No catch can here, stock motor with tune. Does not use oil between changes.

Many years ago we used to race V6 Buicks and we put a K&N breathers on the valve covers. Still ran PCV and pistons were always clean as was intake.

 

Since you won't tell us what you are running or what your plans are, my answer still is it depends. Probably not.

If you are running large ring gaps and a decent amount of boost, then it won't be such a small amount of stuff.


I am currently naturally aspirated, but I don't use PCV because of the amount of oil it introduces into the manifold and intake ports. I have two ports on my engine piped into a tee and dumped straight to the ground.


I would install a centrifugal oil remover and pipe the combustion by products back into the intake, if someone made one, since every method of trying to catch the oil/gas/water from the crankcase hasn't worked for me.


I don't enjoy smelling the fumes, but I prefer to coat the intersection with oil instead of the inside of my intake manifold.

 

Hey MikeGyver,

Thanks for the input.

I intentionally posed the question about the effect of contamination on octane without being specific to my vehicle, because the information I am seeking will not be vehicle specific and therefore more useful to everyone.

Thanks

Kyle

 

Reading Guyers response triggered this EDIT.

CORRECTION. THE PCV IN MOST VEHICLES DOES NOT VENTILATE THE CRANK AT HIGHER RPM’S. IT F7NTIONS ONLY AT IDLE AND WHILE CRUISING AT LOW RPM’s

EDITED ABOVE.....I have read; PCV systems do not return much to the intake system at lower RPM.



The original question remains unanswered.

How much does the PCV that is directed through the intake affect octane?
​​​​​​​

Is it real or marketing by the catch can sellers?

 

I think you are overthinking it, if you want to continue over thinking it, that is your choice and I'd like to see what your conclusion is.

I would approach it more simply. Do I want the coldest, cleanest, air going into my engine? Yes.

 

amen brothuh. Thats why i vent mine

 

The fresh, filtered air for PCV enters the system at the fitting on the throttle body. So the air enters the crankcase through this hose, and is drawn into the intake manifold through another hose with the PCV valve in it. There are only two openings into the crankcase.

So, at larger rpm or engine load, any blow by that exceeds the flow capabilities of the PCV valve will reverse the normal flow. No fresh air enters the crankcase, only blow by comes out.

And is drawn into the intake manifold through the fresh air port in the throttle body.

So regardless of what effect it has on octane, in the stock PCV system any unburnt gas, oil mist, or water vapor must re-enter the engine and coat the inside of the throttle, intake manifold, and intake ports.
ports.

That bothers me enough to eliminate PCV

 

So while I don't know the math, I would have to guess it wouldn't be hard to find. I am sure someone like EFI University has tried this on an engine dyno.

What I do know is, that if you are burning oil, and it is reducing octane; then you are running in knock retard, or risking detonation; assuming the problem is bad enough. With a prepackaged sealed can system w/ check valves running ~$200 these days, it seems like safe insurance to prevent running in retard and losing power, and avoiding the risk of accidentally (or incidentally) ingesting enough oil to cause an issue

Again, if you aren't boosted, just run the stock stuff. It can support moderate/infrequent runs to 6k and is designed to be efficient at peak torque and below. If you are running boost and are arguing the merits of venting the crankcase... well, maybe you should reconsider boost

 

Just to add another layer:

If the oil making its way from the PCV to the intake DECREASES OCTANE of the total charge....

Doesn’t the water vapor from the PCV INCREASE OCTANE of the total charge....



Still have the same basic question from the original post:


How would 1 1/2 oz of oil, introduced as vapor, over 3,000 miles have any measurable effect on octane?

 

I think you are thinking about it the wrong way from a performance perspective. Instead of thinking of it like spread out over whatever long interval, consider the worst possible case. What you should be asking is, how much potential damage could oil ingested into the intake during the worst possible time (high load) do to the engine? In my opinion, for a NA stockish setup making 1hp/ci or thereabout probably not a whole lot honestly. But on a boosted setup making in the 2-3hp/ci range a little bit of oil in the intake, even for only a matter of seconds, would effect the "octane" enough to cause a pre-detonation event and catastrophic failure.