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premix on a rex, im just trying to understand

1st Generation Specific (1979-1985) 1979-1985 Discussion including performance modifications and technical support sections

premix on a rex, im just trying to understand

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Old 09-21-03, 06:07 PM
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premix on a rex, im just trying to understand

ok i did a search and im pretty cofuse right now about the premix deal. i was reading and somepeople use regular engine oil and some people use 2 stroke oil, with one is better? and about the mix, how many ounces you have to really put in the tank? because some people was puting 4 ouces and some about a quart. and what ratio is, i read that is 100:1? sorry im dont understand. why im asking because im gonna eliminate the OMP on my next engine so i have to do premix. thanks
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Old 09-21-03, 06:27 PM
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You don't need to premixes on Rexes. Rexes are 660cc Japan-market Subarus and are not to be confused with RX-7s or any other rotary powered car.

To answer your question as it pertains to rotary engines, you do not want to use motor oil! I've done that in the past but it was because I had nothing else to throw in there. Probably no oil at all is better than using motor oil... motor oil does not burn cleanly. 2-cycle oil does burn cleanly, after all that is what it was designed to do, and it lubricates well when heavily diluted by gasoline, which is also what it was designed to do, and it lubricates well when that fuel/oil mixture is atomized in air, which - surprise! - is what it was designed to do.

Get the picture?

The stock OMP on carbureted cars injected oil (well, dribbled oil down the carb) at a rate of 1:150 to 1:400 oil/fuel, depending on running conditions. When you premix, you cannot vary the strength, so you mix for the worst case scenario, which is roughly 1:150.

Now. The more you stress the engine, the more lubrication you need, so you go a bit richer than that. On the other hand, as noted above the 2-cycle oil has much better properties than regular ol' motor oil for the purposes we care about.

What it boils down to is this... 2-cycle oil comes in convenient 8-ounce and 16-ounce bottles. If you have an SA you can use the 8-ounce bottles because the fuel tank holds about 10 gallons before you are on empty (with 2 or 3 gallon reserve). That's about 1:160 or so. If you have an '81-85 you can use the 16-ounce bottles because you are on empty a fill-up will give you about 16 gallons or more, this is closer to 1:120 which is a bit rich but since it burns cleaner it isn't gonna hurt much.

Also, you always put the oil in *before* the fuel. That way when you fill the tank up, the sloshing and flowing around in there helps mix the oil in with the fuel.
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Old 09-21-03, 06:40 PM
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Nice! I was just going to ask if you are premixing in a can first and then adding to the tank or if you're adding the oil right into the gas tank and filling up.
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Old 09-21-03, 07:23 PM
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Originally posted by peejay

Also, you always put the oil in *before* the fuel. That way when you fill the tank up, the sloshing and flowing around in there helps mix the oil in with the fuel.
Oops, I've been putting it in after I fill up. Guess I'll have to switch around to the othre way. Don't know why I didn't ever think of putting it in first.
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Old 09-21-03, 07:32 PM
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I use 1 oz of MMO per 1 gallon of gas.
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Old 09-21-03, 07:57 PM
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I use 1.5 oz per 1 gallon of gas. Of course my car is used primarily for AutoXing, so it sees lots of hard use. I use Valvoline high RPM 2 stroke oil.
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Old 09-21-03, 08:15 PM
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For getting desired ratio here is a little formula I made:

128 x gal of gas ratio desired = oz of oil to add to fuel

I run 180:1 but am thinking of changing it to 150:1 like peejay said to do.
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Old 09-21-03, 08:31 PM
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It isn't rocket science. Just get the most convenient sized bottle and dump the whole thing in when you fill up.
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Old 09-21-03, 08:32 PM
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for my car that is a gslse i have to use 16 ounces of 2 cicle oil, hmm now i understand. thanks
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Old 09-21-03, 09:09 PM
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I noticed that Amsoil makes synthetic 2 stroke oil. I've never heard of synthetic 2 stroke oil before. Does anyone know if it's any better than the normal stuff?
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Old 09-21-03, 10:14 PM
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how does one remove/disable the OMP?
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Old 09-21-03, 10:41 PM
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Originally posted by mmasid
I noticed that Amsoil makes synthetic 2 stroke oil. I've never heard of synthetic 2 stroke oil before. Does anyone know if it's any better than the normal stuff?
I was using the Universal 2-stroke at Wal-mart until the other day I found a 16oz bottle that was cheaper than two 8oz walmart bottles. It is TC-W3 or something like that. I know it doesn't answer your question but at 1.07 for 16oz, I thought it was a pretty good buy.

