Alan

When I refinished the intake & cam covers in crinkle black I felt it needed some visual interest. yellow has the best contrast with black. If you are deviating from stock IMO you might as well make a bold statement.

Some may not like it... but that makes no difference to me, they can do whatever they like with their cars. My GTS isn't going to a new owner any time soon...;

Alan

ptuomov

I have a thread on crankcase breather system for boosted cars. There may be some ideas for you in that thread. https://rennlist.com/forums/928-foru...ing-right.html

In V-engines that run clockwise (observed from the front), there’s naturally a slightly higher average pressure in the passenger side valve cover than in the driver side valve cover. Chrysler engineer called the driver side the suction side and passenger side the pressure side, and the way he said that makes me believe this is commonly accepted premise in the industry. Because of this natural pressure differential, many (most?) V8 crankcase breather systems come from the factory breathing out from passenger side valve cover and breathing in the replacement air from the driver side valve cover. I am fairly certain that this pressure differential is cause by crankshaft rotation creating small but measurable impact given the angles of the drain channels from the valve covers.

The above is all very generic. Below are some of my thoughts and opinions on the 928 S4 and later crankcase breather systems.

The post-1987 Porsche 928 models have had a number of different kinds of crankcase breather systems installed from the factory. The original conceptual design is described in the US Patent 4,753,214. Figure below is sourced from the patent documents:



There are a number of important features in this original concept. The rear passenger valve cover breather (patent figure 12) has a volume separator inside the valve cover (not displayed in the drawing) and approximately 6mm metering orifice in the elbow. The connection (patent figure 16, 7, 17) has a check valve with approximately 1-1.5 psi cracking pressure. The from passenger valve cover breather (patent figure 10, 9, 6, 5) has no separator and no metering orifice. The connection from the oil filler neck to the intake manifold plenum has an approximately 2mm metering orifice.

Subsequently, multiple different variations of this system were produced. The two later ones are describes in the below figure from the workshop manual:



While the implementation in the early GTS engines on the left is more faithful to the original concept, the implementation on the right for the later GTS engines deviated significantly from the original concept. A connection between valve covers is added, a check valve is eliminated, etc. The GTS engine breather system is broadly considered a failure, but it is not clear whether this is because of the external breather system per se or some other, more fundamental problems with the engine design.

The early S4 crankcase breather system that closely corresponds to the system originally described in the patent documents works somewhat adequately if the engine, including the engine redline, is stock and if the car is run on stock tires at moderate lateral accelerations. If the engine rpm is increased or other changes made, problems often appear. Subsequent GT models had more serious problems in even their stock form, presumably due to higher engine speeds.

The lateral accelerations are relevant for the following reasons. In a 90-degree V8 engine, the oil drains from the cylinder heads are in approximately 45-degree angle relative to the vertical plane. In a 1g lateral acceleration, from the oil drain perspective the engine is effectively turned sideways with the oil drains horizontal on the outside turn bank of the engine. In this situation, there is no gravity drain, and the pressure differential between the crankcase and the head mostly determines in which direction the oil flows. If the crankcase pressurizes and the heads are relieved, it is logical to assume that the oil will flow into the heads instead of the sump.

In addition, the piston pumping pulses create havoc in the oil drains. It is possible that if blow by gasses increase the pressure and density of the crankcase and high rpms then increase the energy delivered by the piston pumping pulses, the oil may be ejected from the oil drains into the heads.
The combination of high blow by rates, high engine speeds, and relatively low pressure in the heads compared to the crankcase may lead to oil drain problems so significant that the oil pump pickup in the sump will be starved of oil, resulting in engine failure.

I believe that the GT and S4 models’ crankcase breathing problems can be mostly resolved by redesigning the external crankcase breather system. Higher the operating rpms and higher the lateral accelerations, more extensive the required internal engine modifications. However, the engine internals of GT and S4 models are adequate for most uses, while improvement are of course possible. This is my opinion, and at this point its not entirely uneducated.

