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Turbulent Air FAB - need advice

patterson

patterson

Well Known Member
Patron
I wanted to raise this question with those of you who understand air flow and the air intake needs of a Lycoming motor. My plane is an RV-4 / late model kit, finished in 2006. I recently removed the Carburetor on my engine and replaced it with AFP fuel injection. The engine has chronic surging now and all fuel and electronic possibilities/sources have been exhausted.

After discussion(s) with Don at AFP, he asked me to send in my one-off air box for air flow testing on his equipment. The box is shaped like Van's. But, we decided to try using no filter in the box (expecting more RAM). We just use a flapper bypass to filtered air for ground operation to protect from debris.

Turns out the air box is the probable cause of the problem. Don said the airflow is so turbulent that it shook the room he was testing it in. I never imagined that direct airflow into the box wasn't going to give me extra RAM and more speed, but that thinking was obviously Wrong!.

So, aside from fitting in a thinner KN filter inside the box (smaller vertical space due to FI servo) what do you think would be the best way to address this? Narrow the intake size? Ramp the air 90 degrees directly into the servo? Any ideas greatly appreciated!
Ron
 
If you don't mind some cowl re work, have a look at Rod Bowers Ram air system. Worked great on my IO 360 with AFP fuel injection.
 
How much room from the bottom of the servo to cowl? You may look and see if you can find or make a cylindrical 90* elbow. Even oval it a bit to deal with limited height (make sure the overall area doesn't change at the ovalled section. You can then bring a tube (same or similar size as servo throat opening) up to meet with the air entry on the cowl and fabricate something that will interface like the FAB. That is a pretty common configuration for aftermarket turbo installation and should eliminate your turbulence issues. Lot's of folk sell aluminum tubing and silicone connectors for this purpose.

The gentler the arc the better on the elbow. If it is sharp, you may need to experiment a bit. Dan can probably provide guidance on how small of a radius you can get away with.

Larry
 
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I am not surprised.

I know of a few instances were people have tried to run their carborated engine with a FAB airbox but with the air filter removed..... doesn't perform very well.
 
Air flow straightener

I had all sorts of problems getting the TBIs to work well on my RV4, and have found an airflow straightener to make an enormous difference. I lifted mine from the front end of an Air Mass Meter from an automotive source. In my case a bosch unit from ebay. Cut the front end off and mount it on the FAB top plate. Worth a try, and cheap/easy to install. The one I have used comes from a Mercedes 200 (Merc' pt#: 000 0940948 Bosch pt#: 0 280217 114) which cost ?10 (cheaper than making one from alloy honeycomb) from ebay.
Have a look at these photos for more info:

https://www.flickr.com/photos/62722241@N00/sets/72157647983156255/

Neil
 
I'd try adding a radius ring to the FM150 inlet (green below). The AFP clamp-on adapter which joins an FM200 to a standard Vans airbox is similar. Fire up a lathe and machine an aluminum ring, tapped in four places for the FM-150's flange screws



Offhand, I'd don't think that airbox is going to generate a big MP increase, but that's another subject.
 
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Don may have an alternative answery

patterson said:
I wanted to raise this question with those of you who understand air flow and the air intake needs of a Lycoming motor. My plane is an RV-4 / late model kit, finished in 2006. I recently removed the Carburetor on my engine and replaced it with AFP fuel injection. The engine has chronic surging now and all fuel and electronic possibilities/sources have been exhausted.

After discussion(s) with Don at AFP, he asked me to send in my one-off air box for air flow testing on his equipment. The box is shaped like Van's. But, we decided to try using no filter in the box (expecting more RAM). We just use a flapper bypass to filtered air for ground operation to protect from debris.

Turns out the air box is the probable cause of the problem. Don said the airflow is so turbulent that it shook the room he was testing it in. I never imagined that direct airflow into the box wasn't going to give me extra RAM and more speed, but that thinking was obviously Wrong!.

So, aside from fitting in a thinner KN filter inside the box (smaller vertical space due to FI servo) what do you think would be the best way to address this? Narrow the intake size? Ramp the air 90 degrees directly into the servo? Any ideas greatly appreciated!
Ron
Click to expand...

You may want to measure the distance from the bottom of your current setup to the base of the engine and then call Don again.

Ask him to tell you about the work he and I just finished.

Might be a help.

James
 
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distance

James,
would you mind clarifying that measurement?
base of my airbox to servo inlet?
thanks
 
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