News

308 Oil pressure?

Discussion in 'Technical Q&A' started by Mr Iceman, Apr 4, 2004.

  1. Mr Iceman

    Mr Iceman Karting

    Mar 3, 2004
    101
    Canterbury, England
    Full Name:
    Scott
    Hi there,
    What oil pressure do you get with a warm engine at idle?

    Mine starts of around 83 lbs/sq.in (half scale deflection on the guage) when I first fire up with a cold engine, however when the engine is warm (around 210'F) the pressure drops to just over quarter scale deflection on the guage (about 55 lbs/sq.in) I know to expect a pressure drop as the oil thins out, and the pressure increases to about 83lbs/sq.in when I rev through 3000rpm but is the low pressure at idle excessive? I've only had the car a month and due to weather have only done just over 100 miles in it and I'm not sure whether the pressure has always been this low.

    The oil level is fine (indicating just over max in fact), your thoughts on this matter would be greatly appreciated.

    Scotty
     
  2. To remove this ad click here.

  3. f355spider

    f355spider F1 World Champ
    Rossa Subscribed Owner

    May 29, 2001
    17,072
    USA
    Full Name:
    Hugh G. Rection
    You're fine...sounds entirely normal....read the owner's manual, there should be some mention of oil pressures, should reassure you.

    Dave
     
  4. Wasco

    Wasco Formula Junior

    Dec 9, 2003
    486
    Salem
    Full Name:
    Randy
    Iceman,

    Mine has been all over the guage. It had been reading real low at even hi r's i.e. 50 lbs at 7k rpms. I was freaking out. I had just changed the speedo sensor and the very next day I noticed low oil pressure. I then changed the oil sending unti which helped greatly. I now have 90 lbs when cold and always 80 at anything above 4k rpms. EXCEPT: now I have experienced intown driving at 80 ambient temps. I now see with Oil temps of near 200 degrees pressure is down to 50 lbs at 4k then 70 at 7k. To low for my taste. I will either try to clean oil cooler as it may be clogged and may try to adjust pressure up on pump. My 78 GTS has 70K miles on the clock so things may just be a little lose for 95 lbs of oil.
     
  5. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    7,008
    The rule of thumb is that you need 10 PSI of oil pressure for each 1000 RPMs. 55 PSI at warm idle is plenty.

    As long as the oil pressure stays above 10 PSI/1000 RPMs and as long as the blow-off valve keeps the oil pressure under 110 PSI at peak RPMs, you will lubricate everything needed to be lubricated, but not carry so much pressure that you won't blow out the oil seals.

    With the pressures you have indicated, you might be better off with a lighter weight oil the next go round. What are you currently running?

    AleeHass (sp?) are you reading this thread <he's our resident oil expert>
     
  6. Wasco

    Wasco Formula Junior

    Dec 9, 2003
    486
    Salem
    Full Name:
    Randy
    I know I should do a search here but I didn't. If you have how about the link?

    Mr. Iceman, my 78 308 GTS which I would think has the same eng. as yours never runs above 210 F Water temp. And rarely I mean rarely runs above 190 F Water temp with a brand new thermostat and coolant. My oil temp on the other hand runs from the cold 140 F to 210 F when I am runnin it the hardest. When the oil temp is at 210 F is where I get the lower pressure readings at near the 10 per 1000 rule. This scares the #ell out of me so I am changing from Castrol 20 50 to Mobil One 20 50. What oil u runin?

    Or where you talking about oil temp "when the engine is warm (around 210'F) the pressure drops"
     
  7. To remove this ad click here.

  8. Wasco

    Wasco Formula Junior

    Dec 9, 2003
    486
    Salem
    Full Name:
    Randy
    Yes 55 at idle is great IMHO. I have been getting only several needle widths off of zero at times at hot temps. This at an idle which I assume on my zero to 170 gauge is about 10 to 20 lbs should still be within the 10 to 1000 rule, {I have her idle at 800}.
     
  9. f355spider

    f355spider F1 World Champ
    Rossa Subscribed Owner

    May 29, 2001
    17,072
    USA
    Full Name:
    Hugh G. Rection
    Just remember you are reading things off of a 26 year old gauge and sending unit...You might want to confirm those pressure readings with a mechanical gauge, or just go ahead and get a new sender. I had weird, very high oil temp readings in my 78 308GTS and a new sender was all that was needed.

