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Understanding the MK7 PCV function, retrofitting the MK8 PCV, and why catch cans are bullshit(IMO)

tigeo

Autocross Champion
Installed the full retrofit today. There was a decent amount of oil in the TIP.

I’ll drive it for a week or so and see if the oil comes back.
I believe some oil in the TIP is a normal condition on these cars, I get some with the MK8 as well but it's not egregious and my hot-side charge hose on the IC never has oil in it...certainly not a major loss point for oil in my car at least.
 
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Klrider44

Go Kart Champion
Location
Bucks county PA
So I can definitely induce smoke on the full retrofit with my vortex XL on E and my street tires (fallen rt660’s). Hard right turn followed by a 3rd gear hard pull, pretty good cloud of smoke.
 

DerHase

Autocross Champion
Location
Hampton Roads, VA
Car(s)
2019 GTI Rabbit
I have a feeling a few things are at play:

1. Some peoples' engines are not as "tight" seal-wise as others - timing cover seals, valve seals, oil cap, rear main seal, etc. If there is ANY air getting past those, then it is that much more volume of air that gets sucked upon by the block passage/PCV system. Even if vacuum target is achieved, there is still a flow of air, and if there is excessive flow of air then it's bound to take some oil with it.

2. Blow-by itself will vary. I'd expect a big turbo to overwhelm the MK8 PCV before an IS20 or IS38. I think it's a pretty solid option on a street driven car in most cases.





In other news there's a local guy who autocrosses with us with a MK7 GTI IS20 S2 EQT tuned that smokes it's ass off when autocrossing. 2015 or 16 MY IIRC with 130k on it. He did the full retrofit and came over last weekend for us to check it over. Confirmed the crankcase is holding vacuum "roughly" the same amount of time as mine when bench tested, but that might not reflect actual running tightness. He DID have a TON of oil from around the oil cap area. He had the old version oil cap that came on the early cars so I gave him a brand new revised version (yellow on the underside). Zero signs of leaking, etc in the week or so he was driving it.

It would be nice to come up with a standardized procedure to test for crankcase "leak-down" essentially to judge how loose or tight the crankcase might be.

Anyway we had an autocross today, it's still happening for him. Zero problems on my own car even on the 28psi tune and being driven much harder, though I DO have the inline can on the car still.

A VW tech friend thinks it might be injectors failing but I'm not so sure of that, though it did not smoke on the 180 U-turn I'd have expected it to happen it. Was more of a long RH W-I-D-E sweeper with little to no brake input prior.

Will continue updates if/as we figure stuff out. Part of me wants to drive his car and see if I induce it. I really wonder if it is far more heavily input-related than we think. (Not as simple as "braking and turning right")
 

Klrider44

Go Kart Champion
Location
Bucks county PA
Inqi
I have a feeling a few things are at play:

1. Some peoples' engines are not as "tight" seal-wise as others - timing cover seals, valve seals, oil cap, rear main seal, etc. If there is ANY air getting past those, then it is that much more volume of air that gets sucked upon by the block passage/PCV system. Even if vacuum target is achieved, there is still a flow of air, and if there is excessive flow of air then it's bound to take some oil with it.

2. Blow-by itself will vary. I'd expect a big turbo to overwhelm the MK8 PCV before an IS20 or IS38. I think it's a pretty solid option on a street driven car in most cases.





In other news there's a local guy who autocrosses with us with a MK7 GTI IS20 S2 EQT tuned that smokes it's ass off when autocrossing. 2015 or 16 MY IIRC with 130k on it. He did the full retrofit and came over last weekend for us to check it over. Confirmed the crankcase is holding vacuum "roughly" the same amount of time as mine when bench tested, but that might not reflect actual running tightness. He DID have a TON of oil from around the oil cap area. He had the old version oil cap that came on the early cars so I gave him a brand new revised version (yellow on the underside). Zero signs of leaking, etc in the week or so he was driving it.

It would be nice to come up with a standardized procedure to test for crankcase "leak-down" essentially to judge how loose or tight the crankcase might be.

Anyway we had an autocross today, it's still happening for him. Zero problems on my own car even on the 28psi tune and being driven much harder, though I DO have the inline can on the car still.

