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-   -   sniffle valve required? (https://vansairforce.net/community/showthread.php?t=178538)

bob woods 01-13-2020 02:00 PM

Sniffle valve
 
I bought mine from airflow performance. They call it a manifold drain. $ 25.00

jcarne 01-13-2020 02:29 PM

Quote:

Originally Posted by bob woods (Post 1399781)
I bought mine from airflow performance. They call it a manifold drain. $ 25.00

Oh I see, hmmm.

Can someone confirm whether it is just a simple brass fitting with a hole in it? If that is the case there is no way I'm going to give Lycoming 115 bucks for one. The one from Airflow Performance just looks like a simple AN flare to NPT fitting.

The Lycoming one found here looks like it has a check valve function to it too.

SmilingJack 01-13-2020 02:46 PM

I bought a piper sniffle valve that has a 90 right Angle drain.

I can?t find the p/n at the moment.

wilddog 01-13-2020 02:46 PM

The one that came with my Titan is a check valve.

DanH 01-13-2020 03:04 PM

Quote:

Originally Posted by jcarne (Post 1399785)
Can someone confirm whether it is just a simple brass fitting with a hole in it?

It is not. It's a check valve. It seals at low manifold pressure, but could just as well be an open fitting at WOT, given a good ram inlet.

flyinga 01-13-2020 03:19 PM

Dan,
To answer your question: yes. Constant flow with no purge valve.

jcarne 01-13-2020 07:46 PM

Thanks guys, you just saved me 100 bucks. After some research I see the one that Airflow Performance sells is indeed a check valve, will be ordering soon.

ronschreck 01-14-2020 05:25 AM

Quote:

Originally Posted by jcarne (Post 1399867)
Thanks guys, you just saved me 100 bucks. After some research I see the one that Airflow Performance sells is indeed a check valve, will be ordering soon.

Jeeeze! Where were you last week when I ordered a sniffle valve from Vans for $120. :mad:

I have been running without a sniffle valve ($120) or manifold drain ($25) for 14 years and over 2200 hours with no problems. I have an Airflow Performance fuel injection with a purge valve. I just installed a sniffle valve because it was noted as "missing" by the well-respected A&P who just did the pre-buy inspection for the person who is buying my RV-8.

I'm not making any judgement about the wisdom of having a sniffle valve, just relaying my experience. When I removed the sump plug to install the valve there was nothing draining from the hole. I am curious to know why the inlet to the sniffle valve is about 1/16 inch diameter and the outflow port is 1/4 inch in diameter! Seems like the slightest bit of crud would render it more useless than it already seems to be.

Sniffle Inlet

Sniffle Outlet

DanH 01-14-2020 08:23 AM

Quote:

Originally Posted by flyinga (Post 1399806)
Dan,
To answer your question: yes. Constant flow with no purge valve.

Then it is common to see a bit of fuel in the air plenum, and from the sniffle.

The engine driven pump creates line pressure by pushing a diaphragm with a big spring. The engine's pump cam (via the pump pushrod and pump lever) compress the spring, then rotates out of the way, leaving the spring free to push the diaphragm.

Now shut down the engine using the mixture control. If the engine stops with the pushrod on the pump cam's base circle, the pump spring is free to push, and 25~30 psi remains applied to the servo inlet. The only thing preventing flow is the mixture disc valve....which has a leak rate. The rate is small, but it's there; it is not a zero flow valve. The leak sends fuel to the divider, onward to the nozzles, into the manifolds. Some of it runs down to the sump plenum. For the most part, the leak rate is offset by evaporation rate, and the difference is minimal.

The disc valve is common to the Bendix RSA and its variations, like the current Avstar controller, and late model Airflow Performance FM-150. Those of us using a "classic" Airflow Performance controller like the FM-200 have a drum type mixture valve. It has a much higher leak rate (1 to 3 lbs per hour) and would rapidly dump fuel into the manifolds in the above scenario. So, AFP units with drum valves also get a purge valve. When opened, the purge valve re-routes fuel from the control, before it reaches the divider, and sends it back to a tank. In consequence, purge valve users see less fuel in the manifolds after shutdown. It's limited to the very small amount which may boil out of the divider and spider lines. That happens with both mixture valve types, purge or no purge.

Anyway, the much reduced fuel dump is why purge valve users see less need for a sniffle.

BTW, a purge valve can be installed with any fuel control. The reason for its existence is per the above, but the serendipitous feature most loved is the ability to flow a large quantity of cool fuel through the entire system prior to cranking.

jcarne 01-14-2020 09:55 AM

Quote:

Originally Posted by ronschreck (Post 1399926)
Jeeeze! Where were you last week when I ordered a sniffle valve from Vans for $120. :mad:

I have been running without a sniffle valve ($120) or manifold drain ($25) for 14 years and over 2200 hours with no problems. I have an Airflow Performance fuel injection with a purge valve. I just installed a sniffle valve because it was noted as "missing" by the well-respected A&P who just did the pre-buy inspection for the person who is buying my RV-8.

I'm not making any judgement about the wisdom of having a sniffle valve, just relaying my experience. When I removed the sump plug to install the valve there was nothing draining from the hole. I am curious to know why the inlet to the sniffle valve is about 1/16 inch diameter and the outflow port is 1/4 inch in diameter! Seems like the slightest bit of crud would render it more useless than it already seems to be.

I hear ya, I have delayed buying that bad boy for a bit now as it just seemed way overpriced to me; like certified market overpriced. :D


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