Optimum Engine Rev's ? ? ? ?

[Gary '79 5C]

I have been reading all the posts and have been wondering as to just what is the optimum RPM's for my engine/trans/rear combination.
I have a Saudi 5C, 6V92 w/ turbo, a 640 trans, and I do not know the ratio in the rear. With assistance I will try to move the bus and count in's and out's.
The engine has larger injectors for additional HP.

I am extremely vigilant of the engine temperature, and always is below 190 deg. The engine and radiators were rebuilt 18K mile ago. After which the engine was dynoed and overheated, Radiators (oversized) were pulled, cleaned, replaced. Another dyno and all went well.

I currently run on interstates, at 68 - 70 mph, with a tach indicating 2100 - 2150.
My long winded question is, Am I running too high of RPM's for a reasonable longevity of this engine. I put about 12,000 miles on per year, 90% interstate driving. I hear as to the life one can expect out of a DD, mine is a non DDEC. I change fuel and oil / filters every 10K.
Any Thoughts...
Gary
And many, many thanks for all the insightful posts.

[RTS/Daytona]

Gary

Check this out ---> http://www.powerlinecomponents.com/literature/detroit_diesel/brochures/6v92ta-tta_automotive.pdf

see fuel per hr at RPMs graph and Horsepower / Torque curves

max fuel economy - 1500 RPMs

rule of thumb for D/D 6V92TA
you can increase fuel efficency approx 1.2% for every 100 rpms less than 2100 down to about
1500 rpms

[luvrbus]

Gary, the best rpms for your engine is where Pete said  it was design for 1500 to 1800 rpm if you do this you will not have as many leaks and get better fuel mileage and a long lasting engine.if you change to a 3.36 gear it will put you in 1800 rpm range at 68-70 mph      have a good day

[Gary '79 5C]

Thanks to both Pete and Luvrbus, I had a problem bringing up the DD page but will try again.
I also will try to determine the ratio currently in the bus. I would imagine it must be higher than the 3.36 you suggest.
Bringing the RPMs down and increasing the mileage would be great, However not sure as to how difficult it will become to climb hills, or mountains out west. I am currently on the East Coast and travel mostly here.
What would a set of gears and change out set me back ? This VS the fuel efficiency increase ? Added life to the engine ?
If it is the difference of getting 200K engine miles @ 2100 RPM, VS 350K engine miles @ 1600 RPM. I will not live to see that difference...

Thanks, I will check the existing ratio, and retry bringing up the DD page.

Gary

[TomC]

Based on 68mph @ 2100 rpm, and 485rpm for 12R-22.5 rubber, that works out to be a 4.33 rear end ratio since the MT640 is a 1:1 ratio in fourth.  If you had a 71 series, I'd say leave it alone, but the 92 series likes to be run slower-as mentioned in the 1500-1800rpm range for cruising.  You NEVER want to rev a 92 series above 2300rpm-because of valve float with the larger valves than the 71 series.  If you want 1800 at 70mph, you can regear to either a 3.21 or 3.07.  3.21 will give you 1816rpm at 70, the 3.07 will give you 1737rpm at 70. Just to make sure, lets say your bus weighs in at a max of 35,000lbs (like if you pull a car).  With 3.07, the 6V-92TA putting out about 900lb/ft at start, 3.49 first gear, 2:1 Torque converter ratio, that works out to be an almost 25% startability.  At Freightliner, we want 16% for on road, 20% for on/off road and 25% for off road.  So the 3.07 should be no problems for you as to startability. As comparison, with my V730 and 4.55 gears, my startability is about the same at 26% and don't have problems starting on grades, even pulling the car.
With the 12,000 miles a year you do, it would be worth it to change out the gears.  But do it all the way. No bigger than 3.21 and no smaller than 3.07.  Your fuel efficiency and your engine life should increase by around 10-20%.  Good Luck, TomC

[Tom Y]

Tom C, I don't think you can get lower than 3.33,3.36 or 3.38 gears for the 5 w/dropbox.  Tom Y

[Gary '79 5C]

Tom C,
You brought it to my attention that I forgot to include that I run 11 24.5 tires. Now I must do the final math as to the total ratio.
But as you indicated I must have a fairly high ratio. The PO had spent a fortune with this bus, Sam Caylor doing a the conversion. The PO might have chosen the higher ratio as he drove frequently to the Denver mountains area ? ? ? I do not believe that he had just left them in if they were original from the Saudi shell.

Anyway thanks, I still think it is worth getting this to a much lower RPM range @ 70MPH.
Gary

[Barn Owl]

Links to the other engines:

http://www.powerlinecomponents.com/literature/detroit_diesel/brochures/

[bobofthenorth]

I didn't want to hijack this thread but check the MPG & RPM numbers in the MPG thread I just started for some 8-92 datapoints.

[TomC]

3.38 gears with 11R-24.5 (478rpm) will give you 1885rpm at 70-still alot better than what you have now! Good Luck, TomC

[Gary '79 5C]

Thanks TomC,
I will scout out a replacement set and depending on cost I am very much inclined that the nearly 300 RPM reduction at highway speeds will be worth it at the pump. I am just under 7 mpg currently. My conversion is not overly heavy, and being a 35'er I am probably under 26,000#. I would guess an increase of 2 - 3 mpg @ 70 mph.

I am kind to the engine in hopest that it is kind to me in longevity.

Thanks, & have a great day.

Gary

[Stan]

As far as I know all MC-5 buses (Saudi may be different) came with a choice of 3.33 for highway bus and 4.10 for shuttle bus.

[Gary '79 5C]

Stan,
You may very well be correct, in my case I believe that mine had the rear curb side door. It could have a 4.11 transit rear ratio.
I did not get to check this as of yet.
Thanks,
Gary