The GTS engine has many issues relating to oil consumption and crankcase breathing, and I believe those aren't caused mainly by the breather system and in any case can't be cured with minor tweaks of the stock breather system. I believe that one problem in the GTS engine is the crankshaft. First, the increased stroke increases the pumping pulse energy by about 18.5% based on a crude back of the envelope formula. Second, the GTS crankshaft counterweights are very poorly designed. They not only cause high bearing loads (not a problem in 928 because of oversized mains) but also block the crankcase gas flow with the addition of the center counterweights and the large fan angles of the counterweights. Finally, the GTS pistons do not have sensible oil drain provisions, leading to poor oil control on the bore walls and significant blow by that may be impossible to resolve with an external breather system. This combination leads to a situation in which crankcase is pressurized by blow by, oil is ejected to the intake manifold, the ejected oil will cause detonation, detonation will cause increased blow by, etc. More radical solutions may be required to cure the GTS engine.

ptuomov

S4 and later versions of the 928 engine have a big problem of splashing too much oil on the bore walls. This is, I believe, the reason why the oil jets cooling the pistons were quickly eliminated from the S4 engines. The problem is not too little oil on the bore walls or undersides of the piston, it's too much oil there. The bottom two piston rings have real problems trying to scrape off all that oil, and fail to do that under some conditions.

In terms of the GTS engine, it is my understanding that the GTS pistons don't have channels that spray on the wrist pins. The S4 pistons do have those small holes in the oil ring groove that do that. The GTS piston pins rely entirely on the splash oiling, which is mostly coming from the rod bearings. Fortunately for the GTS piston pins, there's hugely excessive amount of oil being thrown at the pins because the above mentioned tendency of these engines to overoil anything up the cylinder tower.

Some concerns were raised earlier (maybe a year ago?) about piston wrist pin oiling of GTS engines under crankcase vacuum. I've come to the conclusion that those concerns are entirely unfounded, because of how the crankcase oil flow works. Suppose that we suck a perfect vacuum to the crankcase. What will the oil spray look like from the rod bearings? It will be thrown in all 360 degree directions in a plane that is normal to the crankshaft centerline. Because of the 928 engine geometry, the exposed parts of the piston wrist pin are on those oil spray planes. The piston wrist pins of an open piston design such as the GTS would be really, really well oiled in a 928 engine if there would be perfect vacuum in the crankcase.

Now, suppose that we introduce some positive pressure (and gas density) into the crankcase. The piston pumping pulses will now communicate between cylinders and crankcase bays. This gas flow will disrupt the oil spray disc and the disc will spread oil into a wider area. Less oil will now make it to the exposed part of the wrist pin.

In a GTS engine, lower the gas pressure in the crankcase, the better oiled the wrist pins will be.

This is not necessarily the case in a modern engine. A modern engine doesn't spray excessive amount of oil up the tower. Furthermore, a modern engine will have relatively closed piston design (in extreme, box-in-box structure) and relatively small side clearance between the piston and the rod small end (in extreme, piston guided rods). Because of these reasons, the modern engines often have so called "forced piston oilers" which simply allow the film pressure on the cylinde wall to feed the pin thru a from a ring groove to the wrist pin boss. If we would suck a perfect vacuum in that kind of modern engine, then it's possible that we would get less well oiled wrist pins. But this is irrelevant to the GTS engine because of the fundamental design differences in how the wrist pin is supposed to the be oiled.

ptuomov

Why would crankcase vacuum lead to less oil, instead of more oil as logic dictates, to be sprayed on the exposed part of the wrist pin? If the gas flows don't disturb the oil spray from the rod bearings, it hits almost exactly to the center of the exposed part of the wrist pin. With gas flows, which get stronger with higher crankcase gas pressure/density, the oil spray distributed in a wider area and less of it hits the wrist pin first.

Suppose that it were true that stock GTS engine burns wrist pins. Pulling a vacuum would help with that problem. Allowing the crankcase to build a pressure would make that pressure worse. So isn't the crankcase vacuum pump a solution to any GTS wrist pin oiling issue if those exist?

Pulling a vacuum on modern engine or a race engine built with modern parts can hurt wrist pin oiling. I think those are designed to oil the wrist pin from the bore wall. Vacuum pump reduces the amount of oil on the (front and back) bore walls, and may thus reduce wrist pin oiling. But since the GTS pistons don't oil the wrist pins from the bore walls, this effect doesn't come into play in the GTS engine.

UpFixenDerPorsche

Yes, yes, we must all be individual (Monty Python style):

This is that "experimental" S4 manifold of mine. It was looking a bit second hand after I finished the work, so thought I should at least give it a coat of "something" before re-fitting. Whoooaaaa ... LOL. C3PIO would have been pleased. I have to say the 'gold' look is growing on me.

