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Rotax Airbox

PostPosted: Mon Sep 26, 2016 7:33 pm
by kenryan
None of the 912 powered SuperSTOLs that I have seen use the Rotax airbox. It is my understanding that about 5 more horsepower can be gained by using the Rotax airbox. Does anyone know of SS or Highlanders using the Rotax airbox?

Re: Rotax Airbox

PostPosted: Mon Sep 26, 2016 9:19 pm
by FlyerChief
IMHO the extra 5 HP is an urban myth based on a misunderstanding created by Rotax themselves...

Personally, I don't believe that adding the air box can increase the horsepower of the engine. I know that the Rotax information says, "The tuned air box allows the 912ULS to reach its 100HP maximum power when paired with the tuned exhaust system and includes a selectable cold/hot air intake system for carburetor icing prevention.", but that's like the cereal companies saying their product has 3 grams of protein and 25% of your daily requirement of vitamin D when it's served with a 1/2 cup of milk on it.
It's not the air box that adds the power, it's the well designed exhaust system, but by saying it's both, they can sell more parts... necessary or not!

In theory, from a physics perspective, the exhaust pulses can be tuned to draw out the next cycle of exhaust thereby making the engine breath easier with vacuum pulses rather than back pressure. (This has its own myth attached, since a properly tuned exhaust for a Rotax 912 would have header pipes several feet long.) The intake on the other hand is more efficient when there is better airflow into the carbs. How can you possibly add more obstructions (air box) and cut the air filter size in half (1 filter instead of two) and get more power? Remember that the engine is trying to breath air in through the carbs and is only "powered" by the vacuum created by the piston moving in the cylinder, so any obstruction only makes it harder to breath.

Rotax only supplies the air box on the 914 because the turbo negates the loss of the extra obstructions that are placed in the way with the air box. If it really did make a difference it would likely be supplied on the 912, just as it is on the 914. Look at how K&N designs cold air injection systems... smooth pipes, slow curves and big filters. If the air box adds any power, it's only by getting the intake further from the hot exhaust... in which case you need the carb heat supplied by the air box because icing may be possible then. Maybe the easiest way to add a little more power would be to add a couple of NACA ducts on the cowl to direct more cool air into the intake filters, but only open them when carb ice is not an issue.

If you want to add carb heat using the air box or have the drip trays to keep overflow fuel off the hot exhaust then it's worth having. (I believe these are the only true reasons for having a Rotax air box on an engine that is not turbo charged.)

The above is only my opinion, so do your own research, but I decided the air box was an unnecessary cost when I built my Highlander. There are a lot better things to spend your $1000+ on... bush tires, Stratus 2 and an i-Pad with Foreflight, a bigger prop, etc.... just sayin'...
Highlander #241

Re: Rotax Airbox

PostPosted: Tue Sep 27, 2016 11:30 am
by kenryan
Thanks for your input Dan. Has anybody actually installed the airbox on a SuperSTOL or a Highlander?

Re: Rotax Airbox

PostPosted: Tue Sep 27, 2016 3:55 pm
by av8rps
Here's my two cents...

The Rotax airbox will add power as long as the fresh air intake allows plenty of cold air to the carbs. Cold air to the carbs works better than warm air...what happens in a certified aircraft when you apply carb heat? You lose power. Same holds true for picking up hot air inside an engine compartment verses bringing in cold outside air. Even though the carb filters are pretty much unrestricted, they are getting really warm air to them from not only engine heat but also exhaust heat. Of course, it is almost impossible to get carb icing with all that hot air through the carbs, while you are losing a little power by not having cold air into carbs, you shouldn't get carb ice. And on the contrary, if you get cold air to carbs by using an airbox, you are going to need carb heat hooked up to avoid your engine going silent.

One thing that always puzzled me is that Rotax never says anything about more hp out of the 81 hp 912 by adding an airbox? But common sense says it should pertain just like it does on the 912uls.

A Kitfox guy did a test on his 912ul. It came with a very restricted carb heat airbox that when removed in favor of just filters on the carbs he gained 300 rpm (which others have proved by doing the same - I am one of them). But when he added cold outside air to the carbs through use of two scoops on top the cowl forcing cold air right into the carbs (rather than the hot engine compartment air), he gained another 250-300 rpm. Problem is a 912 will ice easily without carb heat, so he went back to using only air filters off carbs and gave up on cold air.

I always wondered how it might work to use heaters for the carb body or intake flanges that were always on (electrical or coolant) and then applying outside air? But I'm too lazy to do all that so like most everybody else, I just use filters off the carbs and enjoy my airplane that way...