PCV design and theory for turbo cars

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PCV design and theory for turbo cars

Unread postby dleach1407 » Fri Oct 11, 2019 9:53 pm

After partially hijacking a different thread, I thought it would be a good idea to ask some questions about PCV theory on our cars but more specifically blow through turbo forced induction applications. I also wanted to discuss the optimal designs for cars that are using a blow through MAF instead of a draw through design. In my opinion draw through and blow through require different designs unless the user wants to clean a MAF on a regular basis.

So first, lets talk about the stock design. By design, the car has a PCV valve in the rear of the intake manifold that scavenges crankcase gasses from the valley directly into the back of the intake manifold. The PCV is a restricted type valve that only allows a small amount of air to pass through it under vacuum or pressure. The valve cover vent connected to the throttle body or intake pipe on the passenger side allows fresh metered air to pass through the crankcase and engine eventually making it to the pcv and being combusted. This is how the system works most of the time however when at WOT, the PCV has almost no vacuum and some or most of the PCV gasses that are normally sucked directly into the intake through the PCV valve, now pass through the vent tube in to the throttle body and into the intake. The amount of gasses passing through the vent will depend on a few variables such as age of the engine and ring gap since we know older engines and loosely gapped rings can have more blow-by. Regardless of path, the effect is the same, the PCV gasses and oily residue make it into the intake manifold and out the tail pipes. The only real place you see the oily reside is in side the intake manifold and right in front of the throttle body.

With factory turbo cars, the design is essentially the same. All factory turbo cars that run a MAF are draw through and I believe it is because of the PCV requirements. The system essentially works the same however the PCV is usually a check valve that fully closes under boost. I wouldnt doubt some use an orifice type PCV as well. Some cars use a crank case vent on the block, some cars scavenge from the valve cover but all factory turbo cars still have the same PCV design that I am aware of. The only real difference is the vent tube connects somewhere after the MAF but before the turbo. The positive of this design is the MAF never is exposed to the oily PCV gasses but with turbo cars, there is a major draw back to this design. All factory turbo cars eventually fill the intercooler and intake track with oily residue often with oil collecting in low places of the path, especially the intercooler. When you consider these cars are running 14 to 20 lbs of boost, there has to be a lot of crank case gasses escaping through the vent. If you go onto the Focus ST and Suby forums its all over the place and its not due to turbo failure.

So how do we combat this when we run a blow through MAF?

Most of the people I know who run blow through just vent the crank case to atmosphere some use a catch can, some dont. I dont like that because I can smell oil all the time. I designed my "PCV" in a manner that I feel solves all of the issues we run into with blow through.

I currently do not have any type of PCV valve because my intake does not have the provisions for it. I do have my valve cover vented running to a catch can all using 10an hose. The way it is plumbed in, the catch can sees vacuum under boost. It pulls about 1-2 to inches which I confirmed with a vacuum gauge. The outlet of the catch can runs to the inlet of my turbo right in front of the inlet. When the turbo spools, it pulls it down a couple inches. The design eliminates the oil getting into the intake track to help eliminate oil caused detonation, keeps the MAF clean, it helps seal the rings and evacuate the crank case under boost. Under normal driving it has no vacuum so it only catches positive pressure from the valve cover. For me, the biggest concern is coating my MAF with oil making it inaccurate. I also want to avoid oil all through the intake track. I should also point out my catch can has a sintered bronze filter inside of it. These are the same filters used for air compressors to remove oil and water from the air. So far my intercooler, piping, and maf have stayed oil free. I feel like its a win win for blow-thru MAF applications but I am curious what other people are doing.

Decipha, I know you build kits so how do you tackle this in Coyote kits? Do you not worry about the oil getting into the intercooler and piping? Do you not use blow-thru MAF with your kit?
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F303 Cam, 1.6 Miller Mid-Lift rockers
T5 with an XTD clutch that somehow holds my power
80lb Dekas, Stock lines and rails 340LPH in tank
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76mm On3 turbo, HP hotside, Custom coldside
560WHP and 606TQ 8lbs tapering to 6 91 octane
27degrees total timing.
Vehicle 2 Information: 1957 Ford F100
Mustang II front suspension -done
C-notched rear frame - done
Luxury ECU using RZASA - have
Fuel injected explorer 302 - have
4r70W -have

Lots of stuff planned including air bags and twin 55mm turbos.

Re: PCV design and theory for turbo cars

Unread postby decipha » Sat Oct 12, 2019 5:26 am

For safety, everything we do is blow through.

