Converting early gas MH to Duramax? Possible?

[BigRabbitMan]

Can you add an auxiliary radiator and fan any where under it? Or an extra oil cooler some where? I'm sure its still a work in progress so time will heal every thing.

Good luck with it and keep us posted.

The first change will be a different fan. Once I see what that does, then we will go from there.

[FamilyTimeRV]

Thanks for all your efforts and information. We're looking to add an affordable way to rv with 4 children. The FMC (possibly GMC) is our choice with an efficient power plant. We may follow your example.

How's your conversion working out -- no updates in the last few months.

Also, are there any 20-20 hindsights that would benefit a similar conversion?

Kevin H
still in planning & prep stages

[BigRabbitMan]

Thanks for all your efforts and information. We're looking to add an affordable way to rv with 4 children. The FMC (possibly GMC) is our choice with an efficient power plant. We may follow your example.

How's your conversion working out -- no updates in the last few months.

Also, are there any 20-20 hindsights that would benefit a similar conversion?

Kevin H
still in planning & prep stages

Hi Kevin, sorry for the delay in responding. Had things happening that kept me away. I am glad to assist you in your information gathering. The first thing to understand if you are looking at any older motor home, you need to be at a minimum a mechanic to some level as there will be things that need attention/repair/upgrading and it would be cost prohibitive to have that stuff done by a regular shop in most cases. Since you are on this site, I expect that you are covered in that respect.

As to things that I would have done differently, the only one of significance is that I need a larger fan. Another person is in the process of doing the same conversion to his FMC that I did and has secured a better fan. I expect that at some point I will be getting that same fan, but in the meantime I just have drive conservatively going up grades. You can visit both my complete photo album and his (still in progress) on the FMC Owners Club website. The link is: [URL="http://www.fmcowners.com"] . His coach number is 1027 and mine is 1046. He is up to 227 photos in his album and I have 371 in my album. There is also a discussion of both projects in the forum area in the Mechanics section. Feel free to register on that site so that you can post questions in the forum area. He is doing somethings differently than I did but no two people do something like this the same. Not better or worse, just different. He will be doing some suspension things as well as the engine change in his case.

Two major things that I like about the FMC is the low profile and the quietness with the engine in the rear. The GMC also has the low profile, but you do have the engine up front on that one with the attendant engine noise. If you want to convert one to diesel, I think it would be much more of a challenge with a GMC. I am not aware of a compatible engine/tranny for the GMC. There may be one, but I am not aware of one. When you say "efficient engine", I don't know how you define efficiency but if it is more than 8 or so mpg, diesel is the only way you will get there.

There are FMC's that have already been converted to diesel that come up for sale from time to time, but you need to ensure that the conversion process was done properly. That is particularly true of the heavier diesels like the Detroit 8.2 or the Cummins 5.9. Length with the Cummins also causes some issues that have to be addressed.

Traveling with four children raises some questions in my mind. The first is the ages of the children as that will be a factor in how much "space" each one needs. The "A" or dinette model with the optional bunk beds is set up to be able to sleep six people. As they get larger, some have had the teenage children sleep in a tent adjacent to the coach when camping for more than one night.

I haven't had any updates to my thread here as I haven't done anything additional to the coach. In Feb I did travel about 800 miles south to Buellton, CA for a coach rally and back - just had to take it easy going up six percent grades! On the way back I did get 13.6 mpg measured which is very good for a motor home. It is sweet to drive and very quiet with the rear engine and the sound proofing that I did to the firewall. I can hear that it is running, but that is about all.

I am in Oregon, in what state are you located?

[JeepSJ]

Very nice! Are you monitoring your intake air temps? I was wondering how that water/air intercooler was working.

[convert2diesel]

Thanks for all your efforts and information. We're looking to add an affordable way to rv with 4 children. The FMC (possibly GMC) is our choice with an efficient power plant. We may follow your example.

Kevin H
still in planning & prep stages

Kevin:

Just a few thoughts. I am going through the same thought process as you are. Mind you, I don't have to factor in 4 kids.

