Increasing the power of the Detroit Diesel 6-71

[Audiomaker]

Hi everyone, my name is Sean and this is my first post here so hello!

I do not own a bus (sorry), but I do own a commercial vehicle that I am converting to an RV.

I joined this board because countless Google searches pointed to this board as a place where many people have experience with the DD 6-71 motor that my rig is powered by, and also because I am doing a RV conversion (just not from a bus).

To start with, I would like to ask some questions that would have generic answers, but keep the spec’s of my rig and it’s purpose in mind also.

My “rig” is as such:

1983 Hendrickson / Calumet Coach
39’ long
Tandem axle (with inter-axle lockout)
22.5” tires
Curb weight: 34,500lbs
Detroit Diesel 6-71 turbo (not inter or after cooled that i can tell) horizontally mounted near the center of the truck.
Allison 5 speed auto
Engine reported by seller to have been rebuilt in 2000 with around 145,000 current miles.

My experience with Diesel trucks is limited to owning:

1999 Dodge 3500 w Cummins
1990 International Crew hauler with DT466(T)
2000 Freightliner FL-60 with 3126 (300hp/980@12,000lbs)
Plus rentals and other people’s diesel trucks I’ve driven over the years.

I mention these so you know where I’m at.

Now the reason I’m asking the question is because this rig is a dog.  Even in the lowest powered diesel I’ve owned (the DT466), my driving skill allowed me to negotiate the highways.  This vehicle seems so underpowered to me that I feel I need to find the problem, or if there isn’t one, find a way to get at least another 20% out of the motor just to be safe. 
I am not trying to “hot rod” the motor, or pass the semi’s on the passes, and fuel economy matters to me, but there is a point where needing more power is a very mature thing to require.

Currently, the motor seems to run fine.  This is my first two stroke, so “running fine” is just a guess, but it goes pretty smooth without much smoke unless you’re flooring it going slow enough to see it.
It hunts at startup for about 20 seconds and calms down to a nice idle (although high at about 2k).
There doesn’t seem to be anything obviously wrong with it (noises, vibrations, leaks..etc), but it just feels so underpowered that I can’t imagine a truck manufacturer letting it go out the door like this.

It is currently getting about 7mpg avg, but mostly highway (calculated from the 800mi trip it took to get it home).  This, again, is at 34,000lbs.

In the passes, I am by far the slowest thing on the road (well there was that cyclist I passed once).  I have to fight even low percentage grades (1-2 even).
It’s embarrassing and frustrating having obviously loaded Class 8’s have to struggle to pass you to keep their 10mph, and I know they’re tugging 70 plus.

Now the CAT 3126 in my FL-60 would have pulled this entire truck and the Sportchassis together faster, but from my reading, this turbocharged DD should be putting out similar power (300ish).

This is going to be a long first post…sorry…

One thing I noticed on my other TD’s was a lot more turbo whine than I’m hearing with this one.  In fact, I can’t hear the turbo running at all (and there’s no boost gauge as found). Of course, the motor and the turbo are mid-vehicle, so there’s that.
Suspecting this, I pulled over on my first voyage and pulled the intake hoses and did verify that the impeller turns smoothly, and was indeed spinning even at idle…so hmm.
it felt ok for what a big beast it is (man…like twice the size of the one on my CAT).

800mi of sheer torture later, I have it home and have been on the conversion for a couple months, but in the back of my head I’m knowing that I’m going to have to address this power issue before it goes back on the road (and I may end up living in it for awhile so it matters).

Ok, Ok, so here we are (and thanks).

Some Googling about increasing the power of the DD led me to understand that increasing the injector size was the most common route (no idea what I have… I think I have moped injectors).  This in turn provides more boost because of more combustion.

Right off the bat, this strikes me as an antiquated way of increasing power.
You see, every TD I’ve had (well I own a 300hp gas turbo bike too) utilizes a waste gate in it’s turbo setup.  I’ve never owned a TD engine that didn’t.
The idea is that you can over boost the engine to increase horsepower and then manage egt’s and such as required.
Nobody in my former world just dumps more fuel in to see a resulting boost increase after the fact.  That seems so wasteful (or waste-gate-less) in my eyes.

