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BigRabbitMan 06-09-2007 01:08 AM

Converting early gas MH to Duramax? Possible?
I have a '70's era motor home that weighs about 15,000 lbs. and with a tow vehicle gcvw could get to 18,000 lbs. I am going to replace the existing power train with a modern power train. At this point I feel my options are either stay with gas and convert to the GM 8.1L Vortec with the Allison 1000/2000 transmission or convert to the LLY or LBZ with a matched Allison 1000. Given the natural low rpm torque profiles and better performance at altitude of tubo diesel, I would prefer to go to diesel even though it will be more complicated. It also is easier to avoid fuel quality problems in Central America where I will be traveling at some point with diesel vs gas.

With the existing gearing of my unit, in 5th the engine will be turning about 2300-2400 rpm at 65 mph. This seems a little high to me for a cruising rpm for a diesel engine. If the new 6 speed Allison and matched engine can be found, it would turn 1800-1900 rpm at 65mph. That seems better to me as the engine is just turning fewer rpm with less wear and should/would get better fuel economy.

Now the questions:

Is my weight too high as a constant weight/load? What are the different characteristics of the LLY/LBZ at those two rpm levels? What are the torque/economy trade offs? Is the LLY/LBZ happy at the higher rpm as normal rpm? Where can I find a downloadable copy of the torque curves of the LLY and the LBZ?

The weight of the 6.6L is close to the weight of the existing engine and I will completely redesign the cooling system. Others that have reviewed my plans are about evenly divided between thinking it is a great idea to move to one of these modern power/transmission combinations to thinking I am out of my mind. That is probably true, but I won't let it get in the way!

To make the transition, I would purchase a used engine/trany with engine and trany wiring harness and computers and anything else needed. My existing engine had a water pump failure and overheated to the point that major engine damage resulted so now is the time to make a change if I am ever going to do it. The coach is a 1976 FMC.

I thank all you knowledgable Duramax experts for your learned advise.

Dakster 06-09-2007 05:59 AM

I can't help with the intricacies of conversion. Having driven both GAS and DIESEL Motorhomes. I vote for the diesel MH. Since you are upgrading from a 70's era gas motor to a current gas/diesel motor I would be shocked if you didn't get alot more power and better mileage then you are now. Diesel should last a lot longer they are better designed for moving lots of weight. I like the quiet diesel (onan/cummins) generators - again lasts longer and uses a little less fuel per watt.

If money is an issue, the gas motor should cost a lot less to install and should be taken into consideration. Unless you have a friend willing to donate his used DMAX motor...

However, I know many a diesel RV that get in 5-6 MPG range, so I wouldn't expect great fuel mileage numbers. Granted these are 40' RV's that empty weigh 23k and towing a car.

Not that I believe either of these motors are the holly grail of RV motors. But have you researched the most common motors found in large RV's? (Cat / Cummins) I would want all options on the table to pick thru, before I made my choice. Especially since just PARTWISE you are looking at spending some serious cash.

BigRabbitMan 06-09-2007 11:02 AM

Here is a link to a picture of the coach. It is a rear engined unit and since the coach physical structure and weight balance was designed for the weight of the existing 440-I, I am unable to go to a larger power plant without major modifications to the frameing, etc. of the coach. Others in years past have gone to the 1560 lb. 8.2L turbo Detroit Diesel, but had to do much more structural work due to the weight than I am willing to do. That increase in rear end weight also affects the drivability and I like being able to take curves at 10 mph over the posted speed. It is 4 wheel independent suspension with a transverse leaf spring in the front and independent, trailing arm, torsion bar suspension in the rear.

This a 29' coach so it is much lighter than most diesel powered coaches. I know that both gas or diesel will perform well. The 440 would take it up 6% grades at 55 mph. Either of the options will add significant HP, but the higher torque of the diesel at lower rpm will significanly increase the drivability and have other benefits as well. The torque to lbs. ratio of the Duramax is the perfect match IF it can be done one stop short of Fort Knox. I don't want to go to the Vortec engine and then later learn how well the Duramax would have worked and have "Wish I would have's".

I expect a conversion to a Duramax to cost 50% more than to a 8.1L Vortec, but the difference in lower rpm torque and other factors would be worth it to me. Either one will be a time consuming job to get all the harness installed and systems working properly. What ever I do, once the conversion is done it can't be redone to try the other way so this is the one opprotunity to get it right.

