crank case pressure for PA?

There was a thread a while back from someone experimenting using crankcase pressure to pressurize the fuel tank in a PA model. I've lost track of that thread and haven't heard anything recently about those experiments. What's the latest and how's it going? Any conclusions yet?

I put several squeaky lean runs on a Fox stunt 35 trying to get it to run on crankcase pressure before I gave up. I routinely run TD 049's and 09's on crankcase pressure with good success. I have had less luck running Cox Special and Medallion 15's on case pressure. I run muffler pressure whenever I have a muffler.

On the OS .15PF, the only engine I measured, crankcase pressure decreases as RPM increases, which is the wrong gradient for a stable run on the rich side of peak RPM.

Sounds like you just need an inverter

I believe the thread you are referring to was mine regarding the Noblarf and OS 25fp.

The combo never did work right, although I suspect the problem is actually one of fuel foaming.

I am, however running a diesel converted Norvel .06 on backplate pressure in a Baby Pathfinder. It runs great on the ground, but I haven't had a chance to check it in the air yet.

Larry,

I found a one way valve helps prevent an untimed case-tap problem -- backflow on filling and starting.(I have a small handful of KK check valves that aren't yet castor-gummed.) Backflow floods the engine too easily... Note that a checkvalve increases the pressure to the tank to somewhere between untimed and timed tap pressure.

Untimed case pressure sounds good for the big-bathtub inlets on earlier AMEs.

Item: On C** engines, the stock TD needle is a bit coarse for me to hit the setting on untimed case pressure. The KK fine-thread fuel blocks worked fine until they loosened and started drifting. (It WOULD be worse with timed case pressure off the stump below the venturii.)

Item: The NVAs on the older AMEs were too coarse to find or hold a setting on timed or untimed case pressure, or bladders, in tests I ran.

I've also noticed some seepage between pieces of the NorVel build-a-carb NVAs on just muffler pressure.

BTW, I don't recommend muffler pressure for diesels, unless your exhaust oil is always crystal clear. The discoloration (i.e., tarry goop) diesels often spew almost has to be a sooty carbon; diamond is also carbon and cuts most things. I don't know how hard that soot is -- graphite is carbon, and a lube, too -- but don't like the idea of recycling granular carbon thrugh a close fitted engine. I did some tests that showed it DOES discolor and build up in a diesel(ized) engine. Found a cure, but too bulky to use on a small model.

I hope everyone in this thread recognizes the difference between timed and untimed case pressure?

Untimed: tapped into the case and exposed to both intake (up) and transfer (down) stroke pressure changes.

Timed: Only exposed to the transfer stroke -- e.g., the stub nipple on TD housings only opens to case pressure when the shaft valve opening passes it on the downstroke.

A couple of comments. I use a oneway valve with the Cox engines but have not used it with larger engines on crankcase pressure. I tried one when running muffler pressure on a K&B 4011 and didn't like it, for reasons unremembered. The stock TD 09 needle works fine with crankcase pressure.

I have run muffler pressure with no problem on dieselized Tower 40. A properly adjusted diesel does not have a dark exhaust. If it is more than honey-colored, you are overcompressed. (A caviat, I have been told that diesel fuel made up with starter fluid for the ether will run black regardless because of other ingredients in the starter fluid.) Incidentally, your muffler tap should be positioned such that you don't run a bunch of oil into your fuel tank. It is bad enough that you are filling your tank with water vapor under pressure and therefore rapidly degrading your fuel as you fly along.

Just for the record, when I was running the OS 25 fp on backplate pressure I was using a 1-way valve in the pressure line.

As I understand it, diamond requires incredible pressures to form. I think it safe enough for your engine to re-eat a bit of exhaust.

Either that or we should be flying with baggies over our exhausts and selling the goo to a jeweler.