While I wish I could give you numbers, I haven’t yet installed a boost gauge or an EGT gauge (I will), but man… it feels like the truck didn’t come with them because there was no point in it (i.e…. so little boost that it wouldn’t matter)
I am used to a healthy diesel screaming away up a grade… making dogs wince miles away…etc.  With this I hear nada, and that makes me wonder?

I did ask a friend about this who is a diesel freak, and he said the DD’s don’t whine much.  My logic tells me that there isn’t really a way to make a hard working turbocharger “not whine much”, but maybe a lethargic one?

Again, sadly, I have to compare with the CAT 3126.  That motor was getting 10-15 pulling 24k, and as much as 18 unloaded.
Taking a motor that’s getting 7mph and slow as heck, and just dumping more fuel in…I’m really hoping isn’t the best answer.
We are all living in a world now where our MPG matters quite a bit.

That said… first actual question…

Is there a way to increase and manage the boost to the DD 6-71?

I had wondered if there were waste gate turbos that could be installed, and possibly some electronics added for management?  Could one use a 8-71 turbo…do those put out more air?

Secondly,  I’m looking at both water/methanol and propane injection.
Being an in-progress RV conversion, I already have a 30gal propane tank ready to install.

Thirdly, maybe there is something wrong with my motor (as in some system in it not functioning and causing sluggish power or ?
The question here is how does one find that one-eyed master of all things DD who knows what’s wrong with the system as you’re pulling up?
Simply taking it to the local shop, I suspect would end up being pricey and without satisfaction.  Where is the guy who knows how to get the most out of this motor?
With so many around for so many years, there must be one in each state?
(I’m on the west coast by the way).

What I’m looking for is a path…  change this, add intercooler from a Kenworth… get this kit, add water… chant… whatever.

I figured some bus owners have discovered out how to get these things to pull a hill without worsening the fuel economy so here I am!

Thanks for listening!

Sean

[RobSedona]

hello and welcome. Was that a medical coach? as in  Mobile CT Scanning Coach I have seen them used that way it should make a good house car

[luvrbus]

Check the tag on the valve cover the injector size should be there if not pull the valve cover and check the tag on a injector The 6L71T with 75 injectors was 275 to 290 hp with 801 ft lbs of torque 80's will get you close to 300hp, if you have 70's or 65 it will be in the 260 hp range the inter cooler will help but you will need to change turbos or you defeat the purpose of the inter cooler 

Where on the west coast are you located those little guys lose 2% hp for every 1000 ft above sea level fwiw

good luck  

[Audiomaker]

Robsedona,
Correct!  Ex- CT scan mobile.

However, there's a lot to converting these things (and what you end up with), that isn't obvious when you first jump in.
Time will tell if I will decide that converting a bus was a better idea.

Aside from the very odd layouts and onboard systems, they are lead lined (and it's not removable), so they're heavy.  Also... no windows!
I'm all good for the post apocalyptic fallout though... I suppose.

What they DO have that makes it work for me is a side lift gate (patient gate), and I have an aging Great Dane who appreciates this, as well as a motorcycle that rides along.

Beyond that,  it's a beast.  It's the testosterone RV of the park... if you're willing to be open minded and deal with some odd looks, a different RV experience, and some major hurdles in doing a conversion.

Anyway...good guess!

[Audiomaker]

Luvrbus,

Thanks for the response but I'm so new to this that I wouldn't know *what* to change the turbo *to*.



(edit: I'm on the coast...Southern Oregon for the moment)

[luvrbus]

What is the serial number of the engine it may have a after cooler a 6L71T does not a 6L71TA does

good luck

[Audiomaker]

Luvrbus,

I don't have that printed anywhere, and it's dark out now, so crawling under there will have to wait until light.

However, having inspected the engine, I have not come across anything that looked like a air/water heat exchanger.   The intake is pretty straight forward... air cleaner / Turbo / manifold unless they hid something in there pretty good.

