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Discussion Starter #1 (Edited)
This project is a 1990 Fleetwood Flair RV converted to Cummins 4BT power.

The Flair is model 22D. The smallest class A motor home made (that I know of). Flair is 22 ft 10 in long, 8 ft wide and 10 ft high (roof air unit is 1 ft higher).

GVW is 11500. Original delivered weight with all options included is 8,917.

Flair is on a Ford E350 running gear. 4200 lb front twin I beam suspension. 7800 lb rear axle (Dana 70HD) 4.10 ratio (may be changed).

Purchased Flair last week for this project. Came with Ford 460 and C6 3 speed auto trans. Purchase price 5k. Very good condition, 65k miles, two owners.

120 mile trip home from MSP using cruse control at 55 mpg produced 9.5 mpg. Yikes !

The plan is to install 4BTA CPL857 120 hp and a Ford ZF5 5 speed OD manual or a 4L80E 4 speed OD auto transmission.


This Flair is a class A motorhome (it looks like a city bus chopped in half). There is a lot of space to both sides and above in the engine room.

The goal is to get 20 mpg at 55 mph and get a reasonable speed over the mountains when needed.
I have access to a 6BT and there is room to install that, so I may try to install the 4BT and the 6BT to test power and MPG for each one.
Maybe the 4BT will get the MPG but not pull the Flair over the mountains and the 6BT will pull better than the 460 but not get the MPG.
This will be interesting.

Comments, your experience both welcome.
Currently looking for the 4BT.

Added picture of Flair, looks like a bread truck.
DCP_4776.jpg

POST SWAP REVIEW:
This swap is done and tested now. A paragraph of results for those taking a first look.
The swap was a Cummins 6BT not a 4BT. After much advice on this thread and finding a reasonable 6BT (and 1992 Dodge D250 extended cab) decision was made to use the 6BT. Glad I did that, plenty of power. The mechanical part of the swap was straight forward. The Ford twin I beam cross member was used to mount 6BT. On the passenger side stock Dodge Cummins rubber motor mount is bolted to cross member on driver side a V wedge motor mount was added between cross member and Dodge rubber mount. Rear transmission mount was Dodge cross member with welded extension. Dodge drive shaft bolted in without modification. Most connections were no trouble, throttle, shift... Radiator and intercooler mounts were interesting, see thread. Wiring took more time than mechanical, see thread. 1500 miles on swap now (2011.12.05) no problems so far. Mileage is about 15 mpg. The 4.10 axle ratio will change to 3.54 next summer to improve mileage and will change back to 4.10 if there is not a 2 mpg or better improvement.
 

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sounds like a cool project. I'm going to say that the 6BT will not have to work as hard as the 4BT and maybe be a bit better on fuel. And when you need the passing power, the 6BT will be there for you.


make sure you post lots of pictures.
 

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My vote is for the 6B. With that much weight and frontal area that has aerodynamic of a brick the little 4B is going to be working hard all the time. If you have the time and money to try both engines go for it.
Good luck on your quest.
 

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Should be an interesting build, can't wait to see a comparison between the two motors if you decide to take it that far. I bet the 4bt will pull it, maybe turn it up just a little bit and put in an overdrive with those gears.
 

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If this is a poll, my vote is for the 4bt. We have a 35' class A that has a 300hp cummins (6bt vairiant) and when not towing we have gotten 16+ mpg, with plenty of power to climb whatever I point it at. So a 200hp 4bt should scoot that Flair along nicely and get better mpg when just flat cruising. Ours has a 5 speed allison and personally thats the minimum I'd consider. Ive looked into swaping it out for a mh3000 6speed but it just isnt cost affective for the amount is actually gets drove.

Ya I keep forgetting we use a differnt Gal up here. So it we are getting 14mpg (US). aprox
 

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Discussion Starter #6
Thanks for the comments. I think the 4BT power output will be close to good enough, so it is worth a test. The most likely transmission is the ZF 5 speed OD. Some have commented that I must like to shift and there is some truth to that.

I plan to do what I can about aerodynamics. Spoilers at both sides and top in back. I believe the flat slightly rounded front is not bad. Most wind drag is from turbulence in the wake of the vehicle

Screen shot of the gearing table for the ZF 5, stock tires (215/85-16), change to 3.31 axle ratio is attached.

Cruising in 5th at just under 1600 rpm should be a mpg sweet spot, if the 4BT will pull that.
4th will be useful uphill or with a heavy head wind at 55 and 2k rpm.

Could I drive over Donners pass in 4th at 50+ ?
Will it pull 70 mph at 2k rpm in 5th on the level ?

Plan is to try 4BT first and if power is not enough then put 4BT in 1995 Ford F250XL (has a ZF5 now) and put 6BT in Flair.

Thanks for the motorhome 6BT info. I am surprised that the Cummins in RV swap is not more common. Possibly many RV folk are not mechanical.

Flair_gears1.jpg
 

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Mine wasnt a conversion it came that way.

And there is a line in the sand at 2000rpm were the fuel economy drops hard and fast. With my gearing that is just shy of 65mph. If I push it past 2200rpm Im getting 8mpg + or -.
 

