Oil Viscosity and Horsepower

[stormcrow]

The article I just finished reading stated that the type and viscosity of the oil that you use in your engine will affect power output. It reads as follows:

Some enthusiasts automatically use high viscosity oils in high performance engines to offset the drop in viscosity that oils experience when they are heated. However, there is a substantial drawback to this practice, as it takes considerable horespower to pump thick oil through the passages within the engine - as much as 10 or more horsepower at 5500 rpm - over a standard-viscosity oil.

Furthermore, many high peerformance street engines - where oil temperature can sometimes exceed 210 degrees (F) - show no improvements in wear or reliability with high viscosity oils.

The article goes on to say that if oil temperatures are consistently high, or bearing clearances are too loose, a higher viscosity oil may help by reducing oil flow volume that may help reduce detonation.

A thicker oil may also withstand higher temperatures before bearing surface lubrication breaks down and metal to metal contact occurs. But instead of using a thicker oil, consider installing an oil cooler.

Any thoughts about this article?

Steve

[Yargk]

My thoughts are that this might mean that people with high performance water cooled cars shouldn't just go to thicker oils. For instance, just because someone with a new vette adds some turbos which increase heat, he shouldn't put in thicker oil if the manufacturer recommends a different oil. However, Porsche does recommend 20w50 and our cars are air cooled which means MUCH higher head temps. This doesn't apply to us in my opinion.

[masraum]

Also consider the viscosity chart in most owners manuals (in the manual for my '88) and also in most repair manuals. They are usually rated for ambient temperature. Here in Houston I can get away with Mobil 1 15W 50 year round. In Northern Virginia I used to run 20W 50 in my first car except for about 2-3 months when I would go with 10W 40.

Also the age/condition of the engine can affect what you might need. An older higher mileage engine with larger clearances will probably warrant a higher vis oil if you have a pretty equal choice between two. A new car will probably be fine with 5w 30 (as many manuf recommended in the 90's) unless you are in a really hot environment and then I would probably only step up to the 10w 40.

Certainly no expert, but I believe it to be pretty good practice.

[stormcrow]

The article does address the issue of worn engines stating that if bearing clearances are too loose, a higher viscosity oil may help.

With regard to oil temperatures, the article addresses this issue by saying that since lubricant temperature is an important component to reliability, an oil temp gauage is essential in early diagnosis and can even help produce optimum horsepower.

Engines produced optimum peak power when the oil temperature was between 200-220 degrees (F). Above this temperature there is the risk of lubricant failure and below this temperature there is excessive drag from increased oil viscoity.

The indication is that for optimum power, use an oil that is as thin as possible while maintaining sufficient lubrication strenght and make sure the oil reaches full operating temps.

I look up the requirements for my vehicle (78 911 SC Targa) and the recommendation is multigrade - 10 W 40, 10 W 50, 15 W 50, and 20 W 50.

I was running 20 W 50 Valvoline and just started using Havoline and noticed two things. I use less oil and my gas mileage went up. So, I would assume that the higher viscosity oil did have an effect on my gas mileage and as far as the oil usage, I'm not sure why I'm using less.

Steve

[CraigD]

Steve,

What viscosity of Havoline are you now using? What is your oil temp/pressure?

I use Castro 20W-50. During this time of year oil temp doesn't go above 180 unless I push hard for an extended period of time. My oil pressure runs around 15psi/1000rpm.

Personally, I'll take the HP loss if it means my engine is well lubricated and protected. I think the Castrol is up to the job.

[Clark Griswald]

If you track your car, temps are going to hit 230, 240, 250+. In that situation you NEED a high quality high viscosity oil for protection.

Street temps would not normally get this high. But on the street squeezing the last few HP is far less important than maximizing the life of an expensive-to-rebuild flat six. So I would choose the oil that will give maximum protection. I believe the high viscosity oil provides that maximum protection.

And I am not about to change oil everytime I go to and return from a track event. So I think the smart play is to run a high quality high viscosity oil all the time. I use Mobil 1 15-50.

[emcon5]

For relevance to our cars, did the article refer to wet vs. dry sump variations?

Tom

[911pcars]

I'm not going to give a recommendation either way, but we should remember that its not just viscosity that's important. If it were, I'd suggest about 12 quarts of STP or equivalent in the oil sump.

The goal of engine lubrication oils is to remove heat, reduce friction, prevent metal-to-metal contact and numerous other properties for a wide range of operating conditions. Viscosity is only one factor. A high quality 30 weight oil worked fine (even in racing cars) before good multi-viscosity oils were perfected (not too long ago). The early multi-vis oils actually exhibited lower viscosity characteristics at elevated engine temperatures compared with single wt. oils. Now days, most high quality multi-vis oils provide the protection needed for both low and high engine/operating temps; but that's more a function of the synthetics that are now on the market and additives added to dino stock than to viscosity differences.

