this is kind of FC specific, PJ inspired idea.

if i have a stock NA FC, might it be better to put the bigger turbo oil pump in it, and leave the stock oil pressure where it is?

more oil flow from the bigger pump, less energy spent pressurizing liquid?

 

On past na engines I've built that's all I did.

 

admittedly, i belong the school of using higher pressure and when i build, i do what i can (within reason) to increase pressure. however, i honestly don't see anything wrong with that idea of more volume, with stock pressure.

 

When I used to play with 12As with a low-ish powerband, I'd run 5W20. The theory being, once you get to maximum oil pressure, spinning the pump higher is just flowing more oil through the regulator. So, thinner oil means more oil flowing through the bearings and rotors, in theory keeping them cooler.

I'm more worried about keeping the rotors from smacking things, now.

 

yeah ive run 5-20 off and on for years, with no trouble at all.

 

well, with all the Renesis autopsies i've seen floating around the boards, i refuse to run such a thin oil. in fact, whenever i have to pull the engine out of my Rx-8, i plan to go ape-**** on the oiling system! i want more pressure and more volume.

 

I decided to try 5W20 and then 5W30 in my FC. since I drive 100 freeway miles per day at the same speeds, my mileage is consistent. What I found is that between 5W20 and 20W50 (and everything in between) my mileage stays consistent. The whole point of the thinner oil was to increase mileage but that didn't happen. An interesting thing I did notice on the 5W20 though is that the engine only felt smooth for about the first 1000 miles or so after the oil change. Then that smoothness went away until the next oil change. That can't be good. With the 20W50 it feels like the engine is running on a stick of butter and the perceived smoothness doesn't go away like it does on the thinner oil. At least not as fast.

The Renesis does have other issues that are leading to oil related issues though and there is more to do with it than just the thickness of the oil. The S1 RX-8's have the same oil pump at the FC n/a's do. They also run at lower total pressures than the FC's do so basically they don't quite have the oil flow of a stock n/a FC oil system. Their oil coolers don't have fans on them. This may sound like no big deal but when you aren't moving or are moving slowly, there isn't much heat rejection. The FC's on the other hand have the oil cooler mounted against the radiator where the fan pulls air through it. Even when the car isn't moving. The RX-8's oil temps average higher than the FC's do as a result.

Also keep in mind that the Renesis makes more power. This isn't much of a cooling issue for street driving as it only takes so much to keep you moving. However there are 2 other things that are contributing to this. First is that the RX-8 is a few hundred pounds heavier. It takes a little bit more power to keep that car moving. Yes the engine is more efficient but more power produced is also more heat wasted, whether it's out the tailpipe, the coolant, or the oil. The RX-8 is also geared so aggressively that it is always at a higher rpm than the 13B's in RX-7's doing the same speed. This is more load on bearings which is where Renesis engines in those cars have seen oil related problems.

That car has higher stress levels on the oil but with less total oil cooling and flow than the FC's which is where much of the problems lie. Higher oil temps break down oils faster. The hotter it gets, the shorter it's life. Also keep in mind that some oils have a very high volitility which means they evaporate faster. This is formulation dependent. Many RX-8 owners complain about oil in the intake and those equipped with catch cans may have issues with excessively blow off. It is the volitility of their oils combined with higher oil temps that are causing much of their oils to literally vaporize and then recondense once outside the engine. If your oil isn't in liquid form, it's not working! This is why quality oil does matter and not all are created equal. I'm not even going to get into the oil metering system on that car and it's flaws!

The S2 RX-8's fixed many problems. They changed everything about the oil metering system but more importantly with the oiling system itself too. The oil pump now has more capacity and oil pressure has gone WAY up!!! It's the highest ever seen in a rotary and typically even race engines don't use pressures as high as that engine. The oil weight recommendation hasn't changed though but with the corrections made may not be as much of an issue anymore. If mileage doesn't suffer, I can't think of any reason not to run a thicker oil.

Everytime Mazda comes out with a new rotary they do something really good and then mess up another area that they once had right. I don't know why they do this. It seems like every new engineer that comes in doesn't bother to learn about what has already been done and what was found to work.

 

true.

 

Any idea what pressures the S2's run at?

 

Amen. We have that same problem where at the Lab where I work.

 

i'm curious, too. the one number i found quoted over at Rx8Club was like 72 psi, which would be suspect according to what you said, no?

 

I think I found the same thread you did over at the Rx8club forum. I think 72psi was like at 3k or something. Either way, I can't believe Mazda would screw up the S1's oil system. I mean they had to re-cast and re-engineer the side plates to accept the exhaust ports and kept the thicker dowel pin areas like with the 3rd gens (though uneccessary for na) . Yet they design the Renesis for higher reving and felt neccessary to install a lesser flowing oil pump? Fd center outlet OMP system and everything else related to it was proven. My undamaged housing on my 108k original engine was mint. Somebody over their was smoking something.

 

man, there's a lot about the S1 Rx-8 that leaves me shaking my head. by the time i found that they used the N/A FC oiling system, it really wasn't much of a surprise.

 

Peejay said "I'd run 5W20. The theory being, once you get to maximum oil pressure, spinning the pump higher is just flowing more oil through"

Our oil pumps are constant displacement pumps. They do not spin faster when you use thinner oil. The oil pump rpm is a function of engine rpm, it is not a function of oil visocisty. If you use thinner oil, then the pump doesn't have to work as hard to pump it, but the pump will turn the same rpm, and the volume flow rate will be the same at the pump.

Again, the oil flow rate coming out of the pump is a function of the size of the pump.

