Fuel Consumption of 1990 85 hp two-stroke at 3,500-4,000 rpms?

Hi,

Does anyone have any idea what the hourly gas consumption would be on 1990 85 hp two-stroke at about 3,500 to 4,000 rpms? I know wide open throttle would be about 9 gph, but I don't know the rule of thumb in mid idle ranges which should be fine to get my boat up on a plane at 22-24 mph cruising speed.

Just trying to weigh the economics of spending 2K on an older 85 hp vs. 9K on a new f70la 4 stroke.

Thanks.

Re: Fuel Consumption of 1990 85 hp two-stroke at 3,500-4,000 rpms?

depends on what you are pushing with it.

Look at this site and see what you can find on different boats and motors.
You will not find info on something that old but you can get a general idea
Performance Bulletins | Yamaha Outboards

Re: Fuel Consumption of 1990 85 hp two-stroke at 3,500-4,000 rpms?

Your fuel consumption would not likely change with the new motor due to the reduced power and hence the higher throttle setting to maintain the same speed as before. My 75 HP Merc (2S) on a moderately loaded 17 foot Alumacraft with two aboard would burn exactly 7.5 g/hr at WOT as measured with a fuel flow monitor. At 3500 - 4000 RPM it was in the 5 - 5.5 g/hr range. If you attempt to compare fuel consumption between 2- and 4-stroke engines at a given RPM the 4-stroke will always be better. However if you look at rpm vs speed charts for the two engines you will see the 2-stroke is almost always pushing the boat faster than the 4-stroke at the same rpms so fuel consumption is actually a lot closer between the two than advertising makes it out to be. A study I did using Yamaha test data some time ago actually had a 70 HP Yammy 2S getting better fuel economy than a 4S 75 Yammy on the same boat. Therefore the 2-stroke 4-stroke fuel economy numbers must be looked at carefully. At WOT a 2-stroke and 4-stroke on the same boat will be nearly identical except the 2-stroke will be faster.

Re: Fuel Consumption of 1990 85 hp two-stroke at 3,500-4,000 rpms?

Thanks Silvertip and 99 for the advice and feedback. I think I'll stick with a larger, lighter and cheaper 2-stroke especially considering that my boat is 20+ years old and putting a newer 4-stroke on it would probably be a poor investment. The weight limitations of the motor also negate a lot of the fuel economy advantages of a 4-stroke since I will have to run a lower powered motor at higher rpms to achieve the same speeds.

A buddy of mine was down in Baja recently and he noticed that the pangero fisherman mostly had overpowered 4-strokes on their boats. They are relatively poor, so I would assume that the large up front investment they are making in an overpowered 4-stroke probably pays back in better fuel economy over the long haul.

Re: Fuel Consumption of 1990 85 hp two-stroke at 3,500-4,000 rpms?

If one actually "needs" a new engine then the 2-stroke/4-stroke consideration comes into play and you buy whatever lights your fire. But if you buy a 4-stroke over a 2-stroke with hopes the engine pays for itself in fuel savings, that's simply not going to happen. The only time a 4-stroke has a significant advantage over a 2-stroke in fuel economy is at hull speed (trolling for example). A 2-stroke powered boat will be on plane at a lower rpm than a 4-stroke and from there on up the speed range economy differences become closer. It is the commercial folks that put lots of hours on their motors that need to squeeze every nickel until it squeals so to them a very small advantage in fuel economy is a big deal. Recreational boaters in general are not in that situation.