Greetings,

I no longer have an RX-7, but I do fly an experimental plane with a 13B engine. In the airplane world, we almost all premix 1 oz per gallon, and one of the side effects is that it causes the level in the sump to increase over time. One person just reported that he drained a quart of "extra" oil after 50 hours of flight time, and I'd say that's probably about normal for all of us.

As I understand it, the premix oil gets past the side seals, and gets scavenged back to the sump. I searched the archives some, and can't find anyone mentioning this phenomenon in the car. Does it happen in the car, or just to us in the planes? We do operate much differently from the average street vehicle, so it's quite possible that you just won't see it in the car. Understand that we run wide open throttle at 70-100 % power (depending on altitude) for literally hours at a time. Not something you can do in a car :-)

Just curious.

Cheers,
Rusty

 

Do you guys use the stock oil injection system as well as premix, or just premix?

I'm running a NA 13B with just premix, and I gain oil as well. I'd guess it's in the line of a quart every 2000-3000 miles, which is probably about right (I do mostly highway driving, so that's around 50 hours between changes).

I'm not sure what terms to search for (isn't it frustrating, having to know what exactly to search for to find it?), but I know that many people have reported increasing oil levels with premix. It's exactly as you speculate, the premix doesn't fully burn, and blows past the side seals & oil seals over time.

Damn... turbo 13B powered RV-3... *drool* I'd love to see a turbo rotary powered Long-EZ or such. <3 Rutan's designs.

-=Russ=-

 

Thanks Russ. I've heard from a couple car guys now, so I guess it happens to all of us that use premix. One guy speculated that more oil got past the side seals during low power operation, so it may even be worse for street cars.

As for the airplane engine, I just changed my signature. I had a stock S5 turbo on the plane for the first 20 hours or so, but there was no way to control it with the stock wastegate and no muffler. I ultimately decided to remove the turbo, and change to a higher gear ratio gear drive. This is a lot lighter, and I can still get about 200 hp, which is plenty for an RV-3. I'm glad I removed the turbo, because there have been 3 blown stock turbos in the last 6-9 months on airplanes. In each case, the turbine wheel snapped off at the shaft. Fortunately, they were still making enough power to get home safely.

There are a number of those backwards plastic planes (RV joke) you mentioned, including two currently flying turbo Cozy's. I've always loved the canards, but can't tolerate that much fiberglass.

Thanks,
Rusty

 

Are you sure its not fuel dilution into the oil?

 

I premix as well, its just fuel getting into the oil. Normally its just burned off so you dont notice, but when you premix nothing is just burned off so the blowby makes the oil level rise

 

Sorry, but you're not going to convince me this is fuel in the oil. Why would fuel remain in the oil when my normal oil temp is 190-200? Why would the amount of "fuel" that's getting into the oil via blowby change in proportion to the pre-mix ratio (lots of guys have varied to premix ratio and noticed the difference in scavenged oil accumulation)?

I've pretty much accepted the fact that the oil level is going to rise, so I just need to start about a quart below full, then change the oil when it hits the full mark. That should be about 50 hours.

Cheers,
Rusty

 

You could have your oil tested to verify that it is not actually fuel in your oil.
The reason I suggest this is that oil dilution from blowby fuel is already a problem for rotaries.
Premix oil is far more viscous than gasoline: why would oil blow through much more readily than gasoline?
Approximately how much fuel are you going through in 50 hours of use?

Why are you so ready to dismiss that its fuel? Why ask a question only to dismiss the answers?