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Old 10-13-2013, 11:15 AM   #1
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RPM's on a Cummings

Hello all, Got a question I just bought a 1988 Crown Pusher with a Cummings motor & Allison 4 speed auto, now it's been 30 yrs since driving Tractor trailers but my RPM's at 55 were 4000 rpm took it up to 67mph a couple times & it was 4500rpm, is this normal & also are used supposed to manual shift the Alison automatic ? I know it sounds silly but theres a placard telling what maximum speeds to downshift at.
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Old 10-13-2013, 01:12 PM   #2
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Cummings engine speed

The top engine speed on most diesel engine is 2250RPM. The engine fuel pump sets this speed. My Crown is a 1968 with a Cummins. You now have a great bus. Check to notice if tach is cable or electric. Some times the dash unit goes wild. ost diesel tachs have a 3500 max number on the tach. Gas cars have tachs top rpm at 6000- 8500 rpm. Maybe someone changed the tach.
Call me and we can talk. Frank in Idaho 208 423 5969 frank.id.crownatgmail.com
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Old 10-13-2013, 04:51 PM   #3
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Re: RPM's on a Cummings

I know it's farm machinary but we have a 2144 Case combine with a 5.9l cummins (same as the cummins you all know in the Dodge Pickups) and it runs at a jolly 2850 when wide open. This is on the electronic display in the cab but we aslo installed a mech tac back by the engine itself and it reads near 2900 when it's wide open.

There's no way that engine is spinning at 4500 rpm's, it would run away long before that ....
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Old 10-13-2013, 05:44 PM   #4
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Re: RPM's on a Cummings

Thats what I figured it just been so many yrs since I delt with a Diesel I wasn't sure if things had changed think I'll get an extra tack & hook it up at the motor because evan at Idle it show 1600 rpm
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Old 10-14-2013, 03:19 PM   #5
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Re: RPM's on a Cummings

Quote:
Originally Posted by allwthrrider
Thats what I figured it just been so many yrs since I delt with a Diesel I wasn't sure if things had changed think I'll get an extra tack & hook it up at the motor because evan at Idle it show 1600 rpm
I guess when you put it in gear, your bus pops a wheelie, cool.
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Old 10-14-2013, 03:41 PM   #6
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Re: RPM's on a Cummings

look at back of the tach, maybe it is an adjustable one and is on the wrong setting?
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Old 10-16-2013, 03:18 PM   #7
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Re: RPM's on a Cummings

Redline on that thing is at least 5800 RPMs...NOT! Better slow down those RPMs.
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Old 10-18-2013, 08:01 PM   #8
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Re: RPM's on a Cummings

They sell after market tachometers for the cummins engines. I will look up the link if your interested.

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Old 10-20-2013, 02:02 PM   #9
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Re: RPM's on a Cummings

Quote:
Originally Posted by nat_ster
They sell after market tachometers for the cummins engines. I will look up the link if your interested.

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Old 10-20-2013, 10:22 PM   #10
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Re: RPM's on a Cummings

Here is one. This one takes pulses from the injector lines.
http://tinytach.com/

Some use the sensor in the bell housing
Some use pulses from the alternator, but they need to be calibrated.

This one installs at the front of the engine, reading the revaluations off the magnets you epoxy to the dampener. It is a issopro.

http://www.dieselpowerproducts.com//p-7 ... Qgod928AmA
That's the sensor. With that you can use any Issopro tach.

And a nice right up on installing it on a first gen dodge truck.
http://www.dieselbombers.com/dodge-dies ... mount.html


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