Ram air to Bing carbs

You can calculate the probable results.  Dynamic pressure is calculated as 1/2 x p x v^2 = dynamic pressure in Pascals, where p=air density in kg/cubic meter and v=velocity expressed in meters/second.

We can estimate air density at 1.2 kg/m^3 and then plug in what ever velocity you want.

Example 1

120 knots =61.733 meters per second so let’s round that up to 62 m/s.

so… 1/2 x 1.2 x 62^2 =2,306.4 (Pa)

And 2306.04 Pa = 0.681 inHg 

Example 2

150 knots =77 meters per second 

so… 1/2 x 1.2 x 77^2 =3,557.4 

And 3557.4 Pa =1.0505 inHg 

Of course this does not account for the efficiency and placement of the ram air intake. As was pointed out earlier, pressure falls as the square of the speed, so slowing down a little affects pressure a lot. 

Murray Parr wrote:

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.

In terms of max power, CAI for sure, Ram, sometimes. When does one run wide-open-throttle (WOT)? Certainly for the first few minutes of climb, but that's a 60 kts where Ram is 1/4th of 120 kt or ~1/4". Then, perhaps also at 7500-8500' when 65 - 75% power requires WOT. But without a fully-controllable prop most setups will hit an rpm limit and require retarding the throttle at speed anyway. Not only did the Mooney's have a controllable prop, but Lycoming had all the information available to set, say 75%, power using difference combinations of RPM & manifold pressure. On gas engines, the pumping loss caused by a partially close throttle is considerable, so the goal for efficiency is to have the highest allowable MP (most open throttle) with the lowest RPM (reducing friction losses) which gave the desired power setting. Any time the ram boost requires closing the throttle to remain within limits, the gain from it is exactly zero. Rotax doesn't provide the information to play these games wisely, at least not that I've been able to find. Here's my cheat sheet for IO-360 on my old planes as an example: