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Discussion Starter · #1 ·
Howdy from Canada!

I know there's a 4BT Forum out there and also some discussion here on these engines, but I haven't found much history on using them in near 8-ton vehicles like my 75' FMC motorhome.

I'm looking for an opportunity to learn more about the suitability of the 4BT as a re-power for my old 440-I/727 Loadflyte in my 30-ft (15,400/lb GVW) motorhome.

First, for what it's worth there's a wealth of info on these old coaches (only 1050 or so built between 73' to 76') here:
FMC Motorcoach

The FMC's - with the exception of 100 or so transit/shuttle bus versions, some of which had diesels in them - were all configured as rear engined (pusher) units with a Dodge 440-I and a 727 trans.

They are hurting big-time on mpg; 8/mpg or so when they are running good.
There are still an estimated 700 to 800 FMC's still on the road and there's even an FMC Club with a most enthusiastic membership. How cool are they you ask? Clint Eastwood owned one.

Many FMC's have had 8.2 Detroit conversions done to them, some have had Powerstrokes & Cummins conversions done. The 8.2 Detroit is heavy and about as long as the 6BT.

The issue is space.

After researching numerous diesels as perspective candidates, I've stumbled upon and have been doing some study on the venerable 4BT.

At 30" in length and about the same weight as the 440/Rat and with some potential to produce some decent HP/torque numbers, I need to know more from those experienced in using this mill for different applications if it truly would be a good engine to put into my baby?

I'm looking to improve torque substantially as I may pull a small trailer or boat as well as hauling the FMC's nearly 8-ton bulk up long hills at a good clip. The stock 1975 440-Dodge in stock smogger trim puts out about 250 to 275 HP and likely under 400ft.pounds torque.

Could I expect to build a 4BT to produce as much HP and more torque than the Rat-gasser AND be reliable for 100,000 miles or more AND up my fuel economy into the 12 to 15/mpg range?

I've read some guys have been able to get 700 or more HP out of these little beauties. But I don't need that sort of performance. I'm looking for somewhat increased performance (especially on hills), much improved fuel economy, and reliability on our planned 'Around N. America' trip my wife and I are going to do. I'll likely be putting an Allison 1000 (set up to stand-alone) behind her to further improve fuel economy.

Thanks for your time and trouble.

Terry
 

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so that is a 75 foot long motorhome? I don't think a 4BT will get it done :D
Just kidding.
I think you'll get more and better info over at 4btswaps.com
My bottom line is, you'd be asking too much of it. Just the motorhome might be doable, tossing a trailer behind it really puts it into the 6BT range.
The parts you are looking at, the built allison and built 4BT will run over 10K US.
I'd figure a way to put in a 6BT with a 47RH. That would be less expensive and more capable. The 47 transmissions can take the power and lock up the TQ in 3rd and 4th for that fuel economy you seek. My 5th wheel combo runs about 17,500 pounds and is 12.5' tall. I get 13 MPG at most any speed on the highway.
With your lower frontal area you should be way better than that.
It started out with 180 HP and now has closer to 330 with a stock turbo and some injectors and small parts. The engine has not been rebuilt and has 235K miles on it.
You could squeeze a 4BT for that, they are great engines. I just don't think it will be as reliable day in and day out with that kind of duty cycle.
 

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Discussion Starter · #4 ·
Thanks SK!
So, if I'm not mistaken, it would be clear that a 4bt doing the same job as your 6bt could do the job for you, but it would take more fuel (less economy) and have to work 30% harder to produce the same energy and suffer a shorter (maybe a lot shorter) lifespan?

Am I on the right track here?

Thanks!
 

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No, I don't think much more fuel. You'd be pushing the same amount of fuel through 2/3rds of the cylinders, with higher heat to deal with, higher boost (more higher heat), higher RPM to get that power. That will take down the fuel economy a bit, but it is the ability of a 4BT to move that heat. To do so will take a bigger checkbook than a 6BT. A the end of the project you'd have a big bill and not as reliable.
Before anybody chimes in and tears me a new one about 4BT reliability, I am strictly speaking in terms of a constant 14,500 pound rig with ~70 square feet of frontal area. Those numbers are more suited to the 6BT in my opinion.
Can you push the engine out the back and mod the back end?
 

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Discussion Starter · #6 ·
Good advice no doubt. The end-cost of the conversion is huge. That said, I've a bit of a knack at hunting down good-deals and finding ways to save money. I'd obviously try to do that here.

I'd get serious real quick about the 6bt if the cost/performance was like night & day over the 4bt, but I don't know that yet. That's why I'm talkin' to people like you - to learn.

