Excessive blowby

[maybrfd]

We pulled the second engine yesterday.  We have not pulled the pan or heads yet, but did post a photo of the valley pan inversion.  We did a cold leak down test (since he took the oil pan out on a local road with a fairly large dip).  Leak downs varied from a high of 12% to a low of 8%.  Two of the cylinders could not be tested, due to excessive leaking from exhaust ports.  Originally though his lash was too tight, but after checking, we'll need to pull the heads to see if it is a poor valve job (recently performed) or potentially a head gasket leak.  Obviously something going on with this engine so more to come.



  On subject of blowby, I attached a simplified article on subject.  I have some more through technical articles on subject, but unless you really like calculus and complex differential equations, you'll fall a sleep.  If you do want them ask and you shall receive.  While this particular article centers on closed venting systems on an overhead cam, 4 cylinder engine, it will give you a very good understanding as to why we are building too much pressure in our stroker engines and for the most part the equations and illustration, while some are over simplified, will give you a method for designing a venting system, whether closed or atmospheric.

www.106rallye.co.uk/members/dynofiend/breathersystems.pdf‎


All of the Late model MOPARs rely on the head venting through the oil passages and push rod areas (standard wet sump systems).  If you simply calculate the areas associated with the drain holes, assume the pushrods do not exist (allowing max air to flow), and further assume the oil draining back takes up no venting volume, you will find that on the Eldebrock heads you barely have enough area to vent even a really well designed engine (1-3% blowby), with great volumetric efficiencies.  Now add back in the real world where oil (a viscous fluid even at higher temperatures) does take up space, is being highly agitated by both the bottom half of the engine, as well as the top half, push rods take up space, ect.  You simply do not have enough area to properly vent these engines using atmospheric venting without creating significant crankcase pressure.  We have a set of Indy EZ-1 heads; they improve the condition by providing larger drains and volume in their valve covers, but I haven't performed the calculations since the heads are currently inaccessible.  I anticipate I will find a similar condition.

Potential solutions:
increase drain areas and increase venting area
provide additional venting area in other areas of the engine
Go to a closed venting system


Let me know what you think.

Thx!
Scott

[ws23rt]

We pulled the second engine yesterday.  We have not pulled the pan or heads yet, but did post a photo of the valley pan inversion.  We did a cold leak down test (since he took the oil pan out on a local road with a fairly large dip).  Leak downs varied from a high of 12% to a low of 8%.  Two of the cylinders could not be tested, due to excessive leaking from exhaust ports.  Originally though his lash was too tight, but after checking, we'll need to pull the heads to see if it is a poor valve job (recently performed) or potentially a head gasket leak.  Obviously something going on with this engine so more to come.



  On subject of blowby, I attached a simplified article on subject.  I have some more through technical articles on subject, but unless you really like calculus and complex differential equations, you'll fall a sleep.  If you do want them ask and you shall receive.  While this particular article centers on closed venting systems on an overhead cam, 4 cylinder engine, it will give you a very good understanding as to why we are building too much pressure in our stroker engines and for the most part the equations and illustration, while some are over simplified, will give you a method for designing a venting system, whether closed or atmospheric.

www.106rallye.co.uk/members/dynofiend/breathersystems.pdf‎


All of the Late model MOPARs rely on the head venting through the oil passages and push rod areas (standard wet sump systems).  If you simply calculate the areas associated with the drain holes, assume the pushrods do not exist (allowing max air to flow), and further assume the oil draining back takes up no venting volume, you will find that on the Eldebrock heads you barely have enough area to vent even a really well designed engine (1-3% blowby), with great volumetric efficiencies.  Now add back in the real world where oil (a viscous fluid even at higher temperatures) does take up space, is being highly agitated by both the bottom half of the engine, as well as the top half, push rods take up space, ect.  You simply do not have enough area to properly vent these engines using atmospheric venting without creating significant crankcase pressure.  We have a set of Indy EZ-1 heads; they improve the condition by providing larger drains and volume in their valve covers, but I haven't performed the calculations since the heads are currently inaccessible.  I anticipate I will find a similar condition.

Potential solutions:
increase drain areas and increase venting area
provide additional venting area in other areas of the engine
Go to a closed venting system


Let me know what you think.

Thx!
Scott

Since I am unfamiliar with the Eldebrock heads I was unaware that there was so little room for oil drain back and venting.
My experience is with stock heads and the area open to the crankcase is large.
Thanks   Bruce

[maybrfd]

Bruce,


Thanks fro thoughts and experience with igniting gaseous vapors - am sure that was great fun.

Curious as to what you are running in terms of crank, heads pistons and rings.  Have always preferred old school, but am ever so curious as to the why some combos work and others, not so well.  The drain holes in the Eldebrocks measure 11/32", push rod holes slightly larger.  BTW, some crankcase pressure not a bad thing.

We did not see any issues with Carl's car, until we changed crank/piston/ring combo.  He used to bleed some out of his valley and rear mains at 11.47 sec.  The first we noticed this was on my 470 with Indy's.  Puked out of the valve covers and push-on breathers, a little out the rear main seal.  We improved issue when we went to welded risers on the air breathers.  When Ron suggested the atmospheric risers, since we run muffler systems, I calculated exhaust back pressure,  It is enough to close off the exhaust evac check valve at certain RPMs, thus some of the issue.

Not saying it is all to be known, but atmospheric venting on at least the Eldebrocks is insufficient to vent to atmosphere through valve covers alone with higher compressions and larger strokes.  Even with a perfect engine ( high compression and large stroke - work great for most).

Thx!
Scott