You simply put a check valve on the inlet line.

details are in the pre tune write up- http://www.efidynotuning.com/pre.htm
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Re: PCV design and theory for turbo cars

Unread postby PaulC-turbo5.0 » Sat Oct 12, 2019 6:13 am

I did A LOT of research on this subject when I put my car together and I like decipha’s method better then what I came up with because I still have an open breather. 95% of what you find is just open breathers to catch cans which I was against doing.

I have my stock pcv in the lower rear intake ran through a small oil separator from jegs to the inlet elbow before my turbo then an open breather on the valve cover. This has worked fairly well for me as I haven’t gotten a drop of oil in the separator and no crankcase pressure issues. Occasionally I have seen puffs of smoke come from the breather but no noticeable oil burning. Once I saw decipha’s method I was like damn how didn’t I come across this before or think of it myself lol. I will be switching to his method here real soon.

Decipha do you have a recommendation for a good inline check valve?
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Re: PCV design and theory for turbo cars

Unread postby decipha » Sat Oct 12, 2019 7:49 am

I don't have any specific vendor I'm loyal to. When I need some I just do a quick amazon search.

https://www.amazon.com/Check-Valve-Inli ... 131&sr=8-3
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Re: PCV design and theory for turbo cars

Unread postby dleach1407 » Sat Oct 12, 2019 9:35 am

decipha wrote:For safety, everything we do is blow through.

You simply put a check valve on the inlet line.

details are in the pre tune write up- http://www.efidynotuning.com/pre.htm


The problem here is you eventually end up with oil all through the intercooler, piping and coating the MAF. This is just not something I am willing to deal with. I appreciate the reply but in this scenario, I am going to stick with my design.
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Vehicle Information: 1995 Mustang T5 CBAZA/T4M2 Moates QH BE or TpRT
331 10.2:1 compression, TFS 11r 190 56cc heads
HPX MAF 3.25 OD tube with SaxonPC air straightener
F303 Cam, 1.6 Miller Mid-Lift rockers
T5 with an XTD clutch that somehow holds my power
80lb Dekas, Stock lines and rails 340LPH in tank
Minor upgraded T5 with counter gear stiffening plate
76mm On3 turbo, HP hotside, Custom coldside
560WHP and 606TQ 8lbs tapering to 6 91 octane
27degrees total timing.
Vehicle 2 Information: 1957 Ford F100
Mustang II front suspension -done
C-notched rear frame - done
Luxury ECU using RZASA - have
Fuel injected explorer 302 - have
4r70W -have

Lots of stuff planned including air bags and twin 55mm turbos.

Re: PCV design and theory for turbo cars

Unread postby decipha » Sat Oct 12, 2019 10:16 am

You could just simply put another catch can on it. If you have that much blow by you may have some other issues going on.
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Re: PCV design and theory for turbo cars

Unread postby PaulC-turbo5.0 » Sat Oct 12, 2019 10:54 am

If the pcv system is maintained decent I don’t think oil should be a big issue. You could add one of the check valves that decipha linked in between your turbo inlet and catch can and then run a vacuum line from your intake to the catch can with a check valve in between. This way you would always have vacuum on the crankcase. At idle and cruise manifold vacuum would pull through the near check valve then catch can then sucking the far check valve shut so you can’t pull in unmetered air. In boost manifold check valve would close allowing turbo inlet valve to open continuing to pull vacuum on the catch can.

I will run the line from the valve cover to the oil separator for good measure but don’t think I’ll worry about the line from the lower to the intake. I’ll keep an eye on it and can always add a second separator.
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Re: PCV design and theory for turbo cars

Unread postby dleach1407 » Sat Oct 12, 2019 11:38 am

Yeah, I thought about a second catch can on the PCV side. I read over your check valve write up.. Im not sure I am understanding. It seems like you are saying to put it in the vent tube so when the PCV has vacuum, it will suck the check valve closed. Wouldnt that stop all fresh air from entering the engine? Also, under boost wont the PCV be leaking some boost into the engine pressurizing the crank case to some extent and pushing oil vapor out of the vent tube? Seems like you are saying you want the crank case under heavy vacuum except for in boost. Is that true?
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331 10.2:1 compression, TFS 11r 190 56cc heads
HPX MAF 3.25 OD tube with SaxonPC air straightener
F303 Cam, 1.6 Miller Mid-Lift rockers
T5 with an XTD clutch that somehow holds my power
80lb Dekas, Stock lines and rails 340LPH in tank
Minor upgraded T5 with counter gear stiffening plate
76mm On3 turbo, HP hotside, Custom coldside
560WHP and 606TQ 8lbs tapering to 6 91 octane
27degrees total timing.
Vehicle 2 Information: 1957 Ford F100
Mustang II front suspension -done
C-notched rear frame - done
Luxury ECU using RZASA - have
Fuel injected explorer 302 - have
4r70W -have

Lots of stuff planned including air bags and twin 55mm turbos.

