Another good read.

9k, I hope you don't mind the drift. It seems to me we have to have a discusion about what specs are important if we are going to talk specs.

 

George,

Thank you a ton for posting that article. It's confirmed a lot of things I have said in the past about oils but also given me some new ideas.

 

You are welcome!

By the way, that whole "Question of the day" forum on BITOG is very interesting to browse.

 

I've posted a response on the 5w-30 thread on some thoughts from the above mentioned article:

https://www.rx8club.com/showpost.php...&postcount=638

It's very long so please take your time. Some of it is summary but I've tried to apply it more to the rotary engine specifically.

 

Hey guys thanks. Great stuff. This is what it's about instead of screaming 15W-40. So it is fair to say that our cars are in trouble in High load, low RPM situations when running the factory 5W-20?

 

I'm not sure - why do you believe this? Our cars do not make that much tourque, so the low-rpm loading should not be that high.

 

There really isn't any for sure answer. However, taking into account the motivations that are apparent with using an oil of that weight one could conclude that there will be dangers in the long run. With an emphasis on higher fuel economy and more power output, Mazda saw this as a compromise.

If motor failures due to compression loss were never an issue then bearing wear wouldn't have become an issue until the car was way out of warranty.

This is just speculation, but based on the high points of using a 20 weight oil over other viscosities, the benefits from a longevity standpoint are not as great as a sales standpoint.

One of the more interesting parts of that article was the discussion of the Stibeck Curve.



What you're looking at is a chart that shows the Coefficient of Friction (how much friction exists between two surfaces) based on a factor that takes viscosity, rpm and engine load into account.

The chart works like this. Given any position in the chart if you increase the viscosity of the oil then you move towards the right. Decrease the viscosity you move towards the left with all other elements being equal.

Same for RPM. Increase RPM and you move toward the right. Decrease RPM and you move towards the left.

Load works the opposite direction. Decrease the load and you move toward the right. Increase load and you move toward the left.

Now, here's the interesting aspect of using too high of a viscosity. Notice how once you cross from mixed lubrication into the hydrodynamic region that the coefficient of friction increases? This is due to the drag created by the fluid's high viscosity.

Now, "too high" of a viscosity could mean anything as that is all based on bearing clearance, temperature and a host of other factors I talked about in my other post.

Remember that there is no wear in the hydrodynamic region because the metal surfaces are separated by a fluid film. However, you will have a larger amount of shearing and an increased amount of heat as a result. This will yield shorter oil life over the course of operation.

From a racing perspective this doesn't matter. It's all about keeping the engine running and nothing breaking. In a street car it does matter because you have longer change intervals and variations in operating conditions.

 

Actually, I was asking more about why 9k believes we have a high-load, low-rpm situation. Not so much the question about 5w20 specifically, which I agree is specified for all the wrong reasons.

 

We may not have the overall torque a large displacement engine has, but rotor pressures on the E-shaft during peak torque must be at least somewhat similar to what a large piston would see between the connecting rod/crank and crank/mains.

In fact, I personally think it's WORSE. Especially when you consider the following image:



The deflection of the rotor's center of pressure onto the e-shaft always appears so minor that I liken the cranking force on the shaft to be similar to spinning a bar of soap in your hands by squeezing at the appropriate angles. This is why I think the realistic loads saw by the e-shaft is greater than what piston engines typically see. This is also what I think is the biggest inherent design flaw of the rotary that affects performance (more so than high surface area to volume).

 

Interesting - could be true. It would be nice to hear from somebody who has done the math.

Still, peak torque is not a low-rpm affair. So it still doesn't explain 9k's question. I don't know one way or another, but I can't for the life of me see why we would have a low-rpm high-load situation. If somebody can explain I'm all ears.

 

this is what paul and rick mentioned about why the front stationary bearing was wearing in one particular area---"this is where the power pulse of the engine occurs". That --to me---says that is the highest "load" area?
OD

 

My question was based on what I read in the article. I don't necessarily believe on way or another. I am still trying to get my head around a lot of this.

 

OK, cool. My interpretation of a motor with heavy low-speed loads is a diesel - low revs, but high compression and high torque. So, the motor never spins up fast enough to build a thick film, and it's getting pounded to boot, so you have to compensate by using a thick oil.

