13b N/a ?....

[metalhead]

I haven\'t had any trouble accessing ausrotary.

Omp is in my mind superior to premix, although the setup as in the link above, which adapts the omp to suck fresh 2 stroke instead of sump oil is better again.  A couple of things to add to the debate:

When decelerating the omp still injects oil.  On the other hand if no fuel is entering the engine, with premix no oil is entering the combustion chamber, and the seals are exposed to wear.  Also, premix ratios mean more oil is entering the engine, and oil lowers the octane rating of fuel.  This is not a major issue on n/a rotaries, but on cars with forced induction can lead to pinging/detonation.

The omp does need to be checked reasonably regularly to ensure it is working, but I have not had any issue in my cars over several years of having rotary daily drivers, and it saves the hassle of carrying around 2 stroke and trying to explain to anyone who may borrow your car.  Also, as I am now running lpg, I can\'t premix but I\'ll still be running the omp on any petrol fueled rotaries I may own in the future.

[Dominic]

Guess you\'d save quite a few $$£££££\'s in oil costs over time by running the OMP too cos it is a more efficient way of lubricating the chambers using less oil per engine revolution than pre-mix (most of pre-mix oil is burnt with the fuel)

[mullins]

The problem with an OMP is that it injects the oil pipes which take time to reach the combustion chamber where-as premix is instant. An OMP will never match premix. If you give your car a good fang and then slam on the anchors for a set of lights you car will run rich until the supply via the OMP slows down. If you run premix you run the correct ratio 100% of the time and the OMP just can\'t match that! The other big advantage of premix is you can use an oil which is designed to burn and avoid carbon buildup. Premix wins hands down imo.

[oldone]

The problem with an OMP is that it injects the oil pipes which take time to reach the combustion chamber where-as premix is instant. An OMP will never match premix. If you give your car a good fang and then slam on the anchors for a set of lights you car will run rich until the supply via the OMP slows down. If you run premix you run the correct ratio 100% of the time and the OMP just can\'t match that! The other big advantage of premix is you can use an oil which is designed to burn and avoid carbon buildup. Premix wins hands down imo.

I can relate to that fact...:cheers  That is why we used premix in the race cars and not an omp......... poor response out of low rpm corners and long sweeping on\\off sweeps with the omp etc...:guinness

[Dominic]

The problem with an OMP is that it injects the oil pipes which take time to reach the combustion chamber where-as premix is instant. An OMP will never match premix. If you give your car a good fang and then slam on the anchors for a set of lights you car will run rich until the supply via the OMP slows down. If you run premix you run the correct ratio 100% of the time and the OMP just can\'t match that! The other big advantage of premix is you can use an oil which is designed to burn and avoid carbon buildup. Premix wins hands down imo.

I’ve still got much to learn about the rotary engine so please don’t flame me if my spiel below is a load of codswallop :Hammer . Thought I would add my thoughts to the debate anyhow.

I can understand the OMP lag issue but can’t see this being significant or affecting the longevity of the engine. The OMP lines are constantly full with oil (even at low rpm\'s), and the oil in the lines is incompressible, so the delay in getting oil into the chambers when you hit the go pedal will be very very small. I think it would be fair to assume that there would be enough residual oil in the chambers to cope with this very small period of oil starvation. Appreciate that throttle controlled oil injection (i.e. premix) will be more responsive than rpm controlled oil injection (i.e. OMP) due to the fact that throttle position is the precursor to rpm change but surely we only want oil in the chambers when rpm’s rise, not when the throttle butterflies open up.

Mullins, agreed…. with premix you can run a good oil, but I was referring to the link to ausrotary.com that I posted earlier, this discusses the use of a standalone oil tank which the OMP draws from so you can use one type of oil in the sump and another clean burning oil in the combustion chambers.

Not trying to flame pre-mix, just trying to weigh up the two (well.... now 3 options!) i.e.

1) Pre-mix using nice clean oil in the fuel
2) OMP sucking dirty sump oil
3) Modified OMP fed by standalone header tank filled with nice clean oil

Not sure whether I agree with the statement that pre-mix runs the correct ratio 100% of the time, in fact I think there are cases where this could be the opposite. E.g. for your example.... If you blast the car then drop down off the revs approaching lights, as the car is decelerating, with pre-mix, very little oil will be entering the chambers as the throttle will be almost closed....... combustion in the chambers will still be occurring however, therefore during the slow down/coast period, the engine wont be getting much oil. With OMP, as the car comes down off the throttle, even though the throttle is closed the OMP will still inject oil into the chambers because the OMP pump is governed by rpm not throttle position, therefore the engine’s lubricating demands are still met during this period. Surely it\'s more important to govern oil injection on rpm rather than throttle position. I guess this depends heavily on how the oil is injected…..  my comment above may only be correct if oil is injected from the OMP directly into the chambers…… probably wouldn’t be correct though if injected into carb as lack of vacuum would prevent the oil being drawn into the chambers.

As said before, not trying to say which is best... just putting two sides to the argument. TBH though, I think the OMP system has many benefits over pre-mix (although you gotta love the pre-mix smell), such as:

- Less oil used (is this correct???)
- Less complicated operation (fewer idiots like me juggling oil bottles on the gas station forecourt)
- If option 3) of a modified OMP fed by a standalone header tank is used then different oil types can be used in sump and chambers (e.g. synthetic/mineral in sump, nice clean 2 stroke oil in chambers), therefore sharing one of the main benefits of running pre-mix, i.e. not sucking up grim contaminated sump oil for use in the rotor chambers
- RPM controlled oil injection (surely this is preferable to throttle position controlled oil injection?)
- I’m sure there’s another one but I forgot it!

