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People often forget that this is a discussion board and the free exchange of ideas and thoughts are what make it so interesting, entertaining and most importantly, USEFUL! I encourage disagreements and questions, because it forces everyone to get to root of the issue or problem and find solutions. Some here think we are writing a new version of the bible .... I think "they" need to step back and understand why we are all here.
This picture clearly shows what is missing but badly needed for the 928 engine block. Inside the V are the chimneys for pressure equalization between crankcase and cylinder heads. On the outside of the cylinder banks are the oil drain channels similar as for the 928 block but going deeper into the oil pan.
Any good ideas of how to add chimneys to the 928 engine block? Running ITBs will make modification of the block less difficult. The stock intake manifold is taking up too much space inside the V.
Åke http://cdn.stangtv.com/wp-content/bl...rd-351-006.jpg
This picture clearly shows what is missing but badly needed for the 928 engine block. Inside the V are the chimneys for pressure equalization between crankcase and cylinder heads. On the outside of the cylinder banks are the oil drain channels similar as for the 928 block but going deeper into the oil pan.
Any good ideas of how to add chimneys to the 928 engine block? Running ITBs will make modification of the block less difficult. The stock intake manifold is taking up too much space inside the V.
Åke http://cdn.stangtv.com/wp-content/bl...rd-351-006.jpg
One suggestion I made was to tie the head vent tubes and route to the oil temp bung on the oil pan. Equalize pressure and put oil back in the sump.
If you are not using all 3 of the Knock sensor points, then the block might be able to be drilled in the V and plumbed to the head vents to equalize pressure. There are 3 knock sensor attach points on the S4 block if I remember correctly, and only 1-2 are used..
I also tied the passenger head vents to the passenger side base compartment of the metal oil filler that drains into the crankcase. The VAC pump pulls from the other side of that partition.
One suggestion I made was to tie the head vent tubes and route to the oil temp bung on the oil pan. Equalize pressure and put oil back in the sump.
If you are not using all 3 of the Knock sensor points, then the block might be able to be drilled in the V and plumbed to the head vents to equalize pressure. There are 3 knock sensor attach points on the S4 block if I remember correctly, and only 1-2 are used..
I also tied the passenger head vents to the passenger side base compartment of the metal oil filler that drains into the crankcase. The VAC pump pulls from the other side of that partition.
But as I pointed out already this really can't work very well - the breather vents are too high to drain oil except when the heads are already pretty full and you get some lateral acceleration... with a return below the oil level they can't provide an effective parallel equalization path either. I think the vents are also too small - but larger certainly could be fitted - then find a path for equalization to the crank. Without the stock intake it may be possible to add some good additional venting - could be quite short paths too...
Maybe provide some interbay breathing as well. Bore bigger vent holes in the flat bottom of the V - reinforce with plates welded above... I think I've seen pictures of this...?
The stock intake manifold is taking up too much space inside the V.
Maybe provide some interbay breathing as well. Bore bigger vent holes in the flat bottom of the V
On S4+ cars (at least with a stock intake), there's a surprising amount of space between the floor of the V and the bottom of the lower plenum. Certainly enough for some big-*** banjo fittings and -10 or -12 'balancer' hose. It's a giant ocean of space on a car with ITBs or the Threshie intake. If I think of it I'll measure the space this weekend.
Removed the intake / throttle body on my 86.5 928 this weekend and notice the oil accumulation in the intake / throttle body base, and then tried to figure out how the crankcase breather is supposed to work.
The best I could determine is as shown in the included sketch -- vacuum from the throttle body base draws air from one cam cover hose (towards fire wall), the other port on the throttle body draws air from the oil filler base, and this is 'connected' to the other cam cover hose via the other port at the bottom of the filler neck.
Third source of vacuum is from the intake 'T', smaller hose attached to top of the filler neck, also drawing air from inside the engine.
So everything drawing air/gasses/oil from inside the engine to the intake, and the results obvious from oil that accumulates. There does not appear to be a supply of make-up air into the crankcase anywhere.
A more standard system with pcv inlets filtered air into the oil filter cap, and draws it out via a valve cover / pcv valve then into the intake for re-burning. There is a flow of air/gases from atmospheric to combustion.
Would this work on a 928 ? ?
- Connect the two valve cover hoses together, and the then through a pcv valve and into one of the ports on the throttle body (plug the other). Make suck in some oil, not sure.
- The oil filler neck would take in filtered air, and this could be attached to one of the two connections at the filler neck base (plug the other). Because air (maybe ?) going in vs out, then oil is not being sucked into the intake.
- Plug the top small hose fitting on the filler neck. The hose to the intake can remain intact for gas vapor recovery, but not connected to oil filler neck.