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i believe a 120hp 4bt bolted to a zf 5spd wouldn't actually be a bad setup for it... this is the same setup currently available in some versions of the f350 and f550 in south america... but actually i would prefer to get an automatic transmission...
 

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Just for some more numbers, before i ripped my 130hp 4bt outta the oshkosh fed-ex stepvan it was in, i got 15mpg+. That was a 11' high x8'w crackerbox, gvw=15K, scaled at 10K, 4.10 D60, T19 4spd direct. but it only went about 58mph tops w/ those gears. I drove it several hundred miles, from Twin cities to Winona and back several times.

if you're not speed demon, i reckon the 4b will do just fine. but if you'll be towing anything substantial, then you may want to consider the 6b, especially if you already have one.
 

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Discussion Starter #11
Just for some more numbers, before i ripped my 130hp 4bt outta the oshkosh fed-ex stepvan it was in, i got 15mpg+. That was a 11' high x8'w crackerbox, gvw=15K, scaled at 10K, 4.10 D60, T19 4spd direct. but it only went about 58mph tops w/ those gears. I drove it several hundred miles, from Twin cities to Winona and back several times.

if you're not speed demon, i reckon the 4b will do just fine. but if you'll be towing anything substantial, then you may want to consider the 6b, especially if you already have one.
Those are interesting numbers. So 58mph was 2400 rpm in the bread truck ? What speed / rpm were you driving when getting 15mpg ? Both the weight and dimensions of the bread truck are similar to this Flair RV. Max speed of 65 and usual hwy speed of 55 is ok for this rig. Was the 4BT pulling strong in that 10k bread truck at 1500-1600 rpm ?

I plan on being just under 1600 rpm at 55 mph in 5th gear and just over 2000 rpm at 55 mph in 4th gear for mileage or power respectively. I will find out if the 4BT has the same 2k rpm mileage drop finally.... mentioned above.

So your 130 hp 4BT has the inline pump, which is better for max hp after adjustment but not as fuel efficient in stock form as the rotary pump ?
 

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Discussion Starter #12
i believe a 120hp 4bt bolted to a zf 5spd wouldn't actually be a bad setup for it... this is the same setup currently available in some versions of the f350 and f550 in south america... but actually i would prefer to get an automatic transmission...
That is interesting, wonder if a North Am Ford dealer could get a part (like the 4BT to ZF5 adapter) for a South Am F350 ?
 

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That is interesting, wonder if a North Am Ford dealer could get a part (like the 4BT to ZF5 adapter) for a South Am F350 ?
i'm not so sure about it, but if you had the parts # maybe you could try it... back here it works to order parts for grey-import us-spec vehicles from ford at the official dealerships, so maybe can work for you there...
 

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as for rpms at cruising- no tach, i dont know where the governor is set at on my engine, but the pump is still sealed. I must admit it has been 5+ yrs since i drove it, maybe the absolute top end was a bit higher- 62-65? but i know it wasnt really setup well at highway speeds, imho. the mileage figures were from the highway trips i took to winona- via 61, so plenty of small towns en-route, but also a lot of 55 speed limit. one more gear would have been great, the tranny it had was a bit wide from 2nd-3rd.

the engine pulled that truck at 10Klbs up hills like a 14yo boy with a playboy.
 

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Discussion Starter #15
i'm not so sure about it, but if you had the parts # maybe you could try it... back here it works to order parts for grey-import us-spec vehicles from ford at the official dealerships, so maybe can work for you there...
I will try a Ford dealer here. Have you checked the price of a Cummins to small block Ford adapter there ? Or maybe Ford uses the large block pattern, for F350/450 ?
 

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if you go on eBay Brazil and find a 5 speed 4bt truck; try and get a VIN from the auction or seller and take it to the ford dealer.
 

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I will try a Ford dealer here. Have you checked the price of a Cummins to small block Ford adapter there ? Or maybe Ford uses the large block pattern, for F350/450 ?
it's often reported to use the same bolt pattern as the triton 5.4L
 

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Discussion Starter #18 (Edited)
Update

Where did all the 4BT's go ? Had several to pick from this spring then waited to get RV first. Now no local 4BT's available.

So current plan is to begin with 6BT and 47RH trans. Will try to make switching to 4BT later as easy as possible, by moving trans cross member forward or front motor mounts back.

So the rpm-mph table with 47RH and 3.54 axle now looks like this;

Flair_6BT_47RH.jpg
 

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Will try to make switching to 4BT later as easy as possible
are you sure about it? at least for me it would make some sense to retain the 6bt...

and if you want a 4cyl you can also search for other engines such as isuzu or deutz...
 

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Discussion Starter #20 (Edited)
Hi Cr,
I am going to get the 6BT - 47RH (including Dodge D250) today. Then removal of Ford 460 and install of 6BT begins ! I meant that when I install the 6BT I will remain conscious of how I could switch to a 4BT in the future.

If the 6BT reaches the goal of 20 MPG then it will remain in the Flair, no switch needed ! The gearing looks right with 55 mph at 1500 rpm. I would rather have the power of the 6BT if possible.
 
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