I appreciate the info' on the injectors, and I will find out.  Still, that seems to be the standard route of increasing power with these, and to this I am shocked.   I mean, this is a very popular engine even today simply because of the #'s of them out there and I have a hard time believing there isn't some aftermarket system that does more than dump more fuel into it.

Honestly, I can't wait to install a boost gauge... but then again, my perceptions of boost come from the 4 stroke world, and I wouldn't even know how a 2 stroke would react to being force inducted beyond what was considered practical at the time?
I mean... if I install a sensor and it's reading +2psi, I wouldn't know if that's right, or even if adding more would grenade a 2 stroke.

What I do know is that generally the most powerful and fuel efficient engines are getting some serious air, and it doesn't feel like this is, so that's why I started with the questions about alternative turbochargers and perhaps waste gated ones.
If that's not an option, then I'll look at methanol or propane, or whatever anybody else uses to get up the hill.


Sean

[sparkplug188]

Sean,

Welcome to the RV conversion madness   There are members on this board that know the Detroit Diesel 2 stroke engines inside and out.

If you haven't done so already, check the throttle linkage.  I had the same symptoms on my bus-- constant high idle and very little power under load.  The throttle linkage was binding causing the throttle to not open past 50% and then not return to idle.  The Detroit Deisels got a bad reputation for being under powered when the real problem was often poor throttle adjustment.

It may also be worth checking the fuel lines for a partial blockage.   It is possible the pick up tube inside the fuel tank picked up a piece of rust.  A partial blockage can have the same affect as a stuck throttle that only opens 50%.  If you do check the fuel lines, the fuel system will need to be primed before the engine will start again.  Also, keep in mind there are one way check valves on the lines and an orifice on the return line.

Another thing to check-- Make sure the fuel shutoff lever is fully open when the engine is running.  

When diagnosing a problem that can have a million different causes, it is best to start with the inexpensive basics then work your way up as you gather more evidence.  It would be foolish to turn up the HP if it is not getting enough fuel because of a poorly adjusted throttle, half closed fuel shutoff, or a piece of rust stuck in the pickup tube.

Edit:  Also, the tubo won't make much boost if the engine is only getting enough fuel to operate at half throttle.  You really should be able to hear the turbo working on a long steep grade.

 ~Spark

[Audiomaker]

Thanks spark'

I will check all of those.

The thing is though... I'm getting 7mpg highway, and I more or less have the thing floored to get to 60mph on a flat, so that fuel is going somewhere.

I don't know how the busses compare to my 34,000lbs and MPG, but maybe that's normal?  Ack... I hope not.

What I'm afraid of...either by changing the injectors, or freeing up the throttle linkage or lines... is that somehow it's possible to get even less MPG that I currently am.  Right?  I mean "it's fixed", and I can get up a hill, but I'm getting 5mpg?

Put another way, it is the combination of lethargic performance *combined* with poor fuel economy that makes me hope the solution is air related, not fuel.  Seems to me if things were in order, then at 7mpg, I'd be flying up the hills even at 34k (I would be in my FL-60).

Now I don't want to come on here and ask the impossible.  If that's how the DD is, then that's how it is, but I have been on other boards where the ol' truckers claim to be pulling 80k up the grades at *almost* what the 4 strokes were doing, and I can tell you that if this rig weighed 80k it would be going backwards only.

The marine guys report 435hp out of these things, but they never say exactly how they are achieving it.  MPG (or GPM in that case) is important even there.  Are they just over fueling them as well?

I guess I was hoping someone would say "Oh yeah...Banks Power has a kit... 100+hp... we all use it".    Remember, I came from Dodge/Cummins where you threw in a chip. 

One thing though... you say that you hear your turbo.  I don't.  It's part of the reason I really need a gauge.  Because of the placement of the engine, combined with the fact that I'm basically sitting on top of a whining hydraulic fan motor, I can't really be sure.
Trust me, I've stuck my head out the window on the highway and listened for that pitch, but I can't be sure.

