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View Full Version : Crank case pressure and clutch actuation



Flashfish
10-03-2007, 05:15 AM
I just installed an evo slave cylinder and when I finished I tested the lever and a breathing noise started coming from the crank case breather pipe that goes to the airbox. A substantial amount of air passes through the pipe when the clutch is activated, enough of a breeze to feel on my hand (motor off, throttle butterflies closed).

I am just wondering how much the throttle body vaccum assists in the removal of internal crank case pressure? Would re-routing the breather pipe external to the airbox increase pressure due to the removal of the vacuum that is found in the airbox.?

kiddo
10-06-2007, 11:11 PM
The noise you hear from the breather hose when activating the clutch with the engine off is caused by movement of a large diaphram on the clutch. This is normal.

The running engine has positive pressure in the crankcases and considerable air flow out of the breather hose and into the airbox. This too is normal.

The throttle bodies do not suck this air out of the engine. Rather, the engine itself pumps air out of the hose and into the airbox. This air also includes some gases which are burned off when ingested into the engine through the throttle bodies. This is better for our environment.

I too wondered about crankcase breather flow into the airbox and its possible affect on airbox pressurization.

It turns out that the crankcase air flow is too small to make a difference:

On my fully sealed and fully ducted intake, the pressure inside the airbox will increase from negative to atmosphere matching at 50 mph. (80 Kph.)
The maximum airbox pressure of 10" H2O (.024 Bar) is reached at 130 mph
(209 Kph) and did not rise further through 154 mph (247 Kph).

I then removed the crankcase breather hose from the airbox and let it vent into the open atmosphere (above the sidestand) and ran the same tests again. The pressure / speed results were the same.

I hope this information helps to satisfy your curiosity.

kiddo
10-06-2007, 11:25 PM
The noise you hear from the breather hose when activating the clutch with the engine off is caused by movement of a large diaphram on the clutch. This is normal.

The running engine has positive pressure in the crankcases and considerable air flow out of the breather hose and into the airbox. This too is normal.

The throttle bodies do not suck this air out of the engine. Rather, the engine itself pumps air out of the hose and into the airbox. This air also includes some gases which are burned off when ingested into the engine through the throttle bodies. This is better for our environment.

I too wondered about crankcase breather flow into the airbox and its possible affect on airbox pressurization.

It turns out that the crankcase air flow is too small to make a difference:

On my fully sealed and fully ducted intake, the pressure inside the airbox will increase from negative to atmosphere matching at 50 mph. (80 Kph.)
The maximum airbox pressure of 10" H2O (.024 Bar) is reached at 130 mph
(209 Kph) and did not rise further through 154 mph (247 Kph).

I then removed the crankcase breather hose from the airbox and let it vent into the open atmosphere (above the sidestand) and ran the same tests again. The pressure / speed results were the same.

I hope this information helps to satisfy your curiosity.

kzmille
10-06-2007, 11:47 PM
It turns out that the crankcase air flow is too small to make a difference:

On my fully sealed and fully ducted intake, the pressure inside the airbox will increase from negative to atmosphere matching at 50 mph. (80 Kph.)
The maximum airbox pressure of 10" H2O (.024 Bar) is reached at 130 mph
(209 Kph) and did not rise further through 154 mph (247 Kph).

I then removed the crankcase breather hose from the airbox and let it vent into the open atmosphere (above the sidestand) and ran the same tests again. The pressure / speed results were the same.

I hope this information helps to satisfy your curiosity.

Thats great data there kiddo. What type of instrument did you use for your tests.