Engine Oil & Last Century Porsche Road Engines , some light reading
 

2017 , big year at Buchanan Automotive , we have never ever put out so much work & on the bigger jobs , the Ripple Effect is still a significant cause



Ripple Effect ? ,what ripple effect I hear you ask

Answer = Very Simple Physics with incredible damaging effects on last century Porsche engines in our climate , or just about any climate for that matter when using completely stupid Low Viscosity, Low Oil Film Strength engine oils( like the stupid use of a this century 5w-40 viscosity) in last century Porsche engines in our climate, its not as bad as it was 10-20 years ago , but its still bad enough & boy oh boy its costly to the owners & its Completely Avoidable & worst still , the use of these so called synthetic Low Viscosity , Low Oil Film Strength 5w-40 & 10W-40 engine oils has had far reaching consequences over the decades , meaning these last century Porsche engines only needed to be on a Low Oil Film strength engine oil for as little as 10,000 - 20,000 Kms to create accelerated internal engine wear & if that engine ( later ) switched back to a decent Oil Film strength 15w-50 or a 20w-50 the internal wear was slowed or even stopped( relative to how fast the wear was ) at that point in time

During 2017 we had multiple ( new to us ) 944 / 951 / 944S2 / 968 Porsche cars where we were preparing for the new owner to do a bit of club track driving & naturally with these cars typically already travelled 200,000 Kms + & some 23 to 35 years old already , we even had a very nice 968 that was low recorded Kms , but with Shocking amount of Valve Guide Wear & excessive con rod bearing wear , the silliest thing would be to take one of these old cars to the track without some basic preparation , because engine life on a race track with a human behind the steering wheel & the "Red Mist" appears ( it usually does ), engines that are worn / fatigued just go bang & its mostly avoidable & naturally ,preparation is not just the engine / engine bay ( fuel /oil /coolant hoses) , but steering & brakes + ball joints checking & or replace

Note } Red Mist is a saying when a driver gets way too excited on a track & goes all out , even though the driver had no intention at all in doing that in the first place (Male humans in cars Verses other Male humans in their cars = Interesting Mix of engines going BANG with old worn engines )

Three Main Areas of concern that we look / rectify before letting out onto the track , particularly if the previous service records show anything other that 15w-50 or 20w-50 engine oils being used previously over the decades , this applies to all 944 variants & naturally the 928 series ( two 944 engines combined ), but generally only if they are going to the track to be punished

Area of Concern 1 }

Twin Cam 944S / 944S2 / 968 ( in many ways they are basically half a 928S/32v / 928S4 / GT/GTS engine ) , yes I know there are lots of small differences , but in basic terms they a pretty much half a 928 quad cam engine & like what we see on the 928 Quad cam engines . The thin single row timing chain between the two cams , this is on the 944S , 944S2 , 968 & the quad cam 928 series , these tiny ,thin single row chains( one per head ) are quite suitable for what there are designed to do & thats simply to drive the Inlet cam by the Exhaust cam & naturally there is going to be a hydraulic ( Oil Pressure Fed) tensioner with its nylon pads putting sufficient out -ward tension on the chain , now this is where the problems start if the engine oil pressure is too low when the oil is HOT & no I am not talking about the track at the moment at all , I am talking about the years of driving , might be to & from work ( normal street deriving ) in the normal thick / heavy Sydney / Melb / Brisbane traffic snarls , engine oil hot & getting hotter ( all very normal ) except for one small detail , this poor 944 twin cam or 928 quad cam engine is on a Low Viscosity , low Oil Film Strength 5w-40 or 10w-40 so called synthetic engine oil during these few years or so , made even worst still in the simple fact that the very very low Oil Pressure at idle has to supply Oil Pressure to the timing chain tensioners at the TOP of the engine , the oil pressure that is already too low is even lower at the top

So what is the concern I hear you ask ?

