synth bad for rotary engines?

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May 27, 2002
Ocala, Florida
First let me say, great to have you drop in Bret.

You have an interesting question which I have never even put any thought into and wonder what kind of problems do these guys actually see using a synth oil. In looking at the design, it like a motorcylce or toyota engine, has gears and would need a high end lubricant that can withstand shearing unlike a regular motor.. I also see where it could be a problem if oil would get past the seals in the rotor causing fouling of the cyls.

I would suspect because of the seals used in the internals of this engine as that is unlike a conventional engine the barrier that prevents oil from passing into the combustion chamber maybe?

I wonder how old is the mobils data posted against using in a rotary engine and what reasons they used as to why not.

[ June 22, 2002, 01:09 PM: Message edited by: BOBISTHEOILGUY ]

Originally posted by BOBISTHEOILGUY:

I would suspect because of the seals used in the internals of this engine as that is unlike a conventional engine the barrier that prevents oil from passing into the combustion chamber maybe?

I believe this is correct. I'm not familiar with the details, but the RX-7 specifically calls for conventional oil because of a problem that arises in using synthetic on the apex seals of the rotary piston (the triangular thing that spins around).
It is my understanding that SOME synthetics showed SOME problems when used with Mazda rotary engines. Rather than try to specify brands by name, Mazda opted to recommend against use of any synthetics.
It is also my understanding that neither AMSOIL or Mobil 1 fell into this category.
My neighbor, and now his son, have a mid-80s RX-7 which has been running AMSOIL for about 18 years now.
When I owned a RX-7 in the early 90's I used Mobil 1 and had no probs. I even called Mobil at the time and they said that it was ok to use the Mobil 1 synthetic in the Mazda Rotary. I believe I remember hearing that Mazda had some problems with synthetics in their rotaies early on, and from then on advised against all synthetics in the RX-7.
Yea, Mobil mentioned on their webpage that Mazda had problems with synthetics. They said they tested the M1 in these engines with excellent. They then indicated to see your dealer for recommendations. WQhat a crock. They are woried about the wealels (lawyers)
Well maybe if people can see how a rotary engines basic design looks like, it would help understand the lubrication concerns.

Here is what one looks like from the inside.

This is the rotor and housing of a rotary engine from a Mazda RX-7: These parts replace the pistons, cylinders, valves, connecting rods and camshafts found in piston engines.

Now the rotor itself

At the apex of each face is a metal blade that forms a seal to the outside of the combustion chamber. There are also metal rings on each side of the rotor that seal to the sides of the combustion chamber. Notice also there is rubber seals around the center.

The rotor has a set of internal gear teeth cut into the center of one side. These teeth mate with a gear that is fixed to the housing. This gear mating determines the path and direction the rotor takes through the housing.

Now considering these engines have been around for a while, when synth's first appeared, it was easy for a synth oil to cause problems with the older seals where compatibility may have been and issue.

I'd imagine that 3k oil drains were a must given the gear driven parts used in this engine as well and extending drains would have been tuff to do.
Great pics, Bob.

I always thought the concept of the Wankel engine was a great alternative to the reciprocating engine. It had:
1. Compactness
2. Higher engine speed, which resulted in
high power/weight and power/volume ratios. RPM's were 7000+.
3. Inherent balance and smoothness.
2. With two rotors, it was equal to a 6-cylinder
since there were three power strokes/rev/rotor.
3. No power robbing valves or valvetrain were needed.
4. No need to convert riprocating motion to rotational motion.

Disadvantages (which may have been worked out by now) were:

1. Higher heat transfer
2. Sealing and leakage problems.
3. Emissions (EPA almost killed it).
4. Usually used two spark plugs for faster combustion.

My brother-in-law had one of these and used Amsoil 10W40 with no mechanical problems.
It did use oil at about 1-qt./500 miles.
It was very compact, about 1.2 L displacement, compression ratio of about 9.4, and hade a BH
of about 75 HP.

I always enjoyed the sound of it as it revved to
its 7500 rpm redline. Something similar to turbo
winding up.

And you'll love this Bo, the Apex seal had moly insert at the tip.
I bought a RX-4 wagon new in 1974 with 4 speed. No problems with oil as I used a dino. Used a quart in 1500 miles. I built, and put 2 Cap. Discharge units on the engine for a hotter spark, 2 coils. I was working in Santa Clara, Ca then in electronics. The problem with the vehicle is that it came with a Cat converter and Low Lead fuel was just coming on the market. No Lead did not exist then except Amoco Premimun and it was costly. At night when I parked it after a run down Hwy 101, the cat. conv. would be 100% cherry red. I couldn't remove it as it was under warr. plus vehicle inspection in CA. Ran like a scared rabbit though with smooth power. The RX-4 had the new apex seals, so oil useage was less.
Hi y'all,

First post here. This place is a terrific resource!

