Oil catch can pros & cons

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Postby klue » Tue Jun 03, 2008 12:40 am

I just wanted to point out that there is a differnce betweetn the typical pcv VALVE system you see on most cars and the pcv hose that is on ours.

the PCV valve is a emission device, however the pcv hose on our car is not.

the pcv as used on a race engine is to help the seal of the rings.
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Postby ironpanther » Tue Jun 03, 2008 7:13 am

klue would you like to explain how the PCV system helps sealing of the rings?
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Postby klue » Wed Jun 04, 2008 4:12 am

since a race engine usually has a considerable amount of blow by building pressure in the crankcase, this must be overcome to produce power and conquer the pumping loss. when the engine is under high rpm conditions/load the port vacuum becomes high and can easily pull the excess pressure created by the blow by gas. this is turn reduces the pumping loss created by the pressurized block. -> this gives you the performance gain.

if you have the pcv connected in the exhaust with a properly designed stem to creat a vacuum you can get the same effect as running the pcv to the intake(except you dont have to deal with the residues)

my problem with the catch can is that they hamper the flow of the pcv.

as for the sealing of the rings-> it is my understanding that the vacuum that a good operating pcv system produces helps to oil control ring effectively under loaded conditions.
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Postby BriinumsBo » Wed Jun 04, 2008 6:21 am

klue, you can definitely go into more details about each your sentence :smokes:
because for now the only bad point you mentioned is that catch can would be a restriction for the pcv air flow, not that the oil is needed or smth..
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Postby klue » Wed Jun 04, 2008 4:49 pm

sorry im mildly retarded, so please excuse me.
do you not understand the above post, basically the more vacuum you can put on the pcv the better you engine will perform
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Postby BriinumsBo » Wed Jun 04, 2008 8:56 pm

1.so it doesnt matter if i dump the oil or not?
2.and if the catch can is tight and the tubes ar short, its ok to have catch can?
3. what about recirculating blowoff valves? they give pretty big pressure back into PCV and the other tube (EGR?)

its not that i didnt understand, you just said about more different things on each sentence and i just wanted for you to explain more each of them :smokes:
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Postby WarTowels » Wed Jun 04, 2008 9:07 pm

On a related note, what are the effects if the tubing on your catch can is higher than the crankcase?

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Postby ironpanther » Thu Jun 05, 2008 5:01 am

klue wrote:since a race engine usually has a considerable amount of blow by building pressure in the crankcase, this must be overcome to produce power and conquer the pumping loss. when the engine is under high rpm conditions/load the port vacuum becomes high and can easily pull the excess pressure created by the blow by gas. this is turn reduces the pumping loss created by the pressurized block. -> this gives you the performance gain.

if you have the pcv connected in the exhaust with a properly designed stem to creat a vacuum you can get the same effect as running the pcv to the intake(except you dont have to deal with the residues)

my problem with the catch can is that they hamper the flow of the pcv.

as for the sealing of the rings-> it is my understanding that the vacuum that a good operating pcv system produces helps to oil control ring effectively under loaded conditions.


Klue I have to argue your comment about excessive blowby. Any engine, commuter, or race will perform the best being built to the higest tolerances afforded and thus having the least amount of blowby.

Yes I can see the increased vacuum decreasing pumping losses but overall you are not getting a net gain because the engine still is doing the work to create the vacuum. That is simple first law of thermodynamics(output energy is never greater then input). A block breather would actually give you better performance because you are not recirculating blowby gases which displace good cold air and gas going into the engine.

The exhaust concept sounds sound with proper setup. That would include a free flowing exhaust and a check valve to manage the exhaust pulses.

While a catch can may decrease the velocity of the air in the PCV system, if you are using a check valve between the catch can and intake, you will both eliminate that issue and any possibility of problems with bypass valve dumps.

Towels, regarding tubing height it will not affect the performance but considering you are dealing with airflow things like length and sharp bends will greatly affect the performance.
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Postby gt4rcdude » Thu Jun 05, 2008 3:34 pm

I have to agree. The rings seal by pressure differential. The difference between the pressure in the cylinder and the block. Lowering the block pressure a few inches of mercury probably won't do much.