Answer = The Death Rattle , the engine oil viscosity is so low( 5w-40 is a good contender here ) for the conditions ( mentioned above ) it means that at that normal idle speed ( in traffic ) the oil pressure at the hydraulic tensioner/s is so low the tensioner is no longer tensioning & the poor little timing chain is whipping around on and leading to the cam sprockets on the cams , this death rattle also damages the nylon tensioner guides ( shortens their lives remarkably ) and naturally beat the hell out of this thin single row chain because the engine oil is Low Viscosity & Low Oil Film Strength the metal timing chain is able to cut through the oil film( much lower oil film strength ) and bite into the cam chain "CAST" metal sprockets , causing excessive wear to them , which in turn causes more slack to occur ( metal missing from the teeth )& its a recipe for breakage & break they do and its very nasty ( bent valves , hydraulic tensioner ripped from the head & smashes through the magnesium twin cam cam-cover ( what an expensive mess )& completely avoidable

Note 1 } The" twin cam " camshafts chain sprockets are situated in the middle of the camshaft & are not replaceable separately , they are cast & machined finished with the camshaft( all one piece) , its quite a trick centre drive setup , but expensive if worn & they basically do not wear "IF" that are given every possible chance to survive , meaning good OIL PRESSURE hot at idle = 15w-50 or 20w-50 = ( correct chain tension at idle ) & sufficient Oil Film Strength Engine Oil to keep the chain from biting through the oil film & making metal to metal contact

Both Sean & I have heard the death rattle on many occasions ( engine oil hot at idle speed ) , but never when the twin cam 944s/S2/968 or quad cam 928 has been on sufficient oil viscosity for our Australian conditions = 15w-50 or 20w-50 and the basic condition of the timing chains, nylon guides & hydraulic tensioner is in reasonable to good condition

We have lost count how many " New to Us " 928 Quad Cam ( 86-95) , 944 Twin Cam ( 87-95) , 964 911 , 993 911 that had dropped by our workshop ( Mid Summer/hot day ) engine Oil Warning Pressure light Glowing RED , Oil Pressure gauge reading virtually nothing , engine making interestingly bad noises (loose timing chain noises, all at idle ) and all these were on a 5w-40 & even some 10w-40 engine oil viscosities , drained this incompatible oil out of them , changed the oil filter & refilled with 20w-50 & noises dramatically reduced or completely gone & oil pressure warning no longer glowing at idle with that oil temp , but then again is says to use 20w-50 in the owners manual that was printed by Porsche when these cars were new , so its not rocket science

On a 944S , 944 S2 or a 968 ( Twin Cam Engines ) we routinely remove the cams about every130,000 - 150,000 kms & replace the nylon tens pads ( genuine Porsche ) and the timing chain ( Street Car ) but on a more dedicated track & some road driving 944 , then we replace them more often ( cheap insurance ) , removal of the two cams on the 944 series ( 87-95) is much much easier than on a 928 quad cam , access is far better

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Area of Concern 2 }

Valve Guide wear , like on a air /oil cooled 911 engine excessive valve guide wear is always a great concern if the said engine is taken to the track & flogged , there is a reasonable chance the engine could drop a valve head , thats because the heat from the valve head is not getting dispersed through the valve guide & into the cylinder head , instead the excessive valve guide wear is trapping the heat in the valve head just enough that at high RPM it will fail & the valve head will just fall off & into the combustion chamber ( Good Night Nurse ). Naturally the bigger the valve head the more it literally weighs & the valve springs closing force is very high & these single cam ( 2 valve heads ) are slightly more prone to it , so be careful with old Porsche engines going to the track for the first time with unknown valve guide wear ( it can catch you out easily )

As I briefly mentioned above , we had a very nice example of a 968 ( manual trans ) at our workshop recently & during repairs & tests , we had the cams out & I removed 4 or 5 of the 8 exhaust valve hydraulic lifters & I had made years ago a special tool were I can easily do a cursory valve guide wear check ( while the cams were off doing the timing chain and slipper guides ) & I was dumbfounded by the amount of( valve stem side ways movement ) valve guide wear , I even took out some of the inlet valve hydraulic lifters & even found one or two inlet valve guides with way way way too much wear ,we removed the cylinder head this 1992 968 engine ( still had its original wax fibre composite graphite head gasket )which was rotted away( good to get rid of that and install a metal custom head gasket ) , but when we had the 16 valves out , the valve guide wear was out of this world ( way way war too excessive for there Kms travelled ) long story to this car , but in a nut shell the valve guides were worn by the previous use of a low viscosity , low oil film strength engine oil , very very common 15 to 20 years ago to use these oils in these cars , but it was all the rage then ( Emperor's New Clothes ), even though the correct answer was in the owners manual printed by Porsche