I don't have a rotary engined car but someone elsewhere remarked that synth oils are bad for rotaries and should not be used. They said that lots of members at have had problems and that even M1 states that synths should not be used in rotary engines.

Does this ring a bell with anyone? I'm just curious - I couldn't care less about rotary engines but I figured the learned folks here could shed some light on the subject.

I think another reason they didn't become more popular was gas mileage, it wasn't all that great I heard.

I do know it's considered perfectly normal to burn oil at the rate of 1qt per 500mi, that must've scared a lot of people off. How did they avoid massive engine failures? Did they have a low oil light that warned when it was a quart low (like most new cars have now) just to be safe?
Oil consumption on these engines is intentional. Oil is injected at a rate proportional to throttle setting. I forget the exact specs but it can go as high as a quart in 250 miles. Considering that they managed 255 T/C HP from 1300 cc by 1995 or 160 N/A HP in 1991, 17/25 mileage was not bad at all.
That gas mileage doesn't sound too bad at all, I guess I heard wrong!

With oil consumption like that though, you definitely wouldn't want to use expensive synthetic. I wonder if you'd even really need to totally change the oil if it burned that fast? I would imagine just changing the filter every 5-6k and doing a total oil change maybe once every year or two would be more than enough.
I used Mobil 1 when I had my '87 RX-7 and it seemed to use a quart in about 1,500-2,000 miles. I didn't think that made it a big deal to use the Mobil---an extra 4 bucks every 1,500 to 2,000 miles--no biggie.
When I had my RX-4 the only Mobil I available was the 5W20 in the silver metal cans at $4.25 qt. Pricey back then. In 1977 I traded mine for a 1959 Corvette Convt. Red w/white top and like new. I sold the Vette last year for 10X more than I paid for the Mazda RX-4 new. All I did during that period was reg. maint on the Vette. And I got it from an Amsoil Dealer, so it had Amsoil front to rear, including when I sold it.
The rotor was cooled primarily by oil, since no coolant could get to this rotating part. The outer and center housings did have normal antifreeze mix running through it, however.

Personally, I would like to see it return in force and numbers.
personally I'd like to see it stay dead.

- Emmissions issues (you HAD to burn oil for it to work)
- gas hog (a similar weight corvette with today's aluminum V8 gets A LOT better gas milage than those gas hogs did).
- inefficient chamber design. The meare design mandates flame proprogation that can not perform as well as a piston equivelent.

Not to mention the fact that your limited by gasoline how much boost you can run... and in a rotoray engine, your hot-rodding techniques are limited. Adding rotors just increases wight, and there is a limit to how much a rotary can make.

All that, and we havn't even mentioned its durability. Those apex seals were notorious for problems with logevity.

Its a cool idea, and yeah it sounds pretty slick, but it just can't hold up to today's engine standards. Hopefully Mazda will rethink reviving their little project. I know GM's look ed into it a few times, and killed it each time. (GM produced the first rotary engine race car that but out ~600hp... and it needed about 6 rotors to do it. A warmed over V8 could do the same, at less weight and cost. (Although I wish Mazda WOULD bring back the RX-7, the last iteration was very slick).

Originally posted by BOBISTHEOILGUY:

I like the last set of wheels you have there better than the two 4 wheelers!

Me Too Steve, Is that your wife or girl friend???

[ June 24, 2002, 12:22 AM: Message edited by: David ]
Well Mazda is bringing back the rotary with the RX-8... Supposedly the engine has improved design and emissions. Most damage to the rotary engine was caused by the turbos, they tended to overheat and created problems.. NA rotarys were quite reliable. The latest 'Renesis' naturally aspirated version will produce about 250hp/162tq.. not bad for a two rotor wankel with 1.3l displacement. What I like about the design is the engine is very small and can be tucked close to the middle of the car, hence the RX-8 will have 50/50 weight distribution and will weigh about 2900 lbs while accomodating 4 comfortably.
I've owned an '81 coupe & '91 convertable RX-7s, both non-turbo. Used dino in the '81, and M1 10W30 in the '91. Never had a bit of problem with either car; very reliable. Great high-reving engines, but poor low-end torque. Never win a drag race with these! Fuel economy, while fair, was not too good considering the weight of the car. Oil consumption on either was about a quart every 3,000 miles. The amount of oil it injects on startup is quite small. All in all, an enjoyable ride - and very fun to drive.
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