A VW tech friend thinks it might be injectors failing but I'm not so sure of that, though it did not smoke on the 180 U-turn I'd have expected it to happen it. Was more of a long RH W-I-D-E sweeper with little to no brake input prior.

Will continue updates if/as we figure stuff out. Part of me wants to drive his car and see if I induce it. I really wonder if it is far more heavily input-related than we think. (Not as simple as "braking and turning right")
Would you think the BMS catch can could reduce my smoke screen?
 

DerHase

Autocross Champion
Location
Hampton Roads, VA
Car(s)
2019 GTI Rabbit
Inqi

Would you think the BMS catch can could reduce my smoke screen?

I'd be interested in finding out, FWIW my "BMS can" is an ebay knockoff which is identical as confirmed by @mrmatto . $120.

If your smoke continues for a few seconds after getting hard on the throttle, I think there's a good chance it'll help. If it's a single puff that happens when first getting on the brakes, possibly less likely. The difference being whether oil is ingesting via the TIP or the cyl head vacuum passage.

https://www.ebay.com/itm/326003788803


I got it to fit better cutting up my old MK7 hose, plus using 3/4 heater hose from the parts store to replace the one in the kit because it's horribly stiff and I was concerned about it stressing stuff.
 

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Klrider44

Go Kart Champion
Location
Bucks county PA
I'd be interested in finding out, FWIW my "BMS can" is an ebay knockoff which is identical as confirmed by @mrmatto . $120.

If your smoke continues for a few seconds after getting hard on the throttle, I think there's a good chance it'll help. If it's a single puff that happens when first getting on the brakes, possibly less likely. The difference being whether oil is ingesting via the TIP or the cyl head vacuum passage.

https://www.ebay.com/itm/326003788803


I got it to fit better cutting up my old MK7 hose, plus using 3/4 heater hose from the parts store to replace the one in the kit because it's horribly stiff and I was concerned about it stressing stuff.
I might give that a shot, I do get smoke for a bit after the event.
 

MonkeyMD

Autocross Champion
I have a feeling a few things are at play:

1. Some peoples' engines are not as "tight" seal-wise as others - timing cover seals, valve seals, oil cap, rear main seal, etc. If there is ANY air getting past those, then it is that much more volume of air that gets sucked upon by the block passage/PCV system. Even if vacuum target is achieved, there is still a flow of air, and if there is excessive flow of air then it's bound to take some oil with it.

2. Blow-by itself will vary. I'd expect a big turbo to overwhelm the MK8 PCV before an IS20 or IS38. I think it's a pretty solid option on a street driven car in most cases.





In other news there's a local guy who autocrosses with us with a MK7 GTI IS20 S2 EQT tuned that smokes it's ass off when autocrossing. 2015 or 16 MY IIRC with 130k on it. He did the full retrofit and came over last weekend for us to check it over. Confirmed the crankcase is holding vacuum "roughly" the same amount of time as mine when bench tested, but that might not reflect actual running tightness. He DID have a TON of oil from around the oil cap area. He had the old version oil cap that came on the early cars so I gave him a brand new revised version (yellow on the underside). Zero signs of leaking, etc in the week or so he was driving it.

It would be nice to come up with a standardized procedure to test for crankcase "leak-down" essentially to judge how loose or tight the crankcase might be.

Anyway we had an autocross today, it's still happening for him. Zero problems on my own car even on the 28psi tune and being driven much harder, though I DO have the inline can on the car still.

A VW tech friend thinks it might be injectors failing but I'm not so sure of that, though it did not smoke on the 180 U-turn I'd have expected it to happen it. Was more of a long RH W-I-D-E sweeper with little to no brake input prior.

Will continue updates if/as we figure stuff out. Part of me wants to drive his car and see if I induce it. I really wonder if it is far more heavily input-related than we think. (Not as simple as "braking and turning right")
Have him try am AP lol
 

yakev724

Go Kart Champion
Location
NYC
Car(s)
2015 S3
Good news - cars without an SAI port still have live ECU pins for the sensor and we can use this to log mk8 pcvs instead. See below wiring diagram for the pins in the ECU connector to use. These pigtails work perfect - link

I was only able to do a small pull (maybe 12-15psi) and got raw voltage to 2.7V. Going off @DerHase logs of raw voltage looks like I'm in the right ballpark. However I couldn't get the PSI calc to work - is something off in my config?

