Carb Air Intake Opening - How big is big enough ?

[BillSchlatterer]

Just doing finish work on my cowl with the snoot carb air intake on a 7a. Some of the speed guys are extending that snoot toward the prop and closing it in for extra ram effect and less drag?

Sure enough, now as I am spending some time on it, it does look like it is way bigger than it needs to be but ,.... just how big is that?

Has anyone done any research on just how much opening it takes to feed an O-360 ? Is there some math behind it like the horsepower fomulas? I have seen some very small openings.

Currently, it is 2.25" tall by 3.5" wide,.. prox. Frontal area of 7.8 sq inches. I'm thinking that taking it to a rounder taper with about a 2.25 x 2.75 for a frontal area of 6.2 sq inches. That would be about a 20% reduction in intake area but I would not want to get it so small that it wouldn't flow enough air at full power.

A slightly oval shape tends to match the carb air box shape so I thought that staying with that general shape would be best. Closer to the prop seems like that has to help some.... or at least couldn't hurt any.

On the other hand, this all may be a terrible thought so please let me know what you think? Any links appreciated!

Thanks Bill S
7a finishing finishing ,..........

 

[dpansier]

SJ Cowl intake dia.

As a data point the Sam James Cowl has an aluminum intake ring with a 3" dia. opening. The ring ID tapers back at 5 degrees on a side for 1.75".
The initial area is 7.06 sq."

 

[Kevin Horton]

Kent Pacer tried a number of different configurations on his Mustang II, and eventually settled on a long inlet that was just 5/8" behind the prop. The area of the inlet was very small - only 10% larger than the area in the carburetor venturi. He gained about 3 mph speed, but I think this number also includes some air box mods he did.

 

[Tom Martin]

I had the opportunity to spend a few hours with Paul Lipps. He is a very intersting individual and his philosophy is that there is a simple mathematical model for everything we do. I was wondering about the induction air inlet size required for my rocket and the math he used would suggest that at 2700 rpm and 220mph, with your engine, you would need 2.1 square inches. Now in a full power take off, for example, when the forward speed is 90mph you would need 5.0 square inches of opening. Add a bit for internal losses and there you go! You were pretty close with your numbers.

 

[BillSchlatterer]

Hmmmm

When you think about it a little bit, there has to be a maximum limit to how much air a O360 can take through the carb under fixed ram pressure/air speed. That number would be further restricted by the air filter and turning the airflow 90 degrees before it hits the carb throat.

So I suppose there is a forumula that accounts for size of the opening and the relative pressure of the air being pushed/pulled into it less the effects of the air filter and directional changes.

Is there a good way to test if you are getting sufficient flow at full power and low airspeed? Seems like the engine would not turn full RPM if it didn't breathe well?

Comments appreciated.

Bill S

 

[David-aviator]

Just doing finish work on my cowl with the snoot carb air intake on a 7a. Some of the speed guys are extending that snoot toward the prop and closing it in for extra ram effect and less drag?

Sure enough, now as I am spending some time on it, it does look like it is way bigger than it needs to be but ,.... just how big is that?

Has anyone done any research on just how much opening it takes to feed an O-360 ? Is there some math behind it like the horsepower fomulas? I have seen some very small openings.

Currently, it is 2.25" tall by 3.5" wide,.. prox. Frontal area of 7.8 sq inches. I'm thinking that taking it to a rounder taper with about a 2.25 x 2.75 for a frontal area of 6.2 sq inches. That would be about a 20% reduction in intake area but I would not want to get it so small that it wouldn't flow enough air at full power.

A slightly oval shape tends to match the carb air box shape so I thought that staying with that general shape would be best. Closer to the prop seems like that has to help some.... or at least couldn't hurt any.

On the other hand, this all may be a terrible thought so please let me know what you think? Any links appreciated!

Thanks Bill S
7a finishing finishing ,..........

I had a chat with Don at AFP this morning regarding intake considerations. I just spent a week building an intake plenum similar to Van's FAB unit and decided not to use it. There simply are too many air flow restrictions starting with the filter and then numerous curves and turns before the flow arrives at the fuel controller. Same thing is going on with your carb. It is a nice neat convenient internal set up, but it for sure is not the best for air flow.

I don't believe the intake area is all that critical as far as drag is concerned. What would seem to be more important are flow considerations. After talking with Don, I decided to go with the option of using filtered air or going with direct unfiltered ram air straight into the controller. This is accomplished with a flow divider with 2 throttle plates, one at the ram inlet and one at the filter arm. When one is open, the other is closed. This is to prevent ram air from going back through the filter into the engine compartment.

I think the same could be accomplished with a carburetor. The end result being a slight boost in manifold pressure when by-passing the filter. It would seem, unless the inlet has a variable open/close feature, the inlet has to be optimum for take off and climb and the rest of the flight is a compromise. Power is needed for take off and climb and it can not be compromised. Probably the size of the opening was determined by Van's with that in mind. Reducing the opening will not result in much drag reduction, but it could result in less than desirable power for take off.

The AFP inlet is 3.5" (OD). I've decided to go with a 4" Sam James ring for the inlet which has a 3.75" (ID) inlet. It should provide somewhat of a boost in manifold pressure when the filter is by-passed. The trick will be to get the inlet pipe as close as possible to the ring, but not too close, as there will be no mechanical connection between them. We do not need to introduce ram pressure into the lower engine compartment, just into the fuel controller or carb inlet. Don told me about a company in California, Rod Bower Aviation, that builds a beautiful 5" isolator at the intake but it is not cheap.

My cowl is the horizontal FI unit with no inlet. It will have an inlet now that I've decided not to use the snorkle, so the challenge is building an inlet that is not too unfriendly with external air flow. It will be centered around the James 4" ring.