Re: PCV design and theory for turbo cars

Unread postby decipha » Sat Oct 12, 2019 12:00 pm

correct, you want all the vac the pcv line will give you which won't be much.

No, under vac the pcv valve opens and evac's the crank case while the inlet line gets its check valve sucked shut.
Under boost the pcv locks shut and the check valve on the inlet opens and evacuates the crank case.
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Re: PCV design and theory for turbo cars

Unread postby dleach1407 » Sat Oct 12, 2019 1:44 pm

decipha wrote:correct, you want all the vac the pcv line will give you which won't be much.

No, under vac the pcv valve opens and evac's the crank case while the inlet line gets its check valve sucked shut.
Under boost the pcv locks shut and the check valve on the inlet opens and evacuates the crank case.


What is the purpose of stopping the fresh air entering the engine? I dont understand how that is beneficial. It seems to me you would be evacuating less of the contaminants with it sealed. Also, what PCV valve are you using. The ones I get at the autostore leak under pressure.
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Vehicle Information: 1995 Mustang T5 CBAZA/T4M2 Moates QH BE or TpRT
331 10.2:1 compression, TFS 11r 190 56cc heads
HPX MAF 3.25 OD tube with SaxonPC air straightener
F303 Cam, 1.6 Miller Mid-Lift rockers
T5 with an XTD clutch that somehow holds my power
80lb Dekas, Stock lines and rails 340LPH in tank
Minor upgraded T5 with counter gear stiffening plate
76mm On3 turbo, HP hotside, Custom coldside
560WHP and 606TQ 8lbs tapering to 6 91 octane
27degrees total timing.
Vehicle 2 Information: 1957 Ford F100
Mustang II front suspension -done
C-notched rear frame - done
Luxury ECU using RZASA - have
Fuel injected explorer 302 - have
4r70W -have

Lots of stuff planned including air bags and twin 55mm turbos.

Re: PCV design and theory for turbo cars

Unread postby decipha » Sat Oct 12, 2019 2:29 pm

to put the crank case slightly under vac. It reduces windage off the crank same purpose a windage tray is used.

you want the turbo coupe pcv valves 83-88 thunderbird / svo mustang 4cyl turbo
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94 5spd Rionda
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03 Marauder - DyShyKy
00 Explorer 5L - Bernyce
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Re: PCV design and theory for turbo cars

Unread postby dleach1407 » Sat Oct 12, 2019 9:12 pm

The more i think about it the more it makes sense. A PCV valve only flows about 4-5 CFM and the amount of blow by is probably a few CFM so you really arent putting much of a vacuum on the crank case. If there was no valve restriction then it would be different.
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Vehicle Information: 1995 Mustang T5 CBAZA/T4M2 Moates QH BE or TpRT
331 10.2:1 compression, TFS 11r 190 56cc heads
HPX MAF 3.25 OD tube with SaxonPC air straightener
F303 Cam, 1.6 Miller Mid-Lift rockers
T5 with an XTD clutch that somehow holds my power
80lb Dekas, Stock lines and rails 340LPH in tank
Minor upgraded T5 with counter gear stiffening plate
76mm On3 turbo, HP hotside, Custom coldside
560WHP and 606TQ 8lbs tapering to 6 91 octane
27degrees total timing.
Vehicle 2 Information: 1957 Ford F100
Mustang II front suspension -done
C-notched rear frame - done
Luxury ECU using RZASA - have
Fuel injected explorer 302 - have
4r70W -have

Lots of stuff planned including air bags and twin 55mm turbos.

Re: PCV design and theory for turbo cars

Unread postby decipha » Sun Oct 13, 2019 7:21 am

You got it.
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91 4r70w - Skarlett
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93 auto - Danika
94 5spd Rionda
95 auto Aisha
Vehicle 2 Information: Others:
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98 Camaro SS - Bounquisha
02 Harley F-150 - Sasasha
03 Marauder - DyShyKy
00 Explorer 5L - Bernyce
07 GMC 2500HD 6L - Veranafer

Re: PCV design and theory for turbo cars

Unread postby SlowBox » Tue Oct 22, 2019 10:04 pm

I put a pcv valve inline between the oil fill and the front of the plenum, and put the smog pump check valve in the back of the lower intake. Works pretty mint. Someday I'll hook it up to a header evac in my downpipe, but for now it just vents under boost.
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