JET951

Hi john C , ( Up Fixen ) the only time we see oil pressure insufficient ( I take it thats what you combating ) on a last century Porsche 928 ( all versions ) is when one uses a viscosity that is outside the recommendation that was printed by Porsche in the owners manual

So in Australia ( Qld in you're case ) then the Porsche recommendation for ambient temp range of minus 10 deg cel to Unlimited High Ambient Temp = 15w-50 or 20w-50 , its quite clear in all the owners manuals

However if one uses a viscosity of say a 5w-40 ( in you're climate ), then you are in the danger zone of not just insufficient oil pressure at normal core operating temps at idle ( oil pressure warning comes on ) but you're no2 & no 6 conrod bearings are at Extreme risk of damage if you go full power at high RPM

We just do not see any wear to all with 928 or 944/951/968 engines where the engine has been on a decent high oil film strength engine oil with the recommended oil viscosity ( by Porsche in the owners manual )

But we get a lot of engine work( engine failures) that involves engine conrod bearing failure ( big end failure) & or excessive cam lobe & lifter surface wear & way too much thrust bearing wear & massive valve guide wear and in all these cases it was caused by a low oil viscosity oil , low oil film strength oil in these last century engines

But on most 20w-50 oils we see no wear at all ( street use )

We have just assembled a 1987 928S4 engine at work last week , nearly 300,000Kms and leaking / rotten head gaskets ( the normal stuff ) , but the nice thing is ( going by the service records of this S4 over the last two decades ) , this engine has only ever been on a 20w-50 or 15w-50 & the conrod bearings were in fantastic condition , same goes for the cam lobes = No Wear at all , but we are used to seeing this with the use of 20w-50 oils ( as Porsche recommended ) in these last century engines

And its on the track we see it in "time compressed way " , we would see( often too late) owners of 928GTS's , 944S2's , 951's etc turning up at a track day with a engine oil in their sump of 10w-40 & worse a 5w-40 & they would blow the bottom end out of their engines , in some cases on their first track day & YES these are the so called fully synthetic engine oils , but these oils failed & failed very badly , So Much for the Synthetic side of things ?

This is were it got interested in doing what I could in trying to prevent this unwanted destruction & the destruction is caused by the use of a too low oil viscosity for the given oil temp ( core ) & ambient temp range

So now a days we are not seeing this destruction at all , because every one who turn up at these Supersprints with a last century Porsche are now aware of this issue & use what we use = 25W-60 racing , this is just a slightly higher vis with a tad more ZDDP

So the first clue ( alarm to me ) is when someone is having oil pressure issues at idle & I feel sure they have already checked their oil press via a workshop oil pressure gauge ( just to confirm the oil pressure is too low ) be in on any last century Porsche

The damage to the conrod bearings is done with high core temp when the wrong oil viscosity( Too Low Oil Viscosity) engine oil is chosen because the oil pressure becomes so low in conrod bearings , the bearing shells can & will make metal to metal contact with the conrod journal , its that simple

Thats why we do not have any failures with these conrod bearings , in fact we had a Supersprint event today at Eastern Creek ( SMP) & as you know Sean has a 951 which is now a 3.0L twin cam 16V & runs 1.3 to 1.4 Bar boost on E85 fuel & this engine produces 630 - 650 HP & we /Sean runs the standard Glyco 928/944 conrod bearing shells ( yes the so called soft 928 / 944 shells ) & Sean's 951 does not have a dry sump , it retains its wet sump & baffles ( all stock standard in this area ) & with a min viscosity of 25w-60 ( I do mean min ) , Sean won last years Drivers Championship & the Motorkhana Championship & beats most ( 95%) of the GT3's competing & this 951 is a commuting ( drive to work car )

It gets even better , these stock Glyco 928 / 944 conrod bearing shells have been in this engine for three competition seasons ( 3 years ) plus 45,000 + road Kms,

So a 20w-50 engine oil or a 25w-60 oil does not cause wear to conrod bearings ( referring to that stupid comment at the bottom of that chart you posted ) , its the complete opposite , its the wrong low viscosity oil used in a hot climate is the problem in these Porsche engines

ptuomov

Interesting. We haven't had an oil fight for a little while! ;-)

It is self-evident that getting the oil up to the desired operating temperature as quickly as possible and then keeping it there with minimum variation is good for the engine. I am not sure whether the way Rob's engine is plumbed will achieve the former better than the factory system. I am fairly certain that it will achieve the latter better. It's a slam dunk obvious modification for any climate; the fan control, coolant thermostat, and oil thermostat will make it work really well. If that arrangement is not enough to keep the oil temperature under control, adding more external heat exchanger capacity is logical.