Here are a couple of paragraphs out of HPBooks "How to Modify Your Mazda RX-7" which IMHO everyone should have on their shelves who owns an FB.

In some competition applicatoins, it may be necessary or desirable to remove the metering oil pump. If this is done, oil must be mixed with the gas. The resulting premix should be in a ratio of between 100:1 and 160:1. With the rpm levels and engine loads inherent in Camel Light racing, Downing/Atlanta race engines are usually fed a diet of 100:1 premix, which is approximately 6-1/2 ounces of oil per five gallons of fuel. Such a ratio is not appropriate for race engines that are not as highly stressed.

Street engines fed an oil-rich premix will experience an increase in deposits on sparkplug electrodes and possible fouling. For street driving and autocrossing, oil should be administered through the metering pump. (Keep in mind that increasing the percentage of oil in the premix reduces fuel octane.) Premixing is suggested only for endurance-type racing where oil-pump failure could prove catastrophic due to engine operating speed and load.

Note that the oil-metering pump is capable of providing a minimum (oil-rich) fuel-to-oil ratio of about 150:1. Premixing is therefore required if it's necessary to run a richer oil mixture, or up to 100:1.

Jim Mederer says to use a synthetic oil for premix because mineral oils settle out of fuel if it (the mixture) sits for very long. Best choice is a "two-stroke" synthetic which is a good lubricant and it stays in suspension. However, Rick Engman uses mineral oil exclusively and is totally pleased with the results. He states, "Synthetic oil doesn't burn, so it will not have the same affect on reducing fuel octane as does mineral oil. Synthetics also tend to leave the sparkplugs with more of a residue, thus leading to fouling. We get excellent life out of our seals and we don't have a problem using mineral oil in our premix. Maybe I should rephrase that -- if we do have a problem, I don't know about it."
The reason they mention octane is that they were running the lowest octane they could find in their Camel Light race-prepped engine. With running premix they found that the octane went down by minimum of another point (ie 87 dropped to 86) while everyone else in the beginning was running higher octane.

BTW, my gas doesn't stay in the tank long enough to settle out of suspension and I haven't had a problem yet using mineral based oil.
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Old 09-30-03, 02:44 PM
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All I did to stop the omp from pumping oil, was took it off and riped out everything that would come out. Then put it back on and ran a one small tube from one nipple to the other nipple. Still leaks slightly, not noticable on the oil level, but is noticable on the ground if I park in the same spot.

I have heard of people running tubes from the nipples to a "T" connector to a sinlgle tube and then running it back into the engine from the breather on the side of where you pour oil into the engine.

Iam sure their are many different ways to accomplish the same thing.
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Old 09-30-03, 03:51 PM
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Is the an OMP on the Fuel injected cars, or what do fuel injected rotaries use?
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Old 09-30-03, 04:15 PM
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Can someone do a test for me? I would like to see how 2-stroke oil and gas mix.


Put the 2-stroke oil in a transparent bottle/jar and then fill it up with fuel at the right ratio and close the bottle.

See how well it mixes if you don't shake it. Then shake it and watch if after a while(a day or even a week) if the oil and gas are still mixed.


This would give us some real answers.
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Old 09-30-03, 05:15 PM
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All Mazda rotaries have oil metering pumps.

Disable? Mine just wasn't working.

Best way is to get a block off plate and just remove it.
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Old 09-30-03, 06:22 PM
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im kinda new to the world of rx-7s, but what is an oil metering pump(OMP) and what does it do?
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Old 09-30-03, 07:41 PM
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Thumbs up I'm with premix!

You guys need to check out this thread on the great "premix" debate:

OMP/Premix writeup

I think it will answer all your questions and give you a few more!

Definitely stick with TC-W3 2-stroke oil. It mixes very well with gas--no need to worry about how well it mixes.

I add mine in about half way during a fill-up.

The TII engine I'm putting in my car will be premix all the way with no OMP--but to each his own.

Either way I think you're OK. With my current FI SE engine I occassionally premix but don't have to. With the TII engine I will.
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Old 09-30-03, 07:48 PM
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Originally posted by peejay
All Mazda rotaries have oil metering pumps.

Disable? Mine just wasn't working.