Currently I am leaning towards the GMC. The original design was not only brilliant but it has survived the test of time with a large and active owners community. Having said that, for your purposes, shoehorning yourself, significant other and four kids into a GMC would be tough with any of the available floorplans.

Additionally, the front drive system comes out of the Olds Toronado/Cadillac Eldorado of the same vintage. Without major surgery, that means you are stuck with the THM 425 transmission (basically a 400 with the torque convertor and front pump attached to the engine with a chaindrive to the rest of the tranny turned around and going forward. Still a great transmission, especially if you can find a "switch pitch" trans from a 67 or 68 car. Essentially gives you a 6 speed tranny.

The bell housing is a "BOP" bolt pattern so if you use any other engine you would have to get an adapter plate, modify the engine oil pan to accommodate the right hand drive axle and fabricate a front mounted engine mount. There are a number of 6.5 turbo conversions out there as well as a couple of Duramax conversions.

If you are still interested in the GMC, I would suggest you look into the GMC Co-op in Orlando Florida. Jim Bounds is the by far the most knowledgeable person in the community.

http://www.firefightmarine.com/gmccoop/

This thread has me looking at the FMC as an alternative. Love the concept of the independent rear suspension and pusher engine layout. This plus the added room for the family would make sense for your uses and open up your options for engine trans combinations. Unfortunately they are not only the same vintage as the GMC but there were far fewer manufactured. I would question the availability of parts, specific to this platform.

I have also started looking at the Airstream Motorhomes. Lots of them out there with a 40 year production run including some pusher versions later on. Even today, they are still manufacturing a motorhome based on the Mercedes Sprinter platform. Still trying to get some information on the various versions and separating the good from the bad.

From a cost point of view, I would seriously analyse how you are going to use this motorhome and specifically, how much you are going to use it. Upgrading any of the powerplants in these vehicles to a newer engine is far easier and cheaper with a gas conversion than the diesel. In other words, you are going to have to pack on lots of miles to justify a diesel conversion from a cost point of view.

The new "LS" series GM gas engines are plentiful, with an extensive aftermarket support system in place. Properly configured, they can be programmed for very acceptable fuel consumption and with reasonably inexpensive "go fast" parts can lay down some very respectable power. Based upon my research into the GMC, I could do an LS conversion for about $5,000 less than any decent diesel conversion, plus, it can be fixed on the road by any reasonably equipped garage. Even Bubba in lower Buttkiss is probably conversant with this engine family.

Myself, I am leaning towards a GMC with either an AMG 6.5 or LS engine and a switch pitch transmission, upgraded suspension and brakes and a total re-working of the body. I expect the cost to be in the 30 grand area, including the initial purchase. Not much more than a reasonable tow behind trailer.

Hope this helps.

Bill

[BigRabbitMan]

Very nice! Are you monitoring your intake air temps? I was wondering how that water/air intercooler was working.

That question raises one of the little glitches with my system. I installed the Banks system with their IQ monitor/controller with lots of readouts. The problem I have is that it doesn't work with the long extension cord that I have to use to reach the front (30') so that I can monitor things while on the road. It works fine while only using the short cord that comes with the unit but since my engine is in the rear, I can't use it while driving. I will be contacting Banks to see how this can be resolved. In the meantime, I know the water is warm/hot when stopping so it is taking heat out of the air stream but I don't know the actual air temp while under load.

I do have a second issue as the turbo vane position sensor is not functioning with the new sensor. The result is that I am always at full boost so have driven very conservatively. I will be starting a new thread about that issue at some point.

[BigRabbitMan]

I expect the cost to be in the 30 grand area, including the initial purchase. Not much more than a reasonable tow behind trailer. Hope this helps.

Bill

Bill has done an excellent evaluation. His cost estimate is very good for a conversion done properly. In my case, I spent more than needed in some areas and already owned the coach and I ended up with a total cost of about $27,000 for the Duramax conversion. The other person that is currently converting an FMC to a Duramax expects his cost to be the $30,000 number including the purchase price of his coach. I expect that number will be very close to what his actual turns out to be.

