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Crank Evac System and Theory


Crank Evac System and Theory


Background - I spent many months and a lot of money trying to correct oil burning at higher RPM's. I started out thinking it had to be rings, then it had to be valve stem seals, then this and that. This page will give you the steps I went through that corrected my oil containment problem.  From what I understand, this  may not be legal in some states as some of the modifications may affect your emissions. Check your local laws before you proceed. Where I am it doesn't matter.

There are a number of steps that work together to stop or greatly reduce the oil burning in your high rpm Honda. There is also quite a bit of theory behind it. So follow along. This is the exact set up I run in my boosted 2000 SI EM1 running 19lb boost and 314hp of nitrous in 2 stages.

Pressure on the top side (cylinder head side) of the pistons is good. While rings do have to be gapped correctly, the ring tension alone wont seal the cylinder. Its the pressure in the combustion chamber forcing the rings down, and out, that seals it up. 

Likewise, if there is too much pressure on the bottom (crankcase) side of the piston, the rings cant press down and out as hard, and they wont seal the combustion chamber properly. The two pressure forces are fighting each other.

When your car is stock, crankcase pressure is relieved by vacuum in the intake. As the engine is running it creates vacuum. There is a hose from your pcv breather and valve cover that connects to the intake / cold air piping. When the motor is at idle, or freeway cruising (times of high vacuum), the vacuum actually pulls the pressure out of the motor allowing the rings to seal, but, at the expense of a contaminated air / fuel charge, which,  hurts horsepower. Under acceleration (times of low, or no vacuum, or when building  boost) the pressure builds in the crankcase and IS NOT pulled into the intake due to no vacuum. This can be a big problem for those who are running turbos or superchargers as the crankcase pressure has no way to escape.  This is the time where the rings may not seal properly and you get some blow by and oil consumption. Many people try to relieve the pressure by using a small filter off the original valve cover breather hose. This typically wont solve your problem and can leave an oily mess on your motor.

So, you basically need two systems to keep the crankcase pressure low. One for high vacuum conditions, and one for low vacuum conditions.

The system on my car is a sealed system and consists of removing the stock pcv breather, plugging the large hole with the kit from Z10 (shown below), pulling one of the big allen plugs on the back of the block and installing the AN adapter into the hole. I use only one -10 AN line coming from the breather hole closest to the transmission end of the block. This leaves more room to get at the oil filter.


In the first picture you can see what the Moroso Oil / Seperator looked like originally. There is a small fitting on the right side. This was ground off, tapped, and plugged. I welded three aluminum bungs about 1 1/2" above the larger fitting on the left
(close to the top of the can) allowing my to run my three 3/8" x -8 AN fittings for the valve cover breather tubes (one bung is not currently being used). The big -10 AN fitting on the left side is connected to the breather line that comes out of the back of the block. This line works as both the breather tube and an oil drainback tube back to the block. By draining the oil back you don't have to worry about the canister filling up with oil and having to drain it. My car is a daily driver and the last thing I want to do on a trip is have to stop to drain oil out of the oil / air  seperator. The AN fitting on the bottom of the oil / air seperator is for the crankcase evac system. I used a Moroso part number 25900. Its designed for a V8, but will give you an extra set of pipe and check valves in case you change your exhaust or the valve goes bad (I have to replace the check valve every year or so and get replacement valves from Napa). The two breathers shown at the top of the picture and the rubber grommets shown on the left are not used in my system. Drill a hole in the exhaust pipe somewhere after your catalytic convertor (if you still have one) and weld the angled tube in at a 2 o'clock to 3 o'clock position. Make sure the angled part is aimed toward the back of the car and the angle cut is at 90 degrees compared to the exhaust pipe. Screw one of the one way air check valves onto the welded in pipe and tighten down. Now connect the -10 line from the bottom of the oil/ air seperator to this. I just use a hose clamp to hold it onto the valve, this way its easy to remove if you need to drop the exhaust.


On the valve cover I use the original breather tube hole on the backside of the cover, but I have pulled the vent tube out, tapped it and installed a -8 AN fitting. On the front side of the valve cover I have drilled and tapped two holes for AN fittings. The one on the front passenger side will go to the Morosso oil / air seperator. The hole with an AN fitting on the front drivers side will go to a KrankVent. This is a top of the line one direction PCV valve. I used their universal vent with a 1/8" NPT female thread. Cost was about $100 (you dont need the $239 kit thats shown). The KrankVent is vented to atmosphere.


So, connect it all up and this is what happens:

At idle or low RPM driving there typically isnt much exhaust volumn. Therefore the one way air check valve on the exhaust pipe wont open or will only open part way. At times like this the engine can relieve crankcase pressure through the KrankVent. This keeps low pressure in the crankcase.

At higher rpm, or under boost, there is sufficient exhaust volumn to create a negative pressure and open the one way air check valve on the exhaust pipe. This in turn uses the negative pressure to pull the pressure from the Moroso oil / air seperator, which in turn pulls it from the crankcase and valve cover area. During this time the KrankVent closes and all pressure is relieved through the evac system

Now you have low crankcase pressure at low rpm, and low crankcase pressure high rpm and your oil comsumption will be reduce. You may also pick up a couple horsepower as a result of not having a pressuriaed crankcase.


If you have Questions or comments feel free to email me

08/28/2014 23:48