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Coupes & Convertibles Mazda RX-8 Mazda RX-8
3616 messages, Last post on Apr 22, 2009 at 5:06 AM
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Folks -- I will buy a new car soon, have been reading about the RX8 for some time. I know about the flooding issue and oil burning in the 8, but recently read a concern about proper oil weight, that the rotary's parts can suffer if you don't use the right oil. Anyone, what works? And did anyone test drive a Speed6, then choose the RX-8? If yes, how come?
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The thing is - it's not *burning* oil. The engine design is such that it requires internal lubrication of the seals, which means it's closer to a 2-stroke engine in design. There will always be oil consumption in rotary engines.(the same is true with diesels, too. The proper oil to run in it, IMO, is a semi-synthetic blend. Unlike typical engines, the idea of breaking it in and seating the rings doesn't apply. You want as little wear and friction as possible from day one in a rotary, so changing oil is also critically important. A lot of people think that they are fine letting it go longer, because in a 4 stroke engine, if it's burning oil, putting in half a quart every few weeks kind of makes oil changes moot.(filter aside). Not so in a rotary. It's a lot like a motorcycle. Have to warm it up, not rev it to death for the first few minutes, check the oil and fluids every week or so, blip the throttle when shutting it off... But the results you get from the RX-8 are well worth it, IMO.
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Replying to: sonnysayshi (May 03, 2007 9:51 am) The best answer to the question is to drive them both for yourself. They are two completely different vehicles with completely different drivetrains. I really don't think you can compare the two. |
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Replying to: plekto (May 03, 2007 11:44 am) I do intend to drive both the 8 and Speed6, just wondering if others already had done so. |
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No problem  My cars all burn oil at this point - so there's normal "burning" and abnormal "burning"  As for weight, I don't know. I would suggest a bit of oil stabilizer in it, though. It makes a great deal of difference in my truck.(new, 1/4 of a quart would be about enough for a full oil change - mine uses a quart - lol) Oh - I currently have an old 4-runner until I can get something better this fall. |
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This brings up a repeatedly misunderstood issue with the rotary engine. Starting with a piston engine, they do burn oil. If they used no oil the rings and cylinder walls would wear out very quickly. Oil is needed between the rings and cylinder walls for lubrication. It needs to be a very thin layer, or the vehicle would use too much and not meet emissions as well as disappoint owners. Hence the "oil rings" scrap most of it off, but what is left is burnt. Note that on piston engines the rings and cylinder walls are exposed to the crankcase, and oil splashes from there to the cylinder walls (some designs also feed oil directly there). If you measured the volume of oil you drained out at oil change time you would find it was down about a quart (after 3000 mi of use). If it's not down it's because you have gasoline in the oil making up the difference. Now for the rotary engine. There is no way oil can get at the equivalent of the rings (apex, corner, and side seals) and cylinder walls (rotor housing inner surface), as the rotor housings are continuous and the oil is held below them. So oil had to be injected into the engine along with the air and fuel. Getting this mixture "just right" has been quite an engineering adventure. Too much and you carbon up the works (NOT GOOD!). Too little and you quickly wear out the apex seals (the rotor housing surface is chromium and doesn't wear much if at all). Mazda switched to electronics to meter this oil in the 2nd gen RX-7 (1989), and has continued to use this method since. The Renesis engine has improvements over the 3rd gen twin turbo version with better electronics, but mostly in the nozzles that distribute the oil into the intake stream (trying to get the droplets small so the oil arrives more consistently - it's a very small amount of oil and surface tension makes it difficult to distribute the small quantities required). Mazda could have used a separate injection oil tank, like two stroke street motorcycles used just before they went the way of the dinosaurs (my Yamaha RZ350 and DT200 both use this method, with a metering oil pump). I suspect it was considered too complex and required too much faith that owners would keep enough oil in a separate tank, so engine oil was used, from the oil sump. As for the thought that the rotary is some kind of two stroke engine, it is not. It is a "continuous combustion cycle" engine. The misconception is easy to get due to the oil injection, and the fact that there are no valves, that the engine is "piston ported" (rotor ported actually). You can visualize the equivalence of four "cycles" however. Facing the front of the car, the rotors turn clockwise, intake occurs in the 9 o'clock to 11 o'clock part, compression from 11 o'clock to 2 o'clock, ignition from 2 o'clock to 4 o'clock, burning or power from 4 o'clock to 6 o'clock, and exhaust from 6 o'clock to 9 o'clock. The rotor turns at 1/3 the speed of the the "crankshaft" (eccentric shaft). Note the power part or burning of fuel occurs at the bottom of the engine. This was done just in case there was a coolant loss. The hottest part is the last to loose coolant. So, I think you'll find the RX-8 uses just a little more oil than an equivalent piston engine (one with 230 HP). I find it's not necessary to add oil between changes. The amount of oil carried is enough that the injected (and burned) oil decreases the level just enough that at change time you are down to "add oil" levels, but still safe to run. Do keep an eye on the level, however. Certain use methods can use oil faster than others. And you don't want to EVER overheat or run low on oil with a rotary engine! |
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Oh, I *know* it's not a 2-stroke engine. I was using that analogy because most people understand how one works and how it needs to inject a little bit of oil as it runs. And, yes, it goes through more oil than a typical new 4-stroke engine. But that's normal. I find it interesting that owners complain as if the engine is faulty somehow for doing what it should normally do. Mazda needs to put a page in the brochure and manual explaining how it works and why you need to add a little oil regularly(as opposed to every 3-5K miles in a typical 4-stroke engine). The engine also appears to have a much smaller amount of oil in it compared to a typical 4-stroke, so being a quart down is more of a problem whan when you have 8 quarts total. Me? I would have had a separate little tank and used a special oil. Less chance to get the thing messed up if the oil is always "rotary oil"(some specific weight/etc that works best). But I can also see a LOT of people doing zero maintainence on their cars and it dying as a result. Either way, it's reliable and fun if you do the preventative maintainence/checks every weekend.
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Replying to: plekto (May 05, 2007 10:32 am) You're assuming the average owner reads the manual... |
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Replying to: plekto (May 05, 2007 10:32 am) Better look again. The RX-8 holds at least 5 litres of oil. You can't drain it all out, because it's held in the cooler and you also can't fully empty the oil pan. We fully drained ours once, and had to put in the full amount. Shocking! I started calling it "the truck". Also, Mazda has played fast and loose with the dipstick, raising and lowering its' calibrations (full and add marks) a couple of times since introduction. So it's kind of hard to know when you -really- should add oil. If I'm near change time I let it run a little low. Some have stated you can actually run it a quart low with no problems. Don't try to race it like that though! In hard cornering, the oil -could- slop away from the pump pickup. That would be a "bad thing" (tm).
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Replying to: pathstar1 (May 05, 2007 6:04 pm) |
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