Intake ram air tube

[FlyFastJP]

Theoretical scenario. Carb'd O-360 on an RV-4:

I've not tested, but I've heard that the Van's FAB needs to have the air filter in it to keep the engine from coughing and spitting.

If I were to remove the FAB completely, fabricate a direct sealed tube from the cowl scoop to the carb, would that pressure overwhelm the engine's ability to draw fuel from the bowl?

 

[Ron77]

Ram pressure will not affect the operation of the carb. The carb's venturi is what creates the pressure differential that enables the carb to meter the fuel.

Airflow Performance makes a alternate air duct if you want to also have filtered air. Also, Rod Bower Aviation has a system, but $$$$.

Ron B .

 

[FlyFastJP]

Is it just air not flowing properly that causes the engine to run rough with an empty FAB?

 

[DanH]

Is it just air not flowing properly that causes the engine to run rough with an empty FAB?

I'm not sure anyone can say with certainly for the FAB/carb combination, but poor flow following an abrupt turn at the entry has been a problem for other applications.

 

[FlyFastJP]

I'm considering making an aluminum tube with a smooth bend up to the carb. No disruptions, no restrictions.

I just didn't know if it would overwhelm the carbs ability to draw fuel.

 

[Mycool]

Ram pressure will not affect the operation of the carb. The carb's venturi is what creates the pressure differential that enables the carb to meter the fuel.

Airflow Performance makes a alternate air duct if you want to also have filtered air. Also, Rod Bower Aviation has a system, but $$$$.

Ron B .

Been trying to get ahold of Rod bower and his system, his website is no longer valid. I'm trying to get ahold of a sump 90 degree elbow that he sold. Does anyone have leads on getting a similar elbow?

 

[Mike S]

Been trying to get ahold of Rod bower and his system, his website is no longer valid. I'm trying to get ahold of a sump 90 degree elbow that he sold. Does anyone have leads on getting a similar elbow?

Airflow Performance.

https://airflowperformance.com/index.php/product-category/elbows-adapters-spacers/

 

[lr172]

I'm considering making an aluminum tube with a smooth bend up to the carb. No disruptions, no restrictions.

I just didn't know if it would overwhelm the carbs ability to draw fuel.

Apparently you don't trust Ron's answer, so suggest you do some research on carb theory to learn exactly HOW a carb draws fuel. I am not going to describe it here. Dan's post about turbulence at the throat entrance is also relative, but is typically related to disruption of the main circuit bleed port (i.e. static port) and different carbs have these in different places, with some highly influenced by turbulence at the opening and others not so much. There was an RVator article about moving that bleed port on certain models to make it more tolerant of turbulence. I will say that you want to strive for orderly, organized air flow at the carb throat entrance, but pressurized ram air in and of itself is no issue. Hot rodders often put velocity stacks on carbs when no air cleaner is used in order to avoid turbulence created by 100 mph perpendicular wind over the carb.