Cummins 6bt minimum rpm & MPG at lower speed?

[stillsearching]

I'm trying to find information that seems not readily obtainable on dodge cummins related boards because people aren't as hardcore about MPG experiments as here.

I'm hoping to eventually do MPG experiments with a dodge cummins (which I dont have yet) powered truck but am running into some unknowns. I plan to aeromod it, downspeed the engine, play with different driver techniques and maybe even play with more exotic tinkering in the further off future like a hydraulic accumulator hybrid to see just what's possible.

The truck still needs to be capable of towing 10,000lbs and the big paradox so far is the downspeeding problem. It still needs strong axles and i'm exploring transmissions but one obstacle is I can't figure out what the minimum safe RPM without lugging the engine should be - commercial use engines seem to want to spin much faster for instance and suggest slower speeds could be bad for the engine. I'm for instance curious though if it's something like torsional vibrations or just things like needing faster accessory drives.

The other thing is trying to simulate potential MPG improvements. Since aero load goes up with mileage and i'm hoping to simulate downspeeding of higher ratios i'm wondering if anyone has for instance driven a cummins turbodiesel (preferably older 12 valve as thats closer to what i'll be working with) at lower speeds in like 4th and 5th gears (from say 35mph on up) and figured out MPG calculations for it.

[MkVer]

The only concern I can see with lowering your rpms is going to be your turbo. I installed a taller fifth gear on my diesel VW and at certain speeds (low rpms) although the car is driving fine my EGTs start climbing and I get poor economy. (poor fuel/air mixture)

As for your specific questions about the cummins, sorry I just have experience with my VW and powerstroke.

You may have good results with a transmission controller like the ones ATS Diesel sell.

It would allow you to shift the automatic like a manual thus keeping you in the torque band for towing and down low in the rpms for empty.

[user removed]

Originally Posted by stillsearching

I'm trying to find information that seems not readily obtainable on dodge cummins related boards because people aren't as hardcore about MPG experiments as here.

I'm hoping to eventually do MPG experiments with a dodge cummins (which I dont have yet) powered truck but am running into some unknowns. I plan to aeromod it, downspeed the engine, play with different driver techniques and maybe even play with more exotic tinkering in the further off future like a hydraulic accumulator hybrid to see just what's possible.

The truck still needs to be capable of towing 10,000lbs and the big paradox so far is the downspeeding problem. It still needs strong axles and i'm exploring transmissions but one obstacle is I can't figure out what the minimum safe RPM without lugging the engine should be - commercial use engines seem to want to spin much faster for instance and suggest slower speeds could be bad for the engine. I'm for instance curious though if it's something like torsional vibrations or just things like needing faster accessory drives.

The other thing is trying to simulate potential MPG improvements. Since aero load goes up with mileage and i'm hoping to simulate downspeeding of higher ratios i'm wondering if anyone has for instance driven a cummins turbodiesel (preferably older 12 valve as thats closer to what i'll be working with) at lower speeds in like 4th and 5th gears (from say 35mph on up) and figured out MPG calculations for it.

Check board member Diesel Dave's 1300+ mile 35 gallon tank in his Dodge-Cummins.

http://ecomodder.com/forum/showthrea...kup-21766.html

regards
mech

[redpoint5]

I've heard the torque peak comes on around 1600 RPM in my Cummins ISB. If yours is similar that is probably what you should shoot for. I probably wouldn't go any lower than 1000 RPM for steady cruising. You don't want the load so high compared to your RPM that the turbo isn't feeding enough air. Black smoke indicates a rich mixture and wasted fuel economy.

You'll get better economy with a manual, and the automatic transmissions seem to be garbage or people just neglect to maintain them properly. Mine has been going out for tens of thousands of miles now.

There are performance chips you can purchase for the electric fuel pump flavors of the engines that can improve fuel economy, presumably at the expense of meeting emissions requirements. I bought one such tuner and never tapped the fuel pump signal line because some people reported killing the pump shortly afterwards. In other words, I can't tell you from experience what fuel economy improvement can be had by installing a tuner chip.

