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Crankcase ventilation (the good kind)


tommytipover
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I'm so proud of myself. Rather than threadjack the Leakdown thread I started a Crankcase ventilation thread, so I could pick the brains of @Black Magic @mender and @Team Infiniti

about the best way to vent a pcv car that's being raced. Here is what Drew wrote that piqued my interest:

 

"Most of us have blocked some of the stock PCV system, and the average team I would guess has a single 3/8 hose as the only PCV vent. At 150 cubic inches (.087 cubic foot), 6000 rpm, firing all 150 cubic inches every other rev, you are looking at 26 cfm through a 3/8 hole. Lesson learned from blowing out the cam seals, most of us have undersized crankcase vents for bad levels of blowby."

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12 minutes ago, tommytipover said:

I'm so proud of myself. Rather than threadjack the Leakdown thread I started a Crankcase ventilation thread, so I could pick the brains of @Black Magic @mender and @Team Infiniti

about the best way to vent a pcv car that's being raced. Here is what Drew wrote that piqued my interest:

 

"Most of us have blocked some of the stock PCV system, and the average team I would guess has a single 3/8 hose as the only PCV vent. At 150 cubic inches (.087 cubic foot), 6000 rpm, firing all 150 cubic inches every other rev, you are looking at 26 cfm through a 3/8 hole. Lesson learned from blowing out the cam seals, most of us have undersized crankcase vents for bad levels of blowby."

Maybe I’m misunderstanding...

 

The displaced air when a piston moves down goes to the empty space left over as its neighboring piston moves up.. you should not (in theory) be blowing large amounts of air out the PCV vent. 

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9 minutes ago, enginerd said:

Maybe I’m misunderstanding...

 

The displaced air when a piston moves down goes to the empty space left over as its neighboring piston moves up.. you should not (in theory) be blowing large amounts of air out the PCV vent. 

Plus ring blowby, could be minimal if all is healthy.

Edited by Team Infiniti
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16 hours ago, tommytipover said:

So is a hunk of hose from the valve cover to the catch can good enough?

 

The large volume I was quoting in the first thread was doing some rough napkin math on what would have to come out if you had 10% power loss (not 10% reading on a leakdown tester). A good motor at our 150 to 200 hp would most likely be about 2 to 4 cfm actual blow by.

 

Look up vacuum pumps as a power adder. Depending on your ring design pressure in the crankcase will be a detriment to ring sealing, and you will lose some power. There are some other actions going on here, but you will pick up power if the crankcase pressure was high and you vented it better. 

 

The major detriment I have seen from not having enough crankcase venting (a pcv valve at say 1/4" outlet diameter as the only vent was not it) is horrific oil leaks. Almost all of your seals are not meant to handle positive pressure. The crank, cam and valve cover gaskets in the nightmare I lived though pushed out and started leaking even worse than the engine smoked. That engine had a bend ring on assembly and actually burned the piston ringland off in a spot (where the ring was leaking, you need piston seal to trap a insulating layer of air on the piston top or it gets plasma cut). With more venting I would have been able to finish the race (without going full exxon valdez), but the motor would still have burned alot of oil and needed a rebuild. 

 

If you had a dyno you could try adding more vents and see if the power picks up, or you could log the crankcase pressure (what pro's do) and see how high it is. Beyond that, try and make sure you have a 3/8" vent somewhere and add a second if convenient. Make sure to pick an area that is baffled (want air not oil out), or you will just make an oil film version of old faithful.

 

Most of us plug the extra vents, and the "makeup air vent" which is meant to be an inlet, because in a turn or under braking the valve cover dumps oil out that particular port. Run each of your vents to a separate bottle (with some air vents in the top) do a few laps and see where the oil is coming out (if any) and only use vents that get air out, not oil. 

Edited by Black Magic
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I have run a crankcase evaluation system on ours car and have not found a ex check valve that will last!!  We run now just use two big hose one from the crank case and valve cover to a catch can we can gauge oil and drain with a filter breather. I tight piston ring engine should have very low blow by. But you can get times of high blow by when the engine gos in to detonation . We have a post on leak down here  good reading to understand about blow by . 

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