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Old 15 July 2005, 07:01   #11
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Quote:
Originally Posted by kitten
I do belive you are confusing WOT (wide open throttle) with max revs.
correct, there is a difference ?

Pete
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Old 15 July 2005, 07:04   #12
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Why the question mark Pete?
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Old 15 July 2005, 08:26   #13
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Revs are governed by prop size - too big a prop and you will never hit maximum revs!!!

As long as engines aren't run higher than their rev limit then you will be ok. NEVER rev an engine too high until it is properly warmed up. Allow it to idle for a few seconds before switching off.

Lubrication is a big factor - the more oil(within reason) the better a 2 stroke will last - many racing people will use 30:1 despite their engines claiming to be ok on 50 or even 100:1
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Old 15 July 2005, 08:50   #14
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im talkin bout a normal like honda 115 4 stroke and going throttle all the way to the dash until it can go no farther lol
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Old 15 July 2005, 12:22   #15
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This is as I understand it:

1. WOT is a range that the manufacturer sugests that you maintain for best performance and econimy, which is governed by prop size.

2. Maximum revs is usually the upper WOT figure. ie: if your manufacturer suggests that the WOT range is 5000 - 6000rpm them maximum is 6000rpm. Most outboards have some kind of rev limiter to stop over revving if the boat comes out the water anyway.

3. If you under prop, your revs will exceed the WOT and could damage the outboard, but initial acceleration will be very quick.

4. If you over prop, your revs will be under WOT. You'll have low acceleration and be labouring the outboard and potentially will cause more damage than under propping ie: holeing pistons and wrecking piston rods and bearings.

So match the prop to the recommened WOT for your outboard/boat combination and be mindful to the fact that as you load the boat more heavily the maximum revs will drop.

I'm sure that someone will say I'm wrong

Borrow a load of props of different sizes and go do some RIB testing.

Andy

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Old 15 July 2005, 14:59   #16
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Andy

You're confused!

Wide Open Throttle isn't a range, it's a throttle setting. It simply means pushing the lever as far forward as it goes. Nothing to do with the engine's rated rev limit or the prop size. It will probably give the best performance, but will always give lousy economy!

What you are referring to is the engine revs at WOT, which is indeed affected by prop size. As in the following:

- If you have the correct prop, then the engine revs at WOT should be around the maximum recommended for the engine

- If you under prop, your revs at WOT will exceed the maximum rated revs for the engine . . .

- If you over prop, your revs at WOT will be less than the maximum rated revs for the engine . . .

What you're saying is basically right, it's just the terminology that's wrong!

John
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Old 15 July 2005, 15:22   #17
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Quote:
Originally Posted by John Kennett
Andy

More confusion!

Wide Open Throttle isn't a range, it's a throttle setting. It simply means pushing the lever as far forward as it goes. Nothing to do with the engine's rated rev limit or the prop size.

John
Sorry John, not according to my hand book and workshop manual. It states that WOT should be in a range of between 5000 and 6000 rpm and anything between these two figues is within manufacturer tolerances. Yes it is affected by prop size and by load.

I knew that someone would have a go
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Old 15 July 2005, 16:30   #18
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Quote:
Originally Posted by Hightower
Sorry John, not according to my hand book and workshop manual. It states that WOT should be in a range of between 5000 and 6000 rpm and anything between these two figues is within manufacturer tolerances. Yes it is affected by prop size and by load.
If that's really what it says then it's badly worded because it makes no sense!

Does it perhaps actually say "at WOT the revs should be in a range of between 5000 and 6000 rpm"?

John
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Old 15 July 2005, 19:13   #19
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I'll have a look on Monday as they're at work John.

But this is what I think.

So "Wide Open Throttle" is just as it says, butterfly valves at 90 deg to bore. It's a setting at the Carbs/fuel injection...Right?

If there was no load at the drive end, any engine would just rev higher and higher, till something broke...Right?

So if I put a load onto the drive side of the engine, say a prop of a given size and put this into water the engine would settle to a different rpm (chances are it wouldn't blow up)...Right?

If I put a prop of twice the size and stuck it into water you would get a different (lower) rpm....Right?

So I have three examples of an engine running at Wide Open Throttle but With three different revs...Right?

So why can't a Manufacturer state a Wide Open Throttle "Range" for thier outboards to run in?
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Old 16 July 2005, 00:50   #20
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Quote:
Originally Posted by Hightower
So why can't a Manufacturer state a Wide Open Throttle "Range" for thier outboards to run in?
Andy, they do. To quote from my engine manual, "Full throttle opening range (rpm): 4,500 - 5,500"

But you know this, 'cos you've just posted this on another thread...
Quote:
Originally Posted by Hightower
Roy, what rev range does your handbook recommend?

If it was me and your range was 5000-6000rpm I'd be looking for around 5700rpm with full fuel tanks and just yourself as crew. That way if you load heavier (with extra crew) your engine revs at WOT souldn't drop below the lower recommened revs.

Assuming 5000-6000rpm is given as your rev range in your handbook.

Andy
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