View Full Version : blow-by problem

05-16-2008, 09:08 PM
i need some help what causes a blow-by. Installed a catch can for now but its running greddy intake manifold, greddy blow off valve, 850 cc injectors, greddy boost controller, greddy turbo timer, apexi afc neo, apexi head gasket also running its peak 20lbs boost. need help thanks

05-17-2008, 12:42 AM
well you going to get blowby on 20psi cause of crank case pressure.... That is normal... so catch can and try filling your engine to half full instead of all full....

05-17-2008, 01:19 AM
Blow by is what is referred to as compression that is lost during the compression and power stroke when the air/fuel vapors pass by the piston rings and into the crank case causing power loss and a pressurized crank case. In most cases these vapors are routed back into the intake system and consumed by the engine.

05-17-2008, 04:44 AM
^Very true^

Because the first thing you mentioned is the catch can I'll assume you mean what causes the blow-by (oil being sucked into the intake). Yes the catch can is there to trap the oil before it finds it's way into the intake, but at 20psi that's creating a shit load of vacume from the intake side so what's happening is oil is being sucked out of the head through the valve cover. I ran into this problem myself with my rb25det. To cure this problem you can do 1 of 2 things.

1. Do what I did and make a restrictor plug to reduce vacume that will fit into the soft line on the intake side. Make sure the plug has a 1/8th inch hole drilled in the center. Now that the vacume is reduced you won't pull so much vacume/oil from under the valve cover. I made one from aluminum at a machine shop. It was about 1 inch in length.

2. Plug off the port at the intake tube and run a breather style set-up that will pull no vacume.

05-17-2008, 10:56 AM
^^^ Can you be more specific? Not sure what you are referring to soft line and intake and plugging it up.

What are you referrig in 2 as well?

05-17-2008, 01:39 PM

Here's my set-up, look closely and you can see the restrictor plug inside the clear line (check the clear line coming right out of the catch can on intake manifold side). If you still don't see it look at the throttle cable and you see where it crosses the lines to the catch can. Now go about 1 inch towards the catch can and you will see it on the drivers side line.

The other way you can install the can is option #2 in my previous post, to do this you will have a breather style set-up where you pull no vacume. Just picture my set-up without a line from the can to the intake tube (tube from the turbo inlet to the air filter).

05-17-2008, 02:20 PM
Here's some other shots I found in the engine bay thread.

Here's the vacume style installation like mine.

Here's the breather style. Some guys use just a small filter at the valve cover port (CCV will just vent out the filter) and others will stick a line from the valve cover port to a catch can (no line going to intake tube for vacume).

05-19-2008, 11:40 PM
2nd picture on your 2nd post is how I do it and I get oil blowby from my catch can where the breather is... So I got valve cover T to catch to breather.... and I get oil in the can... I run 20PSI so I am more concerned about crank pressure...

06-07-2008, 09:10 AM
i run 22 psi and my catch can fills kind of fast and my rings are good, but like said on above you are gonna have a shit load of blowby at 20 psi or higher