Remember } There is NO Oil Pressure at the valve guides , its just very very minor splash( more of a weep at best ) & in fact the valve stem seal keeps most of it off the valve guide/valve stem , so the valve stem & valve guide relies on High Oil Film Strength engine oil ( Very Simple Physics ) , so a Low Viscosity , Low Oil Film Strength so called synthetic engine oil can easily turn into high wear for mechanical items in these engines that get No Oil Pressure

Basics Physics Time }

A this century low viscosity engine oil , say a normal street emission compliant 5w-40 ( available to the public off the shelf ) is always going to be lower oil film strength than a last century 20w-50 that has good levels of WWII vintage ZDDP AW packages & then if you compare the same 5w-40 emission compliant engine oil up against a 25w-60 racing oil( we use on track prepared 944 etc ) , then the oil film strength difference is greater still

Oil Pressure } the lower the Oil Viscosity the lower the oil pressure in the same test engine at the same oil temps , so in other words a 5w-40 will give ( Hot ) a lower oil pressure in the same last century Porsche engine than a 20w-50 engine oil ( same engine , same oil temp , same RPM Testing / reference points , Very Basic Physics )

Note 2 } In a last century 944 engine ( all ) last century 928 engine ( all ) last century 911air/oil cooled engine ( all ) , here is the list of engine components that get NO ( NO ) Oil Pressure & must rely on Oil Film Strength

A ) 944 & 928 ( all ) tall wide pointy cam lobes striking at high speed against the flat hydraulic lifter face , and the lifter/s is held up with two big valve springs ( no valve bounce with this spring tension ), this is whats known as High Wiping Loads ( High Loaded Flat Tappet Design ) , meaning the force applied is wanting to just wipe the oil clean off the lifter face , if the pointy tall cam lobe/s break through the oil film its instant metal to metal , the oil film is the liquid bearing , naturally its this area that ZDDP pressure induced cross linking AW film forms & with the correct Oil Viscosity , from minus 10 deg cel to unlimited high ambient temps = 20w-50 the cams will survive & as mentioned there is No Oil Pressure here at all , just splash

B ) 911 Air/Oil cooled , rocker shafts to cam lobes , same deal , no oil pressure , just a spray of oil in the general direction & the Oil Film Strength has to be high , same goes for the rockers pivoting on their rocker shafts , the lubrication here ZERO , oil film strength only

C ) 944 twin cam , 928 quad cam , 911 air/oil cooled Timing Chains against their Timing Chain Sprockets , , lots of forward and back shockwaves in this area ( opening and closing valves causes the shock waves ) , No Oil Pressure here to protect from excessive metal to metal contact , just slash & Oil Film Strength

D ) Alloy Pistons in their Alloy Cylinder Bores , be it 944 , 928 , 911 , No Oil Pressure here , just splash & Oil Film Strength

E ) Piston small end bearing ( top of con-rod inside piston ) , just splash feed & Oil Film Strength

F ) Piston Rings against cylinders & inside the piston lands , just splash feed & Oil Film Strength

G ) Piston gudgeon pin ( Wrist Pin in USA speak ) moves inside the alloy piston ( 944 , 928 , 911 ) and must be lubricated , no oil pressure , just a bit of splash feed & Oil Film Strength

H ) Crankshaft Thrust Bearing , this stops the crankshaft from moving forward or back in operation & with a manual transmission 944 , 928 , 911 the load from the clutch arm pushing or pulling ( depending on the design ) will transmit massive forces in one direction & if the thrust bearing is having to cope with low oil film strength engine oils there is always going to be issues with the thrust bearing

Note 3 } The crankshaft thrust bearing has grooves / openings through which the engine oil escaping from the adjacent main bearing must must be let through at the same rate as the other engine main bearings , hence the openings / grooves in the working face of the thrust bearing , in my opinion there is no oil pressure at this thrust bearing surface , and it relies on Oil Film Strength , hence why we see ZERO thrust bearing wear on any last century Porsche if its been on a 20w-50 engine oil at all times , but on lesser oil film strength engine oils we see thrust bearing wear , meaning oils like 5w-40 & some 10w-40 etc in our Australian climate , but then again the answer has always been in the owners manual that came with these last century Porsche cars