    Anytime the readings seem off, it would be worth investigating a bit further. I had very high oil pressure readings at high revs on my 328 (over 115psi, maybe 120 psi) confirmed with a mechanical gauge. My pressure regulator needed to be adjusted.
     
  10. Mr Iceman

    Mr Iceman Karting

    Mar 3, 2004
    101
    Canterbury, England
    Full Name:
    Scott
    Thanks for your replies guys,

    Mitch, I am currently running Agip semi-synthetic 10W 50 (its what they put in at the garage on its service before I collected the car. I think next oil change I will put fully synthetic in, do you mean by lighter weight something like 5W 30?

    Wasco, I was indeed talking about oil temp, I'm interested to see that you aren't swithing the weight of oil on your next change, just the brand, Why so?

    I have done a search on this and found AleeHass' post's very interesting and informative, if I've got it right he is basically saying go for the thinnest oil you can get away with. Is that a correct deduction?

    Thanks again for you responses
    Scotty
     
  11. DN35

    DN35 Formula Junior
    BANNED

    Nov 22, 2003
    599
    Illinois
    Full Name:
    D. Norton
    I've been following all the oil weight discussions and still can't figure it out. I have a '78 308GTS. It currently has 10w40 in it (dealer choice after 60k service). My oil pressure also reads very low at hot idle. Is there a different weight that I can switch to that will increase oil pressure at hot idle?
     
  12. To remove this ad click here.

  13. BigTex

    BigTex Seven Time F1 World Champ
    Rossa Subscribed Owner

    Dec 6, 2002
    73,162
    Houston, Texas
    Full Name:
    Bubba
    Check your engine compartment labels for oil weights.

    The 50W part of 10-50W is essential for the old 308s, I'm told.

    OK to use 20-50W in the heat of Texas!
     
  14. F308 MAN

    F308 MAN F1 Rookie

    Jan 19, 2004
    2,907
    Isle of Man
    Full Name:
    Dave S
    i am using mobil 1, 0w 40 in the tropical heat (we wish) of the UK.....car is a 1980 308 GTS @ 36,000 miles.....oil pressure is always consistent with orig. manual data.....wouldn't change !
    regards, david, isle of man
     
  15. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    7,008
    Based on your current readings, and with the caution of age, you should try a 10W40 or 5W40 or 0W40 first and see what the pressure situation does. If the cruising oil pressure stays up where it used to be, and the idle pressure only drops a little (45PSI or more), then on the subsequent change you might try 0W30 or 5W30. If the oil pressure drops more than minimally, then immediately dump the new oil (like within 10 miles) and put the 10W50 stuff back in.

    For my F355, I have backed down from 20W50 to 5W40 and found that the temperatures on the tract actually dropped in the heat of Texas summers. Sometime this year I am goind to try a 0W30 oil at the track. The lighter weight oil protects better at startup because it is easier to pump around. It protects just as well at the track (as far as I can tell so far) and runs at lower temperature, and after a track session idles with more pressure than the heavier stuff.
     
  16. AEHaas

    AEHaas Formula 3

    May 9, 2003
    1,350
    Osprey, Florida
    Full Name:
    Ali E. Haas
    All oils are too thick at anything less than your engine's normal operating temperature. It does not matter what SAE rating it has on the jar. All oils are too thick at anything less than your engine's normal operating temperature. All oils thicken as they cool.

    It might be best to think of the first number in the rating as the temperature that the oils becomes a solid.

    0W-50 is a 50 wt oil at operating temperature, it freezes at 0 degrees.
    5W-50 is a 50 wt oil at operating temperature, it freezes at 5 degrees.
    10W-50 is a 50 wt oil at operating temperature, it freezes at 10 degrees.
    15W-50 is a 50 wt oil at operating temperature, it freezes at 15 degrees.
    20W-50 is a 50 wt oil at operating temperature, it freezes at 20 degrees.