I think that people have had a lot of success with higher viscosity oils in hot climates. I am not however sure what is the underlying reason for that. There are a number of candidate explanations ranging from the stronger oil film in the bearings to the pump emptying the sump slower to less air mixing it into the oil. Etc. Although we don't seem to know the answer, at least we can have a vigorous debate about it on a periodic basis! ;-)

I, for example, think that as long as enough oil flows into the rod bearings, the pressure at which it was supplied there makes little or no difference. The oil pressure in the rod bearings is not determined by the supply pressure as long as enough oil is there, it's determined by the engine speed and bearing geometry including the clearance. Higher viscosity oil can however typically carry a higher bearing load than low viscosity oil, which has an obvious benefit with the absurdly heavy stock 928 pistons and rods. At a given supply pressure, more low viscosity oil flows thru the bearing though, possibly helping with cooling.

Isn't the late model 928 oil pump really high capacity? I think it starts bypassing at a relatively low rpm regardless of what viscosity oil one is using. If the pump bypass is cycling, doesn't it mean that the oil pressure is the same regardless of the viscosity of the used oil? I am not talking about idle rpms here, but normal driving rpms.

Of course, at the same pressure the system will flow more of the lighter oil. This brings me to another hypothesis: Maybe the problems are caused by the pump sucking the sump empty quicker if using lighter oil while experiencing oil drainback problems in some conditions?

Finally, another thing that I believe is that the oil temperature in the sump is much lower than the oil temperature in the bearings. The bearing load heats up the oil in the bearings pretty dramatically. It's going to be difficult to take the general oil specifications and then exactly relate the sump oil temperature to the oil operating temperature in the bearings for a specific engine. Lower viscosity oil flows at a higher rate, which furthermore cools the bearings more effectively. Higher oil temp coming out of the bearings is not necessarily always a bad thing, since that's heat removed from the bearings. A lot of moving parts and very difficult to make comparisons across engines. This oil temperature issue is a situation where experimenting and experience with a specific engine model is hard to beat.

Not challenging the experimental evidence on what works, just noting that I believe our understanding about why it works is still quite limited.

FredR

Interesting post from Bruce. Porsche Middle East standardised on Mobil 1 [0w40] and advised it was suitable to run in my 928 when I put my S4 motor into my current GTS chassis.

I took this up with Louie Ott and he advised me to dump the stuff pronto which I duly did. In the very short time i had this stuff in my motor the hot idle pressure dropped to 1.5 bar or less and that terrified me to the point where I basically crawled around until I resolved the matter. I went back to a good old Shell super Helix 20W50 and the oil pressure immediately returned to 2 barg even in summer.

Nowadays I throw in a bottle of STP for good measure. I suspect this raises the viscosity to something like the numbers Bruce mentions and I am quite happy to take a hit on a couple of HP if that is the case. I recently had my cam covers off and inspected the cam noses they looked fine and the engine itself looked very clean.

Regards

Fred

ptuomov

Related materials, but you'll have to read the whole thing to know what experiments and simulations are relevant for your case at hand:

http://www.ewp.rpi.edu/hartford/~weg...st_Jan2010.pdf

Alan

I tended to think that the low oil pressures with lower viscosity oils might not be so bad too - seemed likely that hot low viscosity oil will flow faster and the pressure read will therefore be lower.
High oil volume flow seems to be mainly a good thing... unless you empty the sump.

But anyway who wants to see oil pressure warnings on a car with a very expensive & rare engine. So I was of course very concerned to be routinely getting low oil pressure warnings at hot idle every summertime (even with Mobil 1 15W50).

I did everything I could to improve this: replaced the bypass valve, oil thermostat, oil pump, checked the oil cooler flow, replaced the rod bearings... Nothing helped much - small incremental improvement from the rod bearings (some showed some wear).

In the end I installed a temp gauge in the sump and found the temperatures were really spiking when the oil pressure was low. I then became more worried about the high oil temperature degrading the oil - so targeted temperature reduction with the dual cooler and (taking a cue from Kibort - though I was skeptical...) oil that cooled/performed better. these together do the trick - even at well over 115F.

To say you will never have issues with 15W50 oil is a joke: In stock config with Mobil 1 <1.5 bar warnings at hot idle EVERY SINGLE DAY the temps are over 110F - e.g. drive the car at freeway speeds for 15+ mins then sit idling at traffic lights. Unique to my car? - apparently not from many other reports - and others who have installed the dual coolers (or an extra fan assisted cooler). I drive my car every day - I do expect it to work right every day under all conditions.

One additional note - I added a vacuum pump system for crank evacuation - target was to solve the GTS breathing issue. As a side effect it does also seem to incrementally reduce oil temps too.

