John Speake

The front knock sensor is to record knocks on Cyl 1,2,5,6. The rear sensor for 3,4,7,8. That's why the sensors are placed right in the centre of each group. Move at your risk !

Some knock sensor are resonant, and so if the bore is changed, they may not detect detonation. The 928 sensors are untuned.

ptuomov

What gets done to car engines doesn't always have to conform to any sort of evidence or logic, so it's entirely possible that people pay to drill windows in their engines and pay to inspect other people's engines for windows even if the windows lose power. ;-)

In terms of whether they help or not, Ricardo WAVE simulation software shows a curve that says a completely sealed bay with depressed atmosphere and with special rings will yield close to the maximum power. Huge windows and gigantic pan will get very close to that. The worst point in terms of pumping losses is the case where there are small windows and shallow pan, but no complete sealing. That describes the 928 S4 engine in my opinion. From that starting point, there are benefits from increasing the breather windows. Since 944 has a more open and larger sump, I would expect the benefits to be smaller for 944 than for 928. This all from theory and simulations, not practical testing or knowledge of the 928 possessed, used, or claimed.

In terms of practical testing, or more accurately reading about practical testing, I recall reading somewhere that GM found the following with small-block Chevy V8's and later derivatives: For an open and big oil pan V8 engine with traditional main caps, the benefit from practical size main web windows is in the neighborhood of 1hp/liter. For a shallow and small pan engine with a bed-plate main support and without crankcase vacuum pumping, the benefit can be 4hp/liter. Of course, these are pretty stupid and basic rules of thumb. For example, they don't consider peak power rpm, which is obviously a first order issue in terms of windage. Still, stupid and basic as they are, I think that's why the Corvette engines that have the hoodline constraint now have these huge holes in the main webs.

Shallow-pan wet-sump V8 with a bed-plate main support and a binding hoodline constraint... hmm... where have I seen an engine like that before? ;-)

Now here's a totally useless computation: On a 2.5L 944, the above rules of thumb would give 2.5hp gain from adding the windows. On a 6.4L 928 stroker, they would give 26hp gain. This is a useless computation because it doesn't consider rpm and also it can't be calibrated to the existing breather windows: Are the already existing windows good or bad? So maybe we should consider these as estimates of the maximum possible gains and the real gains are likely to be much lower. (Except if they help with reliability, in which case the gains can be much more significant than the above estimates.)

Goes without saying that anything that is being proposed in this thread, form drilling in to the bedplate oil channels to punching holes into the block is done at own risk. Although I'd like take one blown up engine with all the mods in it to Porsche service and request a replacement engine on the warranty just to see how they react.

The knock sensors and detection logic operate based on volume of sound at certain frequencies. If you move the sensors away, they don't hear as well. If you chase the thread and lube it well, then torque it to a higher torque, they hear better. They can't sense the direction in any way, AFAIK.

Even if the sensor isn't tuned to resonate at some frequency, the logic interpreting the output probably has a band-pass filter to let in only the relevant knock resonant frequencies. I say probably because I don't know, I am guessing. If I am guessing right, changing the bore size will still impact the ability to detect the knock, because the band-bass filter is not centered correctly at the correct frequency. If I were changing the bore, I'd investigate the ability to change the knock sensor signal filtering and to tune it to a better to the new knock sound frequency. i don't know if JDS has the ability to do anything about that.

One more thing. I found in my old notes from two years ago that the resonating frequency of a bore for knock detection is approximately, in the neighborhood of 100mm bore: Knock sound frequency in Hz = 10885 - 51.9 * bore diameter in mm. I would trust that formula for anything, but I think I got it from a SAE paper or something so you might find a more reliable formula from there.

AO

Found this little gem on the etherweb. It's a nice read and probably provides some good baseline formula for calculations.

http://www.106rallye.co.uk/members/d...hersystems.pdf

ptuomov

I wonder why nobody has posted that before. ;-)

AO

Sorry I missed that ... And here I thought I was ahead of the curve. Guess not. It's been wicked crazy at work lately.

Vilhuer

No but distance from cylinder its measuring will make difference. Will it be enough big difference to mean anything is good question.

FredR

There is a similar anyalysis of the 928 breather system [Mike Schmidt?] on Louie's website. Seems to fit with the article.

I am convinced the predominant pinging on 6 and 2 is very much connected to this - the question is what causes this? I have wondered if it is connected to windage and proximity to the oil sump, that and blowby gases trying to migrate from 6 cylinders to the chimney above 1/5. One might think this pinging should be predominant on 1/5 but perhaps there is less windage here due to the effect of the side wall slowing it down due to drag?

Regards

Fred

GregBBRD

The point I've been trying to make all along is that all of this stuff is interactive.

The rods bearings are soft, in order to absorb dirt and debris, over many cycles, without damaging the crankshaft. #2 and #6 are the last to get oil and the first to get aerated oil. They therefore "suffer" before other bearings, when the problem is lack of oil/aerated oil. The center cylinders run hotter than the outer cylinders....true in virtually every engine. The center cylinders get most of the oil from the intake manifold.