In the case of the FMC, a conversion/upgrade to modern gas engine/tranny setup would be much cheaper and that is what I would recommend for most people as most motor homes will not get the mileage on them to begin to justify the cost of the diesel conversion. With the FMC, a gas conversion is fairly straightforward with few complications. Mainly engine/tranny mounts and gauges. But again, it takes many miles to justify a conversion verses, in the FMC case, just doing a rework of the existing 440 with possibly upgrading to a fuel injection system. With the 440, I pulled 6% grades at 45 mph passing many other motor homes!

As to parts availability, with both the FMC and GMC there are support groups out there and sources of parts have been identified for most things. As an example, I have just ordered all of the steering linkage parts for my coach as with 160,000 miles it is time for work on that end. They are all off the self items still today as they were/are used on many other vehicles.

Rollin', rollin', rollin',

Stephen H. , FMC #1046

[convert2diesel]

That question raises one of the little glitches with my system. I installed the Banks system with their IQ monitor/controller with lots of readouts. The problem I have is that it doesn't work with the long extension cord that I have to use to reach the front (30') so that I can monitor things while on the road. It works fine while only using the short cord that comes with the unit but since my engine is in the rear, I can't use it while driving. I will be contacting Banks to see how this can be resolved. In the meantime, I know the water is warm/hot when stopping so it is taking heat out of the air stream but I don't know the actual air temp while under load.

I do have a second issue as the turbo vane position sensor is not functioning with the new sensor. The result is that I am always at full boost so have driven very conservatively. I will be starting a new thread about that issue at some point.

Stephen:

I concur with your decision to contact Banks. Your sensors are either resistance or hall effect, both would be adversely effected by length of harness. Same goes for the EGT. There might be a way to calibrate the unit to compensate for the length of wire but only Banks could tell you how. I do know that EGT leads can be purchased with different length/resistance specs.

Good luck

Bill

[Hal StClair]

Not to hyjack a thread but I have a 1977 GMC that I've installed a 2000 H1 6.5TD with 4l80e trans. I used a B/W transfer case from a Revcon MH and went forward to a 2012 9.25" AAM front diff out of a one ton Chevy. We've put about 3000 miles on it since September and have very few nit's to pick so far.
Not a swap for the faint of heart for sure, but a nice match.
Hal

[BigRabbitMan]

The following is a report that I just posted on the FMC Owners Club website. It should bring you up to date as it includes a mileage check on flat ground and one in the mountains.

**********
Since I now have just under 5,000 miles on my conversion, I thought that I would do an update.

The first thing to note is that I am very happy with what I call the "drivability" of the coach. I like the ease of merging with traffic at speed, not slowing down for most hills, engine starting in a revolution or two, butter smooth shifting, a 700 mile fueling range, an even quieter interior and other smaller factors. Many people are interested in what my fuel mileage is as that is a significant factor with any motor home. Historically, with the 440 I got about 7.0 mpg most of the time. That is probably about average for an FMC.

As expected, I have seen a significant increase. Since fuel mileage varies with driving style, weather, terrain etc. I decided to do some close measurements under specific conditions to help define what was occurring rather than just post a series of numbers. So on a recent trip to CA I decided to do a flat land test and a mountain test.

The flat land test was from Redding, CA to Bakersfield then back north to Santa Nella, CA. That was a run of 674 miles over three days time. The first half of the trip was with a storm coming in and a lot of cross wind. The second half was with relatively calm winds. I also made five diversionary trips into towns for various reasons so it was not all straight down the freeway driving. I spent most of the time in the truck lane at 60-63 mph as it was a comfortable driving speed and I didn't want to be in and out of the 70+ mph left lane. The net result was that I got 14.7 mpg for that segment of the trip. I was not towing the Subaru Brat.

