Thursday, March 13, 2008

2 stroke oil

What kind of gas wont hurt my 70cc engine and works best?

I have heard a few good arguments that most small two stroke engines aren't designed to very high specifications and physically can't take advantage of higher octane fuel. I have used different octanes in multiple engines and consistently not noticed any difference.
What I have noticed is fuel from different companies perform differently (I acquired a good/bad station list when I first started driving.)
Speedway consistantly sucks (but their diesel seems okay)
Amoco/BP is good
Shell has always treated the car really well.
Sunoco is always good too (plus they've got lot-o-options on the octane levels)
Marathon has teetered on the fence.

I do practice using, exclusively, either high end or low end though, in hopes of getting fresher fuel. The middle isn't used as often I think (given the stations location) and I've heard it is common practice to mix the 87 with the middle tank (generally 89) if the station mis-estimated how much fuel they would need and ran out of tank space.

That being said I feel better sometimes getting the fancy fuel and even though I know it may not matter at all, I like to be happy, and I like my engine to be happy. Then we ride off into fields of blueberries and stuff outselves silly till we fall asleep in eachothers arms.


The 70cc/80cc engine has a compression ratio of 6.6:1. You can run as low as 70 octane in it with no problem.

Use as low octane as you can: Reason
1. Cheaper
2. More energy content (BTU) per gallon. Higher octane fuel has less energy content
3. Easier starting due to higher volatility. Higher octane fuel is harder to start in cool temps or first start of day. I had run100LL octane fuel in a engine and it was impossible to start when cold. It was a very high compression ratio engine (modified) and needed this fuel. I used starter fluid a lot in this engine.

The only time you need higher octane fuel in any engine (to include cars) is when you experience detonation or preignition and/or you engine is higher in compression (or runs hotter) and is specified in owners manual.
Older engines typically have carbon build on on piston tops and this caused increased compression and hot spots that can cause fuel to ignite prematurely. This is why older cars may run better with higher octane fuel.

If you use high octane fuel for any other reason, you are wasting your money and are being foolish.

Some of my qualifications: Engineer (lots of thermodynamics), maintenance test pilot, 20 years of wrenching on almost any kind of machinery.


As for oil, any TCW-III rated 2 stroke oil should suffice. Type of oil isn't that important, what is important for these engines is the rather high concentration ration of oil to fuel.

With that said, I use the cheap walmart stuff in all of my 2 stroke engines and also in my 4 stroke 1.3L Mazda Rotary engine.

TCW oils: This is an industry developed specification used for outboard marine engines and was specced for "low revving" engines that run cooler temps than air-cooled non-marine engines.

The correct oil to use for a 2 stroke air cooled oil is: API-TC but this specification has not been changed since 1993 and this oil is very hard to find though required by certain 2 stroke engine makers such as bombardier. In fact, Bombardier and Rotax are the only ones who make a 2 stroke oil that is API-TC certified. Many other 2 stroke oils may meet this standard and TCW standard too but they aren't certified that they do and don't display it on their label. Certification to an API standard can easily run into 6 figures for a company.

Now with that said, today's marine engines rev much higher than yesterday's engine and generate more HP then previous verions. Also, many engine makers recommend TCW oils for all of their 2 strokes regardless if they are water cooled or air cooled. Given that TCW has been changed

Here is why I believe we are ok using TCW in our 2 strokes:
1. We are running at a 20-32:1 mixture ratio.
2. If our engines were very high tech and high revving than TCW oil wouldn't be the best thing for it. However, I doubt our 70/80cc engines ever see 10k rpm under load. Let's face it, these chinese 2 strokes don't generator a lot of power for their size. They are restricted by intake, exhaust, stroke and port timing to be very mild.
3. Due to emission reasons, many makers have to recommend a 50-100:1 oil ratio for their engines. A higher quality oil is needed but a lower quality oil at greater ratios should suffice.

With that said, the actual best oil standard is from Europe: It is ISO-L-EGB which tests an oil at a 150:1 ratio in an engine for 3 hours run and then measures various criteria. TCWIII and API-TC oils are tested under a 1 hour interval.

If these chinese engines were a big investment, then go for it and use the better quality oil but for 99.9% of us, TCWIII at such high ratios is sufficent.

I'll let you know how my 3rd bike holds up. I'm milling the head to up the compression ratio and running a mixture that is 80% gasoline (50:1 fuelil ratio) with remainder being 25% methanol mix that contains 20% nitromethane and 18% blend of castor oil and synthetic 2 stroke oil. I live at high altitude and my carb is too rich on regular gasoline. This blend of fuels has been used for past year on my two other bikes with no ill effects and the power is much better at altitude than stock-it doesn't run as rich as before and actually rev's where it should.
If I don't get desired performance with this mix, I'm going to get the grinder out and see if I can alter the port timing which I suspect is very conservative.