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1980's Johnson/Evinrude 20/25hp conversion to 30hp with pics
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<blockquote data-quote="Boat2fast" data-source="post: 344830" data-attributes="member: 13358"><p>A jet pump is like a fixed dynamometer. The more power you have, the more RPM it will spin. It's a water pump...more power in = more water out. That's why a 25 hp will turn it at, say 5200rpm, but a 30hp may turn the exact same pump 5500rpm. </p><p></p><p> This is the same idea behind the factory test props. These make 'loading' an engine, at full throttle, safe. Test-props just 'waste' the power. They do not make thrust, forward or backward. The mechanic doesn't have to balance on the transom at 30mph to check the timing. He could run full throttle, in-gear, right at the dock.</p><p></p><p> The factory manual specifies the right test-prop and the minimum RPM the engine <em>should</em> reach. 1994 OMC factory manual shows the <em>minimum</em> test rpm for the 25hp is 4800rpm, the <em>minimum</em> for the 30hp is 5400rpm. This is using the same test-prop. The stronger the engine, the more rpm it will spin the test prop. By the same reason, stronger engines spin jet pumps faster too.</p></blockquote><p></p>
[QUOTE="Boat2fast, post: 344830, member: 13358"] A jet pump is like a fixed dynamometer. The more power you have, the more RPM it will spin. It's a water pump...more power in = more water out. That's why a 25 hp will turn it at, say 5200rpm, but a 30hp may turn the exact same pump 5500rpm. This is the same idea behind the factory test props. These make 'loading' an engine, at full throttle, safe. Test-props just 'waste' the power. They do not make thrust, forward or backward. The mechanic doesn't have to balance on the transom at 30mph to check the timing. He could run full throttle, in-gear, right at the dock. The factory manual specifies the right test-prop and the minimum RPM the engine [i]should[/i] reach. 1994 OMC factory manual shows the [i]minimum[/i] test rpm for the 25hp is 4800rpm, the [i]minimum[/i] for the 30hp is 5400rpm. This is using the same test-prop. The stronger the engine, the more rpm it will spin the test prop. By the same reason, stronger engines spin jet pumps faster too. [/QUOTE]
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1980's Johnson/Evinrude 20/25hp conversion to 30hp with pics
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