FSH 210 Sport
Jetboaters Fleet Admiral
- Messages
- 7,275
- Reaction score
- 9,031
- Points
- 512
- Location
- Tranquility Base
- Boat Make
- Yamaha
- Year
- 2020
- Boat Model
- FSH Sport
- Boat Length
- 21
When in doubt RTM, read the manual.
To be clear, differing octane levels of fuel contain the same amount of energy / btu’s per gallon. Higher octane gasoline resists detonation / engine knock more than lower octane gasoline at a given cylinder pressure. In other words, higher octane gasoline burns slower at lower cylinder pressures than does a lower octane gasoline. Conversely, low octane gasoline doesn’t burn at high cylinder pressures, it detonates, that’s the knock that can be heard. Higher than required octane gasoline in an engine reduces performance, promotes harder starting, increases heat, and is a waste of money. The higher heat is a result of the higher than required octane burning longer in the cylinder and chasing the piston down the bore, as opposed to a correct octane fuel burning within the first 30* of crank rotation. The power loss comes from the fact the gasoline doesn’t burn within the first 30* of crank rotation, eg highest pressure generated by the burning air fuel mixture in the smallest available space, as opposed to the too high octane fuel not burning completely in the first 30* of crank rotation thereby not achieving maximum pressure. Desired completed fuel burn is 10* after top dead center.
In the United States on a given gasoline pump the octane level of the fuel is posted using RON (research octane number) + MON ( motor octane number) /2 method, this is known as the ATI or anti knock index octane rating.
When doing any testing density altitude must be taken into account to correct for performance loss due to reduced air pressure. Here is a helpful link. density altitude calculator
Just fill out the fields with information taken from the local NWS forecast / current conditions to get not only density altitude, but also the percent of air as it relates to a given location / density altitude.
For reference, Standard atmospheric conditions or Zero Density altitude is 29.97Hg, 60°, and zero humidity.
Example: at an altitude of 4800’ on a 95° day, barometric pressure of 29.92, dew point of 59° the density altitude is 8359’. Therefore your engine will operate as though it was at 8359’.
I have a 2020 210 FSH Sport, Twin TR-1 1050cc with normally aspirated engines, the sticker on the engine states power as 83 KW’s maximum or 111 hp. The shop and owners manuals state fuel requirements as 86 RON/90MON. Using the R+M/2 Method of octane rating found on all pumps in the United States, that equals 88 octane. On a boat like mine with 222 hp total at standard conditions, at 8359’ there will be a hp loss of 55 hp, a significant loss of power.
Air pressure at 8359’ is only 77.74% of standard, or only 11.42 PSI as opposed to 14.7 PSI at standard atmospheric conditions, this results in lower cylinder pressure in a normally aspirated engine. The higher the density altitude the less octane that is required due to the lower cylinder pressures. Standard rule of thumb for normally aspirated modern engines is .5 octane number reduction per 1000’ of elevation gain. In older cars, pre 1988, the rule of thumb reduction in octane was up to 1.5 per 1000’ of elevation gain due to those engines not having the sophisticated electronic engine management and fuel delivery systems found on modern engines.
Speaking of modern engine management systems, whether closed loop EFI or open loop EFI, said systems will adjust the fuel mapping to maintain the target AF ratio based on density altitude. While these adjustments will correct fuel mixture at a given density altitude, said systems will not compensate for too high octane gasoline. As stated previously in this thread, the Yamaha jet boat engines do not have knock sensors to retard timing and adjust fuel mix for a too low octane gasoline. Conversely, the Yamaha jet boat engines will not advance timing based on engine knock sensor input.
When I test drove my boat at 700’ of altitude on a cool rainy day, 20% fuel, two people and no gear (sorry I don’t have the density altitude for those conditions) the boat achieved 7400 rpm and 44 mph. Now when operating at 8359’ density altitude, 50% fuel, water temp of 72*, one person and roughly 100# of gear the boat goes 34 mph and 6800 rpm. Not surprising given the 55 hp loss of power due to the altitude increase. I think that me topping up the fuel tank with 91 octane non ethanol fuel has exacerbated this performance loss. While it is not known what octane fuel was in the boat at the time of delivery, I have to assume that it as at least 88 octane or higher, if I was to wager a guess, I’m betting that it was at least 91 octane as that is what the SVHO motors use. Makes sense from a corporate view point, one fuel that is safe in all of the motors..
Having said all of the above, I’ll be dropping the octane level of the fuel down to the minimum octane level of 88 specified in the manuals by mixing in the correct amount of 85 octane non ethanol fuel to achieve the 88 octane. There are plenty of online mixing formulas and I checked three against each other to confirm consistency and accuracy. Here is a link to one that I am using. Octane Mixture Calculator
As I mentioned above, the rule of thumb octane level reduction is .5 per 1000’ of elevation gain. I gained roughly 5000’ of elevation over standard not compensating for temperature and humidity, so that is a 2.5 octane reduction from the factory 88 octane level or 85.5 octane. I plan on lowering the octane level incrementally to achieve the best possible performance at the various lakes in the area while not sacrificing engine safety.
