Jim Holmgren
Member
I have noticed that there are a lot of questions and misunderstandings regarding carburettors in turbocharged applications and i hope therefor to clarify the subject bit.
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Lets begin on carburettors in an draw through turbo setup.
As most of you know the carb is sitting in front of the turbo compressor inlet in this kind of setup.
Turbo Seals
This means among other things that there will be a vacuum in the turbo when the carb throttle valves is closed and as result most turbo seals will leak oil from the centre housing as they can't handle vacuum.
There are several solutions to this problem.
The most common is to use a turbocharger that has carbon seals in them or to mount carbon seals in a existing unit.
But because turbochargers with carbon seals are often outdated and it is hard to obtain carbon seals for a existing unit, but there are easier solutions.
you can drill a hole from the outside around the seal behind the compressor wheel,
that prevents the formation of negative pressure,
ie a 6mm hole from the center section behind the compressor wheel so close to the center of the turbine shaft you dare without damaging the seal.
This means that it will suck air instead of oil.
A other way is to run a dual throttle valves, one in the carb and one after the turbocharger.
You can also run without valves in the carburettor, and only use one throttle valve after the turbo.
it is also a patent that might interest you, which is about depressurizing the bearing housing, Schwitzers US Patent 3,834,156.
I have read reports of this system should work very well but you will maybe still have some oil leaks, but this is not noticeable.
Carburettor sizers
In a draw through setup you will need a much bigger carb then in a blow through setup.
As the all the air go through the carb unpressurised it will need to be so large that it can support the same power in a N/A engine.
Example if a N/A engine at 450hp needs 750 cfm carb.
A draw through turbo setup at 750 hp will maybe need a 1050 cfm carb since a 750 N/A would need a carb that can flow those numbers as the carb don't know what it sits on.
But in a blow through setup setup you will only need the cfm to support the N/A engine power, so if you have 450hp engine + the boosted power, then a 750 cfm carb can support 1000hp in this example.
Response and blow-off valves
In most draw through turbo setups a blow-off valve will not work as it would operate in vacuum.
The response is in fact faster between shifts in a draw through setup with valve before the turbo, as it is vacuum in the compressor housing the turbo will spin freely at shifts and when the throttler reopened full boost will be had.
F1 cars used this setup with throttle valve before turbo because its fast response!
Many will say that a draw through setup is slow to build boost, but from all people I have head from who have tried this setup have not agreed with this.
Icing and puddling
You can get icing if you have the carb to cold example so could the throttle freeze solid at full throttle, especially in cold climate.
Fuel puddling is also possible, which is self explanatory why it can be dangerous.
To avoid this problems, it may be best to have as short path to the intake as possible.
Intercoolers
The danger in intercoolers in a draw through setup is fuel puddling and hence most systems use water injection.
I have however seen water to air intercoolers in draw through setups but I have no Knowledge of these really, since more testing need to be done on this subject.
___________________________________________________________________
Lets look at carburettors in an blow through turbo setup
Response and blow-off valves
In a blow through setup the compressor turbine will be decelerated by the pressure that will be when you close the throttle valve, so between shifts you will need to build up boost again.
This can be helped with a blow-off valve which relieves the pressure so the compressor wheel will meet less resistance.
fuel management
Since a carburettor do not know the density of air it will provide as much fuel no matter if it is 0 psi or 20 psi overpressure.
To overcome this there are some different ways depending on how the carburettor is constructed.
One way is to manipulate the power valve in the carb to give more fuel as the pressure rise.
You could also use pitot tubes to trick you carb.
Some people use a additional injection system only for the boost.
Other needed things in a blow through carb is good seals and floats that will withstand pressure, also a pressure regulator will be needed, otherwise will the fuel in the carb be push back by the air pressure.
Intercoolers
Intercoolers will work without any problems in a blow through setup.
________________________________________________________________________
Feel freely to post your questions and I hope we all will try to answer them
I hope we can elaborate this thread and make it better as time go, so post supplements and changes!
between I'm from sweden so my english is not the best.
