CentralCoaster

http://forums.corvetteforum.com/zero...=newtopic&id=7

I have an '85 L98 vette, and oily residue getting into the throttle body has always been a problem. Any excess blowby the pcv can't handle will go up the pcv supply line, backing up into the throttle body, and making the passenger side valve cover oily.
The PCV system has to be a closed loop to prevent unmetered gas from entering the system. It's sealed to prevent blowby to escape and pollute.

Now what I'm thinking.. is why not just vent excess pressure to the atmosphere, put a check valve on that side, to prevent the pcv from sucking unmetered air in through it. And... just have the pcv suck from the crankcase.

Is it ok to suck solely from the crankcase? Will this pull in too much oil, or create TOO much crankcase vacuum? I thought crankcase vacuum was always a good thing, something carbed cars with breathers don't have, which is why they burn up more oil.

Here's an explanation of exactly how the PCV system works on TPI vettes if you're not familiar:


quote:
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The drivers side pcv line goes behind the throttle blades, (the vacuum side)
The passenger side pcv vent tube connects ahead of the throttle blades, (the atmospheric pressure side)

So.... flow will go from the passenger side port on the TB, down through the valve cover, head, crankcase, and up through the other side, through the pcv valve and into the plenum.

That is why you can't use one breather on these cars. If you did, the vaccum would just suck a bunch of air from the passenger side breather through the pcv valve on the drivers side... essentially a vacuum leak. (Un-metered air.)

The only time blowby goes UP the passenger side pcv vent line ahead of the TB blades, is under high load condtions. (Low rpm + high throttle = lugging)

This results in lots of cylinder pressure, lots of blowby, and not enough vacuum to pull all the blowby out. Soo.. it goes up the passenger side tube. I think it was designed this way on purpose... so that no blowby gasses could escape from the engine and pollute the atmosphere. Instead... they pollute the whole intake with nasty gooey slime.
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superlund

I've got the oil seperating filters on both PCV lines...The fresh air / high load overflow (blowby) side & the primary line, works great.

LaVidaLoca

I have no PCV valve at all. I put caps on the valve covers with 5/8" fittings on them. Then run a thin flexible tube down to the bottom of the engine compartment so any oil goes on the ground and not in the engine compartment. The engine can breathe as much as it wants with no restriction. I used to have caps with filters in them, but they would drip oil onto the engine after a while.

CentralCoaster

Do you have oil consumption issues with this setup? Apparently crankcase vacuum helps the rings seal better, thus giving higher cylinder pressures, and better performance. I don't exactly understand the reasoning behind it yet.

LaVidaLoca

Nope. Zero oil consumption. The engine only has about 6,000 mi on it and has not needed any oil between changes. BTW breathers on the valve covers used to be the standard before the EPA decided the oil vapors were
bad. So, the PCV is not there to help seal, give pressure or performance. The PCV is there to put the crankcase vapors back down the intake to burn. I capped off the fittings on the TB and manifold so there is no more nasty oil going into my intake.

I just went back and looked at your profile and see that you are too young to remember the pre EPA days :lol:


[Modified by LaVidaLoca, 10:14 PM 9/25/2003]

superlund

You need a vacuum on the back side of the rings to help them seal. Here is a good quote.
"By producing high crankcase vacuum, horsepower is increased by improving ring seal and preventing blow-by, intake-charge contamination and detonation"
From this site... http://www.aerospacecomponents.com/vacp.htm

Toyota took PCV design to a higher level see here... http://www.autoshop101.com/forms/h63.pdf

This site has a valve cover breather with a one way valve that eliminates the vacuum leak / behind the MAF. Only vents outward, allows no air to be drawn in, would work well on our cars. http://www.petersonfluidsys.com/vacsys.html

-Jim

tjwong

Can you describe the oil separtion system you have? I have a LT4 and for some reason it sucked a bunch of oil into the intake from the PCV. I have no idea as to why, there is not a hint of blow by in this engine. And this is the first time that it happened, the PCV line was wet with engine out while the breather line on the opposite side was dry. Nothing in the ventalation system is blocked or is there any signs of blockage.

CentralCoaster

Great info. This is exactly the gizmo I was dreaming up.


One Way Valve - Pop Off
Breather assembly is installed on engine's crankcase in vacuum condition. Valve seals engine from air entering engine, but opens in the event of crankcase pressure, allowing crankcase to vent. Comes complete with -12 AN bung, to weld to aluminum valve cover, and filter.

My only concern is that with this check valve on there, will the PCV pull too much vacuum and suck even more oil into the intake?

