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Discussion Starter · #1 ·
I HAVE A CHEVY 350 THAT I HAVE BORED 30 OVER , WITH A 525 LIFT ROLLER CAM, ALUMINUM ROLLER ROCKERS AND SOME TOP LINE PRO IRON HEADS WITH 2.02/ 1.6 VALVES. ALSO ADDED A 142 WEIAND BLOWER. AND INTAKE. MY PROBLEM IS THAT I STILL HAVE THE ORIGINAL HOLLEY 650 CARB. THIS CARB WORKED FINE WITH THE L98 IRON HEADS I HAD BEFORE, BUT I DONT THINK I AM GETTING ALL I CAN OUT OF THE NEW HEADS AND ROLLER ROCKERS. WHAT WOULD YALL RECOMMEND?
 

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First off I recommend loosing the caps lock, no need to shout.
I would go with a Barry Grant Demon carb, all you have to do is call them up and they`ll recommend a carb for you. I run a road demon Jr and I`m very satisfied with it. A tech agent for BG is also a member of this site, aka [email protected], no other carb company is a member here and that says alot about them, and they try there hardest to make sure the customer gets all the help needed.
 

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what carb?

CID X desired rpm divided by 3456 = CFM

example:
352 ( your displacement with 30 over) X 6000 rpm divided by 3456= 611 CFM.

While I prefer Holley carbs, they are all about the same in quality, I think, so your 650 should work quite well once you get it dialed in.
 

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what carb

I would say thats all thats necessary. The formula I gave you is based on 100% efficiency of the engine and with a supercharger youre about as close to 100% as you are going to get.

You will definately need to jet up the carb some, runnning it on top of a huffer, but that is way past my expertice, as Ive never had any experience with running Roots type superchargers. The only experience Ive had was with a Paxton unit on a Ford 302, and we ran a 500 cfm carb with it and it was quite successful.
 

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The above formula does not work when it comes to blowers. A blown engine will actually be over 100% VE.

I would probably run an 850CFM Holley blower carb (you might be able the get by with a 750). How many pounds of boost are you running? What kind of compression? You can over carb a blown engine and not suffer from it like you would a N/A engine. When I was building by blown engine, I was told on a 6-71 or 8-71 they wouldn't recommend running anything less that two 750's. If your engine is mild then I would go with a 750 blower carb. You didn't list the cam specs so I don't have that info to help me. I would definetly stick with a Holley or Holley style carb (stay away from the Carter/Edelbrock carbs for blown applications). If your car/engine is a little on the wild side I would suggest even running a 950CFM on it. My engine is only 33ci inches bigger that yours and I have 1500CFM worth of carb, my blower is larger so that makes a big difference too.

My engine is a little on the radical side, I run two Holley 750 DP blower carbs, 12lbs of boost, 8.47:1 compression.

I would also suggest you give www.allcarbs.com a call they have great prices and customer service. They proabably have a carb on the shelf that will fit your needs. See what they suggest.

Royce
 

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Max Keith,
I just hope the info helps. That is what this board is all about. It's nice when people post back with results (good or bad) that way we all learn from it. If it wasn't a blown engine your recommendation would have been right on the money.

SIRIGGY,
Any info for us?

Royce
 

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Discussion Starter · #10 ·
Man I really appreciate all the info. My engine has a mild cam dont really know the specs off hand but the lift is 525. Does any one have opinions on the Barry Grant carbs? I think I will go with a Road Demon 850. Any opinions? Thanks. I am trying to get my S-10 in the low 10's. Right now it is running 12.2's with the setup and 1 legged 3:08 's in the rear.
 

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Barry Grant carbs are fine. I don't know if they have a blower specific version. The reason I like the Holley blower carb is because they have boost referenced power valves. Talk with BG and see what they offer. In any case you are under carb'd. Please let us know how it works out (even if it is 6 months from now).

Royce
 

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Technical Support Barry Grant
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Over Carb!!!

It can easily hurt too much to over carb an engine. Carburetors work based upon the air velocity (pressure difference) drawn through them. Going with too large of a carburetor for an application can cause the engine to run rich at idle, and part throttle and lean at wide open throttle.

On a blown application you will want to use a carburetor that has been calibrated, and boost referenced for the blower. We've been setting carburetors up like this for over a decade.

Without knowing more specific information about the combination it would be impossible to make an accurate recommendation, but depending on the rest of your information we'd use either a 650 or 750 Blower Calibrated Mighty Demon carburetor.
 

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The old formula for carbs raises its pretty little head again. It is used as a baseline, only. This subject has come up a hundred times, and it comes down to the same question: What is the intent of the motor?

