LBZ - Target 2200 or 1900 rpm?

[BigRabbitMan]

This forum section seems to have the people that run heavier weights and so will probably have the experience that I need to draw upon. I am getting ready to select the transmission for an LBZ (have engine) installation into my rear engined (currently a gas 440) motor home.

If I use a 5 spd, the rpm at 65mph will be about 2200 rpm.

If I use a 6 spd, the rpm at 65mph will be about 1900 rpm.

The actual GCVW will range from 14K to 18K 100% of the time.

I prefer to lean towards economy vs power but not to the extreme. I want to select the Allison transmission that will be best for the Dmax over the long pull. My subjective preference at this time is the 6 spd with the additional overdrive to bring the rpm down a little. This should help mpg, extend engine life and reduce sound levels as well if my uninformed reasoning is correct.

My concern is that I don't want to bring it down too low and cause engine problems over time. Is a Dmax "happy" at 1900 rpm on level or slight rolling hills at the above gross weight or does it need a few more rpm to live happily ever after?

I don't want to make as important of a decision as this based on a coin toss due to my lack of knowledge.

I thank you for your input. I have done a lot of reading so feel free to write long answers!

[Inspector]

What is your rear end gearing. With that gross weight I wonder if it will stay in 6th over much of the time. I think it really depends on final gearing.
Inspector

[moss6]

Depending on engine model the maximum torque will be between 1,600 and 1,800 RPM; that will be the range of best efficiency.
Randy

[JohnC]

The 6 speed has no disadvantage. You can always lock out 6th, although for the most part the transmission is smarter than the driver It will not allow you to lug the engine and, when you can, the higher gear will give you better mileage because the frictional losses are lower at lower RPMs.

All DM's come with 3.73 axles.

[Inspector]

John C.
You are certainly right. Rabbit is reengining a MH that previously had a 440 gasser in it. The rear end gears most certainly won't be 3.73s. He needs to know what they are.
My Seneca with the LLY and alli has much lower gears than 3.73s. I probably have a road load weight of around 16 to 19K. I will know better after I weight it shortly.
Inspector.

[moss6]

Guys, I think you are missing the obvious. The 3.73 gearing in itself has no bearing on the issue. Think of it more as the ratio of the number of feet the vehicle travels in one revolution to the number of times the drive shaft rotates for that one tire revolution. Stock 3/4 ton Silerado tires are in the neighborhood of 31" tall so with the 3.73's one tire rotation or about 8.1' equals 3.73 drive shaft rotations or a 2.17 to 1 ratio, that is what you should be looking at. If the MH has 40" tires then the rear end ratio should be in the neighbor hood of 4.83 if you want to keep the relative speed to RPM's on a par in both vehicles.
Randy

[Inspector]

I think that is what I was alluding to.

[moss6]

Good to know we are on the same page, sorry I wasn't too sure, but maybe the numbers will get BigRabbit on the right track. I'd be interested in knowing his tire size and pumpkin ratio to see if it was going to work well with his modification. From what I have seen as typical, manufacturers don't match gear ratio's for gassers anywhere near the efficiency range that we see with the Duramax/Alison combo that GM provides.
Randy

[BigRabbitMan]

I thank everyone for their interest and replies.

..... the efficiency range that we see with the Duramax/Alison combo ......
Randy

And what is that efficiency range at a GCVW in the range of 14K to 18K? Is it centered on 1900rpm or 2200 rpm? Those are my two rpm choices for 65 mph.

[Mark Rinker]

Absolutely choose the transmission/gearset that will yield the lower of the two @ 1900. 1700rpm at cruise would even be better. 2200 will waste fuel that is what I ran at with C4500 5spd, 7-8mpg loaded at 62mph, ~27K# GCVW.

[BigRabbitMan]

Absolutely choose the transmission/gearset that will yield the lower of the two @ 1900. 1700rpm at cruise would even be better. 2200 will waste fuel that is what I ran at with C4500 5spd, 8mpg loaded at 62mph.