I know the class 8's have boost gauges in their 6-71 powered tractors... does anyone know what the "normal" boost range is, and has anyone ever replaced the turbo with something that made more?   
I know that my understanding of TD's is that if you give it more fuel, it should be getting more air....usually first... not after the burn.

By the way Spark'... going down the road, I can floor it and max out, but then I can *stand on it* and get a little more, so that throttle linkage sounds like it might be an issue and I'll check.  Still.... 7mpg/34k/slow/flat?

[Audiomaker]

Here is the vehicle we are talking about.

I will have a lot of interesting stuff to share about it in return for everyone's gracious help.

I call this photo "Top Speed"

[RJ]

Sean -

Most 40-foot coaches with 2-stroke Detroits and Allison automatics average 5.5 - 6.5 mpg, both in revenue and RV service.

Thus your rig is in the fuel mileage ballpark.

Torque is what gets you "out of the hole" and "up the mountain" - and a 6L71T is lacking compared to 4-stroke motors.

Marine HP ratings of the 6L71T are accomplished with huge injectors, big turbos and a whole lot more cooling capacity than you could ever stuff in your rig - it's called the ocean!

Rear axle final drive ratio will have an effect on fuel mileage, any idea what you've got?

Wind resistance plays a rather large role, especially when you're trying to push a vehicle with the aerodynamics of a brick thru the air at 65-70 mph.

The combination of 34,000 lbs, an in-line six with no torque and a slushbox transmission = 20 - 25 mph up Rocky Top.  Simple physics.

Depending on the turbo, some produce as little as 5 psi on a 2-stroke (smoke turbo), others can produce 25+ psi of boost.  More boost = more heat = more cooling capacity needed.

Sorry to be the bearer of depressing news, but such is life in the two-stroke Detroit world.

FWIW & HTH. . .

 

[wg4t50]

Most interesting, at this point, I am leaning toward finding a shop/mechanic who KNOWS how to work on the 6-71T engine, it is true, they tend to be on the weak swide, that does not mean dead, yours sounds closer to dead.
Things I would be interested in are why does it idle at 2,000 rpm ?
What is the rear gear ratio.
Your 34,000 lb should not be such a slug, have driven 6-71 non turbo, with 48' trailer, yes it was not pulling grades like a 350 Cummins, but it was not all that bad as your describing, why I feel a proper mechanic is highly needed.
I know it can be made better with the factory standard parts (injectors), would also be looking at the plumbing of turbo, make sure all the exhaust is going thru the turbo, and all the pressurized output if getting into the airbox and no leaks.  Any leaks would surely kill the performance, A boost gauge is nice to see what is going on, Exhaust temp gauge is not a big need at this point.
Everyone has ideas, but being a Detroit Diesel lover, I know it is suffering  from poor mechanics.
Good luck
Dave M

[garhawk]

Hi Sean,

You have all the systems of a less than good performing exhaust track.

Tie a ten gallon garbage bag over the exhaust pipe end.  Have someone hit the starter - you should immediately see the bag fill.

If not, begin your search just before the muffler.

[sparkplug188]

Sean, Some of our buses came with a damper valve plumbed in the air intake for shutting down a runaway engine.  It could be partially closed or getting sucked closed under heavy throttle.  Again, it is worth checking as many possible external problem sources before diving into the internals of the engine.  Like the others have said, a hole in the intake or exhaust can create as much of a problem as a blockage.  When you have eliminated everything simple and cheap, it will be time to hire a set of experienced hands for checking compression, injector adjustment, and power balance.  An oil sample test can give you clues about the condition of the engine before opening it up.  One word of caution, stay out of the governor box. Messing with the governor will cause more harm than good.

By the way, your RV conversion looks like it is ready for battle.  I am jelly 

~ Spark

[Scott Crosby]

My propane injection on my 6v71 is amazing on hills and grades for keeping speed and actually passing vehicles.   The full install is on my site. Here is a video Home made detroit diesel 71 series propane injection system

It's not a risk free solution but I love it on my little 6v71N, a T install should perform even better.