Firstly, my question is... What is the point of having a 3/8" PCV valve when there is a 5/8" open breather on the other end of the rocker cover? Could the PCV be done away with?
.....
I am now in the process of fitting triple Webers and am thinking about removing the check valve from 2 PCV valves and just using them as breather fittings (so I can use 3/8" hoses to my catch can and do away with the 5/8" hose, then run the catch can outlet to the air filter housings on the Webers).
I just can't see the point in having a functional PCV valve fitted when it had an open breather hose of larger diameter fitted next to it. The larger breather would be venting one way or the other long before the PCV valve kicks in.
I don't know if I've explained this properly as I have too many ideas running around in my head atm... If you don't understand where I'm coming from, let me know.
Cheers all,
Zac
The idea behind the pcv was to stop blowby gases from being released into the atmosphere. They were one of the first emission controls to be fitted, and apart from helping with pollution control they also reduce the stinky fumes around the engine and help keep it clean.
With any ventilation system you not only want to get rid of the blowby gases, you ideally want the volume of gas drawn out to be a bit more than the volume of blowby so you can draw some clean, fresh air through. That's one reason for the other line to the air cleaner, it admits the additional clean air and helps keep the concentration of corrosive vapours somewhat diluted. Another reason for it is the fact that it would be practically impossible to make the flow through the pcv always match the blowby volume, and the crankcase would nearly always be either under pressure or vacuum (as you found out).
The stock system works very very well with a stock engine (even if the in and out ports could be a bit better positioned) but it all falls over with anything more than a very mild cam. The change in vacuum makes the pvc behave erratically and it affects engine performance. BTW the pcv doesn't "kick-in" - it's designed to have a roughly constant flow regardless of the vacuum level. With bigger cams I've had good luck using a restrictor in the pcv - I just take the guts out and silver solder a bung in the hose tail that I drill through about 0.030". It looks stock and provides some amount of circulation at idle and light throttle. An oil separator/air filter admits clean air and provides additional venting at wider throttle openings. If you can, also use a vac-u-pan style setup to draw the blowby out at WOT rather than just venting it. These will even work with a full exhaust system and mufflers so long as the system isn't too restrictive. I hate using catch cans (it's like publicly admitting you can't build a decent ring seal) but with a 186 or 202 they're always going to be on the cards.
Crankcase ventilation and oil drainback are very closely related - if drainback is a problem it's good practice to vent the blowby from a position lower on the crankcase rather than up top. That way the oil won't be hindered from dropping through the drain holes by gases flowing upwards through the same holes.
Also be very wary of aftermarket rocker covers - the baffles they use are often shithouse and let a lot of oil through.
If you run some sort of pcv tee it into all 3 runners, then use an additional port in the rear runner for brake vacuum.
Anyhow, there's more than one way to skin a cat but that's what works for me..
