I wonder if there is any reason (other than environmental/emissions or originality reasons) to use a PCV system? Why would someone want to suck hot and oily air into the engine... and also.. at WOT - there's a lot of pressure building up in the crankcase.. but no vacuum in the carb to relief the pressure. Open style breathers (like the K&N style tall breathers) should be much more effective in preventing pressure build up in an engine - especially on an engine that is driven hard...
Looking at basic engine dynamics - I don't see any reason to run a PCV system to prevent preassure buildup.. Is there any reason??

 

have you ever been in an older car and smelled that nauseaous odor that is really oilly and doesn't come off your clothes?? I like my PCV valve system and would not have a daily driver with out one...can't speak to your last question other then to say the system should be vented if for nothing else so fuel vapors don't get trapped inthe engine and cause a problem...

 

GrandSportC3,
Read my post on "Chevyman 77"s thread on PCV valves and tell me what you think!

 

I only ran a PCV system for very short time on my '68.. I initially replaced the PCV system with breathers and later witha crankcase evacuation system through my open exhaust.. I didn't notice any different smell when I removed the PCV system.. but my nose might not be that sensitive...

 

I don't like corrosion in my engines, I use a PCV system on my LT-1 to prevent water build-up in the lubricating oils of my engine. There isn't much of a horsepower penalty and I can then sleep sounder at night knowing I can neglect my Vette just a little longer (1800-2500 mile oil changes using Mobil1 or Castrol Syntec.) I put the PCV system back on when I discovered large amounts of condensation in my "trick" autocross crankcase breather system.

 

Did you use a external overflow tank with a breather?

 

I posted there

 

All I know is after a few miles all engines get some blow by and if you just have breathers on the covers you get oil film buildup around them on the covers and you can also see a little smoke coming out, a PCV system solves these problems

 

I don't have that problem.. My evacuation system blows all the blowby out of the exhaust

 

A PVC System requires a closed system to operate properly. The PVC will draw air into the intake (below the carb) at light throttle (high vaccume). At WOT (low vaccume) the PVC will draw very little. Any pressure build-up will flow up the tube to the air cleaner and into the engine above the carb. This will lower emmisions and keep the oil film off of the motor. If you use a PVC with an open breather (k&N style) it will work at part throttle but will push oil fumes out at WOT. A pvc valve allows such a small amount of air into the motor that it will have no affect on the power output of a well sealed motor. If the motor has exessive blowby then the PCV can not keep up and a large amount of oil vapor is pushed into the carb and can cause problems.
If you do not use a PCV valve then you have to breath the motor somehow. An open style breather will work if again, the motor is sealed very well and has a well designed oil pan. If not you will have oil film out the breathers. The use of a check valve style evac system is generally used when a PCV system can not handle the blowby and an open breather allows to much vapor or mist out. The draw back is that to be maximun effective the check valve should be mounted within 4" of the end of the header system and no mufflers.

Neither a PCV or a check valve style evac will create enough vaccume to create a performance increase. To gain power from a crankcase vaccume system you need to create a minimum of 8" of vaccume. It take a special design vaccume pump or a 4 stage or better dry sump system with valve cover check valves to create that amount. Along with that you need to pay very close attention to cylinder bore finish, a very tight ring land side gap (.0005), prefferably gas port pistons, and a very good designed oil pan. All to control blowby or the pump can not keep up and create a vaccume. High crankcase vaccume will help elimate ring flutter. But ring flutter only starts to accure at over 7000rpm with a 1/16 x 1/16 x 3/16 ring package or over 8000rpm with a .043 x x.043 x 3mm ring package. So ring flutter is not a concern on low RPM street motors.


Open breathers will work fine if the motor has a very good ring seal. If not then a properly plumbed and working PCV will handle any vapors from the motor and still keep the motor clean. If the blowby is too excessive for a PCV and trys to push a large amount of oil out the open breathers then you have to go to a check valve style evac system but it is generally a band-aid to mask a different problem.

 

Hmmm.. just found a quote from Hot Rod Magazine:

I don't know if they consider 7000 - 8000 RPM mid to high engine speeds but maybe - they are wrong...