Note 4 } 928S/S4/GTS Auto Trans version / Front Flex Plate deflected & pushing the bowl shaped flywheel in constantly against the engine thrust bearing , in a nut shell we have seen many of these over the last 15 or so years that the "rear of T Tube" coupling has not been maintained & the single bolt holding it tight has stretched & with the twist of the shaft under acceleration ( shortens the shaft momentarily )& because the front coupling does not loosen as much , the shaft pulls out of the rear coupling and over time you end up with the front flex plate deflected , its not a lot of tension ( you can move the flex plate in with you thumb ) but its a constant inwards tension & with the engine thrust bearing having NO oil pressure to protect it , it has to rely on the Oil Film Strength to protect it & if the oil film strength is low , then bad luck for you , because the engine thrust bearing will wear out & the crankshaft will eventually grind through the thrust bearing & into the crankcase .
BUT we have caught quite a few ( new to us 928S / S4 / GTS 928 Auto ) cars that we have done our T Tube reset & new coupling index bolts & measure the axial play of the crank ( thrust bearing check measurement ) & the ones that are worn ( but not worn out ) we have noted / recored the wear measurement number for that particular 928 , change the engine oil & oil filter to a good quality 20w-50 & then a few years later we go back in to do a T Tube reset & re-measure the wear amount & we have never seen one yet that the wear amount has increased one bit , the wear has completely Stopped , thats because we switched back to a high oil film strength engine oil & looked after the T Tube ( very easy stuff if you know what you are doing )

Remember , the Oil Film Strength varies greatly between most 10w-40 , 5w-40 & even a few 15w-50 in different brands & even great differences between the same brand but different countries

Area of Concern 3 }

In a 944 / 944S / 944S2 / 951 / 968 ( all ) the con-rod bearing journal ( big end bearing ) that will starve of oil pressure at high sustained RPM with normal hot oil temperature with a Low Oil Viscosity ( classic example = 5w-40 ) is journal no 2 & when it starves its all over in a second or two , crankshaft is destroyed & often the con-rod lets go = good night to the engine crankcase & sometimes cylinder head as well ( piston smashes into it , because its now free from constraint )

In a 928, 928S , 928S4 , 928GT, 928GTS ( all ) its con-rod journals no 2 & 6 ( they are on the same crank journal ) and like on a 944 its all over in a few seconds when its at high sustained RPM with the engine oil at normal high temps , the first one we came across was Charles Falzon who destroyed his engine ( on the road ) No 2 & no 6 con-rod bearings staved of oil at high RPM on a famous brand so called synthetic 5w-40 , the so called synthetic side of things did sweet F---k all for that engine ( destroyed / turned to scrap in a second or two ) & since then we have seen a few more , always on a 5w-40 or even a 10w-40

In a air / oil cooled last century 911 > 993 , they will destroy con-rod bearings 2 & 5 , not on the same journals , but in the middle of the crank , same outcome if on the wrong oil viscosity at high sustained RPM & hot operational oil temps with a 5w-40

But Why is it So ?

Answer is quite simple }

The above engines are all road engines & all road engines in last century & this century for that matter have to be functional with BOTH ends of the crankshaft to be used in practical ways , meaning the rear of the crank will need a flywheel , so there is no possible way of doing the Porsche 917 engine thing of having the engine oil pressure feed coming in at that end of the crank( axial feed ) & at the front of the engine we have on road engines pulleys, with a massive bolt in the centre holding the pulleys on that drive fan belts / A/C belts / Power Steer belts etc etc etc , so there is NO possible way of getting engine oil pressure into the front of the crank ( axial feed )

So what are we left with with all of us playing around with ROAD ENGINES , we have to put up with RADIAL feed into the crankshaft to feed oil pressure to the critical con-rod bearings ( big end bearings )


Think about it for a few seconds and it becomes crystal clear ,radial feed on road engines ( 911 , 928 , 944 ) is perfectly fine for most road conditions even at high RPM momentarily , so long as you keep the oil pressure up HIGH ENOUGH to overcome the high spinning speed of the crankshaft throwing the engine oil out ( centrifugal force ) through the very hole the oil is trying to get in to feed the Con Rod bearings along the length crankshaft , its a terrible compromise , but works OK until the Oil Pressure is insufficient & when it is at high RPM , you do not get any warning , its all over in a second or two ( BANG )