I have a good baseline of performance from three outings with the boat in very similar conditions, and I will report back with any changes in performance.
Thanks for taking the time to read this long post!
To be clear, differing octane levels of fuel contain the same amount of energy / btu’s per gallon. Higher octane gasoline resists detonation / engine knock more than lower octane gasoline at a given cylinder pressure. In other words, higher octane gasoline burns slower at lower cylinder pressures than does a lower octane gasoline. Conversely, low octane gasoline doesn’t burn at high cylinder pressures, it detonates, that’s the knock that can be heard. Higher than required octane gasoline in an engine reduces performance, promotes harder starting, increases heat, and is a waste of money. The higher heat is a result of the higher than required octane burning longer in the cylinder and chasing the piston down the bore, as opposed to a correct octane fuel burning within the first 30* of crank rotation. The power loss comes from the fact the gasoline doesn’t burn within the first 30* of crank rotation, eg highest pressure generated by the burning air fuel mixture in the smallest available space, as opposed to the too high octane fuel not burning completely in the first 30* of crank rotation thereby not achieving maximum pressure. Desired completed fuel burn is 10* after top dead center.
In the United States on a given gasoline pump the octane level of the fuel is posted using RON (research octane number) + MON ( motor octane number) /2 method, this is known as the ATI or anti knock index octane rating.
When doing any testing density altitude must be taken into account to correct for performance loss due to reduced air pressure. Here is a helpful link. density altitude calculator
Just fill out the fields with information taken from the local NWS forecast / current conditions to get not only density altitude, but also the percent of air as it relates to a given location / density altitude.
For reference, Standard atmospheric conditions or Zero Density altitude is 29.97Hg, 60°, and zero humidity.
Example: at an altitude of 4800’ on a 95° day, barometric pressure of 29.92, dew point of 59° the density altitude is 8359’. Therefore your engine will operate as though it was at 8359’.
I have a 2020 210 FSH Sport, Twin TR-1 1050cc with normally aspirated engines, the sticker on the engine states power as 83 KW’s maximum or 111 hp. The shop and owners manuals state fuel requirements as 86 RON/90MON. Using the R+M/2 Method of octane rating found on all pumps in the United States, that equals 88 octane. On a boat like mine with 222 hp total at standard conditions, at 8359’ there will be a hp loss of 55 hp, a significant loss of power.
Air pressure at 8359’ is only 77.74% of standard, or only 11.42 PSI as opposed to 14.7 PSI at standard atmospheric conditions, this results in lower cylinder pressure in a normally aspirated engine. The higher the density altitude the less octane that is required due to the lower cylinder pressures. Standard rule of thumb for normally aspirated modern engines is .5 octane number reduction per 1000’ of elevation gain. In older cars, pre 1988, the rule of thumb reduction in octane was up to 1.5 per 1000’ of elevation gain due to those engines not having the sophisticated electronic engine management and fuel delivery systems found on modern engines.
Speaking of modern engine management systems, whether closed loop EFI or open loop EFI, said systems will adjust the fuel mapping to maintain the target AF ratio based on density altitude. While these adjustments will correct fuel mixture at a given density altitude, said systems will not compensate for too high octane gasoline. As stated previously in this thread, the Yamaha jet boat engines do not have knock sensors to retard timing and adjust fuel mix for a too low octane gasoline. Conversely, the Yamaha jet boat engines will not advance timing based on engine knock sensor input.
When I test drove my boat at 700’ of altitude on a cool rainy day, 20% fuel, two people and no gear (sorry I don’t have the density altitude for those conditions) the boat achieved 7400 rpm and 44 mph. Now when operating at 8359’ density altitude, 50% fuel, water temp of 72*, one person and roughly 100# of gear the boat goes 34 mph and 6800 rpm. Not surprising given the 55 hp loss of power due to the altitude increase. I think that me topping up the fuel tank with 91 octane non ethanol fuel has exacerbated this performance loss. While it is not known what octane fuel was in the boat at the time of delivery, I have to assume that it as at least 88 octane or higher, if I was to wager a guess, I’m betting that it was at least 91 octane as that is what the SVHO motors use. Makes sense from a corporate view point, one fuel that is safe in all of the motors..
Having said all of the above, I’ll be dropping the octane level of the fuel down to the minimum octane level of 88 specified in the manuals by mixing in the correct amount of 85 octane non ethanol fuel to achieve the 88 octane. There are plenty of online mixing formulas and I checked three against each other to confirm consistency and accuracy. Here is a link to one that I am using. Octane Mixture Calculator
As I mentioned above, the rule of thumb octane level reduction is .5 per 1000’ of elevation gain. I gained roughly 5000’ of elevation over standard not compensating for temperature and humidity, so that is a 2.5 octane reduction from the factory 88 octane level or 85.5 octane. I plan on lowering the octane level incrementally to achieve the best possible performance at the various lakes in the area while not sacrificing engine safety.
I have a good baseline of performance from three outings with the boat in very similar conditions, and I will report back with any changes in performance.
Thanks for taking the time to read this long post!