Thank you!

That was my thought, but was concerned about the GCVW at that engine rpm. If that is not an issue, it will make for a sweet rig. To change the gearing to get a rpm lower than 1900 @ 65mph with this vehicle would require major changes that would not be worth it.

The 6 speed it is!

[BigRabbitMan]

For those that are interested in the actual numbers involved, here are some to play with.

http://www.700r4.com/speedoCalc/rpmcalc.shtml

Differential ratio - 1:4.625
Tires: Michelin 8R19.5 XZA, Diameter = 33.6", Loaded radius = 15.6" (effective dia of 31.2)
At 65 mph based on the diameter rpm will = 1,834 in 6th.
At 65 mph based on the loaded radius rpm will = 1975 in 6th.

Since my coach is lighter than what the fully loaded tire is rated at, my loaded radius will probably be more than the 15.6 but less than half of the diameter. Hopefully the final 65 mph rpm will be close to the 1900 rpm discussed since Mark feels strongly that the engine doesn't need any more (and could use less) rpm at the GCVW involved.

If you have a contrary opinion, please let us know as the money isn't spent yet!

I thank everyone for their input,

[DmaxMaverick]

No opinion to the contrary, but.....

Disregard the "loaded radius". This figure effects axle height, but only nominally with the drive ratio. The circumference of the tread remains essentially the same, regardless of the loading (actually within 10%, as there is some tread scrub at higher loads or lower pressures). It effects the rollout too little when calculating the final drive ratio. The diameter difference is greater from new rubber to service limit.

I agree with the consensus. Lower RPM is better. I find my best loaded mileage around 1600 RPM's. This with 22K GCW, 33" tires, and 3.73 gears, which is about 60 MPH (CA towing limit is 55) with my combo, on my 2001 LB7/Allison. Trips out of state, where the limit is 65-75, my mileage really suffers. 13-15 MPG in CA, down to 9-11 across the border. I'm rarely in a hurry, so I don't usually drive the speed limit when out of state (sorry....that's me getting your dander up).

[Mark Rinker]

...since Mark feels strongly that the engine doesn't need any more (and could use less) rpm at the GCVW involved...
I thank everyone for their input,

I don't have any of the technical training that others here on the board have, re diesels and automotive tech. My knowledge was formed here on the board, and then proven over many, many miles on the road. Add to that alot of magazine articles over the years...

~1800 is peak torque. You'll do best on mileage to stay close, or slightly under that. The transmission/tire/gearset selection is important! You are smart to investigate all of this up front.

TDP works because of the many, diverse opinions and experiences brought to bear on any given problem - like this one. Glad we could help.

[BigRabbitMan]

I find my best loaded mileage around 1600 RPM's. This with 22K GCW, 33" tires, and 3.73 gears, which is about 60 MPH (CA towing limit is 55) with my combo, on my 2001 LB7/Allison.

This, along with Mark's comments, is what I was looking for -real world experience at higher weights and does the DMax pull ok at the lower rpm level without lugging the engine. Based on your's an Mark's experience it has resolved that issue completely - it does pull my weight at the lower rpm without problems to the engine or operationally! When it is time for tire replacement, I might be able to go with slightly taller tires if the wheels will allow it. After I have made the conversion and have experience with it, there are 22.5 inch wheels for my coach if I feel it needs it bad enough but I think it will be fine with what I have on it now.

Mark, I have and do work with engineers (farm equipment) and they are very good and valuable, but there is nothing that replaces actual experience. After the engineer designs it and the plant builds it, I sell it and deliver it to the user. I then get the report from my customer all about the good, the bad and the ugly! I then take that back to the engineer and we improve the product. Having done that many times, I want to do it just once in this case so I have been taking my time and doing as much research ahead as possible. I think the big questions have all been answered.