Here are some notes written by Hans Mezger ( the designer of quite a few Porsche race engines including the famous 12 cylinder 917 engine }

Because the 911 air/oil cooled engine used for road and competition had Radial feed into the crankshaft to the con-rod bearings , the Oil Pressure needed at 9,000RPM was 70 Lbs Sq " ( 5kp/cm/2) MIN pressure at max racing oil temp

The 908 ( 8 cylinder ) 3.0L air/oil cooled race engine had Radial feed to the con-rod bearings , to overcome con-rod bearing issues in racing the crankshaft required at 9,000RPM was 100 Lbs Sq " ( 7 Kp/cm/2 ) Min oil pressure at Max racing oil temp ( Thats Massive Oil Pressure )

Hence why proper race engines like the 917, 12 cylinder engine had Axial feed & with axial feed there are NO centrifugal forces pushing away the oil trying to get into the crank , so with Axial feed the 917 engine only required at 10,300 RPM was 34Lbs Sq " ( 2.3 Kp/cm/2 ) Oil pressure

So with these last century Porsche road engines we play around with , remember none of them are race engines , yes you can take them to the track , but we need to know why they go BANG & try to avoid it , so obviously we can not turn these into radial feed crank designs ( not going to happen ) , but what we can do is what Porsche did with their 1960's road / race engines , you work with what you have and recognise you need to compensate for the loss of oil pressure going to the con rod bearings ( big ends ) as the crank spins faster & faster ( radial feed has to fight against centrifugal forces ) , its not perfect but we can work with it

Using LOGIC , we know that the lower the oil viscosity , the lower the oil pressure , very single physics & as you can see , last century Porsche road engines need high oil pressure going into the crank & thats why over the last 11 years in the PCNSW we have never lost a single 944 , 944S , 944S2 , 951 , 968 engine that compete at the track ( Supersprints ) & we and our customers have won multiple Drivers Championships & Super Sprint championships & Motorkhana Championships , so to let you into a secret we use at the track a Minium Viscosity of 25w-60 Racing oil in all these 944 variants , two of these Porsche's ( 3.0L 951's on E85 ) put out just on 600 HP and they are all wet sump , no acusump & no dry sump & remember all these Porsche's are road & track cars , meaning they all drive to the track & drive home again , they are not dedicated race cars & remember the tracks we compete at do not have some of the very very long sweeping corners that exist at some tracks in other countries , so we can get away with no dry sump in our circumstances , the closets to that is the vary fast corner 1 at Sydney Motor Sport Park , it is a very fast sweeping corner at high RPM , but we have no issues with it

So remember , prepare a last century Porsche engine for the flogging if its going to the track & the beauty of the 944 engines ( like the 928 engines) , is we can remove the engine sump to change the sump gasket & at the same time remove the individual con-rod caps to check the condition & or replace the bearing shells & cap nuts ( all in the car ) , just like on an old Datsun 1600 or Ford escort , Fantastic Design

The Above is MY OPINION in what I have experienced as a independent Porsche Specialist working on Porsche sports cars 6 days a week since 1977 ( getting on 41 years ) & the reason is I am sick & tired of seeing lovely last century Porsche engines being flogged at the track & even on the road on a way too thin this century engine oil & it all ends in tears ( Con Rod Bearings starvation mostly ) & another Porsche engine going to scrap metal ( lost forever ) , I just hope I can save a few last century Porsche engines from this stupid avoidable destruction

And lastly , the above has NOTHING at all to do with this century Porsche engines , meaning Boxster , 996 , 997 etc etc etc ,I am ONLY talking about oil viscosity & Oil Film strength & Oil pressure issues to the 944 ( all ) , 928 ( all ) and air / oil cooled 911 ( all ) road going Porsche cars from deep last century from when 20w-50 engine oils were the norm

Regards
Bruce Buchanan

Excellent information.
"Area of Concern 3"
J-AZ

Great post providing excellent info.
too many clowns think that 10w40 or 0w40 synthetic is ok for a 944 series engine because 30 years after production Porsche said it was probably ok, not taking into account the interest of emissions compliance and marketing deals with Mobil for the new cars.

very simple rule of 944 oil- if its not freezing outside, 20w50 or better.

Excellent post, should be a sticky.