On the other hand, while our engine does not appear to suffer from such loading, I am more concerned (for the moment ) about the high temps generated at the bearings at high speed. True, the high rotational speed of the crank will make a thick film, but if the oil gets too hot & thin, even breifly, within the bearing it may not matter.

It is interesting that 15w40 and 20w50 are routinely reccomended for the older rotaties, and these two weights have significantly higher viscosity at 150c than 10w40 or anything lighter (unless of course a particular oil manufacturer intentionally modifies the oil to hold viscosity at high temps, such as Redline).

 

'nuther thread.

 

Hey OD, if this is the case then why wasn't this bearing wear or "power pulse" wear seen in the FD RX-7, Higher HP, Highest Load, More Torque, exact same rotor and stationary gear bearings....
Sorry still calling it OIL and OP..

 

Well, you can physically hear the load through the exhaust of any engine. This is most noticeable not only during hard acceleration, but also when "*******". It is perfectly possible that the oil film certain parts in the engine are seeing may not be satisfactory to prevent metal on metal contact under ******* conditions. I can't say that this is an issue in the renesis, but higher visc oil is certainly good insurance.

 

Page 3 of http://www.zddplus.com/TechBrief11%2...ubrication.pdf actually states what I just said about "*******" aka "lugging". High load can be seen in 6th gear in low RPMs and full throttle. MazdaManiac with his knowledge of tuning, could probably tell under what conditions is seeing high loads according to the ECU - that would give us some insight.

 

Not sure how many of you have thought about this, so I'm just throwing this out there:

Since the rotor spins in the same relative direction of the E-shaft but at 1/3 the speed, at 9k e-shaft RPM, the rotor bearings are only seeing 6k RPM.

 

Added Pennzoil Platinum but its weird because it lists the Euro oils and Ultra oils but I cannot find the product data sheets on them.

 

There's nothing complex about where high load situations occur. High loads happen anytime you apply full throttle to the engine. The situation that is being referred to here is a low RPM (say 3,000 or below) and high load.

I pointed out turbo/supercharger applications because the majority of owners I know love the fact that they can go wide open in 6th gear and overtake other cars.

Again, none of this is going to result in damage when you focus on single instances of this happening. If that were the case we'd all be breaking our motors. This is something you'd have to do over the life of the engine that would result in possible long term consequences.

 

Right.. It also doesn't appear that this bearing wear is a major factor in engine failures (right?). The point is, this is where this damage could be happening - and can easily be rectified with higher grade, and higher viscosity oil.

 

Yeah until us out of warranty guys have a chance to pull blown motors apart and really see why they are failing.

 

The majority of the "failing" engines appear from a below spec Vacuum Test or Compression Test.
Main culprits there are badly worn and concaved Apex Seals, probably next would be Carbon Lock of Apex/Side and or Corner Seals...can't be anything else when compressions fail.
A broken seal will fail a C T also, but you will know that usually well in advance.

The Bearing Wear Issue has reared after the engines are naked, at around the 50K + mark and compared to the 12A pics on a motor 3 times that distance...make up your own mind...it is either Viscosity and or Oil Pressure...I am still calling Both, particularly IF the owner does not renew engine oil sooner rather than later...could be low Oil levels also.

Then the variables as to whether the Oil even gets to full operating temperature by some owner use, a 5-10 minute drive to work is another oil and engine Killer.

And as the "scientist" said does not matter if you are using Dino or Synth, both will have a short life under those conditions.

It really is not "Rocket Science" to comprehend what is causing engine failures and or issues, go through all the engine changes in S2, that will tell you what was wrong..

 

Right.... Sooooooooooooo, when you gonna get them there oil pressure readings off of your purrdy motor thang?

 

Not true. Dilluted oil is an engine killer, not the short trips. It is absolutely true that somebody, like myself, who does a lot of short trips would be wise to set his OCI based on the number of trips as opposed to the number of miles, but as long as the oil is changed appropriately there shouldn't be a problem.

After all, everybody who uses their car as a daily driver starts thier car at least twice a day.

Now, carbon build-up may be another issue. I do think we should start a thread on that...