    A 50 wt oil acts as a 50 wt oil at operating temperature, say 212F. It acts as a 60 wt oil at 180F, a 70 wt. oil at 160F, a 80 wt oil at 140F, a 90 wt oil at 120F, a 100 wt oil at 100F, a 120 wt oil at 80 F, a 160 wt oil at 60 F, a 220 wt oil at 40F and it freezes solid at 20F. This is not exact but gives you the idea. Also, it is non-linear.

    A 0W-50 freezes at zero degrees so is less thick at startup than the straight 50 wt oil that freezes at 20. All oils are too thick at anything less than your engine's normal operating temperature. It does not matter what SAE rating it has on the jar. All oils are too thick at anything less than your engine's normal operating temperature. All oils thicken as they cool.

    Ergo, always use the oil that gives you the appropriate pressure at operating temperature but always use the lowest first number, 0W-50 is better than 10W-50 is better than 20W-50. People who state that 20W-50 is good but 0W-50 is too thin have no knowledge of motor oil, none.

    aehaas
     
  17. AEHaas

    AEHaas Formula 3

    May 9, 2003
    1,350
    Osprey, Florida
    Full Name:
    Ali E. Haas
    Just to back step a little - the people here only repeat what their trusted mechanics tell them. It is the auto mechanics whom we trust with our cars that are giving us faulty information.

    I will give them (the auto mechanics) a little credit too. Oils is very difficult to understand. It is certainly the most misunderstood aspect of automobile science.

    I will guess that there are less than a dozen people in this forum that have some understanding. Mitch was a "Doubting Thomas" but he did the experiment and now has a good understanding.

    aehaas
     
  18. Wasco

    Wasco Formula Junior

    Dec 9, 2003
    486
    Salem
    Full Name:
    Randy
    I am changing the brand because of viscosity rates that have been posted here previously that indicated that Mobil 1 50 weight has better viscosity numbers than Castrol 50 weight.

    It has been my understanding that if you have experiened low oil pressures at high temps a thicker oil ( larger number on the end ) will give you higher pressure.
     
  19. parkerfe

    parkerfe F1 World Champ

    Sep 4, 2001
    12,887
    Cumming, Georgia
    Full Name:
    Franklin E. Parker
    AEHass, while I agee that the lower the first number of a oil indicates that it solidafies at a lower temp., the relationship is not as you state, i.e., 0w freezes at 0f, 10w at 10f and so forth.
     
  20. DN35

    DN35 Formula Junior
    BANNED

    Nov 22, 2003
    599
    Illinois
    Full Name:
    D. Norton
    No wonder I got such a low score as an engineer in my grade school vocational aptitude test! So if I have Xw40 in my car and think the hot idle oil pressure is low, I should try an Xw50, as that oil is thicker than an Xw40 at full operating temp and therefore may give me a higher oil pressure at hot idle?

    And as a side issue, I should keep the X number as low as possible so that the oil flows better at cold startup?

    Please tell me I've got it.....
     
  21. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    7,008
    Maybe and maybe not. If the oil is getting hot because the film strength is not keeping part from rubbing, then you need more viscocity. However, if the oil is getting hot because it takes so much pressure in the oil pump to pump it around the engine, then you should use a lower viscocity oil. The only real way to tell is the try a different formulation.

    In my case, I was using a 20W50 recommended by my mechanic and at the track it was getting down to 95 PSI hot and 35 PSI idle while running at 275-280 dF in Texas summertime heat (like 105). AEHass indicated that it might be due to pumping heat, so I tried a lesser viscocity oil (5W40). Low and behold, not only did the oil pressure remain high on the track (95 PSI+), the oil temperature was lower (265 compared to 280), and the idle pressure higher after a long track session. In addition, it seemed that the engine was a little happier and more free with the thinner oil. Finally, the oil pressure comes up in just over 1 second while the thicker stuff would take just under 3 seconds to come up to full pressure.

    Basically, however, the tables of viscocity AEHass posted can give you better detail about the oils under consideration. In his tables, you are looking for the lowest viscocity (centiStokes) at cold that has 10.0 cS or higher viscocity when hot. The lowest cS I have seen in his tables is just over 50 cS cold and just over 10 cS hot. I belive this is a 5W30 oil but it is easy enough for you to do your own research. The comparable 0W30 oil had a higher cold viscocity--which makes me wonder why they don't just publish the cS data and dispense with the W(eight) system.
     