Strosek Ultra

Tuomo, your camping trip must have been very refreshing, you have gathered plenty of strength and energy for writing posts on RL.
Not only GTS pistons do lack of oil return holes at the oil ring groove. I have a factory exchange S4 engine taken apart. The pistons are of the same design as the GTS pistons, no oil return holes at the oil ring groove. It made me curious to see if the lack of oil return holes and no oil spray onto the wrist pin may lead to failed wrist pins. I have examined the pistons carefully and all wrist pins are showing some discoloration bluish. It is not very bad and can for sure be polished off. Comparing the wrist pins with wrist pins from another engine having pistons with oil return holes, no discoloration could be found on the other wrist pins. My conclusion is that the missing oil return holes and thus reduced lubrication of the wrist pin may and will lead to wrist pin failure/discoloration.
I have been in this business working on engines for fifty years plus and I have never before seen pistons having no oil return holes. Even the three small oil return holes at each side of the "normal" designed S4 pistons are questionable whether enough.
In Germany the Porsche Developing Center at Weissach is called "die Denkfabrik" (Think Factory) where they do plenty of thinking and research for the automotive industry including Harley Davidson.
One may ask what they thought of when the GTS piston was designed, maybe it was on a Monday.
Åke

Alan

This is indeed some kind of "Bold Statement"...

Alan

Alan

While the wrist pin oiling scheme may not be ideal on GTS engines (and apparently some other late models), the application of mild crankcase vacuum should really have no negative effects on that. In fact as Tuomo points out there may be benefits Vs. the stock config generally - and esp. at those points where it would pressurize the crank due to excessive blowby through those limited evacuation orifices.

Alan

john gill

I had a set of 85 86 pistons, that I had the holes drilled into for the race engine that i have built (although not run yet), so add another one to the list .

After reading the boosted thread and this one again , I also can see the benefits of the sump vacuum particularly with the secondary air oil separator, that Alan has designed . I also tried the provent circuit , but just kept filling it with oil and overflowing . so it is now sitting on the shelf, I intend taking the car into the dyno and load it up at high rpm , and check to see if I can witness where the oil is coming from.
In my particular circumstances my cars are dedicated to track only , so my solutions need to be more demanding than probably the road going versions .
One point though I believe it is very foolish to discard the combined experience and observations of people like GB and Ake , who work on these cars all the time noting the blueish tinge of the GTS wrist pins, in an endurance race this would a guaranteed failure point, actually catastrophically expensive , (imagine engines seizing at in a high speed, high gear turn flat out)
Does drilling the oil holes in these pistons prevent or alleviate the problem ? particularly in the case of using the vacuum scavenge system on the these engines ?

FredR

Alan,

I can vouch for the fact that when one is dumb enough to convert the S4 breather system to the GTS version the oil consumption increases. Even sadder is the fact that I modified the S4 filler neck rather than the GTS one- durrgh!

Let's hope that I eventually land a system that works [to my satisfaction].

Rgds

Fred

Strosek Ultra

Regarding the wrist pin oiling scheme, I do not think any engines before GTS were supplied with non-drilled pistons from the factory. I believe non-drilled S4 pistons, like for my factory exchange engine, are/were available as spare parts only.
Åke

blau928

MK - (Pistons in a GTS do not have the oil drain holes behind the ring to oil the pins, where the S4 pistons have the holes.) If this is the main issue, then as several have mentioned, pistons with holes should have the GTS behaving like an S4, and the oiling issues should be lowered.

Others:
On another note, if someone wanted to be extra careful, one could pull the motor apart, have the crank and rod bearings drilled to be oiled like a chevy, and drill the rod beams as well so the mains also oil the pins. Couple this with a dry sump, or a vac pump to keep the crankcase under vacuum, and the blowby will also be resolved.

But, it may be simpler/less expensive to just drill the pistons and leave the crank and rods alone. With pistons also drilled, the wall oil film would also be used to oil the pins with splash method.

In my opinion, (OPINION ONLY) another thing to do on cars with no vac pump would be to route the vent lines from cylinders 1-4 to the temp port on the front of the oil pan, as the oil pump pickup is very close to that. If the heads are full of oil, and it has to go somewhere, might as well be back to the pump inlet, and not to the top of a spinning crank that is already whipping oil in the sump...... That would be minimal cost for clamps, hose connectors, and tapping the temp fitting cap on the sump for a fitting. 1 hour on a Saturday with a cold beer...

