Restrictor

[HopefullEngStdt]

Hey all, just finished rebuild of my motor several weeks ago, and since then have not had time to post. But i got a quick question for anyone with an SC and some calipers handy, when I bought the car it did not have a vacuum line restrictor in the vent hose comming from the MFI going to the oil filler neck, I have machined one and guessed the ID size, which means the car runs decently but not perfect cause the ramp up on the distributor is off. Could someone please measure the inner diameter of the restrictor through which it has to flow through so I can machine myself a new one and have my Targa back on the road the way its meant to be .

Thanks

Moritz

[Grady Clay]

Moritz,

Welcome to the Forum.

You have some mutually conflicting (I think) things in tour post. An SC didn’t (usually) come with MFI it came with CIS. If MFI, there isn’t a vacuum connection to the MFI pump – simply an oil supply and an overflow return to the crankcase breather. Both MFI and CIS have vacuum hoses to the distributor but I don’t think there is a “restrictor.”

Let’s start with the basics.
What year and model is your 911?
What is the engine S/N and Type number?
Is this a special 911 or has it been modified?
Please post some images of your situation so language descriptions aren’t an issue.

We can help you sort this out.

Best,
Grady

[Nine9six]

Moritz,
Pour your heart out...You have the attention of one of the Porsche gurus

['78 SC]

The first post is a bit confusing, but here is the restrictor that fits in the line between the oil tank (filler neck) and intake:

[HopefullEngStdt]

Yea thats the piece im looking for, with mfi, i am simply referring to "Mechanical Fuel Injection", Bosch K-Jetronic CIS (Continous Injection System), I just need to know the inner diameter of that restrictor.

[Grady Clay]

Now you have my curiosity up.

First, what is your model & year, second, what injection MFI or CIS?

Why do you feel you need this part if you have MFI? A PET search shows 930.107.289.00 for an ’81-> SC (CIS). I can understand its function with CIS.

In ’78 there was a change that re-routed the breather from the air filter part of the air box to the CIS rubber boot. At the boot there was an “angled connector with orifice and flame guard.” This “… 6.5 mm orifice restricts the crankcase vacuum to between 50 and 100 mm of water under all operating conditions.”

I think in ’81 they either moved the restrictor closer to the oil tank or this is an additional part. PET and the Tech Info I have isn’t particularly clear about this.

In the above picture of 930.107.289.00, the OD needs to fit in the ~26 mm breather hose. I’ll estimate the orifice is about 5.9 mm ±1 mm.

Best,
Grady

[kodabear]

Originally posted by Grady Clay
Moritz,

Welcome to the Forum.

You have some mutually conflicting (I think) things in tour post. An SC didn’t (usually) come with MFI it came with CIS. If MFI, there isn’t a vacuum connection to the MFI pump – simply an oil supply and an overflow return to the crankcase breather. Both MFI and CIS have vacuum hoses to the distributor but I don’t think there is a “restrictor.”

Let’s start with the basics.
What year and model is your 911?
What is the engine S/N and Type number?
Is this a special 911 or has it been modified?
Please post some images of your situation so language descriptions aren’t an issue.

We can help you sort this out.

Best,
Grady

CIS is a type of mechanical fuel injection.

[Eagledriver]

Many people refer to CIS as a form of MFI because it is not electronic controlled. This causes confusion in the 911 world because we have the "real" MFI in the form of the belt driven bosche system.

-Andy

[Huh]

but CIS did become electronically controlled in KE-jetronic and KE-motronic. There was K-jetronic, K-jetronic lambda controlled before the others. The SC has K-jetronic lambda controlled or CIS-lambda. This system is also found in many of the 80's VWs.

[HopefullEngStdt]

Well mine is a 1980 911 SC K-Jetronic CIS (which is a form of MFI but not what is considered the "Bosch MFI"), the lamda sensor has been disconnected for a while, and yes, it needs a restrictor. So if someone has the part, could you please measure it out for me?

Thanks,

Moritz

[Grady Clay]

Chill.

I think we all agree that when we use the term MFI (Mechanical Fuel Injection) we are talking about the ’69-’73 (and some later & earlier) Bosch (and Kugelfischer) belt driven high pressure injection derived from Diesel.

The CIS (Continuous Injection System) was devised by Bosch to better meet the emissions and fuel economy needs starting in the ‘70s.

Both systems are “mechanical/hydraulic” with various electrical sub-systems. That isn’t the issue.

If I would devise a new moniker for CIS, it would be hydraulic and defantly not mechanical. Let’s not confuse the issue.

Each system has its pros and cons that have been hashed over many times.

Thank you for providing the details. it us usually best to do that up front and use accepted terminology to not confuse the process.

Where are you located? I was assuming English wasn't your first language. Sorry if not so.

Best,
Grady

['78 SC]

Originally posted by Grady Clay
Now you have my curiosity up.

In ’78 there was a change that re-routed the breather from the air filter part of the air box to the CIS rubber boot. At the boot there was an “angled connector with orifice and flame guard.” This “… 6.5 mm orifice restricts the crankcase vacuum to between 50 and 100 mm of water under all operating conditions.”

I think in ’81 they either moved the restrictor closer to the oil tank or this is an additional part. PET and the Tech Info I have isn’t particularly clear about this.

I'm certainly not clear on this. I installed this restrictor in my '78 recently because of excessive oil consumption (guzzling, actually) after a rebuild . This was despite good leakdown and compression numbers. The fact that it smoked at odd occasions and not consistently led me suspect it was inhaling oil that worked its way through the breather line. Several Pelican searches confirmed (I thought) that there should be a restrictor in the line. I installed the above-pictured part but wondered (1) how could I have lost it during the rebuild (senior moment?)and (2) why didn't the engine stall when I removed the oil tank cap if there was no restrictor. Now I know the answer to both.

