sparkybean

I appreciate im probably going to get slammed for starting another one of these threads, but im at that point in my build, and it seems everything im reading conflicts with something ive read somewhere else. Ill start with what ive learned, and what i think i know:


-1-
I need a way to vent the gasses that are flowing from the crankcase, due to piston blowby. This is a given.

-2-
The higher the cylinder pressure, the proportionally more blowby. Thus, sizing the system to evacuate the crankcase while boosting hard is wise.

-3-
Having a vacuum placed on the crankcase is a good way to remove the blowby more efficiently, compared to just venting to atmosphere. This also increases power and reduces said blowby (by how much i dont know) by promoting better ring sealing.

-4-
I realise now that crankcase blowby is not just gas, but a gas with some oil vapour and some other nasty byproducts mixed in. The added oil vapour and nasty byproducts lower the octane rating of the intake charge (by how much i dont know), so need seperating if the crankcase gases are going to be routed to the intake. Hence why catchcans are a thing.

-5-
Routing the crankcase byproducts into the exhaust increases tailpipe emissions (by how much i dont know). Routing just the crankcase gases into the exhaust MAY increase tailpipe emissions.

-6-
Engines typically emit about 1% of thier intake air flow rate in blowby, assuming at least half decent compression.

-7-
Flowing fresh air INTO the crankcase is somehow 'a thing' and benificial (i dont know why yet).



Right. Thats about everything i could summise. Im trying to design a system for my turbo'd daily, so if im wrong anywhere, bring the flames. Im shooting for ~230hp, car will be used on the track and the road, thus will have to pass an emissions test every year. I may or may not run a cat (if i need one, ill fit one) but i digress. I am currently thinking about an exhaust slashcut, with the biggest diameter tubing i can reasonably fit, with a catchcan in the line.

Thoughts?

thirdgen

iTrader: (26)

I have a slashcut with a -10 line coming off my breather port in valve cover. No catch can. Seems ok for me.

Chooofoojoo

iTrader: (7)

Options (from my research) :

1) Stick to the Tried and true Stock PCV system WITH a sealed baffled catch can between PCV valve and Intake manifold. Works like stock, keeps the crap out of the intake manifold more or less. Simple, can be modified with a check-valve if PCV valve leaks under boost (a la Brain). Run a little filter on the driver-side VC port. Emissions with inspection? Do this.

2) VTA. Doesn't provide vacuum, but keeps your IM squeaky clean. Simple.

3) Thuuper-duper Slash-Cut. Pulls vacuum (don't think anyone has measured how much though), and can have a catch-can so you don't smoke up exhaust. This is what I run. Two AN10 Breathers off the VC, "Tiny-hole" enlarged, ran to a Sealed Baffled Catch Can to a Vibrant Slash-Cut. Overkill and expensive? Sure is! Stock PCV would have worked perfectly fine for my application . If I remember correctly slash-cuts are not legal in a lot of race series, and heavily frowned upon for inspection emissions. Retrospectively wouldn't do this unless Fae or Soviet level power.

aidandj

iTrader: (5)

Somewhere in faes thread he decided that his slashcuts didn't work. There are a variety of reasons why his didn't work.

18psi

iTrader: (76)

prove to me that it does. on your car
do it.
now

aidandj

iTrader: (5)

I will.



And if it doesn't work I will gladly join your side.

18psi

iTrader: (76)

deezums

iTrader: (3)

Know how a glass bong/pipe works?

The carb/choke/whatever is a source of fresh air in a vacuum.

If there is no source of air, the blowby is never cleared. Look at any OEM PCV setup, miata included.

If there is no flow there is no point even bothering with a catch can and hooking useless tubes to the manifold.

Consequently, I don't understand how someone could have both a intake vacuum driven PCV and exhaust slashcut at the same time, there's only two ports on the valve cover.

Back to the flow situation, if you aren't putting fresh air into the block whatever else you are doing is a waste of time. The quality of slashcut at anything other than ***** out is in question.

All I see is a mess of tubing and check valves and inadequate sources of vac. Aidan don't even run more than 4 pounds on track, so I've no idea what the hell he's doing. Every time you let off you have super duper strong vac ready to clear the "pipe" unless you slashcut...

Why are you making it so hard lol

nitrodann

Hang on back up..

Are you saying:

"People who have both sides of the cam cover vented to a catch can arent doing a good job because you need a way to get fresh air in."

Because if you are, id like to understand that perspective. Genuinely, Im intrigued and would like to learn, no sarcasm at all.

sparkybean

Im not set on the exhaust slashcut idea. Emissions are just measured at the tailpipe here in the uk, if you pass that they couldnt give a damn what else you have done to the car. Im still yet to sit down and read fae's mammoth 180 page wonderthread, but i cant help thinking that if its not effective just use huge lines, say 3/4in. However, if it doesnt work, it doesnt work. Fine.

Im not sure i agree. There is a constant flowrate of blowby from under the pistons, so if a proportionally constant flow rate can carry these away because of a suitable vacuum then no problem, right?

I can only see introducing fresh air into the crankcase necessitating a higher flow rate OUT of the crankcase, as you are now having to remove blowby and an additional air input source.

