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Old 30 December 2007, 18:32   #1
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Fuel Consumption

I own a 25hp X-RNLI tiller (2stroke) I was wondering what the average fuel burn would be on this type of engine, Im planning a trip away and would love to know what she will burn. I think the engine is around 2001.

Pics can be seen at;

http://quiksilver430.blogspot.com/


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Old 30 December 2007, 18:48   #2
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My yam 25 2 stroke used about 1/2 litre a mile on a large SIB or 10 litres an hour roughly.
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Old 31 December 2007, 07:28   #3
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Originally Posted by ian parkes View Post
My yam 25 2 stroke used about 1/2 litre a mile on a large SIB or 10 litres an hour roughly.
I have its baby brother (20 HP - same block I think). And that sounds about right to me -for normal cruising etc... at WOT its almost double that though.
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Old 31 December 2007, 17:09   #4
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I have its baby brother (20 HP - same block I think). And that sounds about right to me -for normal cruising etc... at WOT its almost double that though.

yes i meant to say cruising which was about 18mph half throttle and WOT was about 21

WOT is the point I thought , and sold it
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Old 02 January 2008, 19:56   #5
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The standard consumption to almost any outboard engine is 10% of it's horsepower. Example : a 25 HP 2 stroke engine will consume 2.5 gallons an hour at full throtle. It also will depend on prop being used, gasoline octane, wind, water conditions and how well you boat is inflated.

One good test is to put one gallon, maintain any specific aceleration with out any throtle variation and see hoy much time you run till the engine stops. In this way will have you gas consumtion to that particular aceleration.

Run at cruising speed and se how much time you can cruise with one gallon.

Happy Boating
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Old 03 January 2008, 05:39   #6
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A good tip for consumption is to not use higher octane fuel than you need to. In the US alot of the gasoline is around the 90 RON mark (often lower). The higher the octane, the higher the resistance of the fuel to pre-ignite. Therefore, as long as there is no danger of pre-ignition (pinging/knocking), then a lower octane fuel will ignite easier and run smoother.

So basically, in the UK (and France), don't go using 98 instead of 95 thinking it will work 'better', as unless your outboard specifically demands it (I don't know of any that do), chances are it will work less well.

There are lots of interesting threads on this subject if anyone is interested.......
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Old 03 January 2008, 08:19   #7
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Interesting, I always thought high octane fuel was more likely to go bang, didn't realise it was the reverse. I heard of somebody here a few years ago who was running 100LL aviation fuel in a trials bike, apparently won just about everything but didn't do the engine much good!
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Old 03 January 2008, 12:29   #8
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The higher the octane, the higher the resistance of the fuel to pre-ignite. Therefore, as long as there is no danger of pre-ignition (pinging/knocking),
I think you are confusing resistance to detonation with resistance to pre-ignition.
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Old 03 January 2008, 12:48   #9
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Interesting, I always thought high octane fuel was more likely to go bang, didn't realise it was the reverse. I heard of somebody here a few years ago who was running 100LL aviation fuel in a trials bike, apparently won just about everything but didn't do the engine much good!
No, high octane fuel is LESS likely to go bang.

It is a common misconception.

In the quest to extract more performance from petrol engines, desginers raise the compression ratio (how much the piston squeezes the fuel/air mixture) as high as they can. Theoretically, the higher the compression ratio, the higher the engine efficiency. But as compression increases, the higher the chance that the petrol will 'pre-ignite' (burns before the spark plug fires). This causes an irregular burn, high pressure spots, and the 'pinking' noise.

Higher octane fuels were designed to allow these higher compression engines to function reliably. They are able to resist the higher pressures and temperatures of higher tuned engines before being ignited by the spark plug.

Super tuned race engines have very high compression ratios (either by head design or by turbos, etc.) and therefore NEED high octane fuel (98 up to 130+)

If you look in your car/outboard handbook, you can find your compression ratio. Most petrol engines are in the 8 to 10 / 1 range. In a 4 cylinder, 2 litre engine, a 10/1 ratio means that the piston/cylinder goes from a max.volume of 500cc to a minimum of 50cc. Using 98 RON in such an engine will gain you nothing, and will most likely reduce efficiency, albeit by a v. small amount.

I used to have an Audi S4, one of the few cars out there to officially 'prefer' 98RON, due to turbos, etc. Although I used 98 sometimes, it ran just as well on 95, and I never noticed any pinking.

I am not in the US, but I believe it is quite difficult to buy 95 RON petrol, let alone 98.
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Old 03 January 2008, 12:51   #10
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I think you are confusing resistance to detonation with resistance to pre-ignition.
I agree with you, confusing terms,

Quoted from a Mercury Service Manual:

Use a mayor brand of automotive unleaded gasoine with a minimum posted octane rating of 90 RON. Higher gasolines that contains fuel injector cleaners are preffered for added internal engine cleanliness. The use of higher octane gasolines will not affect the overall engine performance as long as the ignition timing and engine are correctly tuned.

I use 95/97 octane unleaded gasolines with Chevron TCW-3 / 2 stroke oil on my Tohatsu's 05-18-30 HP, and they work fine, the price difference is so small down here compared to 90 RON, so, why not use a cleaner aditive gasoline. It's absolutely true that a lower compression engine requires a lower octane gasoline and viceverse.

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