Crank Case Pressure/Vacuum Theory

[XP_Slinger]

This thread is an extension of the discussion that’s been derailing @tree monkey s 462 thread.

Executive Summary: How the F is fuel supplied to the engine at WOT when negative pressure (impulse signal) is almost non existent?

Steve @Stump Shot was kind enough to connect a gage to saw and post videos for us that proved and disproved some of the discussion content.

First video: static case compression. Note the negative pulse is just as strong as the positive.

More videos coming...just have to find them in the other thread.

 

[XP_Slinger]

Second video: saw running with guage connected to impulse showed constant negative pressure at idle and constant pressure at WOT. The big question here is whether or not the guage is capable of displaying pressure changes at the speed they are occurring accurately. I think it’s showing us something because the needle is twitching by about 2 psi at WOT. If the guage could react fast enough I think this twitch would travel into the negative pressure range.

 

[XP_Slinger]

So at this point in the discussion in the other thread, the accepted theory was that no impulse was needed to supply fuel to the engine because it was being pulled directly from the tank to the high speed circuit. Velocity in the Venturi acting as a siphon which no longer needed the metering diaphragm to supply its fuel.

This video put that theory to the test. If it’s a direct siphon then pinching off the impulse line should have no affect on the saw at WOT...right?

Big thanks to Steve @Stump Shot for taking the time to perform these tests.

 

[XP_Slinger]

I believe that there is still negative pressure present at WOT throttle as shown by the “twitch” on the guage in video number 2. And at WOT, the pulse is so fast that it overcomes the needle springs ability to close it and therefore holds the needle open to supply the high speed circuit with the fuel it needs. Kinda like floating a valve. If this weren’t the case, then pinching off the impulse would’ve had no effect on the saw when Steve tested the theory.

Please discuss...I don’t have the answer, I’m learning.

 

[RIDE-RED 350r]

Without both positive and negative pressure cycle, at any given RPM, I don't see how the engine could even run. Just because it happens extremely fast doesn't mean it ceases to exist.

 

[XP_Slinger]

Without both positive and negative pressure cycle, at any given RPM, I don't see how the engine could even run. Just because it happens extremely fast doesn't mean it ceases to exist.

That’s what I’m thinking Joe. I am thinking that it happens so fast that the needle is held perpetually open to meet the demand for fuel at WOT and all metering takes place at the high speed jet.

Case in point...if you set the metering lever too high it supplies more fuel than the engine needs at idle causing spit back and it won’t idle. But, if you can get past idle to WOT throttle it will typically clear up and tune properly at max rpm. Something to chew on

 

[RIDE-RED 350r]

Reed valves...

 

[RIDE-RED 350r]

That’s what I’m thinking Joe. I am thinking that it happens so fast that the needle is held perpetually open to meet the demand for fuel at WOT.

That's probably the case with the metering valve. But I contend that the pump side valves are still cyclically opening and closing...think reed valves

 

[Stump Shot]

I believe that the tests proves that the impulse is happening properly. Just have to read between the lines of the results, with one being stronger at a given point of the throttle plate. I think this also shows why spitback happens at WOT.

 

[XP_Slinger]

That's probably the case with the metering valve. But I contend that the pump side valves are still cyclically opening and closing...think reed valves

I highly doubt the metering diaphragm and lever spring can react that fast. Don’t ya just love theory? Lmao!

 

[RIDE-RED 350r]

I highly doubt the metering diaphragm and lever spring can react that fast. Don’t ya just love theory? Lmao!

I was saying that I think at high RPM the metering valve probably stays in a floating position but that pump side valves are still opening and closing.

Is the metering diaphragm subjected to the positive side of the impulse? If so, then that changes my thoughts

 

[Fruecrue]

Think sine wave.
Peaks may be higher and lower than the average amplitude (displayed on the gauge).
Gauge buffers the signal.

 

[XP_Slinger]

I believe that the tests proves that the impulse is happening properly. Just have to read between the lines of the results, with one being stronger at a given point of the throttle plate. I think this also shows why spitback happens at WOT.

Spit back at WOT caused by pressure being the dominant force at higher RPM?

Here’s another wrench...I bet the pressure signal could be changed by changing blowdown in one direction or another.

 

[RIDE-RED 350r]

Think sine wave.
Peaks may be higher and lower than the average amplitude (displayed on the gauge).
Gauge buffers the signal.

Persactly.. at the speed the pressure cycle operates, even at idle, I'd bet it would take some pretty sophisticated equipment to measure it accurately

 

[XP_Slinger]

I was saying that I think at high RPM the metering valve probably stays in a floating position but that pump side valves are still opening and closing.

Is the metering diaphragm subjected to the positive side of the impulse? If so, then that changes my thoughts

I don’t believe so, one side gets negative pressure the other gets ambient pressure

 

[Fruecrue]

I was saying that I think at high RPM the metering valve probably stays in a floating position but that pump side valves are still opening and closing.

Is the metering diaphragm subjected to the positive side of the impulse? If so, then that changes my thoughts

Metering diaphragm wouldn’t be subject to impulse would it?

 

[XP_Slinger]

Think sine wave.
Peaks may be higher and lower than the average amplitude (displayed on the gauge).
Gauge buffers the signal.

100% agreed

 

[RIDE-RED 350r]

I don’t believe so, one side gets negative pressure the other gets ambient pressure

Right, vented metering diaphragm cover... Still waking up

 

[XP_Slinger]

I believe the siphon theory is still relevant. With the needle held off its seat, the high speed jet in the Venturi could pull fuel direct from the tank.

 

[RIDE-RED 350r]

I'm not on board with that theory yet