The drawback of the OMP system is the OMP pump can fail, but the units seem very reliable.

Right, off to get my flameproof coat :chat

[mullins]

^ No flame required, good questions/arguements.

Even if the OMP is used the oil is still injected & mixed (with fuel) via the carb so a lot of the "advantages" in your post aren\'t really so (de-acceleration senerio etc).

Although the flow of oil maybe constant if you have a rush of fuel/revs you need the increased oil flow to match the fuel to keep the correct ratio. The increased oil flow has to travel from the bottom of the engine (just above the sump) to the top of the engine almost instantly and physically this isn\'t possible. With premix it\'s already there.

Hope this makes sense. Usual disclaimer applies!:WaveBye

[oldone]

On a N\\A high performance motor in racing conditions or a cheecky road going brapper,you will commit motor suicide if you go with the omp,on a sedate road going motor and driving like a nun,the omp is perfect !!!!:D :cool Remember the omp is gear driven and runs at a given ratio\\ask deliverance,premix is available at any given chosen ratio and imm at that,rather have too much oil on overrun than too little.........:Typing

[alban1973]

I would just like to add, that my car did nearly 30MPG on its last scotland trip. unknow engine mileage and turbo charged, I was not hanging around either...that runs an 1.75 SU, also, the car was actually listed when i rebuilt the carb with parts from Burlan Fuel supplies. SU\'s maybe simple, but they work very well when you know how to set them correctly.   You have to have the needles set at the correct angle, the float level smack on, correct measured seat depth, correct oil, and level. then you can set the mixture and tickover......

[dubber91]

Not sure whether I agree with the statement that pre-mix runs the correct ratio 100% of the time, in fact I think there are cases where this could be the opposite. E.g. for your example.... If you blast the car then drop down off the revs approaching lights, as the car is decelerating, with pre-mix, very little oil will be entering the chambers as the throttle will be almost closed....... combustion in the chambers will still be occurring however, therefore during the slow down/coast period, the engine wont be getting much oil. With OMP, as the car comes down off the throttle, even though the throttle is closed the OMP will still inject oil into the chambers because the OMP pump is governed by rpm not throttle position, therefore the engine’s lubricating demands are still met during this period

if combustion still occurrs...there must be fuel.....if theres fuel theres oil. there is also residual oil from previous induced strokes.  The retardation of ignition timing increases unburned fuels, thus, \'unused\' oil. I think in reality there is very little practical difference in this scenario

Steve

[metalhead]

Not sure whether I agree with the statement that pre-mix runs the correct ratio 100% of the time, in fact I think there are cases where this could be the opposite. E.g. for your example.... If you blast the car then drop down off the revs approaching lights, as the car is decelerating, with pre-mix, very little oil will be entering the chambers as the throttle will be almost closed....... combustion in the chambers will still be occurring however, therefore during the slow down/coast period, the engine wont be getting much oil. With OMP, as the car comes down off the throttle, even though the throttle is closed the OMP will still inject oil into the chambers because the OMP pump is governed by rpm not throttle position, therefore the engine’s lubricating demands are still met during this period

if combustion still occurrs...there must be fuel.....if theres fuel theres oil. there is also residual oil from previous induced strokes.  The retardation of ignition timing increases unburned fuels, thus, \'unused\' oil. I think in reality there is very little practical difference in this scenario

Steve

In a carbed car it is true that a (very) small amount of fuel continues to enter the engine on overrun, however most injection setups cut all fuel until the engine falls below a give rpm (for example 2000rpm) as a means of saving fuel.  Blast the engine up to 8 or 9 thousand rpm, get off the gas and no fuel and (if running premix) no oil enters the engine at all until it falls below a set rpm.  

Also, I find it interesting someone states above that you can vary the amount of oil using premix, but not using the omp?  It was my understanding (though I admit to not having done it myself) that the omp can be adjusted to vary the amount of oil it squirts into the chamber.  

Finally, if a car is not used regularly it is not at all unheard of for the fuel and oil to seperate in the tank, in Australia most of the racing cars that run premix drain their tanks after every meeting and mix up a fresh batch of premix at the next meeting.

There is no doubt that premixing has its benefits and that the omp has its downfalls, the main one being complexity.  Personally though, I feel that a well maintained omp, ideally using an adaptor such as the rotary aviation one so that it draws fresh 2 stroke rather than sump oil, is the best compromise, for me, for the street.

[dubber91]

sorry, i was refering to carbed motors...

premix all the way for me.

steve

[mullins]

Someones started another topic on this if anyone is interested in more debate
http://www.mazdarotaryclub.com/forums/showthread.php?t=33006

[mullins]

Also, I find it interesting someone states above that you can vary the amount of oil using premix, but not using the omp?  It was my understanding (though I admit to not having done it myself) that the omp can be adjusted to vary the amount of oil it squirts into the chamber.

It can be adjusted by shifting the split pin up or down the rod but it\'s a fiddly job. Much easier just to throw more oil in the tank.

[Dominic]

Someones started another topic on this if anyone is interested in more debate
http://www.mazdarotaryclub.com/forums/showthread.php?t=33006

He, he, he, LOL

Just put up the link to this thread on that thread :Giggle

Our thread\'s better

[mullins]

:Giggle It\'s getting outta control....:rollin :chat