Quote:
Originally Posted by 6SPD_FTW
Ya know, I rather take a bit of offense to you saying I am spreading incorrect information because I am not. You are.
Blow-by is what leaks passed the piston rings on the power stroke - "It sucks up blowby gasses/oil and deposits them into the intake tract"....yeah....that stuff. Guess what blow-by is???? 70% unburnt HC's - raw fuel. PCV is to prevent a positive pressure buildup in the crankcase (as a result of blow-by) and to prevent FUEL DILUTION of the engine oil, which causes corrosion of internal engine parts. It's also why PCV valves are designed to close completely on backfires to prevent fuel vapors in the crankcase from being ignited. The fact that a little bit of engine oil gets sucked up into the PCV in the process is the problem that people install catch cans for.
Our engine, as most these days, uses a variable-orifice PCV valve....at low load and high vacuum, it flows less, whereas at high-load and low-vacuum it flows more (because high loads produce much more blow-by), i.e., it accurately matches ventilation flow with blow-by production. Do you REALLY think Nissan (or any manufacturer) is going to design an engine that completely disregards PCV flow into the fuel map? NO. It is correct that no two engines will produce the exact same amount of blow-by at a given load value and there are differences from engine to engine as to blow-by characteristics. Saying the ECM doesn't compensate is just plain dumb. No two injectors deliver EXACTLY the same amount of fuel, no two air filters or intake manifolds deliver EXACTLY the same amount of air, no two engines are EXACTLY alike. That is why we have a closed-loop feedback control system, i.e., an AFR sensor (or O2 sensor depending on your application). It's basically the ENTIRE reason vehicles are controlled by a computer these days. The three-dimensional map of long- and short-term fuel trims is how the ECM compensates for not ONLY PCV variation, but variation in anything that would affect the AFR, e.g., wear and tear.
So yes....if you use a crankcase breather and disrupt the PCV system, the ECM will readjust and it probably won't be enough to upset driveability. But without manifold vacuum continuously cycling fresh air through the crankcase, I wouldn't even want to take the chance of fuel dilution and corrosion of internal engine parts. A catch can is a much smarter idea than just deleting the PCV, and a catch can can be fabricated for <$10 with an air compressor moisture trap from Home Depot or Lowes. Did it myself and it works well. Plus, you have the added benefit of knowing that when the catch can starts catching lots and lots of oil in there that your piston rings need help.
Late,
Trav
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Think about what your saying for a second. "70% of blowby gasses are unburnt hydrocarbons". I'm not sure where you came up with that number as it's likely NOT true. What about all the other non-hydrocarbon gasses that are present? The overwhelming majority of combustion gasses would be non-hydrocarbon gasses. Anything not consumed during combustion will be very minimal. However let's assume it is true.
70% of the blowby gasses (which is a small amount to start with) gets sucked back into the intake tract. Want to guess how much %-wise this unburnt hydrocarbon mixture is vs amount of fuel injected into the cylinder during combustion? Basically negligible. The ECM has NO WAY of measuring this. Your mass airflow sensor tells the engine how much air is getting in through the intake and thats ALL the ECM injects for. So yes I am saying that NISSAN does not account for blowby gasses in the AFR equations. I can also tell you for certain that Subaru does NOT account for blowby gasses in their modern ECM's as well. I have a fair bit of experience working with Opensource tuning on the Subaru's and have seen the effects of running crankecase breathers set to VTA on aggressive and daily driver vehicles. In short there are no problems (at least with AFR anyway...emissions is another story). I'm not as familiar with Mitsubishi, but I'm pretty sure they don't account for it either.
The ECU's AFR correction is also not 3D. Its 2D. Think of it like this. (just guessing on the actual g/s values)
MAF sensor g/s ECU correction %
0-10 x%
11-30 y%
30-50 z%
50+ a%
Oil contamination is not really a concern. The oil gets really hot and the fuel is vaporized. Even with just VTA crankcase breathers, you should really be fine unless you never let your engine get up to operating temps. If anyone is really that concerned about fuel dilution then get an oil analysis done after your first oil change of running VTA breathers.
Also an AFR and an O2 sensor are one in the same. In fact there is no such thing as an AFR sensor. The O2 sensor just measures the ratio of oxygen in the exhaust tract to that of the ambient air. Then calculates the air fuel ratio based on that data.
Additionally your comments about the air filter and the manifold not delivering the same amount of air is meaningless. Only air that makes it past the air filter will be measured by the ECM (which then tells the injectors how much to inject), and all of that air will go into the engine to be burned. Perfect air-Cylinder distribution is inconsequential because there is not an O2 sensor at each cylinder. The system's O2 sensor is only being read far enough downstream that cylinder exhaust will be mixed. I would agree that air-cylinder distribution is important, but its not adjusted for in the ECM.
I'm glad you've had good luck with your setup. I'm not saying all of what you said was miss-information, just the part about the ECM using blowby as part of the air fuel equation.