sniffle valve required?

[theduff]

Originally Posted by DanH View Post

So, want to worry about blowing your sump to bits? The risk is found when cranking at low temperatures, with very, very little liquid fuel on the plenum floor.

Always enjoy reading DanH?s posts as he always does his homework ( his math calcs remind me of some long study sessions in college Algebra and Calculus).
So back to the purpose of the sniffle valve. Dan makes the interesting point that they prevent hydraulic lock and not necessarily to prevent explosions in the Sump. I?m trying to come up with the scenario that caused my Sump (explosion). I?ve had backfires that have been severe enough to split the welds on a stainless exhaust but this incident was different. For a backfire to blow the O rings off the tubes are the same forces at play but just timed differently with the intake valves open ?

[DanH]

Originally Posted by theduff

I?m trying to come up with the scenario that caused my Sump (explosion).

Understandably. Lycomings do pop in the sump. Right now I don't know precisely why.

So anyway, tell us more about the setup....mags or waste spark EI, OAT and engine temperature, priming, cranking speed, etc. Obviously the throttle was closed.

Blowing out the intake tube o-rings is pretty impressive. Each is trapped between two steel rings on the tube, and when the tube is then shoved up into the plenum spigot, there is very little room for escape. Not something I've heard before, but that means little. Is it a common occurrence?

[theduff]

I run a 10:1 compression IO-360 parallel valve with a magnesium Sump from an IO-360-C1C. I machined the front pad to match an AFP FM 200 servo and it is equipped with a purge valve. The intake tubes were modified from the original C1C configuration to both fit a parallel valve engine and fit within the confines of an RV-4 cowl. I’m equipped with one Bendix mag (non-impulse) and a light speed electronic ignition with crank fired mini sensors. I run a fixed pitch wood propeller and a 4 into 1 exhaust. The sky-tec inline starter has always turned the engine over fairly briskly.
The circumstances of the explosion were a hot start after fueling with what I consider a normal start procedure with mixture full lean and throttle forward. I believe I did prime the engine for a few seconds with the boost pump and both levers full forward. Note: I rarely shut the engine down with the purge valve as the mixture control usually provides a positive shut off. I did not use the the purge valve to circulate cool fuel for this start as I prefer not to have the inaccuracies it causes in the fuel flow totalizer.
I hope I’ve remembered all the perimeters for this years old event. This was a one time event and after the installation of a purge valve was never repeated. This is the only explosive Sump occurrence that I’ve witnessed first hand the rest were repeated to me from “ The Front Porch “.
Revised: After thinking about it my start Procedure then was prime than mixture lever full forward and throttle lever at idle. Sorry it’s been a few years.

[DanH]

Originally Posted by theduff

The intake tubes were modified from the original C1C configuration to both fit a parallel valve engine and fit within the confines of an RV-4 cowl.

The outboard o-ring flange was removed so the tube could enter the sump a bit further?

Revised: After thinking about it my start Procedure then was prime than mixture lever full forward and throttle lever at idle. Sorry it?s been a few years.

I hear 'ya brother.

Perhaps prime followed by mixture at ICO and throttle full forward,which would be a standard flooded or salvage start? A hot start at mixture full forward and throttle closed would be unlikely...way too rich.

[DanH]

Originally Posted by Scott Hersha

Dan,
My start timing, running 2 Pmags was set at TDC, but with the newest software version, which I had, the start timing was automatically delay to 4* after TDC. If it really did that, I couldn?t see any way you could get a kickback.

I don't either....if it really does that.

BTW, from your post a few months ago:

After I installed two Pmags on my previous (RV8) airplane, we had two kick backs with a lightweight Whirlwind prop. The second one broke a few teeth off my starter ring gear. I reset the timing to an extra 2* after TDC. It started fine and I never had another kickback event.

http://www.vansairforce.com/communit...97&postcount=8

[theduff]

DanH
?The outboard o-ring flange was removed so the tube could enter the sump a bit further? ?
Negative. The inboard ends the tubes are as manufactured. We went so far as to fabricate some aluminum half circles to hold the intake tube ends perfectly in the center of the Sump sockets as we mocked up and tacked the tubes together. My intake tube inboard ends look exactly like the picture you posted.
Years ago my hot start procedure wasn?t optimal and may be related to the Sump explosion. The only difference between cold start and hot start procedure then was the length of the prime and yes would sometimes require some long cranking.....like 2 or 3 10 second rounds.