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Old 04-08-2015, 02:33 AM   #1
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Default Cummins 8.3 (6 CTA)

Looking for a little help from anybody that has the older Cummins 8.3 (6 CTA) mechanical fueled engine.

I have a 1996 Freightliner FL70 with the Cummins 8.3, Eaton 6306A, 4.33 rear gears and 11R-22.4 tires. Its current setup is a 26í flatbed and GVW is 34k. It buzzes along at 2100 rpm at 60 miles per hour. To me, that just seems too high. The truck has been retired from commercial use and I am converting it to an RV tow rig and I would like to squeeze some fuel economy out of it.

I am trying to find out what the factory recommended cruising RPM should be or several people that are running the same engine to compare it to. I have found tidbits of info that it should run at 1650, 1950, 2150 rpm, or 200 rpm over max torque, or 80% of rpm window. Nothing is consistent and nothing to verify it with.

I can switch transmissions, gears and tires around but I have to find out what the actual cruising rpm should be first.

Any thoughts or ideas?
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Old 04-08-2015, 10:10 AM   #2
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What I have found is that with the lower RPMs comes better economy. You don't want to be low enough to be lugging it either. Get it to where you run the lowest rpm and be able to pick up speed easily with relatively light throttle.
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Old 04-08-2015, 01:26 PM   #3
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The goal is to lower the RPM for improved fuel economy but I don't have a base line of what the factory says it should be.
The flat bed is coming off and the wheelbase is getting shortened so there will be less weight too.
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Old 04-08-2015, 07:47 PM   #4
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I would look for gears to keep the RPMs about 1500+ on your engine. My 5.9 will pull good above that and does quite well at 1600rpm. I get up to 14.5 mpg towing my fiver at that rpm.
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Old 04-15-2015, 02:47 AM   #5
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I heard back from Cummins Customer Support today. Thought I would share this with you and for any searches in the future.

Thank you for that ESN. I was able to pull up the information.
It actually looks like the best RPMs for you would be about 1500-1700 RPMs. Here is what that will give you.
1500 RPMs will give you 172 HP, 602 lb-ft of torque, and you will burn .328 lb/HP-HR of fuel
1600 RPMs will give you 179 HP, 590 lb-ft of torque, and you will burn .330 lb/HP-HR of fuel
1800 RPMs will give you 192 HP, 560 lb-ft of torque, and you will burn .339 lb/HP-HR of fuel.

As a recap, my engine spins 2278 RPM at 65 mph. Customer support recommended 200 rpm more than peak torque. So, it looks like I will be shooting for the 1700 rpm range at 65 mph.

Time to start calculating ratios.
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Old 04-15-2015, 07:55 PM   #6
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Something that would put you about 1550 RPMs for the speed you want to run would be perfect!
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Old 04-15-2015, 09:01 PM   #7
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I'm with Mntom. I think 1700 is kind of high for cruising.
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Old 04-15-2015, 10:19 PM   #8
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Remember that older, especially mechanical, engines were designed to operate differently than newer electronic engines...they do not like to pull down low like new engines can and the result is smoke, poor economy, lack luster performance and shortened engine life. Trust what Cummins recommends. They have all of the data to back up there recommendations....for the most part.
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