One thing that will help is running narrow highway tires instead of mudding tires. It makes a huge difference having a smoother tread pattern and higher PSI in the tires.

[oil pan 4]

If you are even thinking about maybe towing 10k don't use the stock dodge auto transmission. The 47RE. Its rated for 450 ft/lb of torque. A proper 12 valve 5.9L can make 450 torques at idle.
Problem is the converter lockup slips constantly under load with the 700+ foot pounds of torque the 12 valve 5.9L can easily make.

With the manual only problem is the engine can make enough torque to strip the teeth off gears or spin gears on their press fit shafts.

Only answer is the ATS made upgraded transmission.
Which is only 7 or 8 thousand dollars.

Anything that says ATS = $$$$.
Cheap knock off of ATS products are just that, cheap knock offs.
Most if not all ATS products have 5 year 500,000 mile warranties.

Cummins manuals will give safe minimum safe speed and proper oil pressure for operation. The main thing with lugging the engine you have high load and low oil pressure. This causes rod bearings to go bad.
I would say rifle drill the crank, so the rod bearings oil passages get fully interconnected to each main bearing oil port allowing all rod bearing to be oiled through the full 360 degrees of rotation. Most cranks are drilled so each rod bearing gets oil from 1 main bearing oil galley for less than half the rotation.
But that doesn't fix the oil pressure problem at low speed.

Welcome to the diesel towing world. Things wont be cheap. Get over it.
Or you will find your self on the side of the road looking at a huge repair bill that will cost almost as much if not more than doing it right the first time.
If you want cheap stick with a metro.

[stillsearching]

I'd only go low rpm's with a manual, I want one anyway to experiment with pulse and glide. But elsewhere i'd read there were problems either "lugging" (everyone uses the word, not everyone defines it - a torsional vibration is obvious but i'd think that's obvious and am not sure if its lugging or damaging without that vibration) or talk of the accessory drives possibly not spinning fast enough. (like oil pan just clarified it for me but is that the only issue?)

Whats the rated minimum rpm of the cummins for oil pressure reasons? Is there some not too difficult way to supplement the oil pressure at lower rpm maybe some kind of auxillary pump? (I wanted to have some kind of engine prelubrication before start anyways since starting causes most wear, maybe it could be just left on at low rpm) I'm not sure whether ultra-downspeeding is the only way to go though - enough other specific fuel consumption charts. If SFC is the same at 1600rpm as at 1100rpm it just plain makes more sense to turn 1600rpm due to gear loads and everything. Is there a way to measure SFC after mods in the vehicle or is that really expensive? (for mechanical injection)

Running compound turbos is possible to shift the torque peak even a little lower - i'm not looking to go crazy with power so downsizing the little turbo then putting on a big normal turbo would keep EGT's way down and improve torque further to hold the load. None of that solves whether the rod bearings would be getting enough oil. Would rifle drilling make that big of difference?

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PS i'm familiar with diesel dave though he makes his mpg pulse and glide not steady state. If wishes were fishes i'd love to see steady state mpg figures at say 35mph thru 70mph every 5mph or so for different axle ratios, probably in both 4th and 5th since 5th wont go down that low I don't think. I could then calculate "with X aero improvement it should be like Y". But I don't know if anyone has done that. :P

[oil pan 4]

The minimum rated RPM and oil pressure just depends on each individual engine. How much clearance there is between crank, rod and cam bearings.

I think the minimum RPM for lugging the engine is whatever cummins recommends for the minimum oil pressure and at the speed that does not give you that resonance vibration. That vibration will just tear up motor mounts, clutch in a manual transmissions, fatigue any metal line allowing it to break.

Don't worry about spinning the accessories too slow, those are easy fixes that can be fixed more than one way.

