I've had my pump on my car since July 2011. I've driven to sharktober and back (2200 miles each way), and I would say a total of around 20k miles on it to date.

 

I've been trying to correlate the windage problems with engine characteristics with the bit of data that I fell off the truck into my lap. One should expect the GTS to have windage problems that are much worse than those of the S4 because of the crankshaft alone. The counterweight design is terrible and the increased stroke magnifies all problems.

 

Porsche attempted to compensate with the windows in the GTS block to allow for air movement but clearly did not solve the problem.

 

Ding ding ding!

 

As I have discussed in depth with Colin, my S4 motor measures nothing wrong but performance seems a bit down and I have troubles with top end advance. With a stock 90 S4 configuration it seemed to burn oil and more so as the motor was worked. I then modified the breather system to the GTS set up- oil consumption increased. Recently I tried a Pro Vent set up with a big *** vent from the filler neck and a baffle in the filler neck- oil consumption has not reduced- about a litre of oil with every tank full of gas.

This suggests that either I have a deeper routed problem or the breathing issue is at the heart of the problem. The stock system logically puts a small amount of vacuum on the crankcase- exactly how much I do not know but we can only be talking a a few mm of Hg upstream of the throttle butterfly but I have concluded that even that may just be enough to so something positive for the ring seal process nothwithstanding the oil it ingests.

Looking at Jakes system diagram the only thing I note is that on the air out/in connection on the Pro Vent I wonder if it might be more prudent to have a vacuum breaker to allow air into the system if the vacuum pulls too deeply.

If the system works as well as I suspect it may well do, then presumably the amount of oil in the windage cloud will be reduced dramatically, the rings will seal better thus oil is no longer passing the rings and if this happens, the dynamic compression will improve, the fuel octane will not be degraded and thus performance can be optimised.

Suspect those of us interested will be looking to see if this proves to be the case consistently. For sure I am at the point of throwing in the towel given my dustbin truck from hell amuses me highly.

Nonetheless I still enjoy driving my 928 and would really like to crack this oil problem I have.

Regards

Fred

 

Fred -- As you know, there are many ways in which the engine can consume oil. It might be the breather, but it might also be something else. The valve guide seals might leak, the cylinders or rings might be worn, etc. Have you tried routing all crankcase fumes to a catch can and seeing how much oil actually goes out thru the breather lines?

 

Tuomo,

The valve seals were new when we refreshed the top end when fitting my S4 motor into my GTS chassis a few years ago. The bores were very clean with no measurable signs of wear. compression pressures were a constant 185 psig but i have yet to do a lekadown test on this motor.

The John Kuhn baffle seems to catch oil - nothing is seemingly taken out in the Pro Vent that is plumbed to a catch bottle at the moment. Thus my conclusion is that oil is going through the rings- just a question of why. The PO [late 1998] stated that oil consumption was normal- now it is worse than ever and my attempts to convert to GTS system were clearly not a good idea.

Thus I am thinking the problem is either the breather system or tired rings [engine mileage currently 100k miles]. I want to eliminate the former first so this thread of particular interest to me.

Regards

Fred

 

That's a great idea to use a catch can for information gathering. As far as the pump pulling to much vacuum , the oil/air return line size is what limits the vacuum. Larger line or smaller line. You could even use a large line and just add a restrictor or vac control valve

 

Could be the breather system is not flowing as much as it should causing a restriction and pressure builds up forcing oil past the rings.........

 

Do you still have any breather ports connected to the valve head covers and/or the filler neck? Any remaining way for oil to get to the intake?

I do have dual variable VLV's on mine (feeding from the bottom of the airbox) - I don't actually see them being needed for max vacuum limitation (they are set at 12" and I don't see them flowing at all in the current config). I eventually plan to use these set lower to provide fresh air flushing at low blowby flow. But I need to engineer the recirc control to ensure at high blowby cases they stop flowing (mainly just by relieving crank vacuum to below their threshold).

Lots to do.

Alan

 

Alan,

The only recycle connectionI have is the small vent to the idle air asembly. I have retained the connection from the boot to the passenger side rear cam cover vent but that has a check valve in it so as to allow in breathing only as per original design.

I have wondered about trying to design a venturi on the underside to help extract from the Pro Vent.

Trouble with venting to atmosphere is that the crank case pressure ends up slightly positive as the something is needed to drive across the knock out baffle and work its way to atmosphere and no matter how good the baffle design is or is not, ther ewill always be a positive differential pressure across it no matter how small it is.

Thus I suspect my system might just be well suited to a vac pump installation- just not sure how much crank-case pressure is required to cause ring flutter. As I view things a partial vacuum in the crank case can only be a good thing providing the vacuum is not allowed to get too deep and blow crank seals.

Regards

Fred

 

You could easily plumb a simple catch can such that you drain the pro vent into it and vent the top back to the crank case to pull a vac on it. that way you would see how much oil the system is catching.

The self venting system from the pro vent is a clever concept providing the return line is sized correctly- just remember the flow will be vastly different depending on whether the line is liquid flooded or- most likely-not.

My next step is a leak down test- no getting away from that. If that proves OK then more work on the breather system is likely needed. I cannot rule out tired rings.

I guess the obvious question here is why do 928's seemingly have problem and other Vee-8's do not? What does the M5/E39 motor have that is so different? I know it has the oil evacuation system from the cam towers- maybe the 928 problem is because the motor is so compact height wise.

My Vee-12 Jaguar motor sits very low in the chassis, is very compact and does not use a drop of oil.

Regards

Fred

 

Both my V10 M5 and V8 E63 use/used more oil per mile than the twin turbo 928.

 

Tuomo,

How does your TT breathe?

 

My GT used to use a lot of oil. Then I increased the the size of the breather vent through the oil fill port from 1/2" to 3/4". There is a large (up restrictive) 2x90 degree pre-baffle that knocks down the majority of the oil. I believe if the vent is not properly sized, you still generate positive crankcase pressure, ring flutter, pulses, etc.

The addition of the vacuum pump overcomes the need for a large vent, but at what expense? Certainly there some $$$ and parasitic hp loss.

It could also be depending on mileage and engine model, the need for such a system varies. Perhaps engines with worn rings suffer more than fresher ones? (I think so.) Perhaps spark advance also contributes? (Not sure.). Certainly detonation has a huge affect.