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Old 04-15-2016, 11:11 PM   #1
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Cummins 6BT5.9-M

When does the turbo kick in?
Really curious have not been able to get straight answer.
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Old 04-15-2016, 11:40 PM   #2
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Turbos do NOT KICK in. Any time the engine is running the turbo will be spinning unless there is something wrong with it.

Many engines have a control to prevent full fueling untill the turbo is producing a minimum boost level, often around 15 psi. Many do not. The control may be electronic or mechanical depending upon the engine age. It is meant to prevent excessive black smoking upon heavy acceleration. Otherwise you could jam the throttle full at which point the governor will go to full fuel but the engine and the turbo will not be pulling enough air for proper combustion and the engine will smoke heavily untill engine revs and boost build.
It may feel like the turbo KICKED in when the control says ok to the injection pump but in fact the turbo was producing increasing boost as the engine sped up. When that full fuel is allowed it may feel like a kick in.

Electronic engines are somewhat smarter.

The turbo boost is entirely dependent upon the exhaust flow quantity which is dependent upon the load and the fuel setting. If your engine is loafing then the turbo may not be doing much at all. If you crank the engine up, under load, the turbo will produce boost. But keep in mind that without a load there will be no boost.

You could actually run the engine at full throttle out of gear and there will be little to no boost.

There is NO SET RPM level at which the turbo will produce any given level of boost. You will see on some spec. sheets a boost level but note it is given only at full rated load. The load at any given engine rpm can vary greatly boat to boat depending upon the hull and its weight so the mfgr. cannot predict the boost level at other than full load conditions.
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Old 04-16-2016, 01:35 AM   #3
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It is all a function of load. Load produces heat. And heat is what drives the turbo. C Lectric pretty much nailed it.

BUT....to answer your question....as it relates to MY boat. It happens about 1500RPM. You hear them starting to "sing". And then the props and engines start to dig in. It is a wonderful noise. I honestly believe that in most of these applications, the RPM will be similar.
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Old 04-16-2016, 07:28 AM   #4
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All above is correct as I understand it.
Observing my boost gauges, full rpm in neutral no load - produces 0 boost. I start seeing boost pressure at about 1400 rpm and that coincides with the bow starting to rise as the boat is moving above displacement speed. At my cruising speed of 2400 rpm I get about 10 psi boost and 15 kts on full plane.

Boost goes up to about 12-15 psi as the boat is getting over the hump but settles down to 10 psi on plane.
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Old 04-16-2016, 02:25 PM   #5
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Thank you, Roger
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