When I bought my car in 1983 the Porsche dealer insisted I only run Quaker State 10w-40. Did that for about 20K miles before I switched to 20W-50. I may even have done a DE on 10w-40.

Fast forward to this year and at 45K I was getting a slight thump thump thump from cylinder 1. Pull the pan and changed the bearings (they did not look that bad) but noise went away after that.

Bruce,

Great information on the background and considerations that go into choosing an oil for our last century cars. I would be curious if you would be wiling to share the brand of oil you find works the best in these cars when used on the street to avoid all these significant issues? I have a 94 968 located in Northern California (weather not much different than Sydney), so I would be prone to copy any practice you have that has stood the test of time. One extra consideration I have is that I live in a state that still requires bi-annual smog checks (California) , so I have to be concerned with high zinc levels poisoning my cat.

Thanks and Best Regards

Nathan

Great read.... I agree with 98% of it...

my experience driving 8 years and 140k miles... i ran M1 20w50 4.9 bars.... in the summers and M1 0w40 for 5 sub-zero winters flat out in my 968 (cuz that's how i drive: flat out).... Ignorance is bliss. My Porsche tech put in the 0w40 for the winters too apparently not knowin' she'd blow sky high from low oil pressure that (of course) never happened cuz it was always readin' 4.5 bars.... (I musta got the only good engine)...

i then switched finally to their extra awesome 0w50 racing oil (winters, readin' 4.75 bars... ) with even more zinc/ phosphorous than the standard Euro 0w40 oil.... Engine never burned more than a splash of oil all the way to 200k miles. That engine block was spectacular when we pulled it....

did i need more oil pressure running the Mobil winter/euro or their 0w50 Racing oil? I can't say. ....We crazy Yanks treasure the instant flow we get when it's 11F or 3F or minus 5....
(the cold start oil tests with that high viscosity "goo" will make Bob the Oil Guy and you all cringe.... Have you ever tried pouring it out of the little bottle after leaving it out in your car on a ski trip to Mammoth? God.)....

i'm guessin' the op circles with a maniac fringe of street driver/s who rev the engines too high under toxic loads or extra extra badly on tight curves at the track.... When you do that to 30, 35 year old beaters, heat cycled and thrashed, they're going to do what all engines that take that extreme punishment inevitably do: blow up.

i must have been driving slow, cuz mine never did. It drove as advertised: like the near supercar that it is with just 1 turn of the key.


*i run the 0w40 in the LS7. but the 0w50 or 5w50 is a decidedly better winter oil for these 4 bangers. But the "really good" winter oil is expensive, and should be replaced when the cold is over.

Good read. I've always believed it's better to err on the side of a little too thick than a little to thin. I'm also a loyal Mobil 1 user. I put over 130,000 miles on my 16V Scirroco, always going one grade heavier then the recommended range. It was both high revving and always running near the top of the temp gauge (sound familiar) so the extra protection of a heavier weight was cheap insurance. Cold cranking has never been an issue but our winters aren't too brutal here just below the Mason-Dixon Line.

Hi Odur , couldn't be bothered with the rest of you're avatar name , so Odur will do for me , who on earth would try to drive a last century Porsche two wheel drive at minus 25 deg Celsius , and a diesel oil in the petrol ( USA GAS ) power car , it doesn't make any sense at all

Hi Nathan J , nice post & makes sense , in regards to you're post as you probably already know good quality aftermarket Cats are very cheap & very very plentiful ( no shortage here ), but 944 engines in good /excellent near new or in new condition are NOT cheap , it makes sense to look after you're engine & if needed every few years ( may be much longer ) you replace the odd replaceable CAT , think of it like changing a filter

Changing engines are very very very expensive & I do not mean throwing in a 2nd hand cheap that is crap , I mean rebuilding yours with new cam/s , new lifters , new pistons & cylinders bored to suite & it goes on & on , 1000% dearer than a cheap bolt on cat every 5 or 10 years

Regards
Bruce B

Even if a Diesel oil meets the SJ to SN API Service Classification?