For what its worth, id guess an mx5 making 200hp generates 0.5l/sec of blowby past the rings.

sparkybean

Not knowing the reasoning yet behind your 'fresh air in' method, id say this is a great idea. High vac from the manifold when blowby is at its lowest (and for when the emissions test is done, mwuhaha), then the high suction from the exhaust slashcut for when the engine is in boost and for when the slashcut works best, and keeping oil out of the intake. Might even be able to get away without a catchcan if i can keep the exhaust from going all james bond smokescreen...

nitrodann

Best case scenario, maybe.

sparkybean

Righto. Lets call it 2L/sec. Id guess the crankcase holds maybe 20L of air? (this is probably entirely acedemic)

fooger03

iTrader: (2)

The PCV system, with a fresh air intake, introduces cool dry air into the crankcase which is then evacuated by the vacuum in the intake manifold. While that cool dry air is in the crankcase, it warms up (allowing it to carry more humidity) and it gets wet (as in, it picks up high humidity and condensation from within the crankcase.) It then exits the crankcase through the PCV valve as warm moist air.

The ideal combustion process produces two byproducts. Those byproducts are carbon dioxide and water. Any other compounds are formed by contaminants in the oxygen/gasoline mixture (such as that unnecessary and annoying ******* nitrogen molecule)

This means that water enters the crankcase. If this water entered the crankcase merely as humidity, AND if the inside of the crankcase were permanently warm enough to ensure that the humidity did not condense while it was inside of the crankcase, then we simply wouldn't need the P in PCV.

Unfortunately (or fortunately) for us, the combustion process is....well...violent. It's violent enough that it is capable of condensing the humidity in the burn mixture, and separating it from the exhaust air. Some of this water condenses in the barrier regions next to the piston and cylinder walls and does not become exhausted with the exhaust gasses. As a result, this condensation exits past the rings and into the crankcase.

During normal operation on a shiny new tight OEM engine, this condensation, moving slowly past the seal of the cylinder rings, helps prevent exhaust gasses from escaping into the crankcase - the water effectively helps seal the combustion chamber. This also means that it's possible that the only thing escaping into the crankcase is this condensation. If that is true, then there is no air charge from the combustion process to evacuate this moisture out of the crankcase. The moisture will eventually build up in the crankcase causing the sorts of problems generally associated with water in the oil...the condensation isn't an issue when the oil and block temperatures exceed the boiling point of water, but without a flow of air entering the crankcase to evacuate that "stagnant air", that air has the chance to eventually become 250* air at 99% humidity. The problems, then, arise when you turn the key off.

Now, on our fancy pants old or rebuilt engines with "hotdog down a hallway" tolerances, this is perhaps less of an issue, but it's still an issue nonetheless. The likelihood of us building up enough condensation in the oil to cause issues is small, but I would bet a lot of us would start running into issues if we left the car off for several months. For an OEM though, this is a reliability issue - without PCV, their engines simply wouldn't last as long.

Could this be why "because race car that never sees a race track" engines last 40,000 miles and OEM engines routinely make it to 200k? Vacuum pumps and VTA set ups certainly make sense on a race car engine when the rebuild intervals are measured in hours and not miles.

deezums

iTrader: (3)

That is exactly what I am saying.

Like I said before, look at a bong. Smoke is representative of blowby, sucking on the rig is the PCV/Intake, and removing the bowl or freeing the carb is the same as providing fresh air into the block or the exhaust side breather.

No source of fresh air will clear some blowby, but exponentially less than were it free flowing.



If you are constantly producing blowby, yet no airflow you get clouds hanging in the crankspace.

As soon as you introduce fresh air, the whole volume is cleared lickidy-split.

Since a car has infinite lungs, may as well have a source of fresh air all the time, same as OEM.

aidandj

iTrader: (5)

Remember. I'm the guy that bought a twin disc for a stock 1.6.

I'm doing it for science. I'm going to put a map sensor on the crankcase and run with the slash cut and without. Then I can log data and report. I plan on Y-ing the slashcut into the intake side so that there are 2 sources of vacuum. One under boost one under vacuum.

deezums

iTrader: (3)

I know, and i kid

The crank should be reading near barometric, at least that would be ideal, at all times. The vacuum is in the manifold/exhaust and good ol' gravity tries to shove the air, with all the blowby, into either one. Reading map in the crank is kinda useless data, or so I hypothesize. Reading map against a sealed off slashcut won't really tell you anything relevant, either.

You really need to check flow through the hose to the slashcut, but is there any easy way to do that?

aidandj

iTrader: (5)

Well the easy test is what the crank pressure is with and without the slashcut.

Not sure about how to measure flow.

deezums

iTrader: (3)

Yeah, but it's useless test.

It's the same as reading map in the airbox, expecting to see how well stock pcv works. It will show a number, but what's it mean?!

If it reads slightly low vac or the more likely slight pressure, I think it means you should slightly enlarge the exhaust side port.

aidandj

iTrader: (5)

But it will tell me if it is pulling a vacuum or not. Right? I'm not testing if it "works" or if its the "right thing" I'm testing whether or not it will pull vacuum.

Wonder if there are cheap inline flow meters.