blowby question

95tsiawd6blt · Jun 13, 2007

i've got a brand new motor, top and bottom end is all new. With new motors how much blow by do you guys see? my motor has about 30 miles on the rebuild, and i only hit about 5psi during the break in, and i got a couple drops from the catch can. normal? do you guys see the same thing?

fourreGsixty3 · Jun 13, 2007

The oil isnt blow by. Blow by is combustion gasses escaping past the ring's into the crank case. The oil comes from the PCV system removing the blow by, and the oil in combined with the gasses, then releases, and goes into your catch can( or intake pipe). If you want to check your engine for leakage do a leak down test. A good motor can have up to 30% leakage, dont expect 0% leakage since brand new motors from the factor will have around 10% leakage. Also when at TDC dont introduce all the air pressure at once, start out with 10lbs, and slowly let more into the cylinder. This will keep you from throwing the piston straight down quickly if your not exactly at TDC.

Sharadon · Jun 13, 2007

Another thing to remember is a new engine will have a little blow by until the rings brake-in. It should get better with some miles.

I wouldn't worry about it just yet.

fourreGsixty3 · Jun 13, 2007

Also you can break the engine in with more boost. Its a boosted engine, and will break in nicely on 15-16psi. Yes you will get more blow by since the ring's have yet to seat.

nightspeed87 · Jun 14, 2007

On my awd 1g ( old engine never rebuilt or nothing )
with just a hose running off the valve cover breather, and pcv valve ( where there is just a threaded fitting in there now, no pcv. ) I can see combustion smoke coming out of those hoses at idle. I know this engine has blow by because it consumes a quart of oil every month. Also because on a compression test the results were 148, 150, 160, 160 ( The last two maybe due to carbon build up? ) But it took quite a few cranks to build full compression were as in my rebuilt gst hit 155 across the board within four cranks, the laser needed about 8 to get its results. ( Stock 1g pistons on both )
The gst doesnt seem to have any blow by, i broke it in at 15psi under the motoman technique.

95tsiawd6blt · Jun 14, 2007

yeah it had about 2 drops of oil, and looked like some water come out after about 35 miles.

kenamond · Jun 14, 2007

95tsiawd6blt: Please fill out your profile.

nightspeed87: Your PCV system can't do its job at idle without a PCV valve.

PCV 101: At idle, the IM is under vaccuum and the PCV valve (threaded into back-left of VC) opens up. The VC breather (side of VC) provides an inlet to the VC and crankcase (crankcase is connected to top of head by oil return passages). So at idle, air (which has been metered by the MAS if stock MAS is still used) is pulled through the VC breather into the VC/crankcase, and it flushes the fumes out which are pulled out the PCV valve into the IM and through the motor (burned). The PCV also flows differently at varying vaccuum conditions. The car is tuned to this and will not idle properly if the PCV is just replaced with a check valve.

Under boost, the PCV closes (it's a one-way valve). If it didn't, the boost in the IM would go into your VC and crankcase. That part of the motor isn't supposed to be pressurized. To relieve the pressure, the VC breather is needed again, but in the boosted case, the VC breather is a vent for the blowby. It gets blown into the intake pipe where it is then pulled through the motor (burned). Since the blowby was metered air, blowing it back into the intake pipe downstream of the MAS helps, but oil fumes can coat the intake pipe, compressor housing, IC, IC pipes, TB, and IM. That's why folks put a filter in their VC breather line.

There are baffles (metal plates) along the top of the valve cover to keep oil from splashing into the VC breather or PCV valve. This keeps the majority of the oil from going out the PCV (at idle) or VC breather (under boost). The PCV valve even has a snorkel on the end to keep the inlet away from the wall of the VC to help reduce the amount of oil it pulls through.

All of this keeps your crankcase from pressurizing too much under boost, and it also keeps fumes from blowby (these are fuel/air mix *and* exhaust fumes) during idle/vaccuum from hanging around and settling in your oil which would decrease its performance.

So think it through before you remove your PCV, VC breather hose, etc. This is a PCV system, and all of those functions have a purpose. There are many threads on ways to keep oil out of your intake, help with the short-lived OEM PCV valves, etc. that still preserve the PCV system functionality. Might want to check them out if for no other reason than to understand what this system does and what alternatives are good vs. bad ideas.

:thumb:

nightspeed87 · Jun 14, 2007

kenamond said:
95tsiawd6blt: Please fill out your profile.

nightspeed87: Your PCV system can't do its job at idle without a PCV valve.

PCV 101: At idle, the IM is under vaccuum and the PCV valve (threaded into back-left of VC) opens up. The VC breather (side of VC) provides an inlet to the VC and crankcase (crankcase is connected to top of head by oil return passages). So at idle, air (which has been metered by the MAS if stock MAS is still used) is pulled through the VC breather into the VC/crankcase, and it flushes the fumes out which are pulled out the PCV valve into the IM and through the motor (burned). The PCV also flows differently at varying vaccuum conditions. The car is tuned to this and will not idle properly if the PCV is just replaced with a check valve.

Under boost, the PCV closes (it's a one-way valve). If it didn't, the boost in the IM would go into your VC and crankcase. That part of the motor isn't supposed to be pressurized. To relieve the pressure, the VC breather is needed again, but in the boosted case, the VC breather is a vent for the blowby. It gets blown into the intake pipe where it is then pulled through the motor (burned). Since the blowby was metered air, blowing it back into the intake pipe downstream of the MAS helps, but oil fumes can coat the intake pipe, compressor housing, IC, IC pipes, TB, and IM. That's why folks put a filter in their VC breather line.

There are baffles (metal plates) along the top of the valve cover to keep oil from splashing into the VC breather or PCV valve. This keeps the majority of the oil from going out the PCV (at idle) or VC breather (under boost). The PCV valve even has a snorkel on the end to keep the inlet away from the wall of the VC to help reduce the amount of oil it pulls through.

All of this keeps your crankcase from pressurizing too much under boost, and it also keeps fumes from blowby (these are fuel/air mix *and* exhaust fumes) during idle/vaccuum from hanging around and settling in your oil which would decrease its performance.

So think it through before you remove your PCV, VC breather hose, etc. This is a PCV system, and all of those functions have a purpose. There are many threads on ways to keep oil out of your intake, help with the short-lived OEM PCV valves, etc. that still preserve the PCV system functionality. Might want to check them out if for no other reason than to understand what this system does and what alternatives are good vs. bad ideas.

Thanks for taking the time to post all the information.
In reguards to your post I wouldnt be able to put the pcv back on anyways because were it connects to the intake manifold has been welded shut prior to me getting the car. This pcv set up isnt my doing, I just know it elimates the possibility of boost leaks thru it, and leaky pcv valves. Whenever I install my cyclone then I could put it back but not on the currect stock intake manifold its running right now.
But what I can do, supposing it helps at all is connect the vc breather hose back onto the intake snorkle and just slap a 3 dollar fuel filter on it like I did to my gst ..

Therefore would there be any probem with that, hooking the vc breather back up like stock with the addition of a fuel filter, and leaving the pcv part like I have it with a hose hanging down to the bottom of the car, or would hooking the vc line back to the intake while leaving the pcv off be worse than it is now?
(My catch can that the pcv line hangs down into is actualy a water bottle ziptied onto something on the bottom of the fire wall behind the motor...

The Central Hub for DSM Community and Information

blowby question

95tsiawd6blt

fourreGsixty3

Sharadon

fourreGsixty3

nightspeed87

95tsiawd6blt

kenamond

nightspeed87

Similar threads

Latest posts

Build Thread Updates

Vendor Updates

Latest Classifieds