Howdy Folks,

I teach High School Automotives & know the theories but this is the first diesel I've owned. Please excuse me if I ask ignorant questions or supply redundant information as I am just learning about this.

I have a '95 Freightliner with a 5.9 12Valve & a P-pump. The gentleman I purchased it from said this engine (190 hp by the sticker) redlines at 2600. He however installed a 3000 rpm spring kit in the pump. He sounded quite knowledgeable about these engines and claimed to have built several high hp ones. I have a pretty sensitive BS detector and he seemed for real.

My goal is to turn this unit into a motorhome/toy-box hauler/non-commercial service truck. I believe I am running rpm's that are way too high on the freeway. Engine rpm’s are as follows:

It tach's;

2500 @ 70 mph

and

2200 @ 60 mph

I have two pertinent questions:

1. I was under the impression that a redline means that if you run the engine at this speed for very long, you will start poking holes in the block with the connecting rods. How can a fuel delivery change increase the redline of an engine?

2. What should the redline of this engine be?
I have a rear-end surgeon who is going to change my 4.33 ring & Pinion for a 3.42 set. This will decrease my 70 mph rpm to 2K which I am convinced will only be beneficial. Any comment or opinions are welcome and encouraged.

Thanks for your time,

Mikey

P.S.This was the craig's list picture. I'll snap some more as soon as I make some changes.

 

the "redline" that he is talking about is actually a governed speed. changing the springs in the pump allows it to keep fueling past the 2600 RPM. the motor will take 4100 RPM before bad things start to happen. this is a good link for info about the motor. it is a dodge site but they have a section about the 5.9 CTD and the motor is the same. http://dodgeram.org/tech/dsl/Facts/94specs.html changing the gears out should help your highway milage but will be harder on your tranny/clutch if you tow real heavy.

 

The redline is governed and 2600 is normal governed speed. The 5.9 cummins will start to defuel anywhere between 2300 and 2600 rpm. The 3000rpm gov. springs he installed is one of the most common modifications made to the 12 valve cummins. With this spring kit installed the engine will start to defuel somewhere between 2800 and 3200 rpm. If you go over 3600 rpm you will need to install 60lb. valve springs so your valves won't start to float. The 5.9 cummins can run for ever at 3000 rpm. with no damage. You may want to go with somewhere around 3.73 to 3.90 gear ratio for your rear diff. I run 3.54 in my truck and as you say it runs 71mph at 2000 rpm. I am satisfied with these gears now that I have increased my engine power but with the stock 180hp it sucked. I pull a 10000lb 5th wheel and it works well but there are times when I would like more torque. That rpm I gave you is in O.D.
Is your truck a stick shift and how many gears do you have? This will help in deciding what rear diff. gears you will want. How much combined weight do you intend to have when you are done? Are you going to increase power on your engine? what hp/torque range are you building it to?

 

The 5.9 cummins sweet spot for cruising is 2000rpm but if you do not have enough gears you may find that you have problems with having the right torque available for acceleration and pulling power in lower gears. The governor is in the injection pump and the pump will not give you anymore fuel at 2600rpm. It is a gradual fuel decrease and so it actually starts to defuel the engine earlier in the rpm range.

 

Excellent info, Thank you.

The truck will be pulling a relatevily light trailer (compared to what it was built to haul) somewhere around 6-8K depending on how many dirtbikes it contains.

It has an Allison 4 speed automatic tranny behind it. I honestly cannot remember the # but it is a commercial tranny, (Not passenger truck). My tranny shop said it is an overgrown TH400 and is extremely bullet resistant.

This truck is supposed to weigh around 13K as is, prolly add another 2K for the motorhome conversion, and with the trailer have a GVW aorund 22K. So while the load is not that heavy, the total GVW will exceed most pickup truck capacities.

It is inevetable that as I learn more about HP mods It will invariabley get some. It is good to know that 3K will not harm the engine. If what the Gentleman said is true (there is 2K on the current oil) then this is a very clean running diesel. The oil is still transparent, much unlike many of the 3K diesel oil changes I have done in my auto shop.

Thanks again Lifer and Cmac for the info and please keep it comming!

