Crankcase pressure...

[Mark_88]

I was scouting around for an oil separator and went into one of those performance shops to see what they had. The guy behind the counter seemed to know exactly what I wanted and went about researching prices for me. Found one that would work well for under $140...

But in the process he mentioned something I'd never heard of...balancing the crankcase pressure...meaning that when there is too much pressure it actually makes it hard for the pistons to push down and this can severely restrict power. Apparently, it is also not good to have no pressure (which is about where I am right now) and was curious as to how to control or limit the amount of pressure.

I've heard of check valves just before the PCV valve that would allow excess to be blown off while still allowing the PCV valve to function and allow some air back into the vacuum system.

Anyone see any good articles on this? I've searched and found way too much information that will take me days to plough through...

Thanks in advance!

 

[Mikel89us]

Depending on your performance wishes, you may want some pressure or none. Race engines that use a dry sump system actually create a vaccum in the crankcase. The purpose of the PCV system is to remove the blow by gasses from the crank case, it doesnt draw a vaccum on the engine, but it removes most of the pressure. I personally (on my mustang) have a vent tube that exits to the ground. It drips a small amount of oil, but in 5k miles i dont lose even half a quart.

 

[RonD]

The amount of Blow-by in each cylinder increases with age, as the rings wear out.
If you do a compression test on a new engine and it has 180psi and then tested the same engine at 100,000 miles it would have about 165psi or less.
Most engines have the PCV and have a vent hose from the valve cover to the air plenum between the air filter and intake, the vent hose takes up the slack in changing internal pressure, so oil pan and valve cover gasket don't fail, although they still do, lol.

A racing engine is rebuilt often so balancing crankcase pressure would be easy, and the theory behind neutral pressure is sound but not complete.
If you think about it it makes sense, the less pressure behind the piston the less power is used up by pushing against the pressure on the power stroke, but.................
There is always a but, lol, what about the other pistons that are going up in their stroke, the more pressure behind them the less power it requires from the crankshaft.

Not sure how to do the math on that, but just from thinking about it I don't think we are talking about a lot of performance change in a street vehicle.
i.e. I wouldn't bet pink slips on it

 

[Mark_88]

Thanks for the input guys. I know my engine has blow-by but I was wondering, and I should have stated, if anyone knew what kind of pressure it would be in PSI under normal conditions and what it would be if, say, the rings were bad.

I'd also be interested to know how much the actual valves would play in this is the seals were wonky and the valves were not sealing properly.

I was planning on swapping in a rebuilt head but just read some more on blow-by and leak down tests. Apparently if there is pressure out the dipstick then that is blow-by from rings.

I really don't want to waste time with a head swap (and expenses) if the bottom end is really pooched. Maybe I should invest in a leak down test myself to see what the problem is, but I'm pretty sure the smoke alone is from the valve seals...it smokes between shifts and on first start...and it can smoke under high vacuum...all indicating wonky valve seals...and my number 2 plug is contaminated big time with oil but the rest are showing normal...

 

[adsm08]

Depending on your performance wishes, you may want some pressure or none. Race engines that use a dry sump system actually create a vaccum in the crankcase. The purpose of the PCV system is to remove the blow by gasses from the crank case, it doesnt draw a vaccum on the engine, but it removes most of the pressure. I personally (on my mustang) have a vent tube that exits to the ground. It drips a small amount of oil, but in 5k miles i dont lose even half a quart.

Really? You use a road draft tube on a 91? Those things haven't been in use since the 50s.

 

[Mark_88]

Really? You use a road draft tube on a 91? Those things haven't been in use since the 50s.

The PCV system pretty much eliminated road draft tubes...and they are illegal in Canada...which is somewhat close to what I'm running now...only I've put oil separators on them to try to drain off excess oil...but that increases crankcase pressure...especially when the filters get fouled...that's why I was looking into a professional version of the oil separator...and they are pricey...

 

[Mikel89us]

Yes I use a road draft tube, Im not a big fan of putting blow by back into my engine. And here in Indiana they dont test for emissions, so I let it go. I was going to use the oil seperator still, but i converted it from the 8 plug to a 4 plug head and intake (and distributor) and the stock 8 plug separator is taller and didnt fit under the 4 plug intake.

