Blow-Through carbs-- how do they work? benefits?

[Doc Sprocket]

Glad I could make a good discussion happen here, but I already know how superchargers work, and what they do. I know how to buy a bolt-on kit and make it work on a full sized motor, that's not the issue at hand. I am trying to get info on blow-through carbs for small engines, fabrication info, and what benefits they bring. As far as I can tell, putting the carb before the pump is the best way to do it, and draw the air through as on a full size roots blower. Since many smog pumps are just mini roots blowers, adapting a carb north of it and running a belt drive seems to be the way to go. I am wondering why one would go to the trouble of making a blow through system, and how to do so.

Yeah- sorry 'bout that. Sometimes we kinda wander off on a tangent around here.

I'm not the guy to answer this, but perhaps one of the other fellas can.

 

[Desertduler]

You actually HAVE one of those? Wow! They are LEGENDARY! I've never seen one in use, but have seen references to them and pics on the internet. Amazing. Frame that thing!

I forgot the name of the this thing. Colortune is the name of it. I guess I could screw it in my engine and mount my Go Pro up as to see the color of all my carb circuits as I go through them and see where they are at! JK!

 

[Rickracer]

One of the reason a blow through works better is that you can tie fuel enrichment to the amount of boost. For my twin turbo small block Chevy (in a '76 Chevy Vega), I built my own blow-through, E85 carb from a Demon carb, many use Holley DP carbs. I use what's known as a BRPV (boost referenced power valve). Instead of low vacuum (WOT) causing the P/V to open, boost does it, and it's sized to deliver the right amount of fuel for a given amount of boost. Another reason is the size of carb required doesn't change with blow-though, if your motor is say, 450 hp normally aspirated, a 650~750 cfm carb is about right, and it can still make over 1000 hp as a blow-through. For draw-through, it would need to be increased to a Dominator sized carb for that same amount of HP, (1000~1200 cfm). That size carb WILL NOT be responsive at low boost/low HP. Another issue with drawthrough is that you need different/better turbo or supercharger seals for use with fuel.

 

[Desertduler]

And another reason a blow through is better in my opinion is that the float bowls are pressurized with the boost and jetting becomes less of a nightmare.

 

[Doc Sprocket]

And another reason a blow through is better in my opinion is that the float bowls are pressurized with the boost and jetting becomes less of a nightmare.

The bowl is no longer vented to atmospheric pressure? Wouldn't that force fuel up through the nozzles, rather than the venturi effect simply drawing it up?

 

[Desertduler]

If the bowl is vented to atmospheric pressure the air fuel ratio will lean out under boost.
Think about what forces the fuel through the jets it is the drop of atmospheric pressure in the venturi and if the pressure is greater at the fuel nozzles in the venturi the fuel will travel the opposite way.

 

[Desertduler]

So a blow through system richens up under boost.

 

[Doc Sprocket]

If the bowl is vented to atmospheric pressure the air fuel ratio will lean out under boost.
Think about what forces the fuel through the jets it is the drop of atmospheric pressure in the venturi and if the pressure is greater at the fuel nozzles in the venturi the fuel will travel the opposite way.

Oh, jeez. *facepalm*- Cant believe I missed that.

 

[Rickracer]

So a blow through system richens up under boost.

Correct, plus, I use vent tube extensions that go from the bowl vents up into the carb hat, makes a HUGE difference in jetting requirements, as in, much closer to stock jetting...

 

[Desertduler]

Correct, plus, I use vent tube extensions that go from the bowl vents up into the carb hat, makes a HUGE difference in jetting requirements, as in, much closer to stock jetting...

Yes,and the vent extensions pick up on the air flow going through the carb.

 

[Desertduler]

And slightly off the subject but worth knowing is that a carburetors float vent should be subjected to the atmosphere that it is taking on and mixing fuel with in order to maintain the correct air fuel ratio so what I do on my race bikes is I plumb my float bowl vents into the clean side of my air filters so as when the air filter starts getting plugged up on long races the air fuel ratio corrects itself somewhat because the bowl vents are in the available atmosphere that the carburetor has and yes I do lose some power with a plugged filter but not near the amount that I lose by the air fuel ratio going way rich by leaving the vents subjected to the regular atmosphere.

 

[firemanjim]

Glad I could make a good discussion happen here, but I already know how superchargers work, and what they do. I know how to buy a bolt-on kit and make it work on a full sized motor, that's not the issue at hand. I am trying to get info on blow-through carbs for small engines, fabrication info, and what benefits they bring. As far as I can tell, putting the carb before the pump is the best way to do it, and draw the air through as on a full size roots blower. Since many smog pumps are just mini roots blowers, adapting a carb north of it and running a belt drive seems to be the way to go. I am wondering why one would go to the trouble of making a blow through system, and how to do so.

I really don't think you're gonna get an air pressure worth making more hp, from a smog pump. There are a few threading here that talk about using a smog pump. Search for those threads. Just search for smog pump.....