It's too soon to tell if the consumption is down, but I have not observed any smoking and when I removed the line to measure the restrictor orifice, there was clearly oil that had been held back by the restrictor. Here's hoping.

Grady, is there any problem having both restrictors? Your comment quoted above suggests there might have been a period with both. BTW, the opening is 6.3mm. Good eye.

[Grady Clay]

Steve,

This is a very worthwhile Pelican Porsche engineering thread. There are a lot of different issues here. Clearly PAG has been addressing this for a long time.

There is the centrifugal oil-air separator first used on the Turbo and then on the SC and Carrera. These restrictors were just the first step.

I think we should carefully evaluate the progression of 911 breather systems in this era and try and understand what the issues were. Perhaps the latest Factory version is best. Perhaps some non-standard combination of Factory parts is best. Perhaps there is a far better solution yet to be invented given 20-year 20:20 hindsight. It is up to us because PAG won’t do it.


To answer your specific question; I don’t know if Porsche ever used two restrictors at the same time. PET isn’t clear. If they did, the restriction would be increased by 2X, possibly 4X (I’ll have to consult my flow text books.) Can someone shed light on this?

Best,
Grady

[ischmitz]

I always thought the restrictor was to prevent too much of a vacuum leak once you open the oil filler cap. At least that's what it does in my 3.6 - without one the engine almost stumbles to a halt once you open the oil cap to check the level. Once I added the restrictor the ICV (idle control valve) can adjust for the sudden vacuum leak much better and the engine keeps idling, albeit a little rougher. Once the cap goes back on everything is back to normal.

The fact that oily fumes escape from the crank-case and get ingested into the intake is always there. Race setups use oil bottlles and don't introduce the crank-case oily air back into the intake. The downside is that you have to empty the catch-bottle every once in a while.

Ingo

[HopefullEngStdt]

Thanks for the measurement Steve

[Mysterytrain]

Enclosed is a picture from the PET. The part 10a is in red.



More interesting is the drawings from the PET for the Air Sensor boot.
They show 47,47a,47b
911-110-286-00 -connecting pipe
930-107-286-00 -connecting pipe
901-107-729-00 -backfire grid




I'm the second owner of my 78SC. I've had it since 85..and I don't have/never had a backfire grid in place. I'm doubting that the original owner removed it. Looks like I have part 47 in place, which is a 90 degree pipe. It appears that 47a is also a 90 degree pipe but allows for 47b to mate with it. I'm very curious as to why or why not I would need this? Also if I'm following this correctly the restrictor lives in the rubber hose that connects part 47 with the oil filler tube??

['78 SC]

Ron, here's some clarification, in case I have been guilty of shedding darkness on the subject...

I installed the restrictor shown in the photo above because I thought I had left it out during re-assembly. In fact, it was the same PET drawing you posted above as well as some other threads here that led me to believe there should be one there. However, per Grady, 78's had the restriction orifice and screen at the other end of the hose near the CIS boot. I believe that (because Grady said it) and I have never experienced stalling when removing the oil filler cap (just a little roughness) so there must be some sort of restriction in place other than at the tank (as in your lower PET ref.)

My interest in this comes from my belief oil is somehow working its way from the tank to the intake. I keep the level at the lower mark, but the consumption and smoking are excessive. This was not a problem before the rebuild. I'm in the process of plumbing a catch can between the tank and intake to determine if oil is going this route.

My pet theory de jour is that the venturi screen for the oil pump pickup and cam housing flow restrictors I installed as upgrades cause the oil level in the tank to remain higher than before because less inventory accumulates in the crankcase. More oil in the tank, especially when high RPM froths up the scavenged oil could make it more likely to slosh into the breather line. (Am I on thin ice yet?). The restrictor shown above is intalled with the cone towards the tank, which would minimize the opening for any liquid. This restrictor is a later part. Did it come into use around the time the venturi screen was released?

I'm installing my catch can today so I can determine how much oil is passing through the breather.

HopefullEngStdt, you're welcome. Hope you get all your issues resolved.

[Cire]

Originally Posted by ischmitz

I always thought the restrictor was to prevent too much of a vacuum leak once you open the oil filler cap. At least that's what it does in my 3.6 - without one the engine almost stumbles to a halt once you open the oil cap to check the level. Once I added the restrictor the ICV (idle control valve) can adjust for the sudden vacuum leak much better and the engine keeps idling, albeit a little rougher. Once the cap goes back on everything is back to normal.

The fact that oily fumes escape from the crank-case and get ingested into the intake is always there. Race setups use oil bottlles and don't introduce the crank-case oily air back into the intake. The downside is that you have to empty the catch-bottle every once in a while.

Ingo

I take it that there is always going into the catch cans that needs to be drained? Isnt this an issue?

Eric

[Steve@Rennsport]

Originally Posted by '78 SC

My interest in this comes from my belief oil is somehow working its way from the tank to the intake. I keep the level at the lower mark, but the consumption and smoking are excessive. This was not a problem before the rebuild. I'm in the process of plumbing a catch can between the tank and intake to determine if oil is going this route.

Steve:

Dare I say it, but have you performed a leakdown test since you rebuilt the engine?

Excessive crankcase pressure is oftentimes caused by rings that have not seated.

[Cire]

Originally Posted by Steve@Rennsport

Steve:

Dare I say it, but have you performed a leakdown test since you rebuilt the engine?

Excessive crankcase pressure is oftentimes caused by rings that have not seated.

Steve,

How do you perform a leak down test? I did a compression test the other day and that was fairly simple. Is the leak down test, just a matter of timing how long it takes for the air to drain out of the cylinder?

Thanks.

Eric