than a properly tuned 2b holley. The circle track racers already have proven this. The 390 you referred to flows 140 more cfm at the same vacuum as the 500 2b. More air means more power! U will see an increase in RpMs with the same pitch on ur prop.
Just because an engine dynos at xxx HP and xxx torque doesn't mean with more tuning or changing parts that it couldn't make more power. I'm not going to argue with u over this. Bring me ur dd with a 4b intake, adaptor plate and ur 2b on it. Come back and pic it up and if I don't give u more ROMsat same pitch u owe nothing. If i give you an increase you pay for what was done. Nothing will be changed but carb and tuning........... I do what I do and do it very well.
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Gas required Water optional :dontknow:
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Deano
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Re: carbs

Post by Deano »

loudmouse wrote: . . . it is well known that a 4b holley properly tuned will give better mpg than a properly tuned 2b holley. The circle track racers already have proven this . . .
This would not seem like the group of choice to gather Miles Per Gallon data from. :scratch:
Nor do those guys consider 3,000 rpm to be WOT. :lol: Likely, more than double that, which is an entirely different ball game.
skinny99 wrote: . . . In general the smallest carb that will flow enough air to properly feed the engine will make the most torque and turn the most prop. And will do it with the best fuel mileage and throttle mileage and throttle response/snap.

If they weren't so expensive the 390cfm Holley 4 barrel would be the best choice.Flows more than enough air and is very tunable for the times when less than WOT. The 500cfm 2 barrel has some disadvantages but it is very usable and is cheap and easy to find.
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Re: carbs

Post by ddriver »

i have a 14 ft rivermaster with a 1990 350 marine eng. the moter has all the normal things done to it for dd. it has 72" super snapper prop set on 18. i have been running 750 eldo. carb for 10 years an it does well for what it is. it turns 3000 rpm and gets there fast.

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Re: carbs

Post by anahuacbound »

Give Holley a call, they have a few guys that are familiar with airboats. Carb size is chosen compression, displacement, and rpm's. I have a 6.2 LS3 that turns 5,000 rpm's and run a 650cfm. You will loose a lot of low end snap and torque if the carb is too big.
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Re: carbs

Post by lil_armond »

Sorry to say it loudmouse but you are wrong, the math doesn't lie and all an engine is a dynamic vaccum. This is one of the biggest mistakes I see in airboats and cars alike. Skinny is right, a dyno can be fooled and manipulated very eaisly. In the end the only dyno that matters is the prop and it will let you know what you are truly doing.
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Re: carbs

Post by lil_armond »

More air means more power!

Only if everything else in the package is sized correctly so that it can move it out. The original poster asked about proper carb selection for what he has and was given some solid advice. He didn't ask for someone to give him new cam profile, complete head work.
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Re: carbs

Post by jbtn »

sorry everybody got upset over my post about carbs I have got to buy a new carb and just needed some advice, does any body every used one of the summit brand 600 cfm vac sec or would mech sec be better thanks ps play nice its just for fun right

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Re: carbs

Post by Gangreen »

jb call Bo laws( Bo Laws performance ) in Orlando Florida. They built carbs for dirt racing as well as Nascar at one time. They will help over the phone. My carb was almost a grand but damn it does a great job.

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Re: carbs

Post by SpencerMullins »

Hey everyone, I just finished my rivermaster with a new crate 350 dd. i began with the edelbrock 4 barrel 750, it started up great and sounded healthy, however when ever i got on the throttle quickly, it would bog. Also when i went to WOT you could smell raw fuel in the exhaust. I went to a Rodchester (around 500cfm) 2 barrel from a 69 olds. Presently it has the original 41 jets and seems to be a just a tad lean, however i gained snap and 150 rpm from switching. I have to imagine i'm getting better gas milage as well. and yes you motor heads, the larger jets are on order i'm not going to burn it up it never goes above 160*. haha. My stock new 350 with the edelbrock performer intake pushes my 15 ft rivermaster 5 people and a cooler every bit of 40 mph, cruise at 2500. I'm more than happy with my budget build. hope this helps.

