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Discussion Starter #1
I have a 406 sbc. It's a stock blocked splayed. Has 12:1 compression with autotec pistons. Rings are gapped .032 and .034 2nd ring. Has 1.5,1.5,3mm ring pack. Leak down of approx. 5% and cranking compression of 205 psi. My question is at idle it'll puff out the valve cover. When I bring the rpms up the blowby goes almost all away. But as soon as the motor decelerates it had a good amount of blowby come out until the motor stabilizes. This is with a breather on each valve cover. Is this normal? I also just pulled my plugs from lots of street driving and they have oil on threads. Btw this is a nitrous motor for those wondering about the ring gap.
 

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Also would like to know the cause of this. I also have autotec/racetek pistons and i get blow by from the breathers and a small amount from the exhaust during idle. As soon as I start driving it goes away.
 

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Standard tension oil ring or?
Cyl wall finish?
 

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Discussion Starter #4
Standard Tension Ring. I do not remember the finish on the cyl. wall but the machine shop finish honed it for a moly ring.
 

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I measure Blowby on my dyno and contrary to what many people think an engine typically has more blowby at idle than at higher rpms. As the speed goes up, the rings actually seal better and blowby drops.

Joe
 

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Cylinder pressure gets behind the rings and pushes them out to seal better as rpm increases above idle. Blow by doesn't bother me, its when it gets too much to control or changes rapidly in quantity or quality (smoke, steam..........)
 

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My 406 does exact same thing....I put evac tubes to headers and now i cant see it....lol
Yep, works good doesn't it ;).... expell that crap out the pipes rather than vent underhood, keeps things quite a bit cleaner all around
 

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Cylinder walls cant stay round. Wait till you spray it............................
 

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Cylinder walls cant stay round. Wait till you spray it............................
Come-on Jeff, it's a stock block with splayed main caps, what could go wrong............. ;)

Oh, that is why I sold mine when I decided I wanted to go faster, while it was still in one piece!
 

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Cylinder walls cant stay round. Wait till you spray it............................
OEM 400 blocks are pretty frail to say the least.
 

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Discussion Starter #13
Thanks for the responses. Sounds like the amount of blowby is fairly common. I know the stock block is the weak point. I am not going to spray over a 150 shot until I get an aftermarket block. It sees more street time than race time. I am in the process of ordering the parts for a header evac. System Any tips? I've seen breather on one side evac tube on other but no vacuum would be in crankcase. My car has pypes straight thru mufflers as well. 3" x pipe exhaust. Thanks
 

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Have you considered a vacuum pump?
 
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