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Old 06 June 2007, 17:14   #1
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Yamaha USA question.......

http://www.yamaha-motor.com/outboard...s/7/specs.aspx

Take a look at Compression Ratio on 150 hp and 175 hp. I canīt understund that at all....
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Old 06 June 2007, 20:57   #2
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Very odd!
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Old 06 June 2007, 22:00   #3
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Re compression ratio...

This is just a guess but the compression ratio may be affected on those cylinders by the fuel pumps. I haven't looked at a exploded view on the motor but I am thinking the fuel pumps are mounted off of those cylinders.
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Old 07 June 2007, 11:53   #4
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Huh? Compression ratio is unsquished volume to squished volume.

Not sure what fuel pump location would have to do with it? In any case, I think the primary fuel pump is driven off the crank, with the injector pumps coming off the camshafts. Has to be at the top of the engine, I'd think. You may be right about where the pumps mount, but I still see no reason to lower the compression on those cylinders.

The only way to change compression ratios that I know of is to use different pistons in two of the cylinders, have the cylinder heads themselves shaped differently, or have a different throw on the crankshaft journals for those cylinders. Something had to drive the engineers to do so. A glance at the parts list shows that the left and right side pistons are different, but the three are the same per side, so it's got to be either the head or the crank.

The only thing I can think of is that heat was causing detonation problems and they backed off the compression to tame that, but they apparently don't seem to do the same on their higher HP motors, so there may be some other reason (hey, I'm no engineer...)

Weird. First time I've heard of that on any motor.

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Old 07 June 2007, 15:03   #5
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VWīs V5 engine is crazy and then this.... I still donīt understund.
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Old 07 June 2007, 16:10   #6
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They must do something to get the power from each pot the same though or it'd be waaaay out of balance all the time.
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Old 08 June 2007, 14:35   #7
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I Recon it may be for emisons. have to read the manual when i get back to work.


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Old 09 June 2007, 14:54   #8
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Um, NO, No, not even close, V MAx is a two stroke, and no.

it's in the head, has to do with exhaust length, and scaviging effect.
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Old 10 June 2007, 23:13   #9
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So, you're saying that the compression ratio listed is an actual measurement, rather than a theoretical value? That what they are publishing is based on real world compression measurements rather than the uncompressed vs compressed volumes? Somehow, I doubt that.

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Old 11 June 2007, 07:52   #10
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Could be that cyls 5 & 6 are at the bottom of the engine, and as they are closer to sea-level, the air will be denser there, thus obviating the need for the higher compression.
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