Higher temperatures, reduced effective octane from injesting oil, last to get oil to the bearing, first to get air to the bearing.

Now which two cylinders are going to lose the rod bearings first?

This is so obvious that the bigger question would be:

If someone had a bearing failure in other than #2 and #6.....how/why did that happen?

Of the engine failures I've seen and heard about, John Gill one of the only really strange failures that needed to be thought about carefully. I would have loved to have seen those pieces and tried to figure that one out.

ptuomov

There are many reasons why 2/6 could be knocking more often than other cylinders. I would be interested in hearing people's opinions about why that is. (I don't have a strong opinion on this because, for whatever reasons, I don't observe any more knock with 2/6 than other cylinders on my turbo car.)

Some "engine 101" possibilities why 2/6 could knock more often:
(1) Air intake draws oil from the crankcase evacuation system. The intake is designed in a way that the oil in the intake is primarily drawn to cylinders 2 and 6.
(2) The crankshaft somehow sprays more oil in the 2 and 6 bores, and the rink pack oil control is overwhelmed for those cylinders the first, letting oil and causing knock.
(3) Because of the intake runner length, cylinder 2 and 6 fill the best at the mid range points at which the engine is the closest to knock.
(4) The knock sensors are located in a way that they hear the knock of 2/6 cylinders the best, while they miss some knocks on other cylinders.
(5) 2/6 and 3/7 are center cylinders and therefore run the hottest and hot means knock (EDIT: this was added after me reading Greg's above post)

Nothing above is in any way tested with experiments, other than that my Shaktuner logs don't so abnormal knocking tendency at 2/6 and I don't let any oil into the intake, which I guess would support (1) as the underlying cause.

simos

This discussion remains me about one old thread, where Todd installed eight O2 sensors to monitor A/F-ratio on each cylinder separately.
He found that #2 and #6 were running the most lean.

https://rennlist.com/forums/928-foru...ne-spacer.html

"From Todd's data logging he's mentioned 2 & 6 are the most prone to knock due to going lean, in the stock (batch fire) configuration.
He has the fuel trim turned up the most on these two, backed off on #5 more then any other cylinder.

His fuel trim adjustments are not "fixed" through the RPM range. Where one cylinder might be lean at 3,000rpm, it might be rich at 5,500.
This might give an idea of how much time Todd has spent on the dyno tuning each cylinder.
He said #2 is the most efficient, moving the most air consistently throughout the RPM range"


The location and shape of trumpet entries aren't very optimal on #2 & #6, but perhaps they are the most straight pipes...
I would have put my money for cylinder #1, because it has optimal place. entry shape is usually the best and not touching much the plenum walls.

Would be nice if people could check from their ST datalogging if this is really, mostly true or not. I don't see any excessive knocking on #2 and #6 compaerd to others, though.

Sander_Baas

Okay guys,

Quickly my idea about breathing the crankcase and valve covers.

I plan to use all the valve cover connections, to get the most flow as possible.
Try to use a GTS OFN so also this one breathes maximum.

The valve covers and crankcase both end up in the Provent (or catch can)

From here the vacuum line goes to the original vacuum connection after the throttle body (less than WOT = always vac)
And it also joins with the original vacuum connection in the elbow, (makes use of the Bernoulli Effect, so always vac?)
These two connections get separated by one-way valves
Why use both connections? --> Max vacuum generation + maybe always vacuum because of Bernoulli Effect in elbow?

Than what I think is the most important… in this green line (vac out of provent to both vac connections on throttle body)
A heater valve, ( A ) this one is open when there is no vacuum. So in case you drive WOT and both vac connections on the throttle body do not generate any vacuum any more,( but you do build up crankcase pressure!) The valve opens (Because --> no vac) and the crankcase and valve covers can vent to open air thru the provent. This way there is always ventilation.

The provent will dump its oil back in the sump again with a one way valve in place.
The valve covers can get fresh air thru the purple one-way valve

Please shoot as much as possible on my design this is the only way to improve it.

Greetings
Sander Baas

Alan

Those 2 OFN ports aren't big enough to feed your provent - you need a better connection.

You can't create a vacuum leak past the throttle body at idle without compensating.

Once you start generating positive pressure in the crank you don't need vacuum to evacuate it - so the limit case is when the capacity of the lines to the throttle body are exceeded and pressure rises above desireable... You could use a blow-off rather than switched diverter based on losing vacuum.

Could also evacuate using the air pump diverter (differently controlled) into the exhaust... vs atmosphere.

Greg suggests the crank is impossible for high RPM as a sole vent - Not sure about that - but I do think you certainly do need a pre-separator in there - I have a baffle & scrubbies up the GTS OFN before the provent and it still sees quite a bit of oil... (so he may be right). I will do better next time with an improved design - I won't use a stock OFN, I already fill through a new port to the pan so will just construct a better separator box for that purpose only.

Alan

FredR

Yet more points to consider...

