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.
Update on prior conversion and LBZ purchase
Quote:
Originally Posted by
journeyman
My dad has the 2900 R (1974) and quite possibly I will be inheriting it someday.
Jim Black told me that vibration was a problem with diesels in the 2900R and it would need significant body (structural) strengthening that he does. the Duramax may not have those vibration issues (that would be my hope).
Jim Black (of RVS, FMC authorized repair facility - Morgan Hill, CA) and Leslie Hoagland of Motor Coach Restoration in Illinois have only dealt with older diesels: the 8.2L detroit and 6 cyl in line Cummins. Either of those engines will have vibration issues, but more importantly weight issues and a narrow torque curve that do in fact force structual strenghtening. That is why I eliminated them upfront. I know both Jim and Lesle,and respect them both but in this case they are both still thinking old school.
As Journeyman indicated, in the case of the FMC pounds are important and the difference between the Duramax and other modern light weight diesels eliminated all others on that factor alone.
Since my last posting, I was able to make contact with the current owner [he did not do the conversion] of the FMC that was converted a few of years ago and written up on The Diesel Page. He surfaced at an FMC rally held in Colorado last month. While I wasn't able to be at that rally, I have received some very complete descriptions of how the conversion altered the fuctioning of the coach. Those descriptions were all good!
Here are a few:
"The coach has a 5-speed model 1000 transmission. It turns about 1700 rpm at 65 mph." [note: In my opinion that is too slow.]
"When we started up La Veta pass, Jim Evans [440 carb powered] (in the lead) said over the radio that he wouldn't be insulted if anyone passed him, so I [440 fuel injection powered] pulled out and stormed up the hill past everyone [This included a turbo inline 6 cyl Cummins]. Dave Jacobs followed in the Duramax coach, and when I waved him to come on by, he passed us like we were nearly going backwards! It was very impressive - and sounded great, too."
"Coach #735 performed perfectly, getting 11.2 mpg pulling the Bimmer (left in Colo. Sprgs for A/C repair) and 13.8 mpg empty including going over 3 mountain passes over 10,000 feet including Wolf Creek Pass (10,550 ft) which is the curviest and steepest I've ever seen."
"Several miles out from Pagosa Springs, we started the long climb up Wolf Creek Pass (10,857 ft). I suggest that Dave proceed ahead, and they proceeded to disappear again ;-) I didn't push it [440 fuel injected]: we stayed near the 35 mph speed limit at 3500 rpm in 2nd gear (Allison AT545 4-speed) all the way up the climb."
It should be noted that no frame issues were evident with the Duramax converted coach even though it still had the stock frame.
As all of you can understand, having read the above plus additional comments any thoughts of an 8.1 vortec disappeared. It also encouraged me as to the thoughts of converting to a Duramax. I came to two conclusions: for this conversion, it needs to use the 6 speed double overdrive transmission so that the stock 4.62:1 differential can still be used as that combination gives a good transfer of torque to the ground with 65 mph at about 1900 rpm, and that if it would be possible to locate an LBZ it would be best. An LLY would also be fine. The above coach has an LB7.
I then made the mistake last week of typing in Duramax LBZ in the search window of eBay! To shorten the story, I am now the proud owner of a used LBZ that was obtained at an attractive price! I guess I am now commited.
The following is the body of an email I wrote to a fellow FMC owner:
"What this purchase did was commit me to a methodology in that I can no longer look for a matched engine/transmission set. It is now piecemeal and just do the stuff that has to be done to make the pieces work together. That will take awhile. One of the reasons I went ahead is that a matched Duramax LBZ and Allison MH (Motor Home) transmission doesn’t exist. No motor home manufacturer has used the Duramax to this date so they aren’t matched to a MH transmission. Motor home manufactures have either used a gas engine or Cummins or Powerstroke. I already had eliminated those engines so I had effectively eliminated a matched set with the MH transmission. That left the possibility of a matched LBZ with a 6 spd Allison, but one that would have to be converted from a standard arrangement to the MH version with the driveline parking brake. Given the attractive price I paid for this LBZ, I went ahead and set pattern.
"Now the game plan is to obtain a matched ECM (engine control module) and TCM (transmission control module) from a Duramax LBZ / 6 spd Allison equipped wreck including wiring harness(s) if possible. That will be used to control a used Allison 1000MH 6 speed. This will be a challenge to obtain as the 6 speed is 2006 or newer, but this is why the year or more timeline to complete the conversion. In my dreams, I would prefer to use the Allison 2500MH with the lower first and second gear, but the population of those is so small I can most likely forget it. Which ever model Allison (1000, 2200, 2500) I do end up using, I do want it to have a parking pawl. Not all of them have one. [Note: the 2550 has the pawl.]
"Then comes the question of how to control the transmission. One possibility would be to obtain the push button, paddle controller found in some newer motor homes. It is 100% electrical I believe. This could most likely be mounted on the inside of the drivers door (or left side of the drivers seat stand) and, therefore, not be involved with the dashboard or driver/passenger area. Whether or not it could be mounted into the dash in the area of the existing shifter is not known.
"So the “you” part of the “we” is to start looking for the pieces and parts needed to complete the job. Start crawling through the junkyards looking for donor vehicles for the above items.
"Understand that I am not committed to the actual conversion until all component parts have been secured. The Duramax LBZ that is now on the way can be resold if all other components are not located at a reasonable price or if some other issue prevents the conversion."
Who am I kidding!! :D I am now fully addicted to the conversion of my coach to a Duramax LBZ with an Allison 6 speed behind it and ALL issues will be surmounted.
You now have the current status and know what some of the additional parts
needed include. I am going to try very hard to get hold of a 2550MH transmisson. The lower gearing of the first two gears may over time prove to help alleviate any frame issues as the torque is then easily transfered to the wheels when starting out without needing a stiff frame to hold the engine back from rotating.
Wish me luck and if any of you come across any of the items I need (everythng!) please give me an email at Stephen at Heinrichs dot com.
I will post updates as major events happen.
Conversion to 5spd or 6spd better for Duramax?
Quote:
Originally Posted by
BigRabbitMan
The question of the day is whether I should go with a 5 speed or a 6 speed. With my present rear end and wheels, which I will not change, the 5 speed would put the Duramax LBZ at about 2200 rpm at 65mph. The 6 speed would put the engine at about 1900 rpm.
To all of you Duramax folks that run with loads in that range a lot of the time, where should I target my RPM? Reasons?
Have changed the title to reflect the current question.