Xterminator GT
Well-known member
- Joined
- Mar 15, 2012
- Messages
- 48
- Reaction score
- 0
The only reason I ran 91 in this motor is because of it being ethanol free.
TinBoats.net
The original aluminum boat site!
25 Johnson missing
- Thread starter Xterminator GT
- Start date
Help Support TinBoats.net:
- Joined
- May 15, 2010
- Messages
- 3,673
- Reaction score
- 336
- Location
- Central Florida
- LOCATION
- Lake County, Central Florida
Lil Blue Rude.
Without getting into a full blown fuel discussion there are a couple of things that change the ball game where we are concerned.
Although it is generally accepted that higher octane fuels burn at a slower rate that may not always be the case, or the need.
Second, generalizing the need for a higher octane fuel in a two stroke cross-flow engine with a low compression ratio and an extremely high domed piston and lumping that in with an automotive 4cycle engine with flat top pistons and valves is a pretty wide playing field. These two examples and their respective needs are far apart.
Here is a concept that might bring them a little closer together for you though.
Am going to be really general here and this leaves a lot to be desired. Think of a two stroke engine running at 5,000rpm vs a 4cycle running at 10,000 (with its extra wasted cooling (exhaust) cycle. We are just talking combustion cycles here to make things somewhat equal. At that RPM on a 4cycle do you need a fuel that burns fast or slow? Think of something that may run that RPM on a drag strip. Our engines are somewhat similar in that one respect only in that we need a fuel that is resistant to self detonation due to high cylinder pressures and will burn for the length of time needed to produce power. What we (in the 2cycle world) are after in a fuel is complete combustion for the piston shape and cylinder head shape we are using.
Here is an excerpt from an engineer to support this.
The burn rate of a fuel is a measurement of the time required for complete combustion of the air/fuel mixture. The notion that octane ratings affect the burn rate of fuel is about 180-degrees from reality; burn rate is a function of several variables, and the two are completely independent, although there is generally a correlation between octane ratings and burn rates.
To give you a good example of this, we contacted Jim Wurth from Sunoco Race Fuels. He explains, "A perfect example is Sunoco Maximal, which is our fastest burning fuel, and coincidentally one of Sunoco's highest octane fuels at 116 (R+M) / 2. A lot of Pro Stock teams rely on Maximal for those sub-seven second runs. When they are turning 9,000 rpm or more, the fuel has to burn pretty quickly to achieve complete combustion".
This is just the tip of the iceberg and a lot of reading on this subject will bring very little more to the table where we are concerned. This is why I will generally ignore getting into these discussions but I know you have been a good contributor here and seem to have a genuine interest.
Even more reading on 2 stroke design is needed before we can successfully start to target that family of engines and their specific needs vs. automotive production vs. automotive performance/racing.
Without getting into a full blown fuel discussion there are a couple of things that change the ball game where we are concerned.
Although it is generally accepted that higher octane fuels burn at a slower rate that may not always be the case, or the need.
Second, generalizing the need for a higher octane fuel in a two stroke cross-flow engine with a low compression ratio and an extremely high domed piston and lumping that in with an automotive 4cycle engine with flat top pistons and valves is a pretty wide playing field. These two examples and their respective needs are far apart.
Here is a concept that might bring them a little closer together for you though.
Am going to be really general here and this leaves a lot to be desired. Think of a two stroke engine running at 5,000rpm vs a 4cycle running at 10,000 (with its extra wasted cooling (exhaust) cycle. We are just talking combustion cycles here to make things somewhat equal. At that RPM on a 4cycle do you need a fuel that burns fast or slow? Think of something that may run that RPM on a drag strip. Our engines are somewhat similar in that one respect only in that we need a fuel that is resistant to self detonation due to high cylinder pressures and will burn for the length of time needed to produce power. What we (in the 2cycle world) are after in a fuel is complete combustion for the piston shape and cylinder head shape we are using.
Here is an excerpt from an engineer to support this.
The burn rate of a fuel is a measurement of the time required for complete combustion of the air/fuel mixture. The notion that octane ratings affect the burn rate of fuel is about 180-degrees from reality; burn rate is a function of several variables, and the two are completely independent, although there is generally a correlation between octane ratings and burn rates.
To give you a good example of this, we contacted Jim Wurth from Sunoco Race Fuels. He explains, "A perfect example is Sunoco Maximal, which is our fastest burning fuel, and coincidentally one of Sunoco's highest octane fuels at 116 (R+M) / 2. A lot of Pro Stock teams rely on Maximal for those sub-seven second runs. When they are turning 9,000 rpm or more, the fuel has to burn pretty quickly to achieve complete combustion".
This is just the tip of the iceberg and a lot of reading on this subject will bring very little more to the table where we are concerned. This is why I will generally ignore getting into these discussions but I know you have been a good contributor here and seem to have a genuine interest.
Even more reading on 2 stroke design is needed before we can successfully start to target that family of engines and their specific needs vs. automotive production vs. automotive performance/racing.
Xterminator GT
Well-known member
- Joined
- Mar 15, 2012
- Messages
- 48
- Reaction score
- 0
Pappy what is a good ballpark setting for the mixture screw on the front of the carb?
Lil' Blue Rude
Well-known member
I wasn't referring to a 4 stroke vs the cross flow 2 stroke. I'm talking about a loop charged 2 stroke. Which I didn't take the time to figure in different burn rate between the cross flow and a loop charged motor. Not an expert when it comes to fuel, but making a outboard jet go fast.. That's my cup of tea.
- Joined
- May 15, 2010
- Messages
- 3,673
- Reaction score
- 336
- Location
- Central Florida
- LOCATION
- Lake County, Central Florida
[url=https://www.tinboats.net/forum/viewtopic.php?p=334481#p334481 said:
This is exactly why I don't like to get into this type of discussion.
What I was trying to bring out was the fact that:
1. It is not necessarily true that higher octane fuels always burns slower (and all the other revelations that go with that) in fact...that is pretty much yesterday's news
2. That our engines may not necessarily need higher octane fuel to produce power. We need a fuel that will completely combust for our combustion cycle.
3. Since this thread was originally about a 2-cylinder CROSS FLOW engine....I made that comparison.
Am done with this.
To the OP....start at 1.5 turns out and you will probably end up somewhere around 3/4 turn, give or take.
Lil' Blue Rude
Well-known member
Pappy wasn't trying to turn it into a pisssing match either just stating that I was referring to a loop charge motor not a 4 stroke and I realized that his motor being a CROSSFLOW is a different situation. I never stated Higher octane fuels are needed to make power in our motors. I get away with lower octane then most people think you can on my motor due to setup up and tuning. which I'm not going into.
