sniffle valve required?

[DanH]

Annual time, so as promised, I stuck a borescope down the fuel servo and intake throat to look in the sump plenum. Ran it for a hot leakdown, then took the photo 30~60 minutes after shutdown.

For those new to Lycomings, the horizontal intake sump has an upper chamber full of oil, and a lower chamber for intake air. Unlike the vertical draft sumps, the air passages do not pass through the hot oil. Instead, the intake tubes enter the air plenum from the sides.

Part #10 is the sniffle valve.



Intake tube #3 on the left, #4 on the right. Here you can see the threaded sniffle drain passage in the aft quadrant of a small dished depression centered in the plenum floor. That dish would be the lowest point with the aircraft level, like an A-model RV. In addition to the little bit of fuel in the dish, a tailwheel RV will collect fluid at the rear of the plenum floor. Here it's the fuzzy dark streak in the background, behind the tubes. I ran a stick with a swab back there, and got just enough to discolor the swab.



I remain comfortable running this particular setup without a sniffle. However, I shut down with a purge valve, and the airbox has a sump and a drain to divert rainwater when parked, which is not the case with a standard Vans snorkel. And, as Don points out above, not everyone is adept with engine controls.

[vic syracuse]

As much as I respect you, Dan, I wouldn't really consider one shutdown as a reliable indicator. Perhaps after 100 shutdowns?

Vic

[Scott Hersha]

Your need for a sniffle valve may have something to do with your setup. I had a sniffle valve configured on my IO-360-M1B (Superior) with EI and a light weight Whirlwind prop and had two kickbacks, the second one that damaged the starter ring gear and required repair. My sniffle valve was plumbed at the end of about 18? of hard line coming from the forward tap in the forward most location on the intake manifold and sitting back near the exhaust area of the cowl on my RV8. When I discussed the kickback situation with my engine guru, Jimmy (JB Aircraft Engines), he recommended that I install the sniffle valve on the case, where it belongs, and that I install it in the aft most tap on the manifold, especially since it?s a taildragger. It seems that a slug of fuel still residing in the aft portion of the tilted (taildragger) manifold, including the 18? of hard line upstream of the original valve placement, might be enough residual fuel to trigger a POP in the opposite cylinder in a lost spark EI setup. Well, I changed things according to Jimmy?s recommendations and I never experienced another kickback. I don?t know what, if anything, these changes had to do with the results, but I was happy that something worked. Never experienced another problem.

If you have a heavy prop and two mags, you probably don?t need to concern yourself with the kickback scenario as related to the sniffle valve. You have enough flywheel effect from the heavy prop, and a weak enough starting spark with mags, that you shouldn?t have a problem with kickback unless your timing is off.

[rswalden]

Hi, Torque....
I installed mine when the engine was new and haven't had any issues. Easy, cheap installation that takes less time than reading through this thread. Just do it!
Cheers,
Waldo

[DanH]

Originally Posted by Scott Hersha

...It seems that a slug of fuel still residing in the aft portion of the tilted (taildragger) manifold, including the 18? of hard line upstream of the original valve placement, might be enough residual fuel to trigger a POP in the opposite cylinder in a lost spark EI setup.

Scott, consider the waste spark, arriving at the top of the exhaust stroke, or a little before. Both valves are open, thus it's not possible to generate significant cylinder pressure. Even if a combustible mixture is ignited, absent pressure there can be no kickback. Kickback has a single cause, ignition inside the closed cylinder too far before TDC.

The risk to having a lot of fuel or water or chicken soup in the plenum is hydraulic lock...sucking a big slug of incompressible liquid up the pipe and into the combustion chamber. Which is why, way back in post #16, I said I would be henceforth recommending a sniffle for most installations.

Originally Posted by vic syracuse

As much as I respect you, Dan, I wouldn't really consider one shutdown as a reliable indicator. Perhaps after 100 shutdowns?

I respect you too, so tell me, what do you think might happen during one of those hundred shutdowns with a purge valve?

[Z-EDD]

On a related note, if a sniffle valve is fitted, is it still necessary to drill a hole in the bottom of the FAB? I never liked this idea, carefully ensure that the filter seals nicely at the top, but then drill a hole that will allow unfiltered air in at the bottom. Of course I realise why it is there, but doesn't the sniffle do much the same job?

[74-07]

AFP sniffle valve = <$25
Other material required = <$10
Time required = < 2 hours

My "new to me" RV8 now has one. Thanks everyone for the input.

[William Slaughter]

I'd still like to see pictures of a viable method of venting the sniffle valve overboard. I have the same exhaust system as Dan, and every method I've considered as been proven to fail on his aircraft. I've you've got a solution, I'd love to hear about it. Thanks.

[theduff]

[quote=DanH;1403378]Scott, consider the waste spark, arriving at the top of the exhaust stroke, or a little before. Both valves are open, thus it's not possible to generate significant cylinder pressure. Even if a combustible mixture is ignited, absent pressure there can be no kickback. Kickback has a single cause, ignition inside the closed cylinder too far before TDC?

Disagreeing with DanH is sure way to getting in trouble but I must disagree as well. The combustible mixture to be worried about is the residual fuel laying in your Sump. I?ve heard reputable stories of catastrophic events like blowing sumps apart while starting an engine with ponded fuel present. I had my own experience before installing a sniffle valve where the fuel/fumes ignited and caused a very loud boom/explosion and upon inspection I found the pressure from the explosion had pushed 2 of the orange silicone o rings off of their respective intake tubes and their shredded remains were laying in the bottom of the cowl. I think it?s foolish not to have some sort of intake drain on horizontal induction engines. I don?t think the factory would be using them if they didn?t think they were necessary. Just because you haven?t had an explosive event it doesn?t mean you can?t have one. The source of the ponded fuel is another discussion but I?ll just mention some injection systems leak/bypass fuel long after their shut down.
Duff

[Mark Dickens]

Originally Posted by William Slaughter

I'd still like to see pictures of a viable method of venting the sniffle valve overboard. I have the same exhaust system as Dan, and every method I've considered as been proven to fail on his aircraft. I've you've got a solution, I'd love to hear about it. Thanks.

William, I have an elbow attached to my intake manifold port, which is then connected to a short piece of fuel line plumbed to a Lycoming LW-75444 sniffle valve which lets the fuel vent out at the aft end of the lower cowl. I just have a drip pan I put under the cowl after the flight.