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MiracleMax · 08-29-2001, 12:07 AM

There are 2 ways to carb a centrifugal type blower.

A. Blow through
B. Pull through

The easiest is a pull through system, you mount the carb up stream of the blower and let it work like nature intended (@ about 120% volumetric effciency). Lets see here that'd be 306 x 6500 rpm / 3456 x 120% = 690 cfm or a bit bigger and jet accordingly.

A blow through system is a bit tricker. the easiest way to do this would be to mount the carb in a pressure box which negates the need for shaft seals and the like since the carb is operating under equal pressure on both sides obviating the need for extra sealing and simplifying tuning as compared to an exposed carb blow through system. Simply bolting some sort of plenum to the head of the carb makes things super difficult since you've got a positive pressure system flowing through a device designed to operate in the presence of negative pressure. Just like an FMU you have to boost reference the carb for the blower along with the aformentioned need to seal the carb. However a blow through system has its merits mainly from a safety stand point. With a pull through system, fuel is drawn from the carb all the way through the blower and then into the engine. Problem is without some kind of pressure relief on the manifold (a Wilson spacer with blowout protection would be handy here)a backfire travels from the intake all the way back through the blower and up to the carb (lotsa fuel and air to burn there). If the engine is running the
best analogy I can make here is to get on a bike and go down hill pedaling as fast as you can then stick your foot into the spokes on the front wheel and see what happens! Anyway you get the idea. With a blow through system fuel stops at the carb and lessens the damage which could occur. On the flip side, a pull through system gains the evaporative cooling effect of all that fuel turning into a vapor which potentially could add a few ponies and possibly increase the reliability of the blower since everything south of the carb would operate slightly cooler or allow more aggresive timing or compression or both.

08-29-2001, 12:07 AM	#20
MiracleMax Registered Member Join Date: Apr 1999 Location: Hayes, Va, USA Posts: 798	There are 2 ways to carb a centrifugal type blower. A. Blow through B. Pull through The easiest is a pull through system, you mount the carb up stream of the blower and let it work like nature intended (@ about 120% volumetric effciency). Lets see here that'd be 306 x 6500 rpm / 3456 x 120% = 690 cfm or a bit bigger and jet accordingly. A blow through system is a bit tricker. the easiest way to do this would be to mount the carb in a pressure box which negates the need for shaft seals and the like since the carb is operating under equal pressure on both sides obviating the need for extra sealing and simplifying tuning as compared to an exposed carb blow through system. Simply bolting some sort of plenum to the head of the carb makes things super difficult since you've got a positive pressure system flowing through a device designed to operate in the presence of negative pressure. Just like an FMU you have to boost reference the carb for the blower along with the aformentioned need to seal the carb. However a blow through system has its merits mainly from a safety stand point. With a pull through system, fuel is drawn from the carb all the way through the blower and then into the engine. Problem is without some kind of pressure relief on the manifold (a Wilson spacer with blowout protection would be handy here)a backfire travels from the intake all the way back through the blower and up to the carb (lotsa fuel and air to burn there). If the engine is running the best analogy I can make here is to get on a bike and go down hill pedaling as fast as you can then stick your foot into the spokes on the front wheel and see what happens! Anyway you get the idea. With a blow through system fuel stops at the carb and lessens the damage which could occur. On the flip side, a pull through system gains the evaporative cooling effect of all that fuel turning into a vapor which potentially could add a few ponies and possibly increase the reliability of the blower since everything south of the carb would operate slightly cooler or allow more aggresive timing or compression or both.