Speed,Combat,Scale,Racing > Rat Racing and Team Racing
Crankcase pressure
Ron Duly:
Any idea how much timed and crankcase pressure PSI is? Was wondering how a "pump" carb like a Perry would handle the timed/case pressure vs. the Perry pump? Thanks!
BillLee:
From an article in Model Aviation in 1978:
I measured tank pressure. 1.4psi, or 1.6psi when using a check valve in the pressure line.
Article can be seen at https://www.nclra.org/TechTopics/UniflowPhysics.pdf towards the end of the second page.
Dennis Toth:
Bill,
Many years back we use to use crankcase pressure straight to the tank with no check-valve (we didn't have them) and just had the pressure line positioned to the top inside corner of the tank. Not sure how much pressure we got from that but it allowed running much bigger venturi diameters. When I have tried check-valves it seemed to keep increasing pressure and never stabilize. How did you set the check- valve to hold consistent pressure? How would it work with a uniflow tank set-up to the pressure line?
Best, DennisT
Paul Smith:
I my opinion, a check valve is not a pressure regulator, it's just a backflow preventer.
It keeps the fuel from the tank from flooding the engine. This was also accomplished by the "pressure" dome on some ancient hard tanks. The dome was a tiny bowl-shaped chamber soldered to the top of the tank with a tiny hole accessing the fuel tank. This created an air gap to keep tank fuel from getting into the engine.
In combat, we simply hand-pinched the pressure line and started the engine on a prime.
I can see why a check valve would be handy on a racer with a fuel shutoff, seeing that the trapped pressure would pump fuel into the engine via the pressure line.
BillLee:
--- Quote from: Paul Smith on December 04, 2023, 10:39:45 AM ---I my opinion, a check valve is not a pressure regulator, it's just a backflow preventer.
It keeps the fuel from the tank from flooding the engine. This was also accomplished by the "pressure" dome on some ancient hard tanks. The dome was a tiny bowl-shaped chamber soldered to the top of the tank with a tiny hole accessing the fuel tank. This created an air gap to keep tank fuel from getting into the engine.
In combat, we simply hand-pinched the pressure line and started the engine on a prime.
I can see why a check valve would be handy on a racer with a fuel shutoff, seeing that the trapped pressure would pump fuel into the engine via the pressure line.
--- End quote ---
The check valve, in addition to preventing the back-flow of fuel during refueling or after the shut-off is activated, also prevents the back-flow of pressure when running. That's why in my tests from many years ago, the pressure in the tank was measured slightly higher with a check valve installed vs not. Without a check valve you had to run both the fuel line AND the pressure line through the shutoff to keep the residual pressure in the tank from back-flowing into the engine. (That caused all sorts of weird running conditions when trying to shut the engine down.)
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