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

[GTIfan99]

Yeah the BL sucked. I don’t remember if it smoked but I def got knock and oil consumption just driving around normally. Maybe it was bad or maybe it’s incompatible with the CXCA.

Or.......... maybe APR tune runs higher boost that my EQT tune.
Dundundun....

Tune in to the Californians next week to find out...

 

[Mosquito]

I'm looking for an OEM solution to my car smoking on track without having to check and empty a CC after every session.

Well. The MK8+Venturi fixes that. If you still want a catch can because some do, do make sure to use the Tiguan hose and boosted venturi because otherwise you'll break the intended design.
Other than that, everything else out there is a PCV plate that requires emptying a CC at some point. They are more expensive and certainly not even close as good.

 

[mrmatto]

Or.......... maybe APR tune runs higher boost that my EQT tune.
Dundundun....

Tune in to the Californians next week to find out...

I was just driving on the street Wasn’t even doing any high boost pulls.

I still think there’s something to the CXCA/CXCB/DKFA motor differences. Their PCV valves have significant differences so there could be a functionality difference we don’t full understand.

 

[Mosquito]

Or.......... maybe APR tune runs higher boost that my EQT tune.
Dundundun....

Tune in to the Californians next week to find out...

It isn't really a boost issue, at all. Not to say it isn't part of the overall function though.

 

[DerHase]

To be clear: ALL of the OEM PCVs have the same inherent weakness (without the Venturi).

The intake blue path vacuum source goes away when you transition to throttle, and relies 100% on TIP vacuum at that point. If it's not strong enough/available, the crankcase pressure rises due to blow-by.

When crankcase pressure is closer to ambient pressure than it's target vacuum (roughly -0.9psi ish, depending on the specific valve), the diaphragm must open up in order to try and allow the vacuum source (TIP) to draw down the crankcase. This is just how a simple regulator works. That's all that the diaphragm is.

This CAN happen with stock tune, stock TIP, stock turbo, etc etc. It's just far HARDER to make it do so. I think it requires TIME of running with the crankcase depleted for the oil to surge upwards. Longer RH sweepers with part-throttle is going to be the worst part. Rolling onto the throttle slowly (like you do when driving on track at the edge of traction on corner exit) doesn't allow the vacuum to build quickly enough, meanwhile RPMs are high, and when you DO get WOT... there's a sudden hit of boost, pressure in the crankcase, diaphragm is wide open... and then hard suction builds and pulls a volume of oil with it.

As far as the effects of a tune:

When you tune the car you are messing with 3 main things:
1. Blowby. Boost hits harder at lower RPMs. I cannot measure this, but more blowby is reasonable to assume. How much... don't know.
2. TIME. It takes TIME to draw the crankcase vacuum down to acceptable levels where the diaphragm closes. This is why the one TIP can't even generate a vacuum in 2nd gear, but can generate about half of what's needed in 3rd. It's not blowing through the RPMs as quickly so it spends more time with a draw on the TIP that can do work.
3. Little more vacuum draw. This is about the only good thing. BUT it doesn't happen fast enough to counter the big hit to crankcase pressure on initial spool.

Here is a stock TIP, stockish boost levels vs stock TIP, upped boost levels. Basic retrofit (no venturi). This is the exact same effect that'll happen with any of the OEM MK7 valves as well.

Obviously with 10 more psi of boost it's going to be a lot faster. But with the two graphs matched at the same RPMs... you can see that the crankcase isn't getting drawn down appreciably faster. If you look at the PCV vacuum source, there IS more of that. But it doesn't really make a difference before it matters.

edit: and just for funsies... here's the low boost/"stock" boost levels vs the higher boost tuned + Venturi... it keeps the crankcase under control as soon as boost is fully built.

 

[GTIfan99]

Can the venturi be used with my BL?

 

[DerHase]

Can the venturi be used with my BL?

I wouldn’t. You’d be hacking up a $120 hose to save $60 on a PCV.

Also the MK8 diaphragm is notable more robust. I suspect to deal with the higher PCV vacuum levels. It also has a cage which seems to be to align the diaphragm so it can’t cock sideways like it appears all the MK7 valves “failed” from.

 

[Nineeightyone]

Thanks for the continued updates on this, hopefully we wind up with an ezpz solution as I feel there's a fair number of these cars running similar enough setups.