I normally try to stay at about 65mph running the coach on the freeway (as one might need to stop it as some point!), but it tends to creep up as with the rear engine the sound levels are low and the coach handles very well. In the future I will be towing a vehicle ('87 Subaru Brat or '95 Honda Accord) so will have to stay at the 62mph in CA. If interested, here is a link to videos of two other coaches I was following last fall up the coast on Hwy 101. http://www.vimeo.com/user1035559/videos. Note the engine sound level vs my voice sound level.

Again, I thank everyone for their input. As I slowly progress on this project, I will post updates in this thread http://www.thedieselpageforums.com/t...ad.php?t=27875 in the conversion section.

[DmaxMaverick]

I don't "lug" the engine. The Duramax seems very happy tooling along. I drive by gages, with a light, gradual foot. With my setup, the turbo is flat below 1500 RPM's (55 MPH, with an ambient temp below about 75°F), so any more than a slight grade (like an overpass) forces a downshift to accelerate, but it will hold speed with very little power/pedal input, as boost maxes at about 4-5 PSI below 1500 RPM. The turbo response RPM can be as high as 1750 RPM's during hot CA summer towing, with an ambient of 100°+ and A/C on. When maintaining speed, my downshift is determined by EGT and desired speed. When towing or not, I use T/H mode almost exclusively. With my tire size, OD will not automatically engage until 63 MPH, so I have to force an upshift, which can occur above 50 MPH. I do this by tapping the T/H out, then back in (split second time). As long as there is a pedal input, it will upshift, even several seconds after T/H is re-engaged. Driving empty or not towing, I use T/H mode to get the TC to lock early. Tapping the T/H out/in at 2050 RPM forces an upshift, as long as there is pedal input. This allows me to drive with lower RPM's with less energy loss through the torque converter (a very important key to longevity and economy I discovered with my Blazer over many years/miles, attributed to the TC locked ALL the time above 30 MPH). Engine and tranny temps remain lower, as well.

Lugging is defined by an engine's inability to accelerate at a given maximum fuel input. However, if the RPM and fueling is below the turbo envelope, lugging is a non-issue, as far as overworking the engine is concerned. If you monitor the EGT's, lugging will be indicated by a significant increase in temp, w/o and increase of ground speed.

[BigRabbitMan]

Thank you for the above discussion. Being new to diesel powered road vehicles, you are being very helpful to me in describing how to operate/control them. I have run a lot of diesel farm equipment, but over the road systems are different.

I will be saving this discussion.

[JohnC]

The Duramax/Alison programming simply won't allow the engine to be "lugged". It'll downshift first.

The 6 speed is identical to the 5 speed except it has an extra OD gear. Even if you ended up never using it, you'll have the option if you go with the 6 speed whereas with the 5 speed you'd never get the chance.

[Mark Rinker]

'BRM' - Remember, the Duramax is a fly-by-wire system that doesn't provide for simple mechanical substitutions of controls...!!! The more I think about what you are planning, the more questions I have to ask...

• Do you intend to swap over a Duramax gauge and control cluster? (By doing so, you'd have a 'factory' tach, oil/water/tranny temp gauges, warning lights, including SES when codes are set...)
• An OBD-II interface (port) would be verrry advisable for monitoring / tuning ECM/TCM functions - a likely need during the fine tuning of your new drivetrain- which calls for a good portion of the factory wiring harness to be swapped over as well;
• whats the plan for converting the fuel tank(s)? Senders?
• whats the plan for your new accelerator pedal? Cruise?
• by the way...cooling system, intercooler, oil/trans coolers, A/C provisions, etc. etc. etc.
• where are you going to get your vaccuum for the current braking system? Personally, I'd find a air/hydraulic system from a Kodiak and retrofit discs all the way around...

In fact, have you considered buying a wrecked or high mile C4500 Kodiak cab/chassis in its entirety, using the donor engine, cooler stack, HD torque converter, Allision 1000, the complete wiring harness, ECM/TCMs (set up for a 10K# truck with 17500# + GVW expectations and 19.5" wheels/rubber), an awesome 4 wheel disc air-over-hydraulic braking system, often a factory exhaust brake (!!!), dual diesel fuel tanks, dash and gauges, etc, etc, ...?

