Grodus

Hello, I have an old OS .61 SF with a pressure tap in the crankcase plate. Can I use that tap to provide pressure to the fuel tank or do i need to use a muffler tap? I have a tuned pipe and was thinking about using it but I would rather not tap the pipe if I dont have to.
Thanks.

Pete Bergstrom

You can use crankcase prssure to pressurize the tank with the following caveat. The pressure from the crankcase is much higher than it is from the muffler/tuned pipe and I think you might find you will have difficulty in making the engine idle well. Mostly the problem comes from the transition from full throttle to idle. At full throttle there is a ton of pressure in the tank and the engine will likely flood as you transition to the lower throttle position because you have go through that mid-range area which is a compromise between the idle needle setting and the hi-speed needle setting. You will likely be better off tapping into the tuned pipe system.

Pete

Grodus

Ok, thank you very much Pete. Thats what I was looking for.

Cody

ZAGNUT

My Feedback: (1)

there were some SF and VF engines that came with larger bore carbs and pumps that worked on case pressure....if yours is one of those you might have a fuel draw problem without the pump.

hellcat 1

Don't you have to have some sort of a check valve? Crankcase pressure is pressure on the downstroke, but suction on the up stroke.

coriolan

The pulse is stonger on the "down stroke" because the intake is closed, on the "Up stroke" the intake is open and much bigger than the pressure tap so the average is a constant positive pressure higher than with muffler pressure but a lot less than "Timed" crankshaft pressure which is a constant positive pumping action. Crankcase pressure work fine with constant speed engines (c/l,FF)but troublesome when using a carb.

Iflyglow

My Feedback: (79)

You can pressurize your tank with it, and use a check valve and on demand fuel pressure regulator like a Cline, or iron bay. Fuel tank location and going lean are a thing of the past. Os is even marketing these on helicopter engines now.

jessiej

Crankcase pressure used to be almost universal in CL combat. Of course that was a full throttle only proposition.

Jess

Hobbsy

My Feedback: (102)

When using the crankcase pressure to pressurize a tank the tank pressure is an average of the crankcase pumping pressure. The average is very different at full throttle compared to part throttle. Any needle setting would only be correct at a single rpm as Jess mentioned. When using a check valve the peaks of the pressure pulses are trapped making much higher and steadier pressure and a regulator is required. Here is an overdone. just for fun of course, Saito 1.30 with two check valves and two Cline regulators. The beauty of the regulator is that you can set engine close to peak with no worries that it will go lean, it makes for a very smooth running and economical engine. I don't see ever needing on board glow when using a regulator. At 71 I still like to play.