1. That oil starvation is part of the 2/6 failure process must be a given.
2. Having seen what I have experienced [limited as it may be] I am inclined to think that engine knocks may be contributing more to big end failure than perhaps has been given credit for- the knock control system can only do so much and if 2/6 lead the charge then logic says they run out of steam first.
3. The tendency to knock mid range is there but my recent investigations suggested that was no more prevalent than normal with minor events on stockish settings- implication being that oil lifting is not prevalent at that rpm point.
4. There is what seems to me to be a very clear point [5300 rpm] for onset of pings on my motor. That it happens mostly on 6 then 2 is one thing but it seems if those two were not pinging I suspect I could run the stock timing on the other cylinders no problem, my perception being that 6/2 generally lead the charge by 4 or 5 degrees [sometimes].
5. Given the post on the motor with sensors on all 8 cylinders, more air flow thourgh those two cylinders goes part way to explaining the problem but it seemed to me that when I richened up the mix to circa 11.5 to 11.9 it made no difference to the pings. This leads me to conclude that enrichment can only do so much.
6. I have no way of knowing whether 6/2 runs significantly hotter than other cylinders [and for sure one would not expect it to run cooler] -all I can say is that I believe my cooling is optimal if not a little overdone at the moment with the 75C thermostat.
7. I would be interested to know what kind of temperature increase inlet air has to see before that starts to cause some issues. It would be interesting to log inlet air temperature to see what temperature the inlet air temp rises to on its journey through the inlet tract. Much as I like the EIS system a metal shell is going to allow more heat to pick up compared to plastic duct work.
8. it would be interesting to remove the inlet plenum spacers to see if ST2 can detect any signs of a difference [ve or -ve]. I am wondering if these may facilitate an increment of mal-distribution of air compared to the stock set up?

if this oil consumption and the knock problem werre occurring on other cylinders I would be thinking in terms of a knackered motor and although I do not discount that possibility, this thread keeps my hopes alive that it is no more than a breathing problem that just may be soluble without major mechanical intervention.

For sure it is a complex interactive problem and if Porsche could not solve it then enthusiastic DIY hacks like myself have no chance.

Thank goodness we now have some options

Regards

Fred

ptuomov

My plan two years ago was also to use all four valve cover ports to breathe out. I have metering orifices drilled out from from the elbows and an elbow installed in every valve cover port already. However, I changed my mind after thinking about it more, and now I am only planning to equalize pressures and draw air in thru the valve covers, no breathing out of valve covers. Who knows whether it'll work and what I'll think two years from now.

A couple of points about your suggested designs:
- Any design that uses a check valve anywhere in the path depends pretty critically on the type of the check valve. I think that for your purposes you need ones with a very low cracking pressure, and I think (but do not know) that this means a disc-type check valve without a spring. I have ordered a pile of different check valves and will be testing them after I come back from the Caribbeans.
- You need a restrictor somewhere in the line that goes into the intake manifold between the port and the throttle body. Otherwise, you may have idle problems as Alan mentions. I believe that the restrictor should be about 2mm diameter hole, but I am sure more experienced people here can give you better advice.
- You probably want a lot larger ports in the oil filler neck that what already exists in the GTS oil filler neck. I've got a big hose screwed to the plastic filler neck currently. It works but is inelegant. The plastic oil filler neck is hard to work with. Perhaps you could find an early metal filler neck which is easier to play with, and weld a big port to that?
- I agree with Alan that an air-oil separator box would help under the oil filler neck.
- I also agree with Alan that given the shape of the MAF elbow junction, it's never necessary to vent to open atmosphere (as long as you can separate the oil from air.)
- You should also carefully read the ProVent literature. When I read it for the first time, I was surprised to learn about all the features. The ProVent itself has pressure regulation and blow-off safety valve options. Perhaps you could use those for your purposes: http://www.mann-hummel.com.sg/EN/ind...ProVent-en.pdf

FredR

Sander,

For starters I believe you have the connections on the Pro-Vent the wrong way round and this should be mounted as high as possible.

The small line on the side of the filler neck is there to vent the crankcase at tickover/small throttle openings wherein the high vacuum sucks the gases through so I cannot see any point in removing that from the system.

To move any gaseous stream there has to be a pressure differential to drive it. There will be small increments of pressure drop across the air filter but I suspect the key to success here is to design an effective venturi[s] to pull the gases into the fast moving air stream at part/WOT. The pissy little thing there at the moment is probably too small I would hazard a guess. Not sure how much vacuum a venturi can pull- I would have to go back to my books for that one.

Beyond that you need to get fresh air into the system and maybe the stock connection is good enough for that. Cooler[denser] air will want to push the hotter [less dense] gases out of the crankcase but doubt it will stay cool for long in there. If 3% is a good number then there is something to work with for those so inclined.

How you overcome the dynamic slugging back up the cam tower drains can presumably only be down to the amount of differential pressure [read vacuum] generated in the crankcase.

As to what should be plumbed from where I am still on the fence to some extent.

Regards

Fred