  22. Wasco

    Wasco Formula Junior

    Dec 9, 2003
    486
    Salem
    Full Name:
    Randy
    I am no machanic or engineer but oil that is hot from pumping. I know this happens with natural gas pipelines and such but really isnt the heat in the oil from the engine not the oil pump and realted pressures.

    This could be something tho as I have heard folks talk about higher pressures with thiner oil, which is very counter intuitive in my mind.

    DN35, it has alway been my understanding that thicker oil will give you a higher pressure under simular conditions. That is what viscosity is all about. They take a funnel and drip oil thru it, counting the drips. The thicker the oil the less is drips, it would also make sense that it would also take more to pass this oil thru a bearing, giving you higher pressures.

    Very simple in my mind. Think of 90 weight gear lube, that stuff is like molasses. Versus something like gun oil ( 10 W ) that is like water.
     
  23. ham308

    ham308 Formula Junior

    Nov 3, 2003
    358
    NE Switzerland
    Full Name:
    Richard Ham
    These oil threads keep coming up don’t they, so I apologise beforehand for my 2 cents/pence/rappen. My experience is with a large European diesel engine manufacturer so that’s where I’m coming from. (Diesel engines are coming to America you know :) )

    The old monogrades eg SAE30 had natural oil characteristics of sinking viscosity with rising temperature. So basically they were too thin at operating temps (can’t support an oil film, wear problems) and too thick at low temps (low flow rates, high friction). Multigrades attempt to correct this by creating oils which do not follow the natural trend of sinking viscosity with rising temps as strongly. So a 20-50 for example acts like a SAE20 at low temps (so tries to be a thin oil at low temps) and like an SAE50 at high temps (so like a thick oil) If you plot viscosity against temperature for a multigrade it is just a flatter curve.

    Over the past years we’ve been getting thinner oils for fuel economy reasons and it’s absolutely true, less viscosity, less friction, more miles per gallon. The idea being that modern oils with better additives cause less wear even with low oil film thicknesses, so they can be thinner. And modern synthetic oils definitely last longer without breaking down which allows longer service intervals, (like 30-50000 miles on modern trucks)

    Now for my old 77 308, fuel economy doesn’t come into it. It’s either awful or slightly better than awful. Long service intervals are also a farce. It’s has only done 25,000 miles in it’s 27 years. Back in 77 the engine was laid out for a slightly thicker oil ie. 10-50, not 0-40 which is what is generally available now. This means even with a good engine, I’ll get lower pressures than what’s in the book. This is not too bad because it also means higher flow rates and more cooling which is good. However it’s not true that low viscosity doesn’t cause more wear. It’s only marginal, say noticeable over 100-150,000 miles for a modern diesel engine but it’s still a factor.

    In my case, for my low mileage normal road use, if I could get 20-50 easily, I would use it. Even non synthetic. But I can’t. So I buy whatever I can, normally 0-40 part synthetic and put up with the lower oil pressure. It gets changed more or less yearly (only a couple of thousand miles) but more for reasons of corrosion because it’s not really being used.

    I apologise for prattling on, but there really is a lot of hype from the manufacturers, garages and so on. And what I’ve written only applies to limited low mileage applications. Track racing, high mileage ; something completely different.

    If you’ve read this far, well thanks

    Richard
     
  24. Wasco

    Wasco Formula Junior

    Dec 9, 2003
    486
    Salem
    Full Name:
    Randy
    Where is the oil temp sending unit?

    Ham, do you have a brand pref? What pressures are you running?
     
  25. TOM B

    TOM B Formula 3

    Jul 24, 2003
    1,038
    Orange County, NY
    Full Name:
    Thomas Buckley

    I was using Mobil 1 15W50 in my 3.2 Mondial. Owner's manual recommends 10w50. Have now switched to Castrol 5W50. The 1st number as low as possible and the 2nd number at the manufacturer's specification. It seemed logical.
     