The general problem for 928's that ingest oil to the throttle body is that there is no place to extract blow-by that won't already be entrained with oil. Some cars don't have this problem - but they also don't need a solution. Later cars generally have have more severe problems, sometimes perhaps because of longer stroke/bore, different piston oil control, possibly unwise changes to the PVC system over the years etc etc. But for a car with oil ingestion you need a solution to reduce oil entrainment and/or better separate the air & oil on the way out - because otherwise even the best location (the heads - furthest away from the combustion chamber blow-by & rotating crank) isn't good enough to extract from under intake vacuum without also gulping in quantities of oil. Consuming (perhaps lots of) oil this way messes up the intake, deposits carbon on the pistons, causes detonation in the cylinders and creates smoke in the exhaust that may even damage the cats. So solutions really need to actively reduce entrainment or improve separation - if you aren't doing either one deliberately you aren't likely to make anything much better. Just replumbing the system likely won't help at all.
Some ideas:
- Lowering the oil temperature under any given conditions seems to help (extra oil cooler?)
- Some oils behave better at high temperature than others - so chose wisely
- Lowering the net crankcase pressure helps: less positive pressure is good (bigger vents) - negative pressure is better (vacuum pump)
- Drilling out the missing oil passages in the pistons likely helps GTS engines
- Windage trays, oil pan extenders can probably help esp. for longer stroke/bigger counterweight engines
- Extracting through a suitable volume AOS with sump oil return will help (or several AOS stages)
- The main crank vent (oil filler location) is probably best for evacuation via good AOS stage(s)
- Not extracting from the heads probably helps with head oil drain back (avoid starvation)
- Part time fresh air flushing into the heads probably also helps with oil cooling and oil drain down
Thanks Allan,
Great points.
There are several posts on Rennlist about reducing oil consumption when installing a baffling system under the oil filler neck (I ordered one yesterday from 928Rus!).
Given that the intake is sucking in air/gases from both the valve cover and the oil filler neck - and if its true these oil filler baffles noticeably reduce oil intake - I assumed that the majority of the oil is coming from the filler neck.
So....turning the oil filler neck into an air input rather than trying to suck air out should reduce oil intake. It also enables a set up closer to a typical car.
I guess I'd have to try to see if there is any improvement or not...
In a GTS all the oil comes from the head vent (that's pretty much the way its configured - not a smart change by Porsche). I don't expect the heads are actually much different than the crank vent at the filler top anyway. I find the in-crank-vent baffles (even the good ones) help just a little bit. Of course everything depends how bad your car is for ingesting oil in the first place. I do think you will need to think about more mitigation actions as noted but monitor how well each works till you are happy (oil usage may be the easiest measure). If you flush in through the crank vent and evacuate out through the heads (or more likely just the pass head) - be aware the only path for blow-by (or flushing fresh air) out of the pass head is for the blow-by/air flow to go up the oil drains from the heads - there is no other path. This impedes the head oil draining back to the sump. Louis Ott has documented the oil in the heads frothing under high rpm sweeper turns. Highly entrained and/or frothy oil will be severely impeded in drain down anyway - but especially with counter-flow up the drains. I think fresh air flushing the crankcase is a good idea - but the flow should be down the drains - so I'd say vent fresh air into the heads and evacuate NOT from the heads.
Alan -- thank-you. OK, got it flow in through the heads and out the crankcase @ filler neck.
(Blake -- I'll look for that drain, unaware it existed.
A little google and I stumbled on this adjustable pcv valve.https://mewagner.com/?p=444.
With the stock set-up who know how much vacuum the crankcase is under, Adding a pcv valve should regulate that.
Will do some experimenting and get back. At work we use a data logger for pressures // vacuum on large NG generator, I'll data log crankcase vacuum post-mods.
Its a great idea to check it out so you know for sure - but If you have excessive oil usage via ingestion I have a pretty good idea that your crankcase is pressurized under many conditions. Mild crank vacuum is your friend here not your enemy* - I don't think an intake sourced vacuum is likely to need regulating. If you add a vacuum pump then I'd suggest also adding VLV's. *eliminating it will likely make your oil ingestion worse
Out the crankcase at the filler neck - Yes - but via an effective AOS solution - The in-crank vent baffles aren't likely enough alone IMO (but try it first).
Alan -- thank-you. OK, got it flow in through the heads and out the crankcase @ filler neck.
(Blake -- I'll look for that drain, unaware it existed.
A little google and I stumbled on this adjustable pcv valve.https://mewagner.com/?p=444.
With the stock set-up who know how much vacuum the crankcase is under, Adding a pcv valve should regulate that.
Will do some experimenting and get back. At work we use a data logger for pressures // vacuum on large NG generator, I'll data log crankcase vacuum post-mods.
Bob
1986.5 928 Manual
It is a plate on top of the oil pan in the front opposite the oil level sender. Pretty sure it is there up through 86.
06-10-2015, 12:35 PM
thanks!