I don't think you can supplement the engine oil very well. I would bet that the oil pump moves 3 or 4 GPM at idle and about 30GMP at full speed.
The only electric oil pump I have found that might be used is only rated from 2.5 to 3.5 GMP. That may or may not be enough to supply the engine under load.

Another way to increase oil pressure and spool the turbo faster is run an oil restrictor to the turbo.
I ran a .030'' restrictor to my holset HE351VE for over a year until a piece of dirt plugged the nozzle. I punched the restrictor out to 0.050'', doubling the flow, added an inline oil filter and I hooked up the coolant lines.
Have not had any problems since.
I read the holset manual for turbo oiling, the 0.030 restrictor did not give quite enough oil to the turbo per their recommendations but it ran fine. I pulled the return line off the engine and put it in a jug and ran the engine to measure oil flow through the turbo.

[MkVer]

Lugging an engine means a lot of bad things. It all depends on the manufacturer and modifications as for what will die first.

Basically your asking an engine to perform well below it's power peak. RPM's can't supply oil flow, coolant flow, alternator, turbo air/exhaust flow, etc. pretty much all your engine functions are at minimum. Cylinder pressures can spike. All while your asking that engine to act like it's in the power band.

Imagine I give you a really heavy wheelbarrow and ask you to push it up a hill, you're going to move at whatever speed feels best for you. Now imagine I tell you to do it at 1/4 the speed. It's going to hurt and you're not going to like it. Depending on what your life has been your arms might quit first or your ankles in my case my knee due to a previous injury. That's lugging. Its why you see tractor trailers driving at all different speeds up grades, they're putting their engine in the best RPM band for their load.

Hopefully that explains lugging. Its bad.

[gumby79]

As per Dodge TSBTSB 06-01-90 Rev A (technical service bulletin)

GEAR AXLE RATIO
3.54 Axle Minimum Speed 4.10 Axle Minimum Speed
First 5 MPH. 4 MPH
Second 10 MPH 9 MPH
Third 20 MPH 17 MPH
Fourth 30 MPH. 26 MPH
Fifth 45 MPH 40 MPH
If the noise condition is only present at speeds below the MPH on the chart no repair is required as it will not he effective. A road test should be performed with the vehicle owner using the recommended gear speed chart to ensure the owner understands the noise condition is the result of operating the engine below the recommended RPM and not a defect in the transmission/driveline system.
TSB 06-01-90 Rev A

Some 1989 to 1993 Ram trucks equipped with the 5.9L Cummins diesel and manual transmission my exhibit a powertrain induced cyclic groaning or harsh noise condition that appears to be transmission or driveline related. The noise condition is noticeable in all gears, but is especially noticeable in fourth and fifth gears. The noise condition will occur below 1400 RPM during float, coast, or light throttle operation.
The noise is not damaging to the powertrain and is due to the dampening characteristics of the clutch springs. TSB 06-01-94

It looks like the reason 5th gear backoff its nut when lugging a 1st gen
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My 91 auto is stockeverything except the 16cm housing. The stock TCM is set to allow 4 gear (.69 od ) @32.5mph given light enough throttle otherwise by 60mph od . At 55mph normal flats 450-500 egt/1psi boost light hill 5-650egt/2psi 6% grade 725egt /3psi 7% 800egt/ 5psi. This was my run from Helena, MT to anaconda, Mt. This was with the cruise set. If I pull more than 850egt the trans kicks down to 3rd @32.5-59mph @60 and above it'll hold 4th 1000°+
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Turbo is stock h1c w/ 16cm housing and full manifold wrap Exhaust Heat Wrap 1.5 rolls as well as the exhaust housing + a standard turbo blanket/t3-turbo-blanket and one of theas between the turbo and Intercooler
Adding theas insulation parts droped my 1psi from 600egt to 500egt measured befor the turbo
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[oil pan 4]

I should add that low rpms are not the magic key to high fuel economy. I find older 2wd nissan titan have pretty tall high way gears for a truck. Typically turning around 1600rpm at 70mph and still delivering pretty typical pickup truck fuel economy.