RV-4 lower cowl inlet -filter?

Is it detrimental to NOT have a filter installed on the bottom cowl carb inlet?
Others have told me only a screen would suffice to keep debris injestion if flying from strips other than paved runways. Others have said a filter is required. Others have said nothing is needed & that unrestricted airflow into the carb is most important.
Thanks!

The filter helps to keep the engine from sucking in dust, sand, and other abrasives. For that reason alone, I can't image flying without it.

Some years ago, Van published an article in which some testing his company did showed the pressure drop across the airbox was actually lower with an air filter than without. As I recall, it was hypothesized that the filter changed flow characteristics such that the performance was better with the filter than without.

Regards,

Our RV-4 flew for 20 years and to engine TBO without a filter. We always had higher silicon on the oil analysis. We did put a filter on the new engine

Clean Air...

Little Wing RV-4 said:

Is it detrimental to NOT have a filter installed on the bottom cowl carb inlet?
Others have told me only a screen would suffice to keep debris injestion if flying from strips other than paved runways. Others have said a filter is required. Others have said nothing is needed & that unrestricted airflow into the carb is most important.
Thanks!

Click to expand...

JDC
I wouldn't put screen door screen over it as it's very restrictive airflow-wise. A better solution is a Ram-Air Box. The original RV4 plans prior to the invention of the FAB in the late 90's showed plans to "roll your own" airbox. It was simple, light and cheap, like RV's used to be.
The early vinylester cowls had a smaller scoop that only a small filter box would fit. I built mine out of .040 with a flapper door in the front and a 2" SCAT on top. My current RVX has a nearly identical version although it is nice looking and TIG welded. I install a K&N motorcycle filter on top so I can could pull the flapper closed and have filtered air when I wanted, RAM air otherwise.

My original is still working great to this day, 25 years later.

V/R
Smokey


Rev Flow version from Revmaster similar to what I made. Just fabricate a 90 degree 2" Scat with Carb flange welded on and slip this on the end. Connect a cable and "Bob's your uncle".

Thanks! Any pix?
I'm thinking of the Vans filter box and new scoop for the older -4's, but if your's is easier, I might want to try that before commiting the $$ of the box and new scoop.

smokyray said:

JDC
I wouldn't put screen door screen over it as it's very restrictive airflow-wise. A better solution is a Ram-Air Box. The original RV4 plans prior to the invention of the FAB in the late 90's showed plans to "roll your own" airbox. It was simple, light and cheap, like RV's used to be.
The early vinylester cowls had a smaller scoop that only a small filter box would fit. I built mine out of .040 with a flapper door in the front and a 2" SCAT on top. My current RVX has a nearly identical version although it is nice looking and TIG welded. I install a K&N motorcycle filter on top so I can could pull the flapper closed and have filtered air when I wanted, RAM air otherwise.

My original is still working great to this day, 25 years later.

V/R
Smokey

Rev Flow version from Revmaster similar to what I made. Just fabricate a 90 degree 2" Scat with Carb flange welded on and slip this on the end. Connect a cable and "Bob's your uncle".

Click to expand...

Where does your carb heat air go in?

What's the scoop?

sblack said:

Where does your carb heat air go in?

Click to expand...

Scott,
By definition Lycomings have continuous carb heat in drawing their intake air through a sump filled with hot oil. Additionally, the hot air in the cowling surrounding the filter works essentially the same when you pull the lever closed.
When I performed intake air temp testing on my RV4, pulling the lever to draw under cowl air raised the intake air temp 10-40 degrees.

In 1500 hours of flying my RV4, 1000 in my Rocket and another 575 in my RVX, (all with the same setup), I have never had symptoms of Carb Ice. Pulling the lever closed inflight reduces MP 1-2" at 2400 RPM. Pulling it closed after landing on the ground and just before Takeoff thrust is applied is an easy habit pattern to develop.
Using my Ram Air is my own desire to preserve the clean lines of the lower cowl, increase efficiency, save money and improve intake air efficiency. Also avoid glass work at all costs!

V/R
Smokey

smokyray said:

Scott,
By definition Lycomings have continuous carb heat in drawing their intake air through a sump filled with hot oil. Additionally, the hot air in the cowling surrounding the filter works essentially the same when you pull the lever closed.
When I performed intake air temp testing on my RV4, pulling the lever to draw under cowl air raised the intake air temp 10-40 degrees.

In 1500 hours of flying my RV4, 1000 in my Rocket and another 575 in my RVX, (all with the same setup), I have never had symptoms of Carb Ice. Pulling the lever closed inflight reduces MP 1-2" at 2400 RPM. Pulling it closed after landing on the ground and just before Takeoff thrust is applied is an easy habit pattern to develop.
Using my Ram Air is my own desire to preserve the clean lines of the lower cowl, increase efficiency, save money and improve intake air efficiency. Also avoid glass work at all costs!

V/R
Smokey

Click to expand...

Unfortunately it is a regulatory requirement in Canada if the engine is not injected.