Porsche now recommends 15-50 Mobil 1 in their '01 vehicles. This does not mean it's a requirement for the old engines we own. It's probably a good choice, but good dino oils are not extinct.

Sherwood Lee
http://members.rennlist.org/911pcars

[stormcrow]

Craig,

I am about to change my oil and I am going with 10 W 40 Havoline. I have been using the Valvoline 20 W 50.

On cold start up, the oil pressure would be over 5 bar. When the engine finally warmed up, at 1000 rpm the OP would be 1.5 bar. I am concerned that there is too much pressure build up in the engine.

If I am running at 3500 rpm my gauge reads 4.5 and will go to the 5.0 mark if the engine isn't warmed up. I don't disagree that the higher viscosity oils can withstand higher temps better than lower viscosity.

But, if my engine temp is below 200 F then the temp is within range for a lower viscosity oil, so I don't think there would be a problem with going to one grade lower (10 W 40 Vs 20 W 50) and still have the same protection.

This past summer, I checked the temp at the oil filter and was reading 180 F after returning from a short run to warm the engine up.

I don't want to use synthetics because of the problem with leaks, but synthetic oils do a lot better at high temps then dino oil and last a lot longer.

There has been so many discussions about using different grades of oil that I think even though we all drive the same type engine, each engine has its own individuality when it comes to what oil works best in that engine. That may be because of the way we drive them.

As I have stated earlier, since I changed to havoline, my gpm went from 20 to 24 and it's been over 1000 miles since I have had to add oil. With the 20 w 50, I would have already added one quart at about 600 miles.

With the exception of this past weekend, just about all the miles have been on the highway. Not a lot of rpm run up involved with stops and starts.


Anyway, I am going to try the Havoline 10 W 40 for awhile to see how it works out.

Steve

[tobluforu]

I have tried about 10 differnet oils in my 72. The one that seems to work best, temp, leakage, and having to add more is Kendal gt 20-50. Cool color also. Kind of hard to find.

[avorkinn]

One important issue that should not be forgoten, on a flat six, the oil is also a part of the engine cooling system. It is therefore importent to have a good flow, I therefore run with M1 0W-40. I also have to say that I live in Norway where ambient temp seldom reads above 75 F even in the summer.

[911pcars]

Just wanted to add:

Although the best synthetic oils may not turn into mush at 250+ degrees, this doesn't mean you should accept this oil temperature level. Remember, the temp. sensor reads an average oil temperature. Various parts of the engine are usually hotter.

As temperatures elevate inside the engine, parts continue to grow in diameter, pistons to be exact (for example). They'll grow faster than the cylinder they slide in; to the point where there will be metal-to-metal contact with the cylinder (more so with cast versus forged pistons). This will happen despite the quality of the oil.

Put another way, if 95mm pistons sliding in a 95.1 mm cylinders grow an additional 0.15mm from overheating, there's a good chance of ruining the pistons and cylinders. So let's pay more attention to keep our engines cool (around 220ºF is fine) and less with the film strength of synthetic oil at 275.

Sherwood Lee
http://members.rennlist.org/911pcars

[Bill Verburg]

To add to Sherwoods good observations I believe that the head studs on my original 3 liter snapped after a particularly hot day at the track where oil temps briefly saw the high side of 260F. Fortunately upon disassembly there were no other problems.

The viscosity does affect hp and fuel economy. Most newer cars are designed to use the lower viscosity oils for improved fuel economy (the improved hp is only incidental). I cringe whenever I put 5/30 in my Audi but since that is what the factory recommends, that's what it gets.

Until the early 70s the only Porsche factory recommended oil for normal ambient temps was straight 30 weight. The Porsche factory was reluctant to recommend the multi vis oils due to the fact that a dino 20/50 is a light oil with vis improvers to allow it to function like a 50 wt at higher temps(early on this vis extension didn't always work as advertised). Back in that time period there was an endless debate among enthusiasts as to the wt of oil to use). During the 70s the factory finally came around and started to recommend the 20/50 wts as the chemistry of the multi vis oils became more trustworthy. These engines are designed for the heavier weight oils.

The synthetics in addition to their excellent high temp characteristics also offer better flow and easier pumpability at lower temps. Again the hp and fuel economy are benefited.

The thing to remember is that these engines are designed for the heavier weight oils, read your owners manual.