Ram air to Bing carbs

Hi Murray,

At a guess, I would say that my carbs are receiving some RAM (pressure above ambient) air. How much? - no idea.

Why do I think there may be RAM - my fresh air inlet is just inside the front cowl opening, to the outside of the prop spinner. I assume that this is a high air pressure location.

The air inlet is an aluminium horn/tube connected to a, home designed/built, air box - supplying both carbs.

The air does go through a K&N filter, which likly reduces RAM to some degree.

The setup works just fine however if you want a comparative comment on how well it works (better/worse than some other design) - no idea . Sorry😈

Thanks Sean,

I think you probably are receiving a small increase in RAM air pressure; I don't see any problems with this as long as the ambient pressure lines are plumbed into the same pressure source.

Not sure if doing this might cause a need for carb heat is about the only other consideration I can think of.

It's a lot of trouble for little to no gain. It was a factory standard feature on Mooney's thru the J model, including my M20E and C. In spite of being on the nose, bypassing the filter entirely and cruising speeds of 150 knots, it only gave about 1 in-Hg of "boost" or less. With a fancy Lopresti cowl, the gain could be a bit more. Note that dynamic pressure goes up as the square of airspeed, so a typical LSA will be working with 2-3 times less pressure to start with.

Some gain may be experienced via having colder induction air. But most 912 installations have no carb heat and depend on warm engine compartment air for some measure of protection against icing. Using CAI without also installing the Rotax airbox and carb heat would be unwise.

Hi Jeffry,

 "Some gain may be experienced via having colder induction air. But most 912 installations have no carb heat and depend on warm engine compartment air for some measure of protection against icing. Using CAI without also installing the Rotax airbox and carb heat would be unwise".

My last  912ULS, ATEC Zephyr, aircraft had a consistent under cowl temperature of about 10C above ambient and the basic individual conical air filters - Never knowingly experienced carb ice or any change in performance between winter & summer operation.

My new mount, a 912 ULS powered Sonex Legacy, also has about 10C under cowl temperatures. The carburettor each have a Cozycarb heater installed on the manifold side. For a short period I ran individual conical air filters - there was a noticeable difference between winter & summer engine performance. I have reinstalled (with modifications) the home built air box & fresh/cool air induction - could be my imagination, there seems to be  less difference between winter & summer performance. So far no indication of ice 😈

Jeffry Stetson wrote:

It's a lot of trouble for little to no gain. It was a factory standard feature on Mooney's thru the J model, including my M20E and C. In spite of being on the nose, bypassing the filter entirely and cruising speeds of 150 knots, it only gave about 1 in-Hg of "boost" or less. With a fancy Lopresti cowl, the gain could be a bit more. Note that dynamic pressure goes up as the square of airspeed, so a typical LSA will be working with 2-3 times less pressure to start with.

Some gain may be experienced via having colder induction air. But most 912 installations have no carb heat and depend on warm engine compartment air for some measure of protection against icing. Using CAI without also installing the Rotax airbox and carb heat would be unwise.

Thanks Jeffry and Sean

I was also thinking around 1" increase in manifold pressure as my old Mooney F model got. My Sonex is getting close to similar speeds and given how sensitive both the 912ULS and the short winged Sonex is to density altitude, I am sure that extra 1 inch MAP and cooler air intake will make a noticeable difference.

I agree, I will likely need to incorporate some form of carb heat, perhaps as simple as a flap that opens back to the warm cowl air? Lots to consider.