Alan

the flyin' scotsman

We had one last night; always look fwd to our Sat night 'adult' times

JET951

Funny you should mention STP , we use that ( STP Blue) in conjunction with our 951 track / supersprint cars ( customers & our own ) & this is a fantastic combo with the Racing 25w-60 ( extra oil film strength )

ptuomov

As you know, the 951 pan looks like this:



While the 928 pan looks like this:



We 928 drivers have bigger problems than the oil temperatures if we put on slicks and tun the steering wheel! None of this is news to you, I am sure, but just wanted to say it our loud (by typing it, kids will wake up and the wife will become concerned if I start talking to myself).

It's really unfortunate that we can fit in a much deeper and higher volume oil pan. The 951 design isn't nearly as likely to suck wind in the corner as the 928 design.

FLYVMO

Perhaps a baffle similar to this could be made for the 928 oil pan?
This is a oil pan baffle with trap doors for an LS engine (Holley oil pan) but the principle would be the same. Is there sufficient room?

Cheers!
Carl

UpFixenDerPorsche

Good thinking 99.

I've been pondering something similar, using two lightweight 'flaps' sloping down into the oil pan from each side at say 45deg. In a hard RH turn the oil will 'dam' against the LH flap, while oil behind the RH flap lifts the flap and flows to the lhs against the dam, momentarily maintaining the oil level in the centre of the pan.

There's still the issue of slow return of oil from the heads.

The BMW M5 uses a pair of electric oil pumps, one for each side, to pump oil from either head back to the pan under high lateral G loads. An accelerometer triggers the appropriate pump. Otherwise they're turned off.

Rob Edwards

Easier to just use the air pump mount to aspirate the oil out of the heads and pump it back to the pan, rather than trying to adapt some BMW pumps that are probably controlled through some CANBUS computer network:

dr bob

There are lots of threads on the various windage trays and crank scrapers, each has a feature that's almost perfectly offset by a shortcoming in the 928. The Big Issue is oil stranded in the heads, foamed a little, so it doesn't flow back to the sump. No scrapers or trays will solve that, unfortunately. They might delay the effects some, but foamed oil flows slowly, doesn't pump, and doesn't do much lubricatinge when it does. The thicker oils foam less, so stand a better chance of making it back to the sump. Rob's oil scavenging system suckes foam from the heads, through a "separator" that's really de-foaming the oil before it's returned to the sump.

I've used 15W-50 M1 since I got the car with no hot oil pressure issues. But the infrequentl 100º in Los Angeles really is a far cry from 115º+ in Phoenix, where 150º+ pavement temps are common at midnight... It's a different world where it's really too hot and drops all the way down to too hot at night. I'd be looking for that 25W-60 and hoping it was enough, or adding STP to try to keep enough pressure to force flow everywhere at idle.

So thinner oil cools better and transfers heat better, be that good or bad. The older-Harley guys run straight 50 and 60 weight, so the oil doesn't pick up as much heat. If you depend on oil to transfer heat out of the crankcase, thicker may not be the best compromise. Not a serious issue in the 928, fortunately.

Mid-70's today, mid-30's overnight tonight in central Oregon. Vs average 30 overnight, 60 daytime high. Here escaping the heat. Good 15W-50 M1 weather.

ptuomov

If we're going to start talking not only about oil but also about oil drains and crankcase ventilation, this fights going to get out of hand! ;-)



Speaking of oil coolers: Has anyone fitted a larger volume oil cooler in the stock or stockish location? Or is everyone using the stock late model external oil cooler with the factory shroud? To me it looks like there might be room for a dual pass cooler there with a larger heat exchanger core volume. Anybody done that?

UpFixenDerPorsche

No no - not adapting any beemer stuff - was illustrating another approach.

But it is curious that they too have a problem with excess oil in the heads.

I'm thinking of using the air pump mount for a crankcase suction pump.

There are suitably sized electric oil pumps readily availabe to remove oil from the heads.

john gill

No there is not , not unless you want to run a 400 amp alternator , the 928 sump is a **** design , its to shallow the engine makes great power but the high volume high pressure pump can empty the sump into the heads, in the blink of an eye .
My car has placed in the top 3 in class in the local club championship , actually last year it was faster times then a 911 that got second , I couldnt get to all the meetings , the only car that is faster is supercharged 3 litre 944 .
I have 2 stage scavenge crank driven pump to force oil from the heads to the sump . run 50 - 60 w blend oil with a bytron additive and trimetal bearings (resized crank). I also spin the motor to 7000 rpm
THe stock glyco bearings are absolute rubbish , you can scratch the topcoat of with your fingers, I also destroy the oil filter after every event to check for shiny metal flakes. THe ecu has also code to engine flag me for various scenarios and not normal oil pressure during these events.
Even with this the bearings are lasting about a year without issue compared to never with glyco crap.
Locally it has done a 61 at Lakeside and 1.28 at Qr , with working air conditioning .
Our evolving race car build will be 400 kg lighter with same gear learning from the mistakes made with the GT