Mikey

 

You will find that in stock form the oil will stay clean well north of 5k miles. If you increase power the oil will get more soot in it earlier. I have a bypass filter in my truck and run rotela 5,40 synthetic and change my oil one time a year no matter how many miles I go in that year. It is usually between 15 and 20 k. The oil is still not black when I change it . I have increased power to 325-350 hp and have had no problems. Do you frequent the Thumpertalk web site? The Cummins will make near 400 hp with no internal mod needed to the engine. And still be good for towing. If you do not tow there is a lot more hp to tap than that.
Does your trans have a locking torque converter? And is 4th an O.D. ?

 

The very first thing you need to do is fix the Killer Dowel Pin (KDP). It is a dowel in the front timing gear case that can gradually work its way out and fall into the timing gears and more times than not punch a hole in the timing case. If you do a search on KDP you will find a lot of info on this. To fix it after it falls out can be a very expensive and or time consuming job. If done before had it will cost you under $100 and a couple hrs work.

 

Fourth gear is a 1:1 and regretfully no, it does not have a locking torque converter. All tranny Guru's so far have said that while this one is the "best" I am stuck with it's limitations. No OD, and stay with the torque converter I have.

Thanks for the Info on the KDP, I'll check into that ASAP!

Keep em comming!

 

That's a pretty cool rig. Check out www.dodgeram.org for lots more info.

 

From what your telling us I would think pretty hard before changing your gears. You may want to look into an OD unit to put behind the trany instead. If you do change gears I would look at 3.90 or 4.11. If you raise your gears too much I believe you will have a hard time getting going. 1st gear will be pretty high and each shift it will fall on its face.I am talking loaded weight, empty it may perform ok but not great. You may want to make sure your trany is making use of your new found rpm by having the shift pionts raised to take advantage of the rpm. As far as the engine mods, find out the hp and torque limits of the trans and build your engine to stay just a little below that. It should not cost much to reach that and you will be much happier with the truck. By the way, you found a gem of a truck to make into an rv. You will have one of the best drivelines available and a nice looking rig, about the only thing that could be better is a 4 door.

 

My truck when out of OD runs 2500 at 70 mph with my 3.54 gears so my trans gears are much different than yours but the overall ratio must be very close. The more I think about it the more I believe you will be making a big mistake to raise your gears. I strongly recommend you look in to an O.D. unit. They are pricey so you may never break even. With your 3000rpm gsk you really can live without any changes to your gears.
I can tell you that my milage is pretty much the same in or out of OD when pulling a load. The egt's are usually slightly lower when out of OD. Now empty its a different story, OD makes a big difference in milage.
You will be around 15k without a trailer so I believe you will only gain 1-2 mpg with an OD.at the most.
With my 5th wheel and my DRZ400 on the back and loaded with everything I am around 18k lbs. I get between 11 and 12 mpg. In OD.
Out of OD IM probably around 10.5-11 mpg with that same load. I do have a locking T.C. so you will probably be a 1-2 mpg lower than my figures with your rig. Anyway that might give you some idea what you are looking at .

 

Thank you Wcbcruzer. I'll definately check them out.

 

Thank you Lifer. Something I haven't mentioned yet is a noise it starts making around 2500 rpm. It sounds like:

a very high piched (not shrill) alarm,
or air/gasses escapinng from a small diameter tube,
or if you remember the old Star Trek, a perturbed-off Tribble on steroids.

It starts at around 2.5K and gets louder and the pitch travels up with RPM. If I didn't know any better (And I sure DON'T) I would say it is a turbo blow off. It scares me from going any faster than the 2.5K.

 

What state are you in Mikey? Do you have an egt and boost gauge in your truck? You have one of the best fuel pumps to get the engine to do anything you want. Dont go over 3200rpm without 60lb valve springs and dont add fuel or really push it without gauges.

 

Boost and egt gages are a must for diegnosing problems and knowing what your engine is doing. It could be normal depending how your intake is configured. The previous owner may have removed the ring in the turbo , this makes the turbo wistle pretty load. You may have a leak in one of your intake boots or your wastgate might be making the noise from releasing extra pressure. It is possible that the turbo has a problem too.