Look HERE to see where actual testing was done with a NA 383 stroker. The end of the article states:

"Judging from the test results, we'd say that the 750-cfm carburetor seemed to provide the best power curve and throttle response. In the event that the engine made 50 hp or more and was dedicated for racing, we'd be willing to say that the 830-cfm piece with down-leg boosters might add a few extra horsepower and still be livable on the street."

This article has shown that with this particular engine combo that you could go with an 830 cfm, TUNE IT PROPERLY, and it could live on the street.
 

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Technical Support Barry Grant
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Dyno Testing vs. the real world

The problem with this type of test, is it doesn't take into consideration what will happen in the real world. They don't show you the acceleration rates to be able to determine how the HP will be used in the vehicle. Installing the same engine in a 2000 lb kit car vs. a 6000 lb truck would use a different carburetor to get optimal performance.
 

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Then it goes back to the same question as I stated earlier:
What is the intent of the motor?

With what you just stated, then the old formula does not apply then does it? In this example, it would take the 750 Holley if it were used for more street action. But, if it were a weekend cruiser, and went to the track, then the 830 Holley would be the choice.
 

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Re: what carb

Max Keith said:
I would say thats all thats necessary. The formula I gave you is based on 100% efficiency of the engine and with a supercharger youre about as close to 100% as you are going to get.

You will definately need to jet up the carb some, runnning it on top of a huffer, but that is way past my expertice, as Ive never had any experience with running Roots type superchargers. The only experience Ive had was with a Paxton unit on a Ford 302, and we ran a 500 cfm carb with it and it was quite successful.
That formula is a baseline figure. My SBC 400 has a 700 DP on it right now. It is way too small. I am considering going up to an 850. If you plugged 850 into that equation it would never work.

Try plugging a 1250 CFM Dominator into that equation. How about 2 of them. You see what I mean?
 

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Diehard bowtie fanatic

Granted, I have never built a forced induction engine, but I've had quite a few bowties, and nothing beats a Quadrajet for reliability.
All those fancy shiny expensive carbs are great to show your buddies, but give a me a Q-Jet any day, for anything.
Plus, they were made in 750cfm (1-3/32 primary), or 800cfm (1-7/32 primary).
I've always like the off idle response generated by the small primaries, with the huge volume capacity of the secondaries.

Just my opinion. Everyone has their own favorites.
 

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I tried to use the formula on my current engine, and it just was too small. It calls for a 750 if I am at least 95% efficient. That was a joke. I could not get it to idle properly. I could only open up the secondaries so much, or drill a hole in the butterflies on the primaries. I still had a vacuum reading at WOT. I bit the bullet and went up and bought an 850, problem solved. Runs like a charm. Now I had to tune it mind you, set floats, set idle circuits, set curb idle, set electric choke, change out power valves (twice, I found a small vacuum leak at power brake vacuum fitting and after fixing it, my vacuum reading in idle went up). But now that it is set, purrs like a kitten, roars like a tiger to 6000 rpm. Excellent throttle response.

What I was trying to say is there is not "set" rules for carburation. It just depends on the engine combo and what it is happy with, and what you are intending to use it for.
 

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Technical Support Barry Grant
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Carburetor Size Formula

The carburetor size formula is all but worthless with today’s engines, and carburetors. There are way too many variables in an engine that these formulas do not take into consideration. Type of cylinder, running volume, air velocity, and most importantly intended application to name a few. Secondly since not all carburetors flow the same, you wouldn’t be able to plug the information in to be valid. There is not a standard on how carburetor companies or modifiers flow test their products, some don’t even flow test all of their own carburetors in the same manor.

The rating size on our carburetors is a median or average CFM flow number that that carburetor would flow on an engine if it were installed on the type of engine it is designed for. An engine is an air pump, so unless it is overly restricted it will flow roughly the same amount of CFM regardless of what size carburetor you use. This means finding a carburetor that gives the best air velocity, and atomization throughout the intended power range is the goal. Putting a 1295, or even two of them on a stock 350 won’t make it flow anymore air, it will just slow down the air velocity to the point that it won’t be able to pull fuel efficiently.

Bottom line comparing carburetors between manufactures is not a fair or good comparison. When looking for a carburetor for your application your should do a few things:

First, determine what your combination is going to be, and what your expectations are from it.

Second, determine what brand of carburetor you want to work with based upon your goals with the combination

Third, get with that manufacture directly to find out what they recommend. Don’t ask the speed shop, or your buddy, they may know more than you, but they’re not going to know more than the people who designed and engineered it.

Lastly, follow the manufactures recommendations on application, size, and tuning.

The number of people who ask for advice, don’t follow it, and then get discouraged when a product doesn’t work properly’d surprise you.
 
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