Q: What is pre-mix/pre-mixing?
The rotary engine normally uses engine motor oil injected into the combustion chambers for internal lubrication of the rotor housings and the rotor seals. Engine motor oil is not designed for this purpose. Pre-mixing (through the fuel injection system) allows for a superior method of internal lubrication, both in design of the pre-mix itself and in its delivery to the combustion chambers.
Pre-mix fluids are typically measured and poured into the gas tank. The pre-mix then mixes with the fuel which is eventually delivered into the engine via the stock fuel system. Most high-quality pre-mix brands stay in suspension in fuel in the gas tank even after sitting for months. Check to see if the pre-mix brand has the "TC-W3" certification, which will ensure good fuel suspension.
Let's look at the fluids themselves... Engine motor oil is designed strictly as an internal engine lubricant for metal-to-metal contact. Mazda designed the oil injection system to make life simple; it does the job nicely, and it's amazing it can pass emission tests! Burning engine motor oil does not make for clean emissions. Pre-mix is designed to be burned in the combustion chamber, and it is design to lubricate internal engine parts during combustion. Pre-mix burns very cleanly compared to burning engine motor oil; I wouldn't be surprised emissions is decreased.
An obvious by-product of the cleaner burning pre-mix is very clean spark plugs. Tearing down engines running solely on pre-mix awards you with very clean internal parts; the spark plugs and rotor faces are barely covered with a fine layer of soot. Stock oil injection systems injecting engine motor oil into the combustion chambers tend to cake the spark plugs and rotor faces with lots of crusty carbon deposits. These crusty carbon deposits tend to jam seals in some cases. In extreme cases, piece of carbon chip off and score housings and possibly cause apex seal failure!
Now let's look at the difference in delivery... Stock oil injection uses oil injectors that inject engine motor oil through a tiny hole in the rotor housing (primaries) and in the lower intake manifold (secondaries). The oil is then spread out by the apex seals wiping by - keep in mind, this is not a "spray" of oil; the oil injectors are not under very heavy (injection) pressure. Pre-mix is typically mixed in with the fuel, so it's injected under heavy pressure with the fuel in a stock fuel injection system; this gives better dispersion in the combustion chamber.
Even after going through the combustion cycle, the pre-mix is still designed to lubricate the internal engine parts. This is a big advantage over engine motor oil! The combustion by-products of pre-mix actually leave a "lubricant" on the internal engine surfaces. This will be important later on in this text.
The Zenki FC3S uses a mechanical oil injection system that is dependent on throttle position; the mechanical OMP injects more oil into the engine the more the throttle is opened. The Kouki FC3S uses an electronic oil metering pump that is computer controlled by the stock ECU; we do know that this electronic OMP is controlled by a 4-stage stepper motor, so it has 4 levels of oil injection. Unfortunately, we don't know how the oil injection is programmed into the stock ECU for the Kouki FC3S. With the pre-mix, the more fuel shot into the engine, the more pre-mix in the combustion chambers. This works very well in terms of engine load, as you get the highest amount of pre-mix into the engine at the highest engine load - max boost, high RPM's, and at WOT. One draw-back about the pre-mix system is that the stock ECU shuts off the fuel injectors on decel or anytime you lift off the gas pedal and RPM's are dropping to idle speeds. This can be of a concern, as the stock OMP systems are constantly injecting oil all the time, but the pre-mix through the fuel injectors is not. This is where the before-mentioned pre-mix combustion by-product lubrication comes in - with lubrication already laid down from previous combustion cycles (especially under a heavy WOT run at high RPM's), there's very little worry about proper combustion lubrication.
If you do decide to pre-mix, you have the option to remove all the stock oil injection components. The oil injectors are 10mm x 1.00 metric threads, so the appropriate sized bolts can be used to plugs up the holes after removal of the oil injectors. The oil injection lines can all be removed. The vacuum lines (and vacuum splitter) for the oil injectors can be removed, and vacuum fitting plugged on upper intake manifold. And finally, the OMP itself can be removed and blocked off. An OMP block-off plate can be fashioned from a piece of sheet metal. A proper removal of the internal OMP gear would require removal of the front oil cover, but this is too involved for most people. I've heard a pair of U.S. dimes can be inserted into the hole to keep the OMP gear from sliding around inside the front cover before covering everything up with a block-off plate; if this didn't make any sense to you now, it'll be obvious when you take everything apart. One of the big advantages to going with pre-mix is that this allows you to use synthetic engine motor oil inside the "crankcase". Synth oils would normally leave nasty deposits inside your engine (i.e. rotor housings) due to the higher "flash point" (vaporization temperature); we've seen Amsoil 20W50 specifically leave patches of deposits next to the spark plugs holes cause it doesn't burn very well; these deposits cause the apex seal to chatter over them, which causes havoc with sealing and compression between the rotor faces. With the synth oil burning problem out of the way, synth oil in the oil pan does with it does best - just lubricate the engine.
So we're in heavily into this pre-mix - how much do you use? The recommended dosage for street vehicles is about 100:1 to 150:1 ratios. Race cars run a heavier ratio anywhere from 50:1 to 75:1. All these numbers are a bit hard to calculate on the fly. Most pre-mix containers come in convenient quart sized bottles. The easiest way to to use this is to dump half the quart bottle into the gas tank on every fill-up. Half a quart is 16 oz. Your normal gas fill-up (for a 16 gallon max capacity) is about 12 - 14 gallons of fuel. This is right around the 100:1 ratio mark; some might argue it's a bit too "rich" for pre-mix, but I have never had any problems with smoking from the exhaust. Overly "rich" pre-mix ratios will cause exhaust smoking. As a side note, you can run pre-mix with your existing stock oil injection as a pre-caution; these ratios are recommended at around 400:1.
So now you have all of this knowledge of pre-mix - what are some good brands of pre-mix you can buy? On the high-end of the market, there are a number of synthetic pre-mix lubricants by Redline and Royal Purple. You can also check your local dirt motorcycle dealer, as they carry some very pricey stuff used on 2-stroke dirt bikes mades from Klotz and Golden Spectro. My recommendation is that anything with a "TC-W3" certification should be fine. I personally like Valvoline "Multi-Use" pre-mix in the dark blue bottle. These are readily available from your local Kragen/Schucks/Checkers and other big national chain auto parts stores for a little over $2/quart or $7/gallon. Oddly enough, Pep Boys does not carry this Valvoline brand of pre-mix. Another easily available TC-W3 certified pre-mix brands is Quaker Stake (Wal*Mart).