So I've been reading around and I guess if you use too low of an octane on your 8, it causes "engine knocking" (even says so in the manual from what I read)

I'm rather confused, how can you cause knocking in a rotary engine? there's nothing for the rotor to "knock" against, right? or does it just give the rotor so much momentum that it wears out the apex seals really fast? Very curious. thanks!

 

"Knocking" isn't someone rapping on the engine with their knuckles. It's a term that is used due to the sound it makes.

All gaseous mixtures heat up when compressed. The higher the engine's compression, the more the air/fuel mixture heats up under this compression, and the heat can be significant, especially when the combustion chamber's latent heat is helping the matter.

Knocking is when the air/fuel mixture has been heated past it's point of self ignition and the mixture ignites BEFORE the spark plug can ignite it at the appropriate point. The combustion is happening at the wrong point in the cycle and it "slaps" the combustion chamber, making a knocking sound. Rotary vs piston doesn't change this, same principle applies.

Higher octane increases the temperature at which the air/fuel mixture will self ignite, allowing higher compression engines.

The only way that 87 octane is safely usable in a Renesis is if all 6 faces of your engine have already loss compression due to a failing engine. A perfectly healthy Renesis can get destroyed rather quickly by using 87 octane. The ECU might be able to save it by pulling timing and adding lots of fuel, but it's a huge risk.

 

The manual says supreme, just use the highest octane when you fill it up, its better for your engine.

 

Right, I know the name comes from the sound it makes, but that's where I was confused. In a piston engine, it's the piston head slamming around super hard, and I was curious as to how you can have a rotor slam up against something since it turns in a circle and not up and down, but you answered the question. I can see how messing up the timing can destroy the engine rather fast.

btw, been reading a lot of your posts in the new member forum RIWWP, thanks so much for all the useful (and free!) information man! you're really helping us out a lot!

 

pretty much what riwwp said... the effects can be worse or non existent depending on the temperature of where you live, some will never hear it .. others will think a gorilla with a sledge somehow got in the engine.. either way.. cheap gas (low octane) will most likely result in expensive repairs

Posted From RX8Club.com Android App

 

Actually that isn't true. The piston rings still hold the piston against the sides of the cylinder walls equally. It isn't hitting anything. The sound is purely from the combustion wave hitting the piston and cylinder walls sharply with a slap, rather than the slower intended combustion which is a progressive push.

The severity of the knock can change the sounds from anywhere between an inaudible noise that is only detected by non-human sensors to a light pinging noise to an actual metal on metal banging.

 

ahhhh I see, I thought it was the piston slamming against the top of the cylinder harder than what's healthy for it just from too strong of a combustion. Good to know! thanks!

 

verry common mistake... ranks right up there with pineapples growing on trees.

Posted From RX8Club.com Android App

 

pineapples don't grow on trees? thanks everybody!

 

Can anyone tell me what rotary engine knock sounds like and under what speed/load points it actually happens? The combination of very rich open loop operation, low bmep in closed loop, and high engine speed lead me to believe that another noise might be mistaken for knock. Has anyone logged knock feedback on regular vs premium to put some merit to the theory that the engine can't handle regular?

 

In a piston engine, think of the compression cycle in the cylinder, which is an up stroke to compress the air/fuel mixture. If that air/fuel mixture detonates before the down stroke of the piston, think of the pressure placed on the connecting rods. That energy is trying to push down on the piston when mechanically it supposed to be moving up. If the problem is severe enough and given enough time, one or more rods could fail. I am assuming in a rotary when the detonation process occurs the only thing holding the air/fuel mixture is the seals. Again given enough time, you will damage them to the point where they will be useless. Not worth the risk to save a few bucks. Unless you own a decent sized boat or aircraft chances are you are not dropping thousands of dollars for one fill up. The cost of a engine is a pretty large expense to most people. So why people try to be cheap on fuel is beyond me. If you wanted a fuel efficient car, the 8 was not a wise choice.