1719178990066.png
1719179135760.png
1719179162886.png
1719179175870.png
 

DerHase

Autocross Champion
Location
Hampton Roads, VA
Car(s)
2019 GTI Rabbit
Good news - cars without an SAI port still have live ECU pins for the sensor and we can use this to log mk8 pcvs instead. See below wiring diagram for the pins in the ECU connector to use. These pigtails work perfect - link

I was only able to do a small pull (maybe 12-15psi) and got raw voltage to 2.7V. Going off @DerHase logs of raw voltage looks like I'm in the right ballpark. However I couldn't get the PSI calc to work - is something off in my config?

View attachment 306594
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That's awesome news!

What's the full equation?

It looks like the is an xx instead of an x* to me.


Since you have your raw SAI voltage reading, you can assign it to something like "saivolts", and then assign Ambient Pressure to "amb_press" (I think this is the default?

Then for the PCV pressure, it should be referencing the variable

1719190565229.png


This is the equation I've been using because it also factors in ambient pressure (I noticed it reading off when I drove the car about 300ft above sea level a while back).

You will need to fine tune the zero point with key on engine off for the PCV pressure PID. Just change the "- 0.15" at the end to be whatever you need to zero it out.
 

nomunic

Drag Racing Champion
Location
East Coast
Car(s)
MK7
Anyone know a helicoil that will fit the PCV bolt holes, I think mine are all stripped 😭
 

yakev724

Go Kart Champion
Location
NYC
Car(s)
2015 S3
It looks like the is an xx instead of an x* to me.

You were right - android keyboard shenanigans getting in the way. Switched to your formula as well using ambient pressure. Here's a log of a short pull up to 30 psi.

1719680414744.png

1719680459610.png

PCV got down to -4.2psi, not sure how that stacks up with what you've seen. Imagine there is some more room to downside as I've got another 5psi or so of boost headroom. I have a track day coming up in 2 weeks - wonder if I should fit the BMS can as a precautionary measure, but would be cool to do a session with/without it to compare.
 

DerHase

Autocross Champion
Location
Hampton Roads, VA
Car(s)
2019 GTI Rabbit
You were right - android keyboard shenanigans getting in the way. Switched to your formula as well using ambient pressure. Here's a log of a short pull up to 30 psi.

View attachment 306815

View attachment 306816

PCV got down to -4.2psi, not sure how that stacks up with what you've seen. Imagine there is some more room to downside as I've got another 5psi or so of boost headroom. I have a track day coming up in 2 weeks - wonder if I should fit the BMS can as a precautionary measure, but would be cool to do a session with/without it to compare.

Perfect. That's actually better than I was expecting. That's basically better than the IS20 was with a more restrictive TIP (stock, APR, or Unitronic). I'm guessing that the bigger intake you have helps a lot to offset the additional airflow the hybrid makes.

1719693078928.png



Also I've got a few other things going on currently (pending IS38 swap, have to pull the bumper to replace a broken grill, in the middle of DSG tuning myself as well)... but I grabbed one of my old MK8 valves and took a good look at it, I know I tried drilling holes in the bottom side before, but I think there might be one more viable modification that could help it out even more. While *I* have not been able to get any smoking from my own car since the retrofit, I think that it has a good chance of fixing the pressure spike I experienced out of T17.
 

yakev724

Go Kart Champion
Location
NYC
Car(s)
2015 S3
Fwiw I’m running an afe filter in stock intake box, but the larger eqt tip + piping. Full retrofit with charge pipe bung.

Based on your charts looks like full retrofit is key, other variables secondary.

Will do some more highway pulls tomorrow.
 

DerHase

Autocross Champion
Location
Hampton Roads, VA
Car(s)
2019 GTI Rabbit
Fwiw I’m running an afe filter in stock intake box, but the larger eqt tip + piping. Full retrofit with charge pipe bung.

Based on your charts looks like full retrofit is key, other variables secondary.

Will do some more highway pulls tomorrow.

That's basically the gist of it.

For track use, it appears limiting overall PCV source vacuum a bit (BMS instead of OEM TIP) helps stabilize the diaphragm, but for daily use I don't think anyone would ever have an issue.

I went over all the data/got videos of the diaphragm opening and closing while driving here if you haven't seen it already:
 
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