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Re: carbs

Post by lil_armond »

I would go with a mechanical secondary and I promise you Skinny is right. For your Direct Drive application you are shooting yourself in the foot if you go larger than 500 cfm. Torque is king and in a direct drive application that is what you need. This isn't a dirt track car or a dragster so the VE at 5k+ rpm's mean crap to your set up. Go with the smaller carb and you will be happy and throttle response will be crisp not spongy.
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Re: carbs

Post by loudmouse »

Bo-laws is very good advice but I will promise u he will not recommend a 500 2b. Been buying from Bo for about 25 years. To all that don't believe what I'm saying, take my challenge. Power is power whether ur turning a prop, launching a drag car or turning a set of 52" tractor tires. More power will yield more RpMs for the same workload. There is no way I would recommend someone buy a vacuum sec carb(3310) or a 500 2b for a v8 engine. I'm done!
15' Alumitech deckover LSX TT Oxbox 84" JX 4-blade
Gas required Water optional :dontknow:
Built and Tuned by Me
14'X8' JW, Chevrolet 122CID LNF Turbo Engine, Ox box, 80" NGR 3 blade

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Re: carbs

Post by Bdriller »

loudmouse wrote:Bo-laws is very good advice but I will promise u he will not recommend a 500 2b. Been buying from Bo for about 25 years. To all that don't believe what I'm saying, take my challenge. Power is power whether ur turning a prop, launching a drag car or turning a set of 52" tractor tires. More power will yield more RpMs for the same workload. There is no way I would recommend someone buy a vacuum sec carb(3310) or a 500 2b for a v8 engine. I'm done!
Youll slowly come around in time but only if you have the ability to do so, this aint dirt trackers , why dont lycoming aircraft engines have twice more cfm carbs than what they do , i will assume that has been tested and trued to perfection

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Re: carbs

Post by Myakka Rooster »

We groundpounders only run a 500cfm on 470 cubic inches at 2900 RPMs. Just saying

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Re: carbs

Post by lil_armond »

Your challenge is flawed and your response is typical of a car application person. A load is not a load as you have stated, anyone who truly knows anything about an airboat is laughing right about now. The load an airboat engine sees is nothing like what a car sees.

You really should quit giving out bad info, people spend a lot of their hard earned money on these boats and want them to preform best for what they spend. We should be here to help the new guys and the less knowledgeable so they don't meet the same pitfalls as many of us have learned through the years.

"We groundpounders only run a 500cfm on 470 cubic inches at 2900 RPMs. Just saying" Exactly, I cant tell you how many guys I have seen trying to run 650-750 cfm carbs on them and wonder why it was spongy and ran like a turd. Change one over to a 500 cfm and they freak out and can't believe the performance difference and throttle response.
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Re: carbs

Post by loudmouse »

Comparing modern day automobile engines to 1940's aircraft engines is like comparing apples to carrots. Aircraft engines were designed to make a specific HP at a specific rpm with relatively small exhaust and intakes for the cubic inches. This is very old technology that has proven reliable for aircraft. Today's technology allows us the ability to maximize the power with external Bolt-on parts and tuning. With that said if you were to take off running with your mouth closed you will not go far before you will have to start breathing thru your mouth or stop running. The reason, WORKLOAD! A prop works the engine very hard. The harder the workload the more air is required. This is vacuum! And that same vacuum is a power restriction!!!!! Each person can build and set up their boat however they choose but there could be additional power to be gained and would almost cost nothing extra over buying the right parts the first time around.
15' Alumitech deckover LSX TT Oxbox 84" JX 4-blade
Gas required Water optional :dontknow:
Built and Tuned by Me
14'X8' JW, Chevrolet 122CID LNF Turbo Engine, Ox box, 80" NGR 3 blade

Life is short, eat your dessert first! :downtown:

lil_armond
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Re: carbs

Post by lil_armond »

You keep missing the point. The OP has a direct drive rig and asked what carb would best suit what he has. He did not ask what bolt on parts, tuning and head work he needed so that he could run a 750 double pumper. A direct drive rig needs to make all the power it can effeciently as possible from idle to 3000 rpm's and that is it plain and simple. Anything more than a 500 cfm mechanical secondary for his rig is pissing in the perverbial wind. A cam means crap if the carb can't feed it what it wants and the heads cant expell it as needed and vice versa. If the OP wants to utilize his motor as it sets for a direct drive application then the correct carb is needed. On the other hand if he wants to change cam, buy the correct carb for that cam and correctly size the heads to match everything else then he needs to be ready to spend the money.

As far as today's bolt on external parts that optimize performance and add horsepower, well I'll believe it when I see it. Cold air intake does crap with out everything behind it. Headers are good for a little but real horsepower comes from correctly matched sum of all parts working together to provide from intake to exhaust pulse exactly what the math of the build dictates.
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