More carbon with synthetic oil

[Hasbeen]

Some of you may remember me saying I was going to run synthetic oil in my RX/7 even though many said not to. Well, after putting 5,000 miles on the oil I changed it last weekend. The plugs were walnut blasted before I changed the oil as I wanted them to be real clean so I could tell how they would last with the synthetic as far as carbon build up. This car goes through plugs like I change my shorts, every 10,000 you HAVE to change them. Well, there was about 3,500 miles on them when I changed the oil to mobil 1 15W-50, When I took them out to look at them they were carboned up worse than they have ever been. I would have thought that because synthetics generally have a low % of ash, that they would have been cleaner than when I was running dello 400.

The car ran fine durring the 5,000 miles actually I think better than before, but It was starting to miss because of the carbon on the plugs towards the end.

The car is an 87 with 285,000 miles, maybe it`s time for new injectors and the carbon is from fuel.

 

[Jelly]

Can't say I'm incredibly familiar with these engines, but what's the issue with synthetic oil?

The rotary engine injects oil into the combustion chamber as a part of normal operation, correct?

 

[kev99sl]

quote:

---

Originally posted by Hasbeen:
Some of you may remember me saying I was going to run synthetic oil in my RX/7 even though many said not to. Well, after putting 5,000 miles on the oil I changed it last weekend. The plugs were walnut blasted before I changed the oil as I wanted them to be real clean so I could tell how they would last with the synthetic as far as carbon build up. This car goes through plugs like I change my shorts, every 10,000 you HAVE to change them. Well, there was about 3,500 miles on them when I changed the oil to mobil 1 15W-50, When I took them out to look at them they were carboned up worse than they have ever been. I would have thought that because synthetics generally have a low % of ash, that they would have been cleaner than when I was running dello 400.

The car ran fine durring the 5,000 miles actually I think better than before, but It was starting to miss because of the carbon on the plugs towards the end.

The car is an 87 with 285,000 miles, maybe it`s time for new injectors and the carbon is from fuel.

---

I wasn't involved in the original discussion, so I apologize if this was brought up as part of it, but synthetic oil-makers themselves advise against using synths in rotary engines. From Mobil's site:

"Mobil 1 with SuperSyn Technology will not void new car warranties, with the exception of the Mazda rotary engine (Mazda does not recommend any synthetic motor oil)."

 

[Quest]

As I mentioned before: wankel rotary engine (esp. Mazda variants) require a bit of oil to come into contact with the apexes so as to lubricate them. Any oily smoke after the combustion process shall be processed by either a multi-stage catalytic conversion system or, for older Mazda RX-7s, a "post-burner" chamber is allocated so as to rid of any smokes thus making the emissions cleaner.

Knowing that synthetic lubricants burn at a much higher temperature, your observation RE: spark plugs carbon-fouled is of no surprise to me. This is because the combustion chamber isn't hot enough to burn off the sync lube exists for lubricating apexes.

You shall seriously consider mineral-based motor oil with certain degree of esters present. While I do not have the exact specs on this, I know HKS/RevSpec Japan actually carries a special mineral blend motor oil 15W40 specifically designed for Mazda rotary engine's use (contains esters of some sort/degree).

 

[Formyla]

How about a slightly hotter plug?

 

[Jelly]

What about Redline? Seems the polyol (organic) ester base stock with it's solvency and detergency advantages could negate some of the issues here...

 

[williar]

Had a '81 & a '91 (convertible) RX-7. Never had problems with plugs fouling using conventional 10w30 . Maybe it's the high mileage? I sold mine before 100K.

 

[darkdan]

How about switching back to conventional oil since it's bad to use synthetic in a rotary?

 

[mechtech]

How about making a fair comparison and using new plugs. You have compressed carbon into the insulators with the walnut shell method of cleaning. Yes, I know they look cleaner, though. Carbon conducts and will misfire/foul.

 

[Hasbeen]

quote:

---

Originally posted by mechtech:
How about making a fair comparison and using new plugs. You have compressed carbon into the insulators with the walnut shell method of cleaning. Yes, I know they look cleaner, though. Carbon conducts and will misfire/foul.

---

Well, that`s what I`m doing now, I used Amsoil 15W-40 Diesel Marine this time, and new plugs, I`ll check it again in about 5,000 miles and see what is happening. I was having very bad carboning over the years before I switched to the dello 400, the detergents in the diesel oil worked wonders. I`ll go back to it if i find the same issues again. I still had sparkplug fowling issues though. I should post a picture of an RX/7 spark plug, they are different looking and are a surface gap plug.

 

[Kestas]

Rotary engines are notorious for poor sealing. You've got to seal the sides as well as the apex (not to mention the corners), whereas the piston engine only needs to seal the round bore around the piston. I suspect the synthetic oil gets past all these nooks and crannies much like it escapes past leaky seals (or creates leaks).

Another problem that killed the Wankel was the large quench areas in the combustion chamber. This engine became popular just as the EPA was ramping up automotive emission requirements. These large quench areas resulted in incomplete fuel burn, so your thought on carbon coming from the fuel may be a good guess.