- 646
- 12
It's well known that the crankcase gets a small dose of unburned air/fuel from blow-by gasses. The unburned AF mixture entering the crankcase causes several problems such as contamination of the engine oil, it contributes to sludge buildup, and causes corrosion. So, finding a good way to rid the crankcase of the AF mixture adds to the motors reliability. Additionally, there are performance gains to be had. By creating a vacuum in the crankcase you reduce windage losses. You can usually gain a 5% increase in rear wheen HP by putting a vacuum on the crankcase, and in some cases more. There are several ways to do this and they all have their own pros and cons.
1) Intake Manifold Vacuum Assisted Ventilation
An example of this would be a PCV valve. It works, but it has it's flaws. First of all, you want the amount of vaccum applied to the crankcase to increase as the RPM of the motor increases. Since there is no vacuum at WOT, there will be no vacuum in the crankcase at WOT. During low load conditions the vacuum is high (as high as it will get anyways), so basically you get the opposite of ideal with the PCV system. On top of that, you contaminate the intake charge with oil and unburnt fuel from the blow-by which makes for a gummed up upper intake plenum. With the 302s and 351s all the crankcase vapors get dumped down the cylinder #8 runner which causes another list of problems. All in all, this system blows (no pun intended).
The only good thing about the vacuum assisted system is that it is cheap and it induces little engine bay clutter. It is also good because it keeps these gasses from entering the atmosphere. So in terms of performance and reliability it isn't that great but from an environmental standpoint it scores high.
2) Road Draft Tube
This method consists of hooking up a slash cut tube to the valve cover. The tube must hang down low enough to catch the air that passes below the vehicle when the vehicle is traveling in the foreward motion. There must be a breather installed to allow air to enter the crankcase as well (as is the case with any method). This system relies on the Bernoulli effect; as air passes over the tube a vacuum is created as long as the slash-cut opening is placed in the opposite direction of the airflow. The faster the vehice travels in the foreward motion, the greater the vacuum. This is good. But, there will be no vacuum when the vehicle is still or moving very slow. From a reliability standpoint that is it's main downfall. The amount of vacuum that gets generated will be very low untill you reach high speeds.
Bottom line: This system is good for rigs that spend all their time on the freeway, and rigs that have a full exhaust system (see the next method) but otherwise it is less than ideal.
3) Exhaust Driven Crankcase Evacuation
This is very similar to the road draft tube. This system uses the exhaust gasses to create the Bernoulli effect as opposed to the using the air that passes under the vehicle. So you have exhaust that passes over a slash cut tube that is welded into the exhaust system. This tube is connected via a hose to the valve cover. The crankcase gasses get drawn into the exhuast stream via the vacuum that gets generated and thats all there is to it. This is better than the road draft tube because there is exhaust gas traveling through the exhaust pipes as long as the motor is running which means that there will always be a vacuum in the crankcase.....even at idle. As RPM goes up, exhaust gas velocity goes up and thus vacuum in the crankcase goes up; this system works good in that respect as well. However, a catalytic converter may not respond too well to this and that is the main flaw. As long as you have an open exhaust system, this is the way to go. It's cheap, reletively clutter free, easy to install, maintenance free and it works well. Back pressure in the exhaust would not be a good thing, so some people use one way check valves to prevent exhaust from entering the crankcase. If you have a free flowing exhuast (no cats, straight through muffers / no mufflers) you don't need the check valve(s).
I had a road draft tube on my van and I have since switched to the exhaust driven system.
4) Electric Vacuum pump
Getting the amount of vacuum generated to correspond to engine load / RPM is quite difficult plus getting an off the shelf pump to draw exactly the amount of vacuum you want it to when you want it to can be even more difficult.
The electric pumps that I found (generic brake booster vacuum pumps) draw about 15 inches of vacuum at 13.5 volts and about 18 inches at 16 volts. You don't want any more than 15 inchs of vacuum ever in the crankcase.
I could not find any other pump suitable for the job and I have not yet found a way to make this work.
5) Open Breather Ventilation
Just put a mushroom filter on the valve cover(s) and thats it. It works in the sence that the vapors are able to escape, but it does not PULL out the vapors. This is not the way to go.
6) Mechanical Vacuum Pump
One way to do this is to take a thermactor pump and hook it up to the valve cover and make sure there is a breather up there there too. Some people do this with good results. You have to find a way to measure the amount of vacuum being placed on the crankcase and restrict down some how so that the vacuum does not get too high. But, since the air pump was not made to have air/fuel and oil in it, it wont last long. Plan on having to replace it / rebuild it fairly often. For some people, rebuilding one is no issue.
