Skip to main content ↑ ↓
Topic: Governed Speed vs Overspeed RPM? (Read 2253 times) previous topic - next topic

Governed Speed vs Overspeed RPM?

1998 U-320 36' M11 Celect Plus 450E+

Several threads have discussed mountain driving by changing gears and using the retarder.  Our engines have RPM limits, but the tachometers on the dash do not show a redline.

With the engine there are three RPM specs which come into play:
    • Idle Speed
    • Governed Speed
    • Overspeed

Idle Speed RPM, and Governed Speed RPM, can both be found in the Cummins engine owner's manual under specifications, and also on the data plate on the side of the engine itself.

Overspeed is not specifically listed.

The Cummins engine manual for our M11 Celect Plus 450 has these two warnings in bold letters in the section titled "Operating Instructions":
    • To prevent damage to the camshaft and the valve train when using an engine compression brake, do not exceed governed speed.
    • The engine speed must not exceed 2,300 RPM under any circumstances.

When I called Cummins' support line and gave them the serial number for our engine I was told the Overspeed RPM was 4,200 RPM for 15 seconds.

What is Overspeed?
If our engine's speed should never exceed 2,300 RPM, and if when using a compression brake it should never exceed 2,100 RPM, then what does Cummins' Overspeed RPM refer to?

Thank you,
Chris

Re: Governed Speed vs Overspeed RPM?

Reply #1 –
Chris,

First, I would REALLY question  4,200 RPM even for 15 seconds..  Wonder if that means at 16 seconds rods would start flying out the side of the block??? The good news is that the Allison would not allow you to get close to that figure without up-shifting.

Maybe I missed it, but what is governed RPM?  It makes sense for it to be a little lower than max no-load speed.
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: Governed Speed vs Overspeed RPM?

Reply #2 –
Quote
    The engine speed must not exceed 2,300 RPM under any circumstances.
When I called Cummins' support line and gave them the serial number for our engine I was told the Overspeed RPM was 4,200 RPM for 15 seconds.

This is pretty interesting. I called Cummins on our M11 (400hp). I knew the governed speed was 2,000 and I asked him what the absolute limit was and he told me it was 2,300 rpm. I've always used that (on our 450hp M11 too) as the "everything will come apart and blow up" point so I make sure not to exceed it when stuck in gear and accelerating going down hill. I can't imagine that thing turning 4,200 rpm. Seems to be confusion on this issue. Hopefully others that have called Cummins will chime in.
or
93 225
95 300
97 270
99 320

Re: Governed Speed vs Overspeed RPM?

Reply #3 –
Brett,

The Governed speed is 2,100 RPM for the M11 Celect Plus 450E+

I re-edited my post because some of the details had gotten chopped off.

I was very surprised by the 4,200 RPM response too.  With the maximum RPM being 2,300, I thought that would be the Overspeed RPM for 15-30 seconds.  4,200 RPMs seem like it would make for a less than a good day.

According to the Allison operator's manual, when going down hill in a lower gear, the transmission may up-shift if the governed speed (2,100 RPM) is exceeded.

Chris

Re: Governed Speed vs Overspeed RPM?

Reply #4 –
Then,  I would be very comfortable with:

2,100 RPM as highest RPM under load (i.e. on the throttle).
2.300 RPM as highest RPM under no-load (i.e. throttle closed).
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: Governed Speed vs Overspeed RPM?

Reply #5 –
Quote from: Numbers –

According to the Allison operator's manual, when going down hill in a lower gear, the transmission may up-shift if the governed speed (2,100 RPM) is exceeded.

Chris


Would that happen even when we have manually selected a lower gear? Say manually selected 4th on downgrade, will the Allison upshift to 5th between 2100 and 2300 ISM  rpm, even though 4th is slected? My guess is yes, it will still upshift.
Dan - Full timing since 2009
2003 U320 40' Tag 2 slide

Re: Governed Speed vs Overspeed RPM?

Reply #6 –
No one would give you a dime for a core deposit on an engine anywhere close to 4200 RPM. Seems to me that a few years ago, a member over revved his 8.3 Cummins on a downgrade and bent pushrods. The Allison didn't shift up in time. The rough running engine was only partially repaired in the next town but destroyed itself several hundred miles later to the tune of just over $30K.

Why come anywhere close to limits? Fuel cutoff on ours is 2130 RPM but I never let it get past 2000 RPM on the downgrades. Conservative rev limits don't cost much time on the downgrades and my 200 degree limits on temp are good insurance for any wet sleeve engines like Detroits and 8.3/11 liter Cummins on the upgrades. Just imagine sitting by the side of the road with a disabled coach and a $30,000 waiting after an expensive tow. No stuffing Pandora back in the block.

Pierce
Pierce and Gaylie Stewart
'93 U300/36 WTBI
Detroit 6V-92TA Jake
1140 watts on the roof
SBFD (ret)

Re: Governed Speed vs Overspeed RPM?

Reply #7 –
Quote from: AC7880 –
Would that happen even when we have manually selected a lower gear? Say manually selected 4th on downgrade, will the Allison upshift to 5th between 2100 and 2300 ISM  rpm, even though 4th is slected? My guess is yes, it will still upshift.


YES, it would automatically upshift.  No different than if you down arrow to second while driving 60 MPH.  The Allison ECU is "idiot-proof" in protecting the transmission.
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020