Thinner or thicker oil in an high torque diesel?

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first of all congratulations for the forum that I found through another forum under advice of a friend. I own an Alfa Romeo GT (turbo diesel with 170hp), i tuned the car bringing it to 200cv, now has a torque of 380nm at only 2200rpm (on a total of 4300rpm). I can use 5w30 with ACEA B5 (lower HTHS) or ACEA B3 (higher HTHS). So i know that B5 oil are thinner than B3 oil. I read many advices about the statement that thinner is better. But in a car with very high load at very low RPM, is better go to a 0/5w30 thinner b5 oil or 0/5w30 thiker b4? Is the statement "thinner that i can get and thick as necessary" still true? I see that this statement is always used about gasoline engine with normal/high rpm. But in a turbo charged diesel scenario (high load/low rpm)? I have read also a paper where in a high load and low rpm engine is better go to thicker oil (obviously still being among those specified by the manufacturer). Where is the truth? Remember that both specifications are allowed by manufacturer. I drive at medium speed in highway but i have always great torque at low rpm and often I drive hard with many short journeys. So which oil you choose? Thinner 5w30 (B5) or thicker 5w30 (B3)?
 
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Marco, Considering you have chipped the car to 200 HP from the stock 170, it may well be that the manufacturer's recommendations are no longer suitable for your Alfa. In other words, with the engine generating more power, torque, and presumably heat, you may need a more robust oil than originally specified. I'd consider using a "thicker" synthetic 5W-30 or moving up to a 5W-40. I've owned several Alfas over the years and have found that one can seldom go wrong in choosing a stout motor oil for them. Especially as it sounds as if you will be driving the car as it was intended to be driven--con brio. Ciao, Mark
 
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Yes, Mark is correct. The SAE J300 specification for compression ignition engines specifies a minimum HTHS viscosity of 3.5, which is also the specification for A3/B3-B4 oils. Even though Alfa allows for a B5 oil, I would opt for a high HTHS. Engines that are capable of high torque at low RPM are exactly the application where the high HTHS is needed. The oil wedge in the bearings is thinner at low RPM, so the viscosity of the oil becomes more important. If I owned your car, I would also agree with Mark and look into the many quality 0W-40 or 5W-40 oils you have available. Ed
 
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Originally Posted By: MarcoBatiato
Is the statement "thinner that i can get and thick as necessary" still true? I see that this statement is always used about gasoline engine with normal/high rpm. But in a turbo charged diesel scenario (high load/low rpm)? I have read also a paper where in a high load and low rpm engine is better go to thicker oil (obviously still being among those specified by the manufacturer). Where is the truth?
The actual lubrication axiom is "as thin as possible, as thick as necessary" and it applies to all IC engines including low rpm, high torque deisel engines. It is also true that the highest viscosity demamds of an engine like in a deisel will be at low rpms when the torque is high. But there is no contradiction here. The HTHSV rating of an oil is given at 150C but the actual HTHSV of an oil is still temperature dependant. For example a light B5 oil may have a HTHSV rating of 3.1cP but at 90C it's actual HTHSV will be typically around 9cP and still over 4cP at 130C. So the only way to know for sure if a lighter B5 oil is more than thick enough for your increased boost application is based on your oil temp's. Since you have modified your engine you should have an oil pressure and/or oil temperature gauge installed. An OP gauge will tell you what your operational viscosity at all times your engine is running. Until the oil is up to temperature even the lightest B5 oil you can find is still heavier than optimum and quite likely will still be at normal operating temp's. What you need to know is the test OP spec' for you engine as provided by the engine manufacturer. Two spec's are usually provided, one on idle and a second one at some designated elevated rpm such as 55 psi at 2,500 rpm. In all likelyhood you actual OP will always be above the test spec' on the specified oil; you can consider that your safety margin. Since you've increased your engines power that will increase the minimum viscosity demand slightly so you could raise the minimum OP you're willing to accept to say 60 psi. In other words you're narrowing or increasing your own safety margin and you won't likely need to run a heavier A3/B3 grade of oil to maintain that higher minimum OP level. With an OP gauge you'll also learn of your oil pump by-pass level. It is usually set in the 6 to 8 bar range. Even with a light B5 oil you will not be able to use high rev's without the oil pump going into by-pass mode until the oil tep's are close to fully hot. Running a heavier oil will mean that the oil pump will stay in by-pass longer during the warming up period and if it stays in by pass at normal operating temp's you're running an oil way heavier than necessary for your application. So as you can see, running a heavier oil may not be necessary and doing so may prove to be counter-productive.
 
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If by "safer" you are asking if there's more "margin" for when things don't go to plan, then yes, a 5W-40 is "safer" that a 5W-30.
 
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