I think a 2WD C4500 or even C5500 Kodiak would be a cheaper and a better donor for all the needed parts than any K3500 or K4500 truck which will have '4WD-isms' (including a transfer case you don't need) built into the chassis, ECM/TCM programming...etc...etc...

MorePower's experiences swapping a Duramax drivetrain into an 80s truck (LilRed) will be invaluable for this project...starting with an engine, and working your way out may be the most expensive way to approach this swap.

Swapping in a Duramax drivetrain, electronics and controls is more of 'setting your old motorhome over the top of a new drivetrain' than it is about swapping an engine into the place of a tired 440ci gasser.

Think MONSTER GARAGE... Then ask yourself - WWJJD? (What would Jesse James do?)

[BigRabbitMan]

'BRM' - Remember, the Duramax is a fly-by-wire system that doesn't provide for simple mechanical substitutions of controls...!!! The more I think about what you are planning, the more questions I have to ask...

• Do you intend to swap over a Duramax gauge and control cluster? (By doing so, you'd have a 'factory' tach, oil/water/tranny temp gauges, warning lights, including SES when codes are set...)
• An OBD-II interface (port) would be verrry advisable for monitoring / tuning ECM/TCM functions - a likely need during the fine tuning of your new drivetrain- which calls for a good portion of the factory wiring harness to be swapped over as well;
• whats the plan for converting the fuel tank(s)? Senders?
• whats the plan for your new accelerator pedal? Cruise?
• by the way...cooling system, intercooler, oil/trans coolers, A/C provisions, etc. etc. etc.
• where are you going to get your vaccuum for the current braking system? Personally, I'd find a air/hydraulic system from a Kodiak and retrofit discs all the way around...

In fact, have you considered buying a wrecked or high mile C4500 Kodiak cab/chassis in its entirety, using the donor engine, cooler stack, HD torque converter, Allision 1000, the complete wiring harness, ECM/TCMs (set up for a 10K# truck with 17500# + GVW expectations and 19.5" wheels/rubber), an awesome 4 wheel disc air-over-hydraulic braking system, often a factory exhaust brake (!!!), dual diesel fuel tanks, dash and gauges, etc, etc, ...?

I think a 2WD C4500 or even C5500 Kodiak would be a cheaper and a better donor for all the needed parts than any K3500 or K4500 truck which will have '4WD-isms' (including a transfer case you don't need) built into the chassis, ECM/TCM programming...etc...etc...

MorePower's experiences swapping a Duramax drivetrain into an 80s truck (LilRed) will be invaluable for this project...starting with an engine, and working your way out may be the most expensive way to approach this swap.

Swapping in a Duramax drivetrain, electronics and controls is more of 'setting your old motorhome over the top of a new drivetrain' than it is about swapping an engine into the place of a tired 440ci gasser.

Think MONSTER GARAGE... Then ask yourself - WWJJD? (What would Jesse James do?)

Mark,

The above post is right on target!

Moving forward a year and half, I have purchased a "lightly rolled" 2006 GMC k25 with an LBZ and 6spd Allison with 49,000 miles on the clock. Virtually all parts are available for transfer except the air filter box! The Kodiak would be better for the reasons listed, but I needed to move forward with what was available.

I will be following the methodology that you have described above in that the dash cluster, steering wheel contols, fuel sender, etc. etc. will be moved from the donor to the FMC coach. I do have to deal with the 2 wheel drive vs 4 wheel drive issues, but that is in process. That has a little more complication due to the requirement for a driveline parking brake that attaches to the rear of the transmission as it does now with the AT545 that is currently in the coach.

Here is a link to the thread where the conversion itself has been discussed. That thread gives more particulars of the coach and will be continued as the conversion continues.
http://www.duramaxdiesels.com/forum/...d=1#post422358
That thread includes pictures of my coach so you can see what the coach actually looks like.

Again, thanks to everyone who has contributed to this thread,