2 stroke leaded fuel

[Bradkitcher]

Hi Norso Lubricants

Can you please give me advice on racing fuel, oil and plugs for a tohatsu 2-stroke 15hp

Regards

brad at kitchergroup dot com

[Rotary John]

The primary benifit of Av gas is the higher octane rating; 100 mim. It used to be 130 octane. The higher octane allows higher compression ratios and advanced timing (more power) because the higher octane prevent detonation. We used to run Av gas in our 2-cycle racing engines back in the 70's even when auto gas had lead in it also. Lead also acts as a lubricant for the rings, but leads to more and harder deposits. When lead was removed from auto gas in the 70's, problems with coated rings and rubber parts were numurous, along with reduced power to prevent detonation. My '79 Corvette 350 had a wopping 170 HP. Vitually all modern engines are designed and tested to run on non-leaded gas, including outboards. An octane booster will allow the same engine modifications that Av gas allowed and a much lower cost.

[Mark75H]

What about using it in stock motors with 6:1 compression ratios?

[Rotary John]

What about using it in stock motors with 6:1 compression ratios?

Don't get me wrong, AV gas won't hurt yor engine. I suspect a race engine is torn down and inspected on a regular basis, so if piston deposits are getting heavy, they can be cleaned off. I guess my point was if you don't need the higher octane to prevent knock and detonation, there isn't any benifit in running Av gas in modern engines.

[calvin]

Some have warned against using hi-test in 2 strokes because of why they could not explain..would this be an issue?

[Mark75H]

The problem is not necessarily 2 strokes, but low compression motors.

Most 2 stroke fishing motors are generally fairly low compression motors that can not take advantage of higher octane. Some, but not all low compression motors running on some but not all high octane fuels will make less power. Some will make no difference. None will make more power by switching to high octane if the compression is low or medium.

Using high octane gas in a motor that doesn't need it, takes a chance on making less power than running on the correct octane rated fuel for that motor.

[Bradkitcher]

What is high and what is low compression. I tried to work it out and my motor is 12:1. Does that sound correct. Is this high or low.

Advancing timing apparently helps with higher octane fuel.

Does anyone have any recommendation for time advance, why it works, what fuel to use and any oil recommendations.

Thanks guys

[Mark75H]

High varies with bore size and chamber shape. On a two stroke, it is often better to compute "effective compression ratio" rather than full stroke compression ratio as calculated for four strokes.

[Bradkitcher]

Could somebody please calculate my compression ratio or the other type which is called the effective compression ration. Then I would greatly appreciate input to if this is high or low.

I heard that these outboard have a higher compression than outboards of a similar size.

Per Cylinder

Bore 55mm
Stroke 52mm
Capacity per cylinder 123.5cc
Cylinder head height 21.75mm
Cylinder head capacity with spark fitted

Thank you all for your help so far.

[Mark75H]

Need to know the height of your exhaust port