Premix - is it worth it?

[nsiddiqui]

Hi All, newly taken ownership of a 2001 CLK320 Coupe and have moved over from owning RX8s for 7 years.

In the RX8 i used to premix with specific 2-stroke oil which burnt away cleanly but allowed extra lubrication as it was mixed with the fuel.

Is it worth premixing in the 112 engine? Is anyone doing it? or is it a big No-NO?

 

[martyz]

You have a proper engine now,just add fuel.

 

[markjay]

I know it's done on some Diesel engines, not perol, though in the case of the RX8 it might be a rotary engine thing...?

I would suggest to simply stick to Premium Fuels (Shell V-Power / BP Ultimate), of failing that use fuel additives (Redex, Forte, Millers etc).

 

[nsiddiqui]

Hah, I was expecting some flack for having previously had a rotary they aren't all that bad.

I always use premium fuel anyway, so I'll just stick with that for now then. It all seems to be working fine so as they say, if it works, why fix it. Just thought it might be worth asking the question to see if there was any benefit at all - it seems not.

 

[markjay]

The last time I had any exposure to rotary engines was when the NSU Ro-80 was introduced... and the demise of NSU as an independent car manufacturere soon followed.

So if their short-lived engines is anything to go by.... they could do with all the extra lubrication they can get...

I would have thought though that the RX7 (and later incarnation RX8) overcome these issues.

 

[nsiddiqui]

It's more of a precautionary measure than anything else really. Many, probably most, owners don't premix at all. The main reason I used to premix for when I was doing track days.

 

[Dave Barlow]

I own and ride '70s 2 stroke motorbikes that have pumps instead of premix. In the day, lads would disregard the oil pump as either unreliable or power sapping, it was neither, and premix the fuel.
Every millilitre of oil added meant a millilitre less fuel.
Ratios of 20/1 may seem minimal, but led to piston seizures, holed pistons, and depending on bike model, big end/crank failure, due to weak mixture.

 

[190]

Adding 2 stroke oil as as an upper cylinder lubricant is done with petrol as well as diesel. I've used it for years on an old motorcycle and whatever else may be claimed as a benefit I'm in no doubt that over along period of time it has cleaned up my combustion chambers and piston tops.

It has to be TCW-3 spec 2 stroke oil which is ashless but even then I'm not sure I'd add it to a car with a cat.

 

[nsiddiqui]

Well the RX8 has a cat and it worked fine there. Of course, we were very specific about the oil used to ensure it was mineral 2 stroke oil so it was combustable rather than end up with a coked up chamber. The opposite effect of what we were looking for.

As for mixture, it wouldn't be any more than 120ml per full tank of fuel

 

[190]

I use the commonly published ratio of 1 part oil to 640 parts petrol. I hadn't heard that it matters if the oil is mineral or synthetic although the oil I use is mineral. The TC-W3 spec. is what make it burn without deposits. It's intended for marine 2 stroke engines and contains none of the metallic additive compounds commonly found in other oils. I guess that would make it kinder to a cat.

 

[Bellow]

Any oil added to petrol will lower the petrol's octane rating.
Oil will not vapourise as effectively as fuel meaning effectively a weaker A/F mixture during the main combustion event.

Now study how the ECU reacts to lower octane fuel and a slower burning mixture.

 

[nsiddiqui]

i'm no expert on octane ratings etc. but i wonder if i fill 97RON fuel and add 120ml in approx 55l when i fill up, then how much would it reduce the octane rating? would it be significant enough to make a difference to the ECU?

This is hypothetical for now as i'm not convinced it is needed in this engine as compared to my previous rotary.

it's also interesting how on rotary forums its always a question of how much and which premix to use and not how it lowers the octane rating of fuel etc. like we are moving into here. of course, two different types of engines and all.

 

[boxbrownie]

If your not interested in wearing the same new outfit as the King wore, just fill up at Asda/Morrisons/Tesco etc

 

[MancMike]

Hi, OP.

I used to have an RX8 too, and yes premixing two stroke really helped the smoothness of the engine and undoubtedly increased engine lifetime and raised compression slightly.

I've only ever done it once in a piston engine, just to use the last of my 2-stoke. It did pretty much the same as in the rotary, just made the engine feel/sound slightly smoother. I don't think it's needed in a piston engine at all, so wouldn't bother, but you could give it a go.

 

[nsiddiqui]

another ex-8 owner, small world.

I did only really ever ask this question as i have a full 1L bottle of premix sitting at home and wondered if i could use it here - though opinion is firmly against the idea as it isn't needed.

i shall have to find another home for that unopened bottle of royal purple premix

 

[190]

Any oil added to petrol will lower the petrol's octane rating.

I know that's true in theory but the reason I first tried dosing with 2 stroke oil in my old motorcycle is because I'd read that it can reduce pinging which appears to be a contrary effect to lowering octane. My experience over several years of use is that it does indeed eliminate mild pinging. I don't have a explanation for that other than it must somehow slow down the combustion process. Could that also explain reports of increased smoothness.

No ECU's (thankfully) in 1979 just points ignition and centrifugal ignition advance in an engine originally designed to run on higher octane leaded fuel.

 

[Bellow]

I know that's true in theory but the reason I first tried dosing with 2 stroke oil in my old motorcycle is because I'd read that it can reduce pinging which appears to be a contrary effect to lowering octane. My experience over several years of use is that it does indeed eliminate mild pinging. I don't have a explanation for that other than it must somehow slow down the combustion process. .

Lower octane fuel will always detonate before higher octane - there is no controversy in that. And fast combustion (not slow) defeats detonation (by burning the mixture before it has the opportunity to detonate (due to advancing pressure wave and radiated heat)).
If, you experienced reduced detonation then it was due to some secondary, peripheral effect. Unburnt oil cooling the exhaust valve as the gas exited the cylinder being a potential reason or the piston running cooler due to reduced friction due to the additional oil.