I just saw that you live in PA, im sorry for you!

 

[RonD]

Thanks for the input guys. I know my engine has blow-by but I was wondering, and I should have stated, if anyone knew what kind of pressure it would be in PSI under normal conditions and what it would be if, say, the rings were bad.

I'd also be interested to know how much the actual valves would play in this is the seals were wonky and the valves were not sealing properly.

I was planning on swapping in a rebuilt head but just read some more on blow-by and leak down tests. Apparently if there is pressure out the dipstick then that is blow-by from rings.

I really don't want to waste time with a head swap (and expenses) if the bottom end is really pooched. Maybe I should invest in a leak down test myself to see what the problem is, but I'm pretty sure the smoke alone is from the valve seals...it smokes between shifts and on first start...and it can smoke under high vacuum...all indicating wonky valve seals...and my number 2 plug is contaminated big time with oil but the rest are showing normal...

There is always blow-by, that is seen in all piston engines, even brand new.
There is just more as engine rings have normal wear.
Excessive blow-by will usually cause oil to build up in the vent hose and air plenum tube, this is the rubber hose that comes out of a valve cover or "oil filler" tube and runs to the hard plastic Intake plenum.
Remove and inspect these, if you see liquid oil build up then probably time to due the rings, slight oil vapor residue is normal in older engines.



No reason to remove the head to put in new oil seals on the valves, that can be done with only the valve covers removed.
You will need:
new seals
a valve spring compressor
Small diameter longer rubber hose, like vacuum line from engine
small balloon, fits in spark plug hole when deflated
vise grips

Remove all spark plugs
Remove valve covers
Remove rockers
Do 1 cylinder at a time, manually rotate that 1 cylinder so it is at it's TDC
Insert one end of rubber hose into balloon's hole, tape it up well to hold pressure and not fall off.

insert balloon/hose into spark plug hole
blow it up, pinch hose closed with vise grips, we are just holding the vales in place with the balloon so it doesn't require a lot of pressure, valves are very light weight.
remove the springs and keepers
replace seals
re-install springs and keepers
deflate balloon, suck it flat if needed.

Rotate crank for next set of valves

Anytime you are working on an engine with the valve covers off it is a good idea to plug the head's oil drain holes, a few shop towels;
this will prevent the "oh no's" when a needed bolt or part succumbs to gravity in a Murphy's Law manner.

You can use compressed air in each cylinder instead of a balloon to hold the valves in place if you have a compressor and spark plug adapter, but if you had that you probably wouldn't have said you would "pay for a leak down test", so I am just assuming you don't have that equipment.

 

[Mark_88]

Thanks Ron...no, I don't have the tools but I know someone who does...

The main problem is I think I cracked the head with an over heat issue. The engine has been getting progressively worse since then in terms of oil consumption and blow by...could also have damaged the rings in the process of over heating, but the motor actually runs very strong...other than the oil burning and blowing all over the place.

My oil catch can hasn't seemed to have actually trapped any oil (as in more than oil film) and the tubes are gunked up with foam from using Lucas (I watched the video someone posted about that).

When I slam the engine with a bottle of "no smoke" stop leak the smoke clears up for a few days...and then starts all over again...running thicker oil has increased the amount of time the oil stays in the engine, but there are other issues with valve cover leaks (among others).

I do have a block that has only 105,000 km (just over 60,000 miles) on it but was hesitant to swap the blocks (still have to go get the block etc etc)...

Anyway, I had the head rebuilt already...just a matter of tearing it apart and putting it back together. Since I'm doing it outside I was hoping for a couple of days of sunshine and warmer weather to do it...should only take two to four hours in total since I've already done this three times in the last ten years...and my swap time was just under 3 hours last time...IIRC...

 

[adsm08]

I just saw that you live in PA, im sorry for you!

Why? My vehicles are registered in Northumberland County. As long as everything is there and looks like it might have half a chance of working I pass.

Also, I am a licesned inspector, so as long as it isn't horridly dangerous (which is a state I don't allow my vehicles to get to) it passes weather is it stricly within the letter of the rules or not.

 

[Mikel89us]

Well good for you! I thought all counties in PA were strict. I know most of them are. I dont even have to have anything stock, if i want it i can have it. including the long tube header and no cat. No EGR, no PCV.