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Carb Size Requirements...Blow Thru vs Draw Thru - Is There a Difference?

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Old 05-15-02, 09:24 PM
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Carb Size Requirements...Blow Thru vs Draw Thru - Is There a Difference?

I read somewhere (possibly here) that in a supercharger draw thru carb set-up, the carb has to be much larger than stock to gain anything...I.E You'll max out a small carb;
However on a blow thru carb set-up, the carb size was'nt going to be an issue so long as the fuel jets were large enough to deliver.

Is this true? Is it becausen in a blow thru the fuel is pressurized as much as the rest of the carb?
Help me to understand this.

I'm starting to worry that my Yaw might be too small for my Eaton M-62 - 12A project.
However, according to a graph on the Eaton site, the model blower I have pushes only slightly more than my carb can flow at atmospheric. (Carb can flow 425 cfm, blower 450 cfm @ 5 psi @ 14K RPMs.)
BTW, with these numbers, does it sound like I'll have decent power?

AHHHH! The more I learn, the more questions I have!
The biggest thing I've learned is that all is not absolute in a project like this. Mathmatically, and theoretically, things seem to work out so much better on paper than in real life!
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Old 05-15-02, 10:03 PM
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A blow thru turbo already has created pressure before the carb. Its air inlet is unobstructed. It forces the air thru the small venturis. Much more than they will pass on their own.
A suck thru system has to create its pressure after the carb. So in order to feed the turbo the air it requires to boost large amounts of air must flow thru the venturis first. Hence the large bore requirements.
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Old 05-15-02, 10:25 PM
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One of the probs PY said the Nikki carb might expierience when set up as a blow thru is the fuel being forced out under pressure from areas normally under vacuum.

An option I saw was at Mazdaracing.com or something like that. The had a pressurized box the carb literally sits inside of. When outside pressure is the same as inside, the vacumm is restored to normal. The carb functions correct. But it is sitting in a positive pressure area.

One thing I really like is that you can intercool doing the blow thru.
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Old 05-15-02, 11:44 PM
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Re: Carb Size Requirements...Blow Thru vs Draw Thru - Is There a Difference?

Originally posted by Sterling
I'm starting to worry that my Yaw might be too small for my Eaton M-62 - 12A project.
However, according to a graph on the Eaton site, the model blower I have pushes only slightly more than my carb can flow at atmospheric. (Carb can flow 425 cfm, blower 450 cfm @ 5 psi @ 14K RPMs.)
BTW, with these numbers, does it sound like I'll have decent power?
Just because your carb can flow 425 cfm, doesn't mean that it will. And is the 450cfm the outlet volume or inlet volume? If it's 450 cubic feet at 5psi, that seems like it will be plenty of air.
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Old 05-16-02, 06:55 AM
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This is for a carb-in-a-box set-up, particularly so that I can use an IC.

Paul Yaw flow tested his carb as he developed the mods for it.
According to his flowbench, it will flow "415-425 cfm".
(Unlike a Holley 600, where I know of no charts demonstrating that it will in fact flow 600 cfm.)
Is this what you mean by,

"Just because your carb can flow 425 cfm, doesn't mean that it will."?

It's 440-460 cfm @ the inlet.

http://www.eaton.com/supercharger/index.html
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Old 05-16-02, 02:45 PM
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Sterling:

Yahoo Forum on BLOW-THROUGH Turbo systems.

Yahoo Forum on DRAW-THROUGH Turbo systems

Yahoo Forum on TURBO FB's (1st Gen RX-7 Turbos)

I know they are listed for Turbos, but there is lots of information and members with Superchargers. They like it nice and simple too for the most part.

Good luck.
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Old 05-16-02, 05:04 PM
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Try this link. It has Carburetor blow through systems for rotaries...

http://www.mazdaracing.com/carbs.htm

I ordered me a system from them....
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Old 05-16-02, 05:27 PM
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Blow-thru can be the same size or smaller than stock. This is because the air is denser (pressurized, under boost, etc) as it goes through the carb. Any pressure drop through the carburetor will be slight, and you can compensate by cranking the boost up a little more. Besides, the pressure loss also means a temperature loss, no bad thing.

A draw-through must have a much larger carburetor, since the air is at ambient pressure. Otherwise, flow through the carb will stall and you'll find yourself hitting a wall so to speak. For example Paul Yaw says the stock carb can only flow enough air to support 145 or so HP. Put that on a drawthrough turbo setup and it won't make much more than 145hp either, although you'll get that power at a much lower RPM because the turbo/blower is able to force 145hp worth of air into the engine at a lower RPM.
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Old 05-16-02, 09:50 PM
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Originally posted by Sterling
This is for a carb-in-a-box set-up, particularly so that I can use an IC.

Paul Yaw flow tested his carb as he developed the mods for it.
According to his flowbench, it will flow "415-425 cfm".
(Unlike a Holley 600, where I know of no charts demonstrating that it will in fact flow 600 cfm.)
Is this what you mean by,

"Just because your carb can flow 425 cfm, doesn't mean that it will."?

It's 440-460 cfm @ the inlet.

http://www.eaton.com/supercharger/index.html
I meant that you're just concentrating on the peak flow at peak rpm when you should be looking at flow through the range you plan to use. Your engine cannot flow 425cfm at idle, and it's possible that at the peak rpm you'll use it still would not flow 425cfm of unpressurized air. Once you feed it 5 or 10 psi the same amount of air will fit into a smaller space, this means that the carb can flow much more air under pressure than it can under atmospheric conditions.

Let's say that at 6000rpm the engine can flow 425cfm but the M62 only flows 400 cfm. Expansion would occur and a pressure drop would take place. Your main concern might be that the M62 does not flow enough air for your application. But that's just a theory.
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