 

Actually 7000 -8500 rpm would be considered mid range RPM, over 8500 would be considered high rpm. Anything under 7000 is considered low RPM. So for once Hot Rod had a correct statement. (which isn't very often)

Pete

 

Just found some more information.. actually it's common sense but I didn't think about it... I was always wondering why stroker engines would have in genereal more blowby and ring flutter than none-stroked engines.. I just read a few tech articles on that subject... Stroker engines have a longer stroke and therefore higher piston speed. This means that a 383 stroker has a much higher piston speed than a 350... Using RPM to determine the RPM that flutter occurs isn't really accurate... For example - there's a huge difference of the RPM that ring flutter occurs between a 302 engine with 4 inch bore and very short stroke and a 383 with a 4 inch bore and very long stroke.. If the rings would flutter on the 302 at 7000 - 8000 RPM - they would already flutter below 5000 RPM on a 383.. It's all based on piston speed and not RPM... Therefore - stroker engines will need more evacuation than a regular PCV system can provide.. Just look at the size of the vacuum hose going to the carb.. It can hardly pick up any pressure.. The hoses that go to my evac. system can flow about 10 times as much as the hoses that are used in the PCV system..

 

If you need 10 times the flow of a PCV valve hose to handle the amount of blowby you have, then you need to look at the inside of your engine and find out why.

A stroker motor does not produce more blowby than short stroke motor. I build 420 inch motors and at 8000rpm they will not have any more blowby than a 355 inch motor. I monitor that very closely when I dyno motors. But I will say that I am not comparing apples to apples here. To do this it takes a $1400.00 set of pistons, $600.00 set of rings, a $800.00 oil pan and a very experienced hone operator. All of these factors are not realistic to do on a low RPM street motor.


A 3.25 stroke motor will have piston speed of 4333fpm at 8000rpm.
A 3.75 stroke motor will have piston speed of 4333fpm at 6934rpm. Not under 5000rpm as you say. But piston speed as only a small part of the ring flutter problem. At 8000rpm an cylinder will fire 66 times a second. At 5000rpm it will only fire 41 times a second. It is this combustion process that the top ring has to control. As the piston reaches TDC the upper ring is loaded by compression and cumbustion. As it starts to travel back down, if there is pressure on the bottom side of the ring it will rock in the ring land until the pressure on top is the ring forces the ring back down. This process (ring flutter) continuis down the bore until the piston reaches BDC amd starts the exhaust stroke. By creating a vaccume in the motor it will help draw the top ring to the bottom of the ring land to help contol this flutter. It require a minimum of 8in of vaccume to show any effect. Well above what a check valve evac sytem or a PCV valve can create.

The only part of the equation that piston speed comes into play is on the inertia factor of the weight of the top ring. Hence the trend to go to .043 x .043 x 3mm ring packages to reduce the weight of the top ring.

To really understand this concept you need to go to AR when they dyno your new motor. When you monitor blowby very carfully you can see exactly when ring flutter occures. On a given motor you may not have ring flutter at 8300RPM but you will at 8400rpm. Lower the RPM back to 8300 and the flutter goes away.

Pete

 

Pete79L82,
First of all, I don't know much so this is no argument!

I have a question though!

What will 8in. of vacuum do below if you have one crap load of inches of pressure above the ring?

 

Pete,
Good info.

How do you measure blowby?

 

I need to clarify that I am talking about PSI not inches of water.

It is a combination of ring drag, combustion trapped between the two upper rings and any oil that the oil rings did not control that will force the upper rirng to the top of the ring land as the pistion is being force down the bore. The head of the piston has much greater force applied to it because of surface area than the rings do. The exposed surface area of the ring is only the piston clearence times the bore diameter, which is less than a .25 square inch on a 4.0 bore. By creating a vaccume on the bottom side of the rings you help to remove any gases left over from the last combustion cycle and start to pull the excess oil off of the cylinder walls to reduce the load put on the oil rings. As you increase the amount of vaccume (along with other things) you will raise the RPM at which ring flutter occures. On our Busch and Cup motors we are exceeding 25 PSI of vaccume.


Pete

 

There are various means to monitor blowby.
A gas flow meter, similar to what the welding industry uses will give you a direct CFM readout of blowby. I use a pressure transducer that is sampled by a computor to give me a reading of the variation between the flow of air coming out of the motor and the barometric pressure in the dyno room.


Pete

 

I didn't say that I need that size it's just the size that comes with the evacuation kit..... Anyway - the bigger the hose - the more vacuum can be achieved.. Vacuum is a better thing than pressure.. I race the car and it is proven that the crankcase evac. system will give you better sealing rings etc...Why else would most race cars at the track use it? My engine doesn't smoke etc. that would indicate any problem.. because if excessive blowby would get past the rings - you'd get blue smoke...

 

I just have to give a big on that...

Hose size means nothing unless you are moving a LOT of volume