If using a vac pump, it can be limited with vacuum limit valves that only allow a certain amount of pressure drop in the crankcase. However, based on some elementary physics, splash should not be affected by vacuum in a static environment. In a dynamic environment, the pressure differential might also cause the oil particles not in suspension to go out the crankcase via the vac pump depending on the differential pressure. Ergo, vacuum limiting valves may be a good idea.. (I think so..)

One thing I am not sure about, is how many on this forum knew that when Porsche raced the 928, they absolutely knew that there was an oiling issue with the heads being full of oil. Instead of overdoing it with lots of stuff and extra parts on the race car, they simply changed the restrictor size in the oil line to the heads, so that there was less volume of oil going to the heads in the first place..... I believe it was a 5 Deutsche Mark part and significantly controlled the oiling issue in the heads. This would then not require dry sumping the heads as the flow is restricted to the head, and oil stays in the sump.

In a perfect world, for a race motor, I would dry sump the motor, install either piston squirters or drill the crank/rods/rod bearings for the pin oiling and be done with it. If I was concerned with ring seal, I would also get gas ports in the pistons to control that. Expensive, but racing is not cheap.....

For a street car also used for once in a while track duty/DE, drilled pistons with a vacuum pump and limiting restrictor seems to be the cost effective ticket.

For a USA street car that will be cruising at 2,000 rpm 90% of its life at 60-70mph and 4,000 rpm to overtake someone for 10-15 seconds.... Leave it alone.... As much as many like to think they are on the autobahn, the USA is not Germany, and we cannot drive at wot for minutes on end.

In my case I have a vac pump on my S4 with no issues to date. However, I only have about 1200 miles on the vac pump system of my own design with a much larger oil/air separator than a Provent 200. I use a PV400, as the 200 was overwhelmed by the vac pump at higher rpm (>2500). The return from the PV400 is to the temp fitting on the front of the oil pan/sump. It's tapped for AN-8 tap (0.5"/12.7mm). The vacuum in the crankcase generated by the vac pump also draws oil from the PV400 into the sump.... The 1-4 head is vented to the 2 part metal oil filler so oil from the heads go into the crankcase. The other side of the oil filler also connected to the crankcase is attached to the vac pump inlet for evacuating the case. The PV400 vents to atmosphere currently, and I plan to try venting the PV 400 to the 5-8 head and do some pressure/vac testing once I have wired the car fully with the datalogger and can track pressures at any point I choose.

With that said, I have 305,000 + miles on my S4, and the motor has never been apart, and it does not burn oil. It has seen many track days, but has been sitting for a couple years since my shoulder injury. It started right up last weekend, and did not smoke at the SITP event in Berkeley. Aside from ticking lifters that were not pumped up as the car sat for 2 years, the car runs fine. 5 minutes of idling solved the lifter ticks, and it is now back in the warehouse awaiting some supercharger surgery.

$0.02

blau928

Another thing that can affect a vac pump is the pulley ratio between the pump pulley and the crank pulley....

I am not sure what ratio people set their pullies at, but it's another way to control the amount of vacuum and evac flow from the crankcase....

R

hacker-pschorr

In an attempt to clean up and keep this very interesting discussion on topic, I've moved the side chatter to this thread:

https://rennlist.com/forums/928-foru...the-world.html

If you find your post missing and you did not receive an PM explaining why it was deleted, chances are it was moved to the above linked thread as it was tied to that discussion.

Alan

Using the cam cover breather vent ports as currently configured would not provide much of an oil path back to the pan. If you plumb all ports, rotate the elbows 180 degrees and drill them out as large as possible you have a marginally sized path that will drain oil only under severe conditions - the heads would already have to be quite full and you would have to be getting lateral G force to make the 45 degree vent ports act like side ports (but only on one side at a time). Generally the port location is wrong. If you had an internal pickup attached to the inner vent - that dropped to the low side of the cam cover/head top to allow active pumped scavenging of the oil collecting there that would work better and supplement the oil drains - similar to what Greg has done in his top line system. I'm certain that helps normally - but it needs to also be able to handle the worst case oil volumes, but under any conditions it buys more time.

What might also help in a similar concept is extra breather port passages between the heads and crank above the oil level - it's possible all 4 cam vents could be big enough to make a difference (better elbows/ports would help). This would provide flow paths independent to the oil drains for better equalization. Then the oil can just drain down the drains unimpeded as intended. It would have been nice if Porsche had provided for this, (and better interbay breathing) in the original or updated basic design - but they didn't. Finding a good place to connect to the crank above the oil level with nice short paths is challenging however (esp. because you still need a separate crank evacuation path).

Alan