 

[mrmatto]

TBH, the full retrofit is pretty simple. As long as the data continues to support it, that's looking like the ticket for most. And, it seems that if you do the full retrofit (with the Venturi), you can use an aftermarket TIP too because the Venturi ensures a good vacuum.

 

[Nineeightyone]

TBH, the full retrofit is pretty simple. As long as the data continues to support it, that's looking like the ticket for most. And, it seems that if you do the full retrofit (with the Venturi), you can use an aftermarket TIP too because the Venturi ensures a good vacuum.

Now I just wait for someone to sell a kit... paging @StorableComa

 

[Klrider44]

Legend - my harness is on its way and just finished fabbing up the venturi to my charge pipe hose. Curious to measure the impact.

In case anyone needs this setup (though keep in mind not many aftermarket charge pipes have a larger silicone hose like EQT): bung fitting 1 fitting 2 hose

View attachment 297564

View attachment 297565

How is this holding up? Was thinking about going this route since I have the EQT pipe and don’t have a Venturi yet. Just the MK8 valve and Tiguan hose

 

[avenali312]

The intake blue path vacuum source goes away when you transition to throttle, and relies 100% on TIP vacuum at that point. If it's not strong enough/available, the crankcase pressure rises due to blow-by.

I just replaced mine at about 74k miles (the one I removed was an AH and it was replaced by an AP). Stock car. It has never seen the track, so never a smokescreen incident, but could my MST hose and TIP have caused the PCV failure? Maybe not "cause," but added to it. There are definitely signs of increased crankcase pressure, but I know that a lot of people change their PCV in this mileage range just as a maintenance item, so I just figured it was time. I'm going to keep the TIP and hose regardless for the whooshies, but just curious haha.

 

[DerHase]

TBH, the full retrofit is pretty simple. As long as the data continues to support it, that's looking like the ticket for most. And, it seems that if you do the full retrofit (with the Venturi), you can use an aftermarket TIP too because the Venturi ensures a good vacuum.

Honestly an aftermarket TIP (the ones that pull very little vacuum... like the BMS, and likely the CTS V2) will probably pair with it even better (without "need" to restrict the Venturi boost source).

BMS TIP Full retrofit: Darker lines
OEM TIP Full retrofit: Lighter lines



I'm going to play with restricting the Venturi boost flow down a little bit, and once that's figured out will swap the BMS back on to the effects on it without changing the restriction. Basically the goal is to see if restricting the Venturi becomes a "need to tune" for specific TIP/boost level thing... or if a blanket "just make sure it's not flowing TOO much" kinda deal will work to fix all TIPs. Which I suspect will be the case based on how little boost is required to make it start working.

FWIW in 2nd gear there IS a small difference in the rate of draw... but IMO the main thing to focus on is the fact it never gets out of hand (-0.2 or -0.1psi)... instead going back towards target before peaking at -0.6 to -0.7psi. Both are still way better than without Venturi regardless.

 

[DerHase]

I just replaced mine at about 74k miles (the one I removed was an AH and it was replaced by an AP). Stock car. It has never seen the track, so never a smokescreen incident, but could my MST hose and TIP have caused the PCV failure? Maybe not "cause," but added to it. There are definitely signs of increased crankcase pressure, but I know that a lot of people change their PCV in this mileage range just as a maintenance item, so I just figured it was time. I'm going to keep the TIP and hose regardless for the whooshies, but just curious haha.

I don't think it necessarily "causes" failure... but reduces the PCVs ability to do it's job properly (and giving signs of PCV failure as a result, even though the PCV isn't the problem).

A quick google shows it looks almost identical to the BMS... so probably really terrible with an OEM PCV honestly.

 

[avenali312]

I don't think it necessarily "causes" failure... but reduces the PCVs ability to do it's job properly (and giving signs of PCV failure as a result, even though the PCV isn't the problem).

A quick google shows it looks almost identical to the BMS... so probably really terrible with an OEM PCV honestly.

I'll probably leave it on just because I don't want to deal with that hose from the PCV to the TIP ever again

Some photos of when I installed in in 2020:
https://www.golfmk7.com/forums/inde...venali312s-oem-gti.343666/page-7#post-7396763