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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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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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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). **** You just made my point.  The design of the engine tolerates a low oil condition much less well than a typical engine. So you have to check the oil every couple of weeks. Of course, if you have a bike or an old classic car, this is perfectly normal. A little extra work and you're good to go. But the RX-8 isn't a put gas and forget it Honda Civic, either.(and it's not washing machine bland, either
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Replying to: plekto (May 07, 2007 4:44 pm) The oil temp rises as there is less mass getting cooled in the pan. The anti-wear additives are being worked X times as fast (depending on how low the oil is). |
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Hi guys. It's good to be back on the board and read about RX-8 owners' words . I had been on this board a couple of times, and last time I posted, I was about to ship my car to Korea from US. Now I have settled in my home country. I was a little concerned about bringing my RX-8 into Korea since there is no official dealer/service center. I still couldn't part from the red hottie, so took the risk. Overall, I am happy driving this exotic looker in the city where 90% of luxury cars are BMW. Just hoping there won't be any major issue. I am about to make an oil change for the first time since I came to Korea and have a little bit of problem  . The manual says I am supposed to use 5W20 and I remember sometime ago on this board that synthetic oil is not good for rotary engines. Well, there is the problem. Apparently, it is not easy to purchase regular petroleum 5W20 here. Synthetic 5W20's are sold in a specialized stores for racing car tuning but not petroleum oil. Just wondering if I can use synthetic oil at all. Or, petroleum 5W30, which I can also purchase here. If neither, I should order oil from overseas (which is whole another problem). Thanks.
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Replying to: articist (May 11, 2007 5:59 am) Please note, synthetic is also petroleum oil. It's just got certain petroleum molecules in it that don't occur in nature. They have to be converted from "normal" oil. In fact, these days, most synthetic oils have moved back to being very close to normal oil. It's expensive to convert to the group 4 molecules and they are using group III more often now (eg Mobil 1 has changed from group IV back to group III last I heard). I think Royal Purple, Redline, and Amsoil still make group IV, and the German Castrol was group IV as well. Don't know if it's still available. Only reason I mention this is many think the group IV works best in the synthetic world. Mazda allege synthetics will harden certain seal materials in the engine. Lots of people use synthetic oils with good results. As always, YMMV.
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Replying to: pathstar1 (May 12, 2007 6:45 am) Anyway, after some extensive search for 5W20 non-synthetic, I did find a website which says they import such. I know you are in Canada, and this particular brand is Petro Canada. Is it a good oil? I think they have SL and ILSAC GF-3 seal. Thanks again.
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