Converting early gas MH to Duramax? Possible?

[BigRabbitMan]

Mark,

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]

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]

......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]

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]

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

[JeepSJ]

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.

[BigRabbitMan]

Yes, in 2004 and it is no longer online. I would be more interested in finding out who the builder/owner was/is to see what the long term report has to say than how he did the conversion.

I see the conversion just as a tedious process once all the needed hardware is identified and obtained. Every conversion has differences that reflect the ideas and preferences of the person doing the conversion. I would hope to do a good job of documenting what I do so that others who are wanting to prolong the life of and moderize their FMC will have a path to follow. Of the 1050 of them built from 1972-76, it is estimated that 700-800 of them are still on the road.

My coach (as all of them) originally had the Chrysler 440-I (Industrial or Marine version) and a Loadflite 3 spd transmission. I previously swapped the transmission to an Allison 545 and replaced the carburator with an Edelbrock MPI fuel injection system so I am no stranger to conversion or modification. Here is a picture that shows a different modification I did. That is a satellite internet dish deployed while we stopped at a beach in southern California.


With that engine having toasted due to water pump failure, now is the time to look at going to the next level. I may swap in a used 440 that I am aware of to be able to use the coach while I am assembling and designing the conversion over to the Duramax and modern Allison.

Thank you for your assistance and interest,

[BillCarry]

I have a 6.5 TD 30 ' motorhome which I bought new in 1994 (one of the first 6.5 front engine units built by Georgie Boy and perhaps the industry). Loaded it weighs around 15,000 pounds.

I just passed 150,000 miles.

I have accurately calculated mileage on four or five trips from MIchigan to Florida and back and I consistently get between 11.5 and 12 mpg. I drive 60-62 mph on the interstates. With three previous, slightly smaller gas motorhomes (one GM 454 TBI and two Dodge 440 carb) I consistently got 7.5 to 8 mpg. My oil consumption has not changed since the engine was broken in and runs a quart about every 1800 miles. The over the road performance feels about the same as the gas jobs.

I'd say if you can pull this off with the Duramax you'll be real happy with what you get.

Bill Carry

[BigRabbitMan]

Bill,

Thank you for the input. As you did, with the 440 I am getting 7.5-8.0 mpg and would expect it to go to about 11-12 mpg. That lines up with your experience. I will get it done, but I do know that it will be a long involved project. That is why I went ahead and got a replacement 440 installed so that I can use the coach while I am gathering all of the parts.

What I am looking forward to is the expected improvement in general performace particularly in the mountains. That is where I expect to havest the benefits of the higher torque of the Duramax linked to the gearing of the 6 speed Allison through my 4:62-1 rear end.

Once installed it will be off to Mexico and Alaska!!

[BigRabbitMan]

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

A double overdrive would make for sweet traveling at 80 mph!!!!!

[hogmotors]

I believe that a stock DMAX w/3.73 gear & stock 2.45 75R16's runs about 2400 @ 75mph. My 99 sub 6.5L ran that & I expect the DMAX to do about the same when it hits the road since I didn't change gears.

[Dakster]

BigRabbitMan,

Have you made any decisions or started on the project?

[journeyman]

My dad has the 2900 R (1974) and quite possibly I will be inheriting it someday.

We have the 454/4L80e in it and got 8 MPG with the TH400 and yet to get enough driving data with the 4L80e.

I also would like to consider the LMM/Allison 6 speed. Weight on the engine is comparable. 1000 should be similar to the 545.

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).

I am impressed with the 454, and possibly will keep it in favor of diesel... But may inject it to improve efficiency