Maybe this is tjwong's problem. Maybe his problem is a clogged pcv vent line on the passenger side.

superlund

IMHO, I don't think you can pull too much vacuum; the issue is separating the oil out before it is drawn into the intake. Just look at those hi performance vacuum pumps. Call Peterson up on the valve cover breather, they have long been a leader in oiling systems. The simple fact they designed it tells me they know the issues.

TJ could have a plugged vent line, but states his ventilation system / side is working...

I'll post some pics of my system w/ separators this evening.



[Modified by superlund, 7:25 AM 9/26/2003]

superlund

OK sorry for the delay, found time to upload some pics, added some lines & text. Forgive the mess, I can't drive a wheel-ball optical mouse very well... you may need to zoom in to read.

First pic shows the oil separator on the driver’s side, simple air tool type filter from Lowe's, just routed the lines from the intake vacuum & PCV ports as required. Blue line is from PCV valve & white line is to Vacuum port. Notice it has a small amount of oil trapped at present.



Second pic shows the oil separator on the pass side, same type filter. You can see how I routed the line from the valve cover, by following the yellow line. The filter is directly in front of the HVAC blower motor.


Third pic is another shot of the pass side & shows the filter in better detail & you can follow the yellow line on to the T-body inlet.


-Jim



[Modified by superlund, 10:28 AM 9/28/2003]

vinnies87

Have 87 L98 and have noticed the same thing, oil on the valvue cover, but haven't noticed any excess in the throttle body... but wondering what would be a good solution to this...

flynhi

When I first installed the 383 in my 71 I was using quite a bit of oil. This could have been due to the rings not being seated yet.

I installed PCV valves on the hoses in both valve covers. I don't know what other effects that has had, but it has stopped the oil burning.

Am I screwing something up by doing this?

87NassauBlue

After reading all of these posts, there have been a lot of different interesting views of the pvc system. You didn't mention how many miles your engine has on it. If you getting an excessive amount of oil in your throttle body, you are either running your car very hard, your engine has more blowby that the pvc can handle or both. The pvc system is an emissions system, not a performance enhancement. Before PVC systems were common, the crankcase vapors were vented to the atmosphere. Think about it, your crankcase has a volume of gases in it that are displaced each time a piston goes down. Oil mist is also present. These gases have to go somewhere when the pistion are on the down stroke. At some point in the life of your engine, your rings will wear to the point that the blowby will become excessive and force more of this oil and vapor into the intake. That is why you are getting oil into your throttle body.

The website that is selling high vacuum crankcase pumps does have some valid racing points. A high vacuum on the crankcase will allow less resistance when the pistons come down because of the less dense gases. The part abou t sealing the rings better, could only be attributed to preventing ring float. but this would only be at high vacuume. I didn't read how much vaccume this racing pump can pull on the crankcase, but a typical engine will not have a high vaccume on the crankcase. You might be due for a rebuild. I'm going through it right now my self.

flynhi

87 NassauBlue
At the same time crankcase pressure is being increased due to a piston moving down the bore, another piston is moving up its bore an equal amount.
So it would seem that the pressure would reamin roughly equal and there would not be a significant amount of crankcase pressure due to piston movement and it mus be coming from loose rings.

87NassauBlue

:iagree: I pretty much agree with you. At higher rpms, there is a lot of gases moving around in there and some can get forced out because the gases don't always go exactly from the higher pressure area to the lower pressure area. The engine vacuum should be able to handle the excess without getting a lot of oil in the intake. If your getting a lot of oil in the intake, you probably have weak rings. I agree with you.

I really thought that high vacuum crankcase racing pump was interesting, but they didn't note any volume or pressure specs on the site. I wonder if there are any test results on these pumps.

JUAN J SANCHEZ

Here's a picture of my air pump converted to vacuum pump on my 89. I was concerned about sucking oil and non beneficial contaminants that may lower my octane rating, among other things, into my intake. After doing some research, it led me to GZ motorsports's Greg who gave me a quick explanation of PCV/Crankcase systems and convinced me that I could make a vacuum pump work on the street with little maintenance. I was gonna get rid of the air pump anyway but as some of you might know you need some kind of pulley in it's place to make the rest of the serpentine set-up work! Why not make it beneficial? The key to making the pump live is to keep it lubricated with oil. I'm gonna feed the pump oil from an oil pressure port and regulate with a nozzle/jet type set-up. I'll keep ya posted ! I've mocked everything up and it lines up perfectly. I had to get a custom pulley because my stock air pump pulley 6" was too small and would have spun the pump too fast (you want about 70% of crank speed) so Greg fabricated a 8" pulley. I'll let ya know how it works ! Let me know what you think?


[Modified by JUAN J SANCHEZ, 2:25 AM 9/30/2003]


[Modified by JUAN J SANCHEZ, 2:26 AM 9/30/2003]