Ok, i was linked to
this thread on miataturbo.net yesterday, and had a couple of longish chats with Rob and Brian (m2cupcar and neogenesis2004) about it. after that i had a think and stare at some pictures and came up with the following comment that i've pasted here for completeness :
Quote:
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Originally Posted by I
I couldnt find those pics that i thought i took. perhaps i never took them. either way, it doesnt matter as yours are of the utmost quality anyway :-)
Further to discussing this with both Rob and Brian last night, i was thinking about what i might do for one when i get back.
Currently i just have the longer labyrinth with the short oem hose hanging in mid air doing nothing much at all, and the oem pcv setup to my manifold much like stock.
For a number of reasons i believe its beneficial to have the pcv valve in place and functioning in some place or other. the pcv gasses can be greasy and oily with no negative impact on anything much, so the small labyrinth is probably best left supplying that in the long term.
its apparent from Robs fine pics that the longer labyrinth and its associated escape route are rather small and restrictive. most likely totally fine for normal NA use, but if you double the power it would be reasonable to double the blowby and therefore with nothing else changed double the velocity of gas through that port under load. given a tripling of power, probably a similar tripling of flow would be needed to keep pressures oem low.
in order to achieve tripple the flow at the same near 100kpa pressure you would need approximately tripple the area to pass it. thats not going to happen through that stock rear port.
my proposed solution is this :
oem pcv as it stands
rear port enlarged as much as reasonably possible
inside of rocker at the front of the head ported where that gas enters the labyrinth to slow the flow sufficiently to allow the oil to fall out
some porting of the baffles in the big labyrinth to ease velocity in there.
and lastly :
having had a think about it and a good look at Robs pics and my own, it would appear that the only decent place to put an extra port on the fe3 cam cover is at the rear of the intake cam. they were nice enough to put a cast in wall across in front of it to keep the oil away, and because its adjacent to the end of the cam no oil will be being flung there are high revs.
an exhaust scavenging system installed on this new port, or just another catch can and VTA (vent to atmo) setup.
given these mods you end up with the following situation :
throttle closed at idle/trailing throttle :
pcv draws dirty (wet) gas out of the crank case and clean air enters the original feed that goes to the turbo intake or vta. either the check valve on the exhaust scavenger is closed and no flow occurs or it too is scavenging some gas and disposing of it.
effect : clean air entering the case on one side, and dirty air exiting where it doesnt matter
result : engine oil stays cleaner and more effective longer because moisture is scavenged out at idle and on trailing throttle
throttle open wide scenario one :
pcv closed due to pressure in the manifold being 100kpa or greater
exhaust scavenging pulls so much gas out that clean air flows in through vta/vtt (vent to turbo) and crank case is kept clean under throttle too.
throttle open wide scenario two :
pcv closed due to pressure in the manifold being 100kpa or greater
exhaust scavenging pulls in exact gas that is being blown by and nothing flows through vta/vtt
throttle open wide scenario three :
pcv closed due to pressure in the manifold being 100kpa or greater
exhaust scavenging pulls in a bunch of gas, but not enough to prevent pressure buildup or more flow.
VTT/VTA dumps excess to atmo or turbo inlet
in this way you end up with low pressure at all times and clean air flowing in to replace wet blow by whenever opportune.
its possible that the exhaust scavenger would always be enough, this could be tested with the vta/vtt blocked and a sensitive pressure guage installed in the cabin and watched while thrashing the car hard.
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after discussing this with them a faint memory of spots of oil that had exited my open pipe at speed came back to me. if they were exiting at speed then there was significant volume coming out. not surprising when you consider the more than doubling in power.
currently that seems to me to be optimum.
if the vent gas can be kept clean enough, then feeding it to the turbo is just fine. if it cant, a can with filter would be better so the intercooler stays dry.
fred.