  26. AEHaas

    AEHaas Formula 3

    May 9, 2003
    1,350
    Osprey, Florida
    Full Name:
    Ali E. Haas
    Tom, I would agree with your choice of Castrol 5W50 instead of the Mobil One 15W-50 based on viscosity. You may want to consider also the Pennzoil synthetic 5W-50.

    The reason that people get confused is when they talk about the lower SAE number. When you say it acts as a 20 wt oil people think it is too thin. A 20 wt oil has a thickness of around 50 cS at 104F, I would guess a thickness of 75 cS at 70F and 100 cS at 32F.
    A 50 wt oil has a viscosity of 130 cS at 104 F, probably 200 cS at 70F and 300 cS at 32F.

    So a 20W-50 acts as a 20 wt oil at start up (104F) and has a viscosity or 50 cS. This is still too thick if your engine needs 18 cS to operate correctly. The same 20W-50 oil has an operating temp. viscosity of 18 cS at 212F. It is less thick (than a straight 50 wt oil) at start up and more thick (than a 20 wt oil) at operating temperature.

    A 100 wt. gear oil has the same vicsosity as a 30 wt engine oil. Look at the charts from previous. Do not ask me why they choose to do it that way.

    aehaas

    PS. Soon I will post recommened oils based on pressure data. I am waiting for more data from Valvoline but it is harder than pulling teeth (I have done this in facial trauma cases).
     
  27. AEHaas

    AEHaas Formula 3

    May 9, 2003
    1,350
    Osprey, Florida
    Full Name:
    Ali E. Haas
    I want to go back to pressure of your oil. The pressure has (basically) little to do with lubrication. It is the flow of oil that lubricates moving parts. If pressure was the main item then we should all be using a 90 wt oil. It would have a start up (104F) viscosity of 400 cS plus, and at an operating temperature (212F) it would have a viscosity of 60 cS.

    The viscosity of 60 cS is the same as a 10W-30 wt oil's start up temperature viscosity of 60 cS. Using the 90 wt oil would be like using the 10W-30 oil at ice cold temperatures even though your engine was at full operating temperature.

    Oil flow is lubrication but some pressure is needed. That is 10 PSI per 1,000 RPM. The best oil for your car gives you this pressure at the peak of HP on that curve (peak torque actually, and usually around 5,500 RPM). Idle pressure is not as important as high RPM, high torque conditions.

    My 575 Maranello manual states the pressure should be 75 PSI at 6,000 RPM. They do not state what it should be at idle as it is of no concern. Actually I get 30 PSI at 650 RPM idle. This is with 0W-20 wt Mobil 1 at operating temperature.

    The reason that Mitch's idle pressure went up using the 40 wt oil instead of the 50 wt oil is because his baseline operating temperature went down. Even though he used a thinner oil it did not thin as much because his engine temperature was lower. There was less friction.

    aehaas
     
  28. AEHaas

    AEHaas Formula 3

    May 9, 2003
    1,350
    Osprey, Florida
    Full Name:
    Ali E. Haas
    I want to go back to pressure of your oil. The pressure has (basically) little to do with lubrication. It is the flow of oil that lubricates moving parts. If pressure was the main item then we should all be using a 90 wt oil. It would have a start up (104F) viscosity of 400 cS plus, and at an operating temperature (212F) it would have a viscosity of 60 cS.

    The viscosity of 60 cS is the same as a 10W-30 wt oil's start up temperature viscosity of 60 cS. Using the 90 wt oil would be like using the 10W-30 oil at ice cold temperatures even though your engine was at full operating temperature.

    Oil flow is lubrication but some pressure is needed. That is 10 PSI per 1,000 RPM. The best oil for your car gives you this pressure at the peak of HP on that curve (peak torque actually, and usually around 5,500 RPM). Idle pressure is not as important as high RPM, high torque conditions.

    My 575 Maranello manual states the pressure should be 75 PSI at 6,000 RPM. They do not state what it should be at idle as it is of no concern. Actually I get 30 PSI at 650 RPM idle. This is with 0W-20 wt Mobil 1 at operating temperature.

    The reason that Mitch's idle pressure went up using the 40 wt oil instead of the 50 wt oil is because his baseline operating temperature went down. Even though he used a thinner oil it did not thin as much because his engine temperature was lower. There was less friction.

    aehaas
     

Share This Page