________________________________________________________________________
Lets begin on carburettors in an draw through turbo setup.
As most of you know the carb is sitting in front of the turbo compressor inlet in this kind of setup.
Turbo Seals
This means among other things that there will be a vacuum in the turbo when the carb throttle valves is closed and as result most turbo seals will leak oil from the centre housing as they can't handle vacuum.
There are several solutions to this problem.
The most common is to use a turbocharger that has carbon seals in them or to mount carbon seals in a existing unit.
But because turbochargers with carbon seals are often outdated and it is hard to obtain carbon seals for a existing unit, but there are easier solutions.
you can drill a hole from the outside around the seal behind the compressor wheel,
that prevents the formation of negative pressure,
ie a 6mm hole from the center section behind the compressor wheel so close to the center of the turbine shaft you dare without damaging the seal.
This means that it will suck air instead of oil.
A other way is to run a dual throttle valves, one in the carb and one after the turbocharger.
You can also run without valves in the carburettor, and only use one throttle valve after the turbo.
it is also a patent that might interest you, which is about depressurizing the bearing housing, Schwitzers US Patent 3,834,156.
I have read reports of this system should work very well but you will maybe still have some oil leaks, but this is not noticeable.
Carburettor sizers
In a draw through setup you will need a much bigger carb then in a blow through setup.
As the all the air go through the carb unpressurised it will need to be so large that it can support the same power in a N/A engine.
Example if a N/A engine at 450hp needs 750 cfm carb.
A draw through turbo setup at 750 hp will maybe need a 1050 cfm carb since a 750 N/A would need a carb that can flow those numbers as the carb don't know what it sits on.
But in a blow through setup setup you will only need the cfm to support the N/A engine power, so if you have 450hp engine + the boosted power, then a 750 cfm carb can support 1000hp in this example.
Response and blow-off valves
In most draw through turbo setups a blow-off valve will not work as it would operate in vacuum.
The response is in fact faster between shifts in a draw through setup with valve before the turbo, as it is vacuum in the compressor housing the turbo will spin freely at shifts and when the throttler reopened full boost will be had.
F1 cars used this setup with throttle valve before turbo because its fast response!
Many will say that a draw through setup is slow to build boost, but from all people I have head from who have tried this setup have not agreed with this.
Icing and puddling
You can get icing if you have the carb to cold example so could the throttle freeze solid at full throttle, especially in cold climate.
Fuel puddling is also possible, which is self explanatory why it can be dangerous.
To avoid this problems, it may be best to have as short path to the intake as possible.
Intercoolers
The danger in intercoolers in a draw through setup is fuel puddling and hence most systems use water injection.
I have however seen water to air intercoolers in draw through setups but I have no Knowledge of these really, since more testing need to be done on this subject.
___________________________________________________________________
Lets look at carburettors in an blow through turbo setup
Response and blow-off valves
In a blow through setup the compressor turbine will be decelerated by the pressure that will be when you close the throttle valve, so between shifts you will need to build up boost again.
This can be helped with a blow-off valve which relieves the pressure so the compressor wheel will meet less resistance.
fuel management
Since a carburettor do not know the density of air it will provide as much fuel no matter if it is 0 psi or 20 psi overpressure.
To overcome this there are some different ways depending on how the carburettor is constructed.
One way is to manipulate the power valve in the carb to give more fuel as the pressure rise.
You could also use pitot tubes to trick you carb.
Some people use a additional injection system only for the boost.
Other needed things in a blow through carb is good seals and floats that will withstand pressure, also a pressure regulator will be needed, otherwise will the fuel in the carb be push back by the air pressure.
Intercoolers
Intercoolers will work without any problems in a blow through setup.
________________________________________________________________________
Feel freely to post your questions and I hope we all will try to answer them
I hope we can elaborate this thread and make it better as time go, so post supplements and changes!
between I'm from sweden so my english is not the best.
