The M30 puzzle of the day - question

[Jeff N.]

Martin was supposed to post this but I guess he's got CRS syndrom... :)

Soooo....on the M30...there's a 3mm (?) vacuum line that runs from the base of the intake manifold and T's into the valve cover breather hose.

What the heck is >that< for?

Jeff

[Hector]

we can enjoy the buzz with this other question. Hope you don't mind the piggy-back ride or intrusion, Jeff. For the m30, what is the purpose of the hose (similar in size to the fuel line) coming from the bottom of the intake manifold, connecting to a dangling white L-shaped or elbow connector with another piece of hose hooking this elbow to the master cylinder, I think? What is the purpose of this elbow anyhow?

[Jeff N.]

That's a vacuum check valve. Sits 'tween the intake manifold and booster. Keeps air from leaking back into the engine when you go from a low vacuum to high vacuum situation within the brake booster.

BTW, insidious place for a vacuum leak. Make sure the takeoff is capped and there are no cracks in it (like mine had a long time ago).

Jeff




we can enjoy the buzz with this other question. Hope you don't mind the piggy-back ride or intrusion, Jeff. For the m30, what is the purpose of the hose (similar in size to the fuel line) coming from the bottom of the intake manifold, connecting to a dangling white L-shaped or elbow connector with another piece of hose hooking this elbow to the master cylinder, I think? What is the purpose of this elbow anyhow?

[Bill R.]

closed, say your coasting down from high rpm, then the crankcase ventilation system doesn't suck anything if it depends on vacum in front of the throttle plate.... but by tapping into manifold vacum with a small tap is supplies a little suction to the crankcase whether you're accelerating or decelerating.... It always keeps some suction in the crankcase.... So, is there a prize?







Martin was supposed to post this but I guess he's got CRS syndrom... :)

Soooo....on the M30...there's a 3mm (?) vacuum line that runs from the base of the intake manifold and T's into the valve cover breather hose.

What the heck is >that< for?

Jeff

[Paul in NZ]

your prize is our undying gratitude,adulation,and awe of the depth and breadth of your knowledge and your willingness to share with us :D

[Jeff N.]

The Baconator swears his car runs better with the openings plugged. He's mumbling something about vacuum leaks and all that. Must be the large amount of blowby he's got...messing with his mixture and all.. :D

Sooo...short of sucking out noxious gasses out of the crankcase and pumping them back into the engine, is there any other point to keeping a slight vacuum in the crank case?

What would happen if you plugged up that vacuum line in question AND the valve cover cover inlet into the intake? Run the valve cover vent to a catch can or similar... You'd have neutral to postive crankcase pressure and no connection of the intake track to the engine.

...prize? Wazzat? :p



closed, say your coasting down from high rpm, then the crankcase ventilation system doesn't suck anything if it depends on vacum in front of the throttle plate.... but by tapping into manifold vacum with a small tap is supplies a little suction to the crankcase whether you're accelerating or decelerating.... It always keeps some suction in the crankcase.... So, is there a prize?

[Bill R.]

The crankcase ventilation system which started out as a pollution control device has turned out to be nothing but good, there is not downside to it.. by scavenging all the combustion gases and unburnt fuel that gets down past the rings and scavenging condensation.. it keeps the bearing alive much longer , same for the other parts... no acids, no sludging ..
Removing the crankcase ventilation system is one of the last things i would do... jmo....
Also when you get postive crankcase pressure then is when the front and rear seals go... and if the engine is tight then its not a very big vacum leak anyway...






The Baconator swears his car runs better with the openings plugged. He's mumbling something about vacuum leaks and all that. Must be the large amount of blowby he's got...messing with his mixture and all.. :D

Sooo...short of sucking out noxious gasses out of the crankcase and pumping them back into the engine, is there any other point to keeping a slight vacuum in the crank case?

What would happen if you plugged up that vacuum line in question AND the valve cover cover inlet into the intake? Run the valve cover vent to a catch can or similar... You'd have neutral to postive crankcase pressure and no connection of the intake track to the engine.

...prize? Wazzat? :p

[JoeS]

how I interpret BillR's post is that the vaccum line pull comes from the intake manifold and into the valve cover.

I thought the vaccum pull went the other way ...were the vaccum pulled from the head into the intake manifold.

I thought this because how else does the oil shitty fudge get inside your intake manifold. The air that comes thru the air filter should be relatively clean. shouldn't it?

[winfred]

i think a slight vacuum helps the rings seal better also, years ago i read about a dyno test where they installed a pump to vac down the motor while doing a pull, it had good results up to a point, it was so long ago i can't remember how good it was

[Jeff N.]

i think a slight vacuum helps the rings seal better also, years ago i read about a dyno test where they installed a pump to vac down the motor while doing a pull, it had good results up to a point, it was so long ago i can't remember how good it was

[Jeff N.]

I think it best applies when you're using a low friction racing ringset. In the Summitt Racing catalog they sell vacuum pumps for block. Think it does just what you said...



i think a slight vacuum helps the rings seal better also, years ago i read about a dyno test where they installed a pump to vac down the motor while doing a pull, it had good results up to a point, it was so long ago i can't remember how good it was

[Jeff N.]

The high vacuum will be in the intake manifold behind the thottle plate. I think what Bill's saying is that the purpose of the line is as follows:

When the car is at idle, the vacuum in front of the throttle plate is very low (close to atmosphere). During this state, there is very little vacuum available to suck fumes from the valve cover vent. The vacuum line from the manifold corrects this by "transfering" vacuum from behind the throttle plate to infront of the throttle plate.

As the car goes off idle and the manifold pressure moves to atmospheric, this transfer tube has less and less effect.

I think - since the all aspects of the vacuum line are behind the AFM - all air is "measured" and therefore does not effect the a/f mixture.

...hmmm...ok. Maybe that's about right.


how I interpret BillR's post is that the vaccum line pull comes from the intake manifold and into the valve cover.

I thought the vaccum pull went the other way ...were the vaccum pulled from the head into the intake manifold.

I thought this because how else does the oil shitty fudge get inside your intake manifold. The air that comes thru the air filter should be relatively clean. shouldn't it?

[Bill R.]

there supplies some crankcase vacum... at large throttle openings the velocity in front of the throttle body is high so then you have high vacum there evacuating the crankcase.. At large throttle openings there is no manifold vacum anyway so the small line has no effect on the engine... At idle speed its only a 3mm leak sucking into whats a fairly tightly sealed crankcase depending on how much blow by you have...











The high vacuum will be in the intake manifold behind the thottle plate. I think what Bill's saying is that the purpose of the line is as follows:

When the car is at idle, the vacuum in front of the throttle plate is very low (close to atmosphere). During this state, there is very little vacuum available to suck fumes from the valve cover vent. The vacuum line from the manifold corrects this by "transfering" vacuum from behind the throttle plate to infront of the throttle plate.

As the car goes off idle and the manifold pressure moves to atmospheric, this transfer tube has less and less effect.

I think - since the all aspects of the vacuum line are behind the AFM - all air is "measured" and therefore does not effect the a/f mixture.

...hmmm...ok. Maybe that's about right.

[Hector]

for the vacuum leak tip.

Correct me if I'm wrong Bill but what you just said about the air velocity, "at large throttle openings the velocity in front of the throttle body is high so then you have high vacum there evacuating the crankcase." is just a manifestation of Venturi's principle. Cool!! Man, science is great.