For the mountain segment, I went from Redding, CA home to Cottage Grove, OR. The beginning and end of the trip were at a similar altitude with an initial gradual climb from Redding to Weed at 3,900 ft. It then dropped down to the Shasta river and back up to 3,000 ft then back down to the Klamath river. Those were 4 or 5 mile 5 & 6% grades after some flat stretches. It then went from the Klamath river down in the canyon up to the top of the Siskiyou pass in Oregon at a little over 4,100 ft which is the highest point on I-5. After dropping down the 7 mile 6% grade into Ashland it became a series of ridge crossing with the first several at the 2,000 ft level then into the valleys in between. As I continued north, the ups and downs became less significant until I reached home in Cottage Grove. Again, I was primarily in the truck lane except when passing trucks on grades. When descending 6% grades, I slipped the Allison 6 speed tranny into third gear (which is the same gearing as second gear in the 727 tranny) and came down the grades at 50-55 mph without needing to use the brakes.

The mountain portion of the trip was 301 miles and the coach got 13.3 mpg which is a respectable number for mountain driving in my opinion.

With those two numbers in mind and the conditions that created them, I expect my long term number to be somewhere in between them. Towing the Brat will probably reduce those numbers by a mile per gallon. Even with this kind of fuel mileage, it will take a lot of miles to recover most of the cost of the conversion excluding the unpaid labor cost for myself and the others who invested significant hours as well.

But, yes, I am glad that I did it and hope to see all of you on the road or at a future rally. Schedule yourself for the June Mega Rally in Carthage, MO and you may have the opportunity to take my coach for a test drive.

*********
There is now a second FMC owner converting his coach to a Duramax with 6 speed Allison. Here is the link to his thread on the FMC Owners Club website. He, also, has received good information from this site and its members. http://fmcowners.com/mbbs22/forums/t...sts=54&start=1

[BigRabbitMan]

In the above post I mentioned that I was going to a rally in Carthage, MO from my home in Oregon. Well, I went to that rally and then continued on to the East coast including SC and Virginia, on up into Canada through Detroit, then back via MN and South Dakota. In all, from my door back to my door was 8,424.6 miles! It was a great trip and it gave me a chance to drive the coach extensively under different conditions and see how it performed.

It met all of my expectations and much more. At my Canada stop, we tweaked the adjustment of the front end elements and got the last of the play out of the system. It now tracks like it is on rails.

Up until the Canadian stop, I ran the Banks tuner on Normal or the stock tune. At that point I changed it to the Economy tune just to see what, if any, difference it would make. Up until that point, I had 11,289 miles on the conversion with an average of 14.02 mpg. From that point home via the northern route with a couple of diversions along the way, the coach averaged 15.08 mpg over that 2,907 miles.

Yes, I am very surprised and pleased to be getting mileage like that in a 15,000 lb. motor home. I will continue to monitor fuel mileage for the next year or so just for the record.

I thank the group for its support of this project,

[a5150nut]

Glad to hear of your success. That's a long ways from Morgan Hill where it all started.

[BigRabbitMan]

Time for an update. The coach was running a little warm when climbing hills so I converted the fan to a larger unit and changed the radiator back to the original FMC stock radiator which is a five row brass unit. I can now take 6% grades at 60 mph and not have the temp gauge move at all.

I have also gotten a lot more miles on it as I have now put 48,000 on the conversion in the three years since it was completed. My measured fuel mileage over that distance has settled in at 13.43 mpg. That includes all fuel after the initial fill up.

Driving my coach with the D-Max LBZ and 6sp Allison is a sweet experience.

I am a happy traveler!

[DieselDavy]

Those are some pretty good numbers for as much as that thing weighs!
Enjoy!

[arveetek]

In the above post I mentioned that I was going to a rally in Carthage, MO from my home in Oregon. Well, I went to that rally and then continued on to the East coast including SC and Virginia, on up into Canada through Detroit, then back via MN and South Dakota. In all, from my door back to my door was 8,424.6 miles!

If you should ever find yourself coming through Carthage, MO, again, please let me know. I work for an RV dealer here in Carthage, and would love to see your rig.

Casey