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Old 01-03-2010, 08:35 PM   #1
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RPM Limits

When going down hill a steep hill lets just say I put it in manuel 3rd gear with the engine brake on how far should I let the RPMs go??? 25RPM or less ? more? I have a 2001 Monico Knight with a 300ISB Cummins and 6 speed Allison trans.
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Old 01-03-2010, 08:42 PM   #2
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I am no expert and am sure there are definitive answers to this question as well as opinions. In my case, I use both the engine brake and downshift to control the speed and the RPMs. I like to keep it at no more than 90% of Max RPM.
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Old 01-03-2010, 08:52 PM   #3
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What is your 90%
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Old 01-03-2010, 09:04 PM   #4
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2,400 is the governed RPM on my Cat C7, I assume the Cummins is the same.
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Old 01-03-2010, 09:13 PM   #5
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On my ISB, maximum stock BHP is at 2700 RPM with fuel shutoff at 3250 RPM, and the tach is redlined accordingly. According to Cummins, valve float sets in at 4000 RPM, although exhaust back pressure may induce valve float earlier with the exhaust brake engaged.

With that said, I'd keep it below the governed RPM (3250 RPM in my case). The higher the RPM, the more braking force the exhaust brake will produce.

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Old 01-03-2010, 09:43 PM   #6
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I have the same truck as Rusty and I try to keep it below 2700-2900. At that point I'll use the brakes to slow it down well below that and shift to a lower gear to control my speed.
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Old 01-04-2010, 07:29 AM   #7
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With the engine brake on, the transmission will, or should, automatically downshift as soon as it is within safe limits. Some are programed to do this down to 2nd gear and some down to 4th gear. Manually downshifting doesn't affect this sequence. Even when the engine brake is not on downshifting should automatically occur when RPMs are within safe limits. A call to Allison for your transmission would be in order to be 100% sure of this.
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