Cummins 8.3 (6CTA)

Lugnutz · Apr 7, 2015

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 26’ 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?

greasytshirt · Apr 8, 2015

What rpm is peak torque?

Lugnutz · Apr 9, 2015

I dont have any verifiable information or charts as if yet. Its a new truck to me and I have only driven it one time so far. I was trying to get factory info on it for a baseline. Its amazing that the information is so hard to find on the net. I have an email into Cummins but have not got a responce yet.

greasytshirt · Apr 9, 2015

Whats the CPL code?

Rated horsepower?

tknight · Apr 9, 2015

My 8.3 cruised with an Allison automatic 4 speed at 1750 all day long, this was in a 29,000 40' Motorhome

Lugnutz · Apr 10, 2015

The tag on the timing gear cover shows a CPL 2101 and I also found a place where it showed advertised horsepower rating 210 @ 2200 RPM.
I had a chance to drive it again this morning. Seems like the power starts pulling about 11 to 1200 RPM and than hits a wall at 21 to 2200 RPM. Don't know if that helps.

greasytshirt · Apr 10, 2015

Do you know whether the transmission is a FS-6306A or FSO-6306A? They have different gearing.

Lugnutz · Apr 11, 2015

It's the FS-6306A. Non overdrive and 1:1 ratio in 6th gear.

greasytshirt · Apr 11, 2015

So I've been theorizing a bit.
Generally speaking, the best BSFC numbers are present right before peak torque on these engines. This is based on various graphs I've looked at. It'd be nice to find the one specifically for this calibration though. BSFC tapers off past the torque peak, first slowly then more quickly at peak HP, then even more rapidly as the engine hits rated RPM.
But you don't want to run the engine directly at peak torque. If the slightest load is encountered, you'll have to downshift. A few hundred rpm past peak torque is needed to give the vehicle time to react when a load is encountered. In theory, not having overdrive is a benefit in this case. Since your target is to find the best rpm at cruising speed, you wouldn't need overdrive to do that, and there will be fewer frictional losses in the transmission by running it in direct.

It sounds like the torque starts really winding up around 1200, but where the peak is, I don't know. I'm assuming that the answer lies in the 1300-1600 range. Certainly much lower than the current 2100 at 60 mph.

greasytshirt · Apr 11, 2015

That has a Bosch P7100 pump on it, right? Well, that makes this deal a lot easier because there's a few easy things you can do to that pump to add power.

Lugnutz · Apr 11, 2015

You are theorizing the same questions that have been running through my mind. I can see where in some cases these engines would need to run the higher RPM’s. Perhaps, industrial generators, Marine applications, Farm equipment to name a few or if my truck was under a constant load, but what I am wanting to do with it I don’t believe it’s necessary for the higher RPM’s.

Tknight mentioned his 29k, 40’ MH cruised at 1750 rpm. I would be curious to know the trans and axle ratio and the other variables.

It does have the P7100 fuel pump. Yes, they can be turn up for more fuel and higher RPM but I’m trying to go the other way. Plus, the truck is subject to emission testing.

On another not, I finally got a reply from Cummins and they are asking for the serial number on the engine. I will send that over to them and see what they come back with.

sewmun · Apr 11, 2015

I bought a 01 FL70 5yrs ago & it has the ICS electronic version of the 8.3 & it came w/a 4.11 gear that I changed to 3.42 since,the changed 70mpg from 2450 to 2000, I usually run 63mph & 1700 rpm getting over 10mpg.I also have a tall box 13'6" which kills the mpg the faster I go.Best cruising rpm I've found is around 1600 for your engine,mine with the 4-valve head needs to be spun alittle faster.Your peak torque is probably the same as mine @ 1400.

sewmun · Apr 11, 2015

I found this:
http://www.cummins-engine.com/tag/c-series

Lugnutz · Apr 11, 2015

sewmun said:
I found this:
http://www.cummins-engine.com/tag/c-series

I saw that in my searches but nothing is listed for the 210hp engine. It sure does look promising for figuring out the cruising RPM. According to the email I got back from Cummins the best RPMs for the engine is around 200 RPMs more than the Peak Torque specs. He asked for a serial number and I have emailed the info back to him. Lets see what comes back.

Lugnutz · Apr 11, 2015

sewmun said:
I bought a 01 FL70 5yrs ago & it has the ICS electronic version of the 8.3 & it came w/a 4.11 gear that I changed to 3.42 since,the changed 70mpg from 2450 to 2000, I usually run 63mph & 1700 rpm getting over 10mpg.I also have a tall box 13'6" which kills the mpg the faster I go.Best cruising rpm I've found is around 1600 for your engine,mine with the 4-valve head needs to be spun alittle faster.Your peak torque is probably the same as mine @ 1400.

That's interesting, everything I found regarding the two newer engines (ISC, ISL, 24 valve head) said they didn't spin as fast as the 6CTA engines. The explanation was because of the fuel system and 24 valve head.

tknight · Apr 12, 2015

All I know is Allison told me it was a 4 speed automatic that is commonly used in garbage trucks and city snow plows basically it's bullet proof don't recall the model as my ex wife stole it and sold it when we got a divorce..... Women ya can't kill em or sell them for parts
At least not in this country....

Lugnutz · Apr 15, 2015

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.