Rpm's on a Cummings

[Stu & Filo. T]

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.

[opus]

I'm thinking that is a 220 Cummins, which should top out at 2200. Look on the engine plate and see what the rpms are there.

[Stu & Filo. T]

Found a plate on the trans that says it's a model 647 , can't find a plate on the motor

[usmcbay]

55 - 60 My 96 12v is running 2300 - 2400rpm

Your RPM's are probably reading wrong... (99% sure of it) You would have to have a High Power aftermarket 4k Governer Spring Kit to achieve those kinds of RPM's and I'm talking HOT ROD upgrades to handle it. You should not be able to go over 2500 2600 RPM's...

[Stu & Filo. T]

Originally Posted by usmcbay

55 - 60 My 96 12v is running 2300 - 2400rpm

Your RPM's are probably reading wrong... (99% sure of it) You would have to have a High Power aftermarket 4k Governer Spring Kit to achieve those kinds of RPM's and I'm talking HOT ROD upgrades to handle it. You should not be able to go over 2500 2600 RPM's...

Thats what I figured just wanted to make sure things hadn't changed from 30 yrs ago, I've got the maintenance records from the beginning going to have to go page by page to see what was done, I looked at new tach for diesels & they have DIP switches in under the covers did stock gages have them also I wonder

[usmcbay]

Originally Posted by allwthrrider

Thats what I figured just wanted to make sure things hadn't changed from 30 yrs ago, I've got the maintenance records from the beginning going to have to go page by page to see what was done, I looked at new tach for diesels & they have DIP switches in under the covers did stock gages have them also I wonder

I don't know - but here's a local fleet maintenance and repair shop for Buses here in the Houston Area... these guys have been pretty knowledgeable when I needed their help... you may have to call them back though...

www.chalks.com

(713) 672-6344

[usmcbay]

That's a GREAT LOOKING BUS btw~

[nat_ster]

Is this a double thread?

Nat

[Stu & Filo. T]

No this is the one dicussing RPM's

[Crown_Guy]

As you said it's obviously a rear mounted engine, I can tell also by the radiator on the left side
at the rear of the bus. I own and know a great deal about this style of Crown with the rear mounted
engines. Crown originally used Detroit Diesel 6V-53's (which mine have) for a customized bus
developed exclusively for Embree Buses in Pasadena Ca. I bought and at one time owned all seven of
the original eight oredered by Embree. I worked for Embree and put many thousands of miles on
each of these buses while in service and when they came available I managed get all that were left.
The reason for the 6V-53 was purely a question of space available and it had barely enough power
to do the job. They ran at a governed speed of 2850rpm and a road speed of just 63MPH. I would
guess and I have heard that when Crown tried mainlining the rear engine configuration they looked
for a more powerful engine and found a good one in the Cummins. Please don't quote me on this
because I wasn't around much with the later designs but I heard that they were using the 6BT
series turbo-charged and after-cooled to get as much as they could out of it. It's the same engine
series as in all the Dodge Ram trucks and they do indeed turn at upwards of 4000rpms and run
just fine. My guess is yours only has about 200hp or so as my non-turbo 6V-53's with big injectors
and smoking all the time for extra power only put out 195hp. In the end though it was always the
torque that made them weak. Your Cummins probably has a little more useable torque and an ability
to reach and maintain a decent road speed. Take the time and get it to a Cummins engine shop and
have someone specifically ID what you have before you start changing instrumentation settings.
Also rest easy knowing that whatever you find you want to do anywhere on the bus you will be able
to get parts for repairs or upgrade. By the way does yours have full air-ride suspension or is it
sprung? The Embree specials were all full air-ride and rode like a dream. Most school districts did
not spend the money for air-ride but just curious.

[nat_ster]

If it is a Cummins, it's mechanically governed meaning that you can't over rev it if you tried. Once you hit the governed speed, it triggers the defuel, and cuts the fuel to the Injection Pump till the engine has dropped back to a safe speed.

Nat

[Stu & Filo. T]

Pretty sure it's either the tach or the sensor just at idle it read 1600 rpm's, i found a black screw on the back of the tach and played with it a bit I was able to get the reading down to 900 rpm's at idle & as high as 2700rpm at idle. should they idle around 500rpm?