My MAJOR question is what happens/will happen if the Allison 5 spd is used and the coach is geared to cruise at the 2300-2400 rpm range. What will this do to a LLY? Nothing? Cause a cooling problem? Burn fuel like crazy?

At 15,000 lbs. what milage could I expect under cruise control with that weight and rpm?

At that weight and rpm, what degree grade would trigger a downshift from 5th to 4th? 2% grade? 3% grade? 4% grade?

A 10,000 lb. trailer plus 3500 dually will weigh as much or more than my coach. In reading posts here, it appears that there are a number of people that run this type of rig on a regular or contunuous basis. Their experiences should parallel what I could expect if I make the conversion. What is the normal rpm of a 2500/3500 with the Allison 5 spd on cruise control at 65 mph?

I am now focusing on the LLY with the 5 spd as the 6 spd is just too new with too few of numbers on the road to be showing up with salvageable engine/tranys in salvage yards yet. Everyone is still driving carefully with the new trucks!

If the LLY with the Allison 5 spd appears to be a good fit at the gearing described above, then I can move on to the other issues such as RELATIVE cost, technical aspects, availability, etc., etc..

This forum is where the experience and knowledge of how a Duramax operates and performs is located. That is why I am asking these questions here and not in an RV forum where I would just get speculation. I have been speculating too much on my own.! Here are the people that actually drive them!!

I appreaciate everyone's assistance,

Mark Rinker 06-09-2007 05:58 PM

Have you considered swapping in the drivetrain from a 6.5TD motorhome?

BigRabbitMan 06-09-2007 09:05 PM


No I haven't and won't for a couple of reasons. The first is that if I am going to go through the tremendous amount of work that it will take to put any diesel into my coach, I want to be as current in the technology as possible. The second is the constrants of space and weight to power issues with my coach. I am not wanting to diesel the coach just to be "dieseling it". I am looking at some specific attributes that the Duramax has that are not there with earlier engines or other brands.

If I can't get any answers to the questions I posed above, I will go with the 8.1L Vortec. It's an easier conversion. I am not quite sure why some relatively straight forward questions such as normal rpm at 65 mph are going unanswered.

Thank you for your interest,

a5150nut 06-09-2007 09:50 PM

IIRC, the Allison 5 speed of certain varieties has an aftermarket conversion to make it 6 speed double over. Might check with John Kennedy on that. Be patient, some one will comew along with your answers. In the mean time, gear ratio and tire size might help. I remember a link arround here to a conversion table to determine rpm at a a given speed.

Good luck and happy motoring.

A quick search and I found 1 Gear/RPM Calculator

Hope this helps. If not there was another one on 4x4 site.

DmaxMaverick 06-09-2007 10:49 PM


Originally Posted by BigRabbitMan
......I am not quite sure why some relatively straight forward questions such as normal rpm at 65 mph are going unanswered.

Thank you for your interest,

It's not straight forward. Your MH will have a drive ratio unique the the MH. Very different than the vast majority discussed here. Your RPMs will be the same in direct (Allison 4th gear), regardless of the tranny. The 5/6 speed Allisons will have an OD of .71 in 5th, and .61 in 6th. Just do some math (or ask for help with it) and compare it to what you have. Unless you plan on changing the tire size and/or drive ratio, it will be the same, with an addition of OD(s). A 6 speed may not be the best answer if you have no OD now. If you have a 70's era auto tranny (TH400?), it will have a direct final gear. Adding any OD will be a bonus, and a lockup converter won't hurt, either. With the weight you are dealing with, a double OD won't help unless you have a deep enough final drive ratio.

Dakster 06-10-2007 06:03 AM

Nice looking coach. I can tell you take really good care of it and have alot of modern conviences added... To your fuel mileage question, I will sum it up based on what the RV salesman told me. "No one buys these things for the mileage they get." Gas or Diesel I doubt you'll break 10mpg. based on your description of your driving habits. This is of course, my opinion, and is based upon my unique experiences.

BTW, I drive RV's between 75-80 mph too... So don't take the comment as a flame.

Duramaster 06-10-2007 12:22 PM

I don't know much about "repowering" M/H but I can say from experience STAY AWAY FROM THE 6.5 TURBO DIESEL! :D The DMAX or the 8.1 is the best way to go!!

JeepSJ 06-10-2007 05:17 PM

Wasn't there an article on here a while ago about an older FMC motorhome that was converted to a DMax? That one was also rear engine.

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