A friend of mine ran his PCV line into his exhaust on his airplane. You have to fabricate it in such a way as to create a vacuum using the exhaust pulse and velocity. He made a small venturi in the exhaust pipe and welded a fitting at an angle just down stream of the smallest part of the venturi. No check valve required. It worked great. Not a good plan if you have a catalytic converter downline. Emissions may be affected too since you'll be burning oil.
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Postby BriinumsBo » Thu Jun 05, 2008 5:34 pm

so is it ok to use oil catch can? im not using a breather filter for the can, everything is sealed tight! its ok filter out the oil? (its absolutely not needed for valves and rings?)
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Postby gt4rcdude » Fri Jun 06, 2008 2:22 am

It can't hurt, it may help. No harm, no foul. Best oil runs in an engine, not through it. Not counting two strokes. I've run one for 12k miles with no ill effects.
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Postby ironpanther » Sat Jun 07, 2008 2:44 pm

yes it is just fine to run a catch can.
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Postby klue » Sat Jun 07, 2008 5:08 pm

BriinumsBo wrote:1.so it doesnt matter if i dump the oil or not?
2.and if the catch can is tight and the tubes ar short, its ok to have catch can?
3. what about recirculating blowoff valves? they give pretty big pressure back into PCV and the other tube (EGR?)

its not that i didnt understand, you just said about more different things on each sentence and i just wanted for you to explain more each of them :smokes:


1 dont use the oil from the catch can its garbage, the additives are destroyed.
2. if you want to use one, use the biggest hose you can, with nothing smaller than that diameter in the whole system(short and sweet, minimal bends)
3. i dont recirc bovs, and i dont use egr.(no comment)
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Postby klue » Sat Jun 07, 2008 5:12 pm

ironpanther wrote:
klue wrote:since a race engine usually has a considerable amount of blow by building pressure in the crankcase, this must be overcome to produce power and conquer the pumping loss. when the engine is under high rpm conditions/load the port vacuum becomes high and can easily pull the excess pressure created by the blow by gas. this is turn reduces the pumping loss created by the pressurized block. -> this gives you the performance gain.

if you have the pcv connected in the exhaust with a properly designed stem to creat a vacuum you can get the same effect as running the pcv to the intake(except you dont have to deal with the residues)

my problem with the catch can is that they hamper the flow of the pcv.

as for the sealing of the rings-> it is my understanding that the vacuum that a good operating pcv system produces helps to oil control ring effectively under loaded conditions.


Klue I have to argue your comment about excessive blowby. Any engine, commuter, or race will perform the best being built to the higest tolerances afforded and thus having the least amount of blowby.

Yes I can see the increased vacuum decreasing pumping losses but overall you are not getting a net gain because the engine still is doing the work to create the vacuum. That is simple first law of thermodynamics(output energy is never greater then input). A block breather would actually give you better performance because you are not recirculating blowby gases which displace good cold air and gas going into the engine.

The exhaust concept sounds sound with proper setup. That would include a free flowing exhaust and a check valve to manage the exhaust pulses.

While a catch can may decrease the velocity of the air in the PCV system, if you are using a check valve between the catch can and intake, you will both eliminate that issue and any possibility of problems with bypass valve dumps.

Towels, regarding tubing height it will not affect the performance but considering you are dealing with airflow things like length and sharp bends will greatly affect the performance.



i dont need that thermodynamic mumbo jumbo. It works, thats all that matters to me.

you dont need a check valve, the turbine controls the pulses enough

and yes the engine must create that vacuum weather you use it or not is up to you.


-> dont get me wrong catch can is great, its keeps the intercooler clean but my greddy can needed some heavy mods to get it to work effectively.

BUT, there are better ways to control blowby ie: exhaust. you dont need no fancy tubes on ventrui's, just look at the design of the pcv nipple that tucks into the intake tubing. basically a short pipe cut at a angle(to generate the vacuum from the pressure differential) placed in the exhaust stream, where its still hot and at maximum velocity.
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Postby bridge47 » Thu Sep 11, 2008 9:01 pm

How did I miss this debate? You all make some good points. Hot Rod mag did dyno testing on PCV routed to exh. some years ago. No venturis b/c exh was very free flowing and it pulled vacuum. Can't remember exact #s but it was around 500hp small block and there was definitely a gain. I think their recommendation was that it was not worth it at OE power levels/street driving.

My experience with venting valve cover line to K&N filter (no check valve I know) is that oil gets dirtier faster but I change it often anyway. Also noticed hesitation from no boost to boost b/c, I thought, that was some unmetered air that ECU was counting. I'd be willing to try catch can if I get her back on the street. Probably a Moroso one, just because.
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