Air intake reducer set
 

Hi

I have an HPI Savage Octane, gas truck.
Between the air filter and the carb, HPI has put something they call "air intake reducer set", parts 112655 in the attached pic.

I don't think HPI would put that stuff in there for no good reason (but on the other hand they did put the air filter right behind the wheel for no good reason...)

But anyway,
a) Why put obstacles in the air intake path?
b) What would happen if I were to remove the a and b parts? Short term effects, long term effects?

I plan to replace this whole guacamole with a velocity stack and a Losi 5IVE filter directly on the carb, to get it out of the way of the wheel spray.

A carb has what's called a Venturi, the Venturi carries fuel to the path of the air going throug the carb. As air rushes past the Venturi fuel is drawn out and atomized. How much air, it's speed and direction as it goes past the Venturi determines how strong a signal there is and how the fuel is atomized.

HPI likely put these items there to manipulate the signal on the Venturi of an existing carb instead designing their own carb. Take those items out and you're going to be in no man's land figuring out the tune assuming you can get a tune on it at all. Maybe it'll work, maybe it won't.

Personally I'd try to find a different air filter or tubing to attach to that assembly to reroute the filter. The tune will be less effected that way while still achieving the results you desire. In reading the Octane thread it seems tuning has its difficulties with the stock setup, removing that assembly will likely only increase those difficulties.

Hi

Thanks for your response. And yepp, you are probably correct. I decided not to make any changes to the air filter right now, due to these odd things HPI put in there, they really don't give a quality impression, if I say so.

In any case, as I am still in break in phase with all the issues mentioned in other threads, I will not add more problems by changing these things.

Once the car is broken in and running fine as it is, I will have a go just to see how it works out.

Looking at the picture you posted, it looks like part 'b' is a spring steel flap, and acts as a reed valve of sorts. I will bet that it only reduces airflow at idle, but as airflow increases, it bends downward into the channel below to allow more air thru.

I agree. A reed valve of sorts because it covers about half the carb opening. At lower engine speeds it's definitely blocking more of the airflow. But while the total volume of air is reduced at lower engine speeds this flap also raises the velocity of the air going past since the engine is still trying to pull the same amount of air in regardless of the flap. This increased velocity increases the signal on the Venturi. As engine speed increases the flap moves allowing a greater volume of air in while maintaining a more constant velocity across the rpm range. A more constant velocity should make fuel metering easier with less of a transition from low to high speed needles. I'm not a carb scientist so I could be wrong about the last part though. Definitely sounds like something you'd want to keep though, at least with this carb.

Hi

Well, the b part is made of thin plastic, similar to body material I would say, but a bit thinner and feels a bit more brittle.

When fitted, it is pointing slightly downwards, as much as it can as it leans on the angled surface of the c part. In order for it to "close" there need to be some flow in the opposite direction.

I was a bit surprised about that because I thought it would be pressed up against the a part when it was fitted and pressed together, bit it was still pointing downwards.

I'll see if I can take it apart again and take a photo of what it looks like in static position.