There is one big problem with "dedicated" Diesel engine oils & thats they have more detergents in them ( for Diesel Engines ) , this higher detergent levels tend / will foam as the engine revs go higher & higher , now on a Big Rig Truck engine with dedicated diesel engine oil , for example }

• Caterpillar – '07 Cat C15s, including Multi-Torque models rated at 1,750 pounds-feet or more, should be geared for 1,325 rpm at 65 mph. C15s making less than 1,750 pounds-feet should run slightly faster – 1,400 rpm at 65 mph. And the C15 Heavy Haul, for rigs grossing 90,000 pounds or more, should spin at 1,500 to 1,650 rpm at 65 mph. As with the '04 ACERT engines, operators of '07 Cats have a wide operating range, with peak torque at 1,200 rpm but luggable to as low as 1,100 rpm. In high-range gears, transmissions should be upshifted at 1,500 rpm to get maximum fuel economy.


The above is just an example of dedicated late model interstate big rig trucks operational engine RPM & its operational RPM is very low ( fuel economy driven reasons ) , now with a petrol ( gas USA speak ) Porsche , be it last century or this century , its always a bit odd people thinking , gee I know I will use a dedicated Diesel only engine oil in my petrol/gas high revving possible engine ( lets say 944 engine or a 928 engine that can easily be revved to 6,700 RPM ) , naturally thats a lot higher than 1,650 RPM

Maximum Torque of my 1993 968 is at 4,100 RPM & on a dedicated diesel engine oil ( high detergent as they all are ) the tendency to foam is extremely high = risking air to be drawn into the oil pump ( foaming oil ) , what a disaster

If oil companies thought this would a great idea , they wouldn't sell engine oil designed for petrol / gas powered cars in the first place , it just does not make any sense at all

Now some interesting info for you =

Question } how did this strange non logical " lets use a high detergent high foaming dedicated diesel engine oil that foams at high RPM in my petrol / gas powered car engine" human thinking behaviour start ?

Answer } It all started around the time when humans were noticing cam lobe wear / lifter face wear ( be it OHC or push rod ) some twenty plus years ago or so particularly when humans were switching from classic last century engine oils that still has high oil film strength( 15w-50/20w-50 engine oils) to the latest ( 20 years ago latest ) so called synthetic lower viscosity engine oils ( all hell broke loose with worn cams in all sorts of last century car makes ) , not a lot was known publicly about ZDDP , but a few humans thought that they had heard some where that ( 20 years ago ) that diesel oils then ( not now ) still had very high levels of this thing called ZDDP & the word got around

So be very careful with dedicated Diesel engine oils in high revving petrol / gas powered engines

Sub Note } Dedicated diesel engine oils contain "Anti Foaming additives , this is to try to contain the higher ( dedicated diesel oil ) detergents or soaps levels that they have , and these anti foaming additives are Silicon Fluids ,which have a high molecular weight ( these silicon fluids do not other work in the oil and are not lubricants such ) , These anti foaming additives try to disrupt the air bubble film , but only the bubbles on the surface of the oil , however its generally recognised that these silicon fluids may have a tendency to negatively affect the release of "AIR" entrained in the oil itself , which on a high revving petrol/gas powered sports car is not a good idea at all

The above is just a culmination & my opinion of working on Porsche sports cars , 6 day a week for 40 + years ( not 40 minutes )
Regards
Bruce B

Thank you for this, Bruce. Diesel oil always seems antithetical to me. But that pesky SM rating gets me.

Lots of motorcycle guys run diesel oils in their motors.

But those only rev to 10k+.

Diesel engine oil in a petrol/gas powered motorcycle engine ! , amazing " el-cheapo" silly thing to do , I had to wait to stop laughing before I googled up why some one would do it & I found it in about 30 seconds , on the Oil Depot / Amsoil dealer site it explains why you wouldn't , I have had motorcycles ever since I was 15 years of age ( 46 years & still own two motorcycles ) & I can honestly say this is the first time I have heard such a funny thing , particularly when there are so many very high quality dedicated motorcycle engine oils

Is Diesel Engine Oil OK For Motorcycles Engines and Transmissions?

September 2, 2017 By Marc Roden

I've done it more times than i can count, as my car was driven 150miles into the mountains of colorado regularly and cold soaked overnight at temps as low as -25f at an altitude of 10,000ft. With 0w40 in the pan it always started the next morning. It was a slow start on the super cold days, but she fired up and was outputting heat in the cabin pretty quickly. These cars are fantastic in the snow with proper tires. That said, i'm 15w-50 the rest of the year, 0-40 is waaaay to thin for warm/hot days.