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Discussion Starter #1
I have a 144 B&M blower, with a crower supercharger cam 449 or 459 lift and 288 advanced duration 9.5 compression ratio, with 76cc stock heads. I also have a 750 cfm (*secondary vacuum*) carb. Will this carb be enough to support this engine setup? I've been getting a lot of criticism on whether it'll work or not, let me know what you think, thanks.
 

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Discussion Starter #5
my bad, it's a 350 sbc and i made a mistake it's 8.5 compression, but will a secondary vac carb be enough in either case? thanks.
 

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He doesn't need a blow through carb for a 144 supercharger...

On a street driven 350 that should be just about the right size of carburetor...The vaccum secondaries will work fine, allowing the engine to decide how much air it needs...The only time it would cause a problem is if you are running a lot of boost and/or high RPM and there is enough vaccum under the carb at WOT to actually close the secondaries or the powervalve...Use a vaccum gauge hooked to the carb base at WOT, run it up to your "redline" and see if you get more than 4" vaccum...If you get 4" or more THEN you should start thinking about a bigger carb...
 

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I would be running a larger carb than that. I also have a 350 with a weiand 142 blower, and I am running a 800 cfm Edelbrock 1413 and it is fine. The problem with the Holley is you need one that has a boost referenced power valve circuit or it will not run right. It could run lean and burn things up, or run way to rich. The reason I went with the Edelbrock is because it was way cheaper than a blower series Holley. I had a perfectly good 750 vac sec holley around from my old non-forced induction setup that I had to get rid of. You do not need to run a blow through type carb because the blower is sucking through the carb.

Subman
 

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It is a pretty simple matter to convert a standard Holley carb to use on a roots blower. You basically just reference the power valve to a manifold vacuum source (below/after the blower). For best performance, any carb used on a blower should have this done. I think that for that blower and cam, a 750 would work fine.
 

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Discussion Starter #11 (Edited)
Sorry, but I'm kind of a noob here with carbs. I got a vacuum hose ont he back of the carb which has the electronic choke and a hose from the valve cover there for brakes. There's another very small valve which has a very small red cap on it. Which one if any of those would the power valve be? And where would it need to be connected to?

I'm trying not to spend a lot of money on this, even thought I dumped a couple g's a lready, but if it's possible to convert this carb to work, I'd like to, but if it's a lot safer to just buy a new carb like the edelbrock 1413, let me know, thanks.
 

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The power valve references through a small hole in the botom plate of the carb, by the throttle butterflys. You cant just hook a tube up to it. The carbs that are made for blowers can have a tube hooked up to them, but not the standard ones.

Subman
 

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Discussion Starter #13
My blower has a hole int he very top of it which is plugged right now, is that what you're talking about?
 

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I nabbed a couple pictures from a book I have. Super Tuning and Modifying Holley Carburetors by Dave Emanuel. If you want to learn about carbs, this book is pretty good. They show what is done to boost-reference the power valve.
 

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The Smell of Nitro in the morn
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Try the Knowledge Base on this Forum.
No, I would not use a vacuum carb on any forced air motor.

How does a engine control the vacuum secondaries when its on a blower, please explain ?
At WOT there is no vacuum when a motor is under pressure from a supercharger / turbo. :confused:
 

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As engine speed increases, airflow increases. The vacuum signal for Holley carbs comes from a small hole drilled in the primary venturi. Airflow through the venturi creates a pressure drop, or vacuum, which pulls open the secondaries. On Quadrajets, Edelbrocks, Carters the secondary throttle plate is opened mechanically, and the air doors/flap are pulled open when the engine is pulling enough air past them to overcome the spring tension/ weight holding them shut. A vacuum secondary carb with work on a roots-blown engine just as well as it will on a naturally aspirated one.
 

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-Blower Drive Service-

Q: What carbs do I need to run and why 2 of them?

A: The size of carb(s) or CFM required for a given application can be calculated by the following formula A: {(CID x RPM) / 3456} x {Boost: 14.7) + 1} = CFM required. The amount of CFM required will determine carburetor size and quantity. If you try to use a carb with less CFM than required, performance and economy may be greatly reduced.
 
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