7) Dry Sump
This is the best way to go......hands down. With this metod you can get more than the standard 5% increase in power from the motor. But, this method gets very expensive. I never really looked into it because it wasn't an option for me, but in addition to getting spendy, you have to change the oil pump set up and the oil pan, so it is labor intensive as well as expensive. I don't fully understand exactly how it works since I never looked into it, but I am told that it is the best way to evacuate the crankcase.
1) Intake Manifold Vacuum Assisted Ventilation
An example of this would be a PCV valve. It works, but it has it's flaws. First of all, you want the amount of vaccum applied to the crankcase to increase as the RPM of the motor increases. Since there is no vacuum at WOT, there will be no vacuum in the crankcase at WOT. During low load conditions the vacuum is high (as high as it will get anyways), so basically you get the opposite of ideal with the PCV system. On top of that, you contaminate the intake charge with oil and unburnt fuel from the blow-by which makes for a gummed up upper intake plenum. With the 302s and 351s all the crankcase vapors get dumped down the cylinder #8 runner which causes another list of problems. All in all, this system blows (no pun intended).
The only good thing about the vacuum assisted system is that it is cheap and it induces little engine bay clutter. It is also good because it keeps these gasses from entering the atmosphere. So in terms of performance and reliability it isn't that great but from an environmental standpoint it scores high.
2) Road Draft Tube
This method consists of hooking up a slash cut tube to the valve cover. The tube must hang down low enough to catch the air that passes below the vehicle when the vehicle is traveling in the foreward motion. There must be a breather installed to allow air to enter the crankcase as well (as is the case with any method). This system relies on the Bernoulli effect; as air passes over the tube a vacuum is created as long as the slash-cut opening is placed in the opposite direction of the airflow. The faster the vehice travels in the foreward motion, the greater the vacuum. This is good. But, there will be no vacuum when the vehicle is still or moving very slow. From a reliability standpoint that is it's main downfall. The amount of vacuum that gets generated will be very low untill you reach high speeds.
Bottom line: This system is good for rigs that spend all their time on the freeway, and rigs that have a full exhaust system (see the next method) but otherwise it is less than ideal.
3) Exhaust Driven Crankcase Evacuation
This is very similar to the road draft tube. This system uses the exhaust gasses to create the Bernoulli effect as opposed to the using the air that passes under the vehicle. So you have exhaust that passes over a slash cut tube that is welded into the exhaust system. This tube is connected via a hose to the valve cover. The crankcase gasses get drawn into the exhuast stream via the vacuum that gets generated and thats all there is to it. This is better than the road draft tube because there is exhaust gas traveling through the exhaust pipes as long as the motor is running which means that there will always be a vacuum in the crankcase.....even at idle. As RPM goes up, exhaust gas velocity goes up and thus vacuum in the crankcase goes up; this system works good in that respect as well. However, a catalytic converter may not respond too well to this and that is the main flaw. As long as you have an open exhaust system, this is the way to go. It's cheap, reletively clutter free, easy to install, maintenance free and it works well. Back pressure in the exhaust would not be a good thing, so some people use one way check valves to prevent exhaust from entering the crankcase. If you have a free flowing exhuast (no cats, straight through muffers / no mufflers) you don't need the check valve(s).
I had a road draft tube on my van and I have since switched to the exhaust driven system.
4) Electric Vacuum pump
Getting the amount of vacuum generated to correspond to engine load / RPM is quite difficult plus getting an off the shelf pump to draw exactly the amount of vacuum you want it to when you want it to can be even more difficult.
The electric pumps that I found (generic brake booster vacuum pumps) draw about 15 inches of vacuum at 13.5 volts and about 18 inches at 16 volts. You don't want any more than 15 inchs of vacuum ever in the crankcase.
I could not find any other pump suitable for the job and I have not yet found a way to make this work.
5) Open Breather Ventilation
Just put a mushroom filter on the valve cover(s) and thats it. It works in the sence that the vapors are able to escape, but it does not PULL out the vapors. This is not the way to go.
6) Mechanical Vacuum Pump
One way to do this is to take a thermactor pump and hook it up to the valve cover and make sure there is a breather up there there too. Some people do this with good results. You have to find a way to measure the amount of vacuum being placed on the crankcase and restrict down some how so that the vacuum does not get too high. But, since the air pump was not made to have air/fuel and oil in it, it wont last long. Plan on having to replace it / rebuild it fairly often. For some people, rebuilding one is no issue.
7) Dry Sump
This is the best way to go......hands down. With this metod you can get more than the standard 5% increase in power from the motor. But, this method gets very expensive. I never really looked into it because it wasn't an option for me, but in addition to getting spendy, you have to change the oil pump set up and the oil pan, so it is labor intensive as well as expensive. I don't fully understand exactly how it works since I never looked into it, but I am told that it is the best way to evacuate the crankcase.