 

I think what Peejay is getting at is that a thicker oil can't flow through the bearing clearances as quickly as a thin one can. Because you are exactly right, the oil pump is of the positive displacement type, the total system flow basically only changes as a function of RPM. Where Peejays point comes in is because you have the same overall flow and the the thicker oil cant flow through the bearings as quickly, more of the pumped oil gets 'wasted' by flowing through the reg to maintain a constant pressure instead of going through the bearings. So yes, the overall oil flow doesn't change, but more of it goes through the bearings and less through the reg.

A problem will arise at very low RPM if the oil is so thin that it can flow through the bearings faster than the pump can supply it, giving very little, if any oil pressure.

 

I ran a TII oil pump on a high rpm prepped n/a engine. 100 PSI regulator, Id see 50-60 psi at 850 rpm idle, and 100 psi by 3k rpm.

As for oil weight, heavier is better for the engine. Theres a thermostat in the oil cooler, so the oil temps never drop below that point when its warm. Been a while since I looked at the numbers, but I believe is was 160 degrees. Heavier oils deal with consistently elevated oil temps better than lighter oils.

 

I think you completely missed my point.

If I hit the regulated pressure at 3000rpm, then essentiallty that is all the oil that will flow through the engine. When the pump is spinning faster (because the engine is spinning faster) then it's not flowing any more oil through the engine, it's just working the pump against the regulator spring.

I was running thinner oil for the purpose of flowing more oil through the engine. If it didn't build maximum pressure until 4000rpm, then (assuming a 100% volumetrically efficient pump) 33% more oil was actually flowing through the engine once up to regulated pressure. I wanted more oil flowing through the bearings and the rotors because I was using the beehive oil "cooler" and if I couldn't get the oil temps down, the least I could do was to flow more oil through and maybe pull more heat out that way.

Yes, I know the math is imperfect, but that's just for explanation of theory. In practice, pumps are never 100% volumetrically efficient, that VE is dependent on oil viscosity, and journal pressure bleed is somewhat dependent on RPM...

 

I'm going to throw a few oil facts out there just to mess some people up. Actually I'm going to ask questions and I want to see the general consensus.

1: What is harder on an oil, lugging it slowly at low rpms at high loads or high rpm full throttle driving?

2: If an engine requires an oil with a thickness of 10 cSt at 100*C to be adequately lubricated, does this mean that 10 cSt viscosity is all that is needed at any temperature?

3: Will a thicker oil will help keep things cooler?

4: What is harder on oil, gears or bearings?

5: Does your engine require a thinner or thicker oil at hotter temperatures?

6: What is the lowest viscosity that an oil is generally considered to still be effective at?

Have fun with those and I'll post the answers after a few responses come in.

 

With or without adequate oil cooling?

Should be just the opposite. The reason we run oil through under pressure (well, in engines that don't rely on the oil wedge to keep things in alignment) is to flow oil through the bearings to pull heat away.

You can tell just by the oil if an engine has a geardrive or a chain drive for the camshaft. The geardrives leave a lot of crap in the oil.

That's not being hard on the oil, just dumping junk in the oil. Bearings put a shear loads on the oil which is harder on the actual oil molecules.

How long is a piece of string? There are far too many variables to deal with to condense it to a simple answer.

When you ask these questions, are you taking into account that viscosity changes with temperature? If I was continuously running my VW at 250 degrees oil temp, I'd want something really thick in there since even 20W50 gets too thin to keep the lifters pumped up. If I was just idling it around in the wintertime, slam some 5W20 in that thing! But, wintertime 5W20 is thicker than steaming hot 20W50.

 

1: Lugging at high loads and low rpms is harder on an oil than flat out high rpms. That is a stronger test of its strength.

2: The cooler an engine, the thicker the oil has to be in order to adequately protect it. This is only true to a point though but the fact that an oil at 100*C is only 10 cSt in viscosity does not mean that only 10 cSt is needed. If the engine is cooler, it needs more. Again to a point. If you get so thick that it can't get anywhere, it does no good.

3: Bearings are harder on the oil than anything else.

4: Thinner oils absorb and reject heat faster than thicker oils.

5: Answer is related to #2.

6: 2 cSt. Keep in mind this doesn't mean you should aim for 2 cSt. It depends on your clearances, operating temperatures, and driving habits.

Incidentally the more acidic an oil is, the better it protects. The old Timken bearing test which was popular for a while would illustrate this very well. If you poured Coke, yes the drink, in the bearing and pulled down on the bar, it wouldn't score the bearing! This doesn't mean it is an adequate lubricant though so don't use it. There's more to it than that test.

AGMA has standards for oils that calculate based on temperature and load what viscosity of oil should be used. If your oil has an operating temperature of 80*F, it isn't going to be the same oil as one designed to operate at 180*F and the higher temp one isn't necessarily thicker! It might be but it doesn't have to be. In some cases going thicker makes things worse. In others it doesn't.

I've actually got some neat charts at work that show all of this and even have a little spreadsheet where I can enter in the viscosity of any oil at 40*C and 100*C and it will show me what the viscosity should be at any temperature. We have charts here for our large pumps that show what oils we recommend depending on climate and operating temperatures. The chart for conventional oils shows that many different weights are needed across the temperature range than synthetics. We may only recommend 3 different synthetics depending on usage but if conventionals are needed we have up to 6 different choices of weight. They just aren't as strong. We also recommend changing them more often.

Many things about oils are backwards from what most people understand and even BITOG has many things wrong. There are misconceptions about additives and when to use certain types of oils.