Best way is to get a block off plate and just remove it.
Nor mine. At least I have never tested it, but I have also never noticed any oil in the tubes or my oil level drop any across 1000 miles. So I pre-mix for safety and piece of mind besides the 2-stroke being cleaner too.
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Old 09-30-03, 11:06 PM
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ok, but heres a question for ya, there are diffrent ratios on bottles of 2 stroke... 100:1, 50:1...which am i supposed to use? Or am I just going crazy?
Also, im guessing synthetic two stroke would be best for a TII because of the octane rating? Wouldnt you want to preserve the octane rating in a Forced air car?
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Old 10-01-03, 08:51 AM
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Originally posted by oni-dori
im kinda new to the world of rx-7s, but what is an oil metering pump(OMP) and what does it do?
Rotary engines have no way to get oil to the apex seals at the tips of the rotors as the ride around the trichoid of the outer housing. So the answer was to inject small amounts of oil into the fuel/air mixture a la a 2 cycle and that would keep the seals nice and lubed.

The oil metering pump is the mechanism by which that was accomplished so owners did not have to dump oil into their gas (and hence feel like they owned a 2 cycle engined car). It works OK except for a couple problems. First, they leak, and leak, and leak and leak. The second is that it needs to be checked and adjusted (who bothers with that - what a pain). The final, and probably most serious problem is that they would fail and the owner would have no way of knowing it - the end result being no oil at the apex seal as they are dragged across the outer housings at a high rate.

I am sure there is more that one could say but that is the basic gist of it.
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Old 10-01-03, 11:33 PM
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Originally posted by CheddaKing
ok, but heres a question for ya, there are diffrent ratios on bottles of 2 stroke... 100:1, 50:1...which am i supposed to use? Or am I just going crazy?
Also, im guessing synthetic two stroke would be best for a TII because of the octane rating? Wouldnt you want to preserve the octane rating in a Forced air car?
Yup, with forced induction, you'd want to preserve the octane rating so the synthetic 2-stroke would be better though in my previous post they mention the possibility of synthetic 2-stroke tending to foul out the plugs more often.

As far as mixture, if you keep with the minimum of 150 (I am running just slightly under 120 I think on my NA 12a) you would mix in 12.8 oz of two-stroke for 15 gallons of gas or 0.85oz per gallon.
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Old 10-02-03, 07:43 AM
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Regarding synth deposites on plugs, inspecting and cleaning plugs should be part of very routine mainatainance on a forced induction rotary, IMO. You should be paying close attention to things like cracked insulators, ect.

Peejay- Are there any crank oil additives that will help the crank oil be more conducive to being used as an IC lubricant?

It'd be great if you could put 2 cycle in the crank!(I know it's not formulated to do this.) But is'nt there a way you could get the best of both worlds? Are there any other funky crank oils out there that might still do their job as a crank oil, but allow mixing with 2 cycle?

Also, I'm prepping an Rx-4 carb for my Rx-7, and the oil inlet lines are fed directly into the bowls. Pros/cons as compared to Rx-7 oil "dribble" injection?

Picked up a copy of the Rx-3 Haynes, and the OMP output is the same as for the Rx-7, BTW.
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Old 10-02-03, 01:11 PM
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Originally posted by Sterling
Also, I'm prepping an Rx-4 carb for my Rx-7, and the oil inlet lines are fed directly into the bowls. Pros/cons as compared to Rx-7 oil "dribble" injection?

Picked up a copy of the Rx-3 Haynes, and the OMP output is the same as for the Rx-7, BTW.
As a pro, the gas and oil would be mixed better and should provide better atomization at the inlet to the rotor.

Cons? Bowls cannot hold as much gas? Are the bowls calibrated then for the added weight of the oil too?

Are you also swapping in the RX-4 intake? One of the 12a intakes during the 1970's had larger ports than the 12a in the RX-7's. The ports were shrunk for emmisions reasons.
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Old 12-04-03, 04:17 PM
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i use mine as a daily driver...as most do... it's an FB what is the OMP? i haven't mixing two stroke with my gas.....uhm is this a bad thing? so all you do is buy two stroke oil for the FB it's about 8oz.? add it in before you fill up and you're done? i'm so confused!!! i was fine until i stumbled into this site, joined, learned a few things....and now i'm lost this whole oil thing is a big deal? sombody just give me a simple answer in lamons terms PLEASE! ~signed rotofire (a noob but workin on it)
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