I do know of at least one FMC with a 6bt in it. They had to bolt the aft trans u-joint directly to the Rockwell diff. u-joint, eliminating the slip-yoke assembly entirely - in order to fit it in.

I don't want to change the beautiful profile of the FMC's -end.
Would you tinker with Sofia Vergara's butt to make her a little stronger?
 

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It is simple math. the 4BT is 2/3rds. If you have an example to go by on the 6BT already, by all means do that, and improve on what they did. That is what it is all about.
 

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All the b series have ~1 liter per cylinder accept the newer ones. 4bt is a 6bt with -2 cyl. A 4bt would work but for something that grosses 15k minimum I would go with the 6bt just for the sake of driveability. The 4bt was used in box vans that could weigh upwards of 10-15k but they also topped out at less than 60 usually. A lot of them had 4.56 gearing. And that 727 isnt helping. Not real familiar with Allison but ive heard mostly good. It would help mileage enough be noticeable if it has Torque Converter Lockup.
 

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Discussion Starter · #9 ·
Thx Dan. I agree, the FMC has low gears in the diff (4.65 I think) and the 727 gets it all rolling with 2.75:1 1st-gear and a 1:1 final drive in 3rd.

The Allison 1000 might just be the 'right-boy' to take the 4BT to the dance.

The Ally starts it all rollin' with a beefy 3.1:1 first and has an overdrive (2 of em' with the 6speed) WITH lock-up.

I'm wondering if, with the right trans behind it (or in front of it in the FMC's case) that the 4BT could do the job the 440 does with 20 or 30% more torque and 50% or more fuel economy?

If, in the opinion of guys who know these mills, this could be done with the 4BT, I'd really be on to something here and I know hundreds of other FMC owners who'd like to hear about it. It's not been done before as far as I know.

I'm not looking to be novel here, I don't have the coin to play around or experiment, it has to be worth doing otherwise I'd just put the Rat on propane or shoe-horn in an over-drive or another diesel (6bt or 7.3 Powerstroke ect) somehow.

That's why I'm tryin' to learn from those who've bin-dare-dun-dat.
 

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More info on 4bt swaps at 4btswaps.com. One guy was going to go 4Bt in his smaller class C but when he got time to do it, the engines had dried up. He got a 6BT donor truck and yanked it all out and is in the process of installing it right now.
 

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Discussion Starter · #11 ·
Yeah, I joined that forum then tried to post and it wouldn't let me, citing my reference to another web 'link' which it claimed I wasn't allowed to do till I had at least 10 posts. There was no such reference in my post so after trying to edit it several times I gave up. Fug-em'.
 

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After re reading closer, It could be possible to get 12-15 aerodynamics permitting. How fast do you typically run and could you take a picture of the engine compartment and post.
 

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Discussion Starter · #13 ·
Thanks!
Not sure how to put pics up here yet but I can provide a link to an FMC site which has everything you could ever want to know about FMC's.

If you clik the 'Manuals' tab, it'll take you to the Parts & Service menus where you should be able to whatever you need to see.

Hope this helps: FMC Motorcoach

One more of the reasons I like FMC's, there's still a wealth of information and a parts & service infrastructure out there.
 

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I see why you want a 4bt. The 4bt with a few mods could work. Neighbor in La has a tractor with a ve pump 4bt, 6000 hours of bush hog work. Finally just parked it.
 

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I think some simple math here will clear most of this up. Your goal is to move a lot of weight with a lot of frontal area without too much effort. Look at what our trucks weigh and haul with a 6bt. How does it compare to what you're talking about? Now, subtract 1/3 of the engine capability. Do you think it's a reasonable goal for 2/3 of the 6bt?
 

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Discussion Starter · #16 ·
Thanks for the reply V8.

Again, I'm a carpenter, so I'll try not sound like I know what I'm talking about, because I don't and that's why I'm here: to learn.

Maybe though deduction, mathematically like you suggested, I might be able to confirm what I've suspected all along: that a properly built 4BT could reliably match or come quite close to my 440's 275hp and considerably better it's sub 400/lbs. ft. torque while bettering the fuel economy by 50% or more (12/mpg).

Also, with respect to aerodynamics and frontal area, the FMC is one of the most streamlined motorhomes ever built; not a 60's XKE Jag by any stretch, but it's way slipperier than a cabover Mack and only stands just over 9/ft high.
I'm totally up for a wake-up call here. But before you trash this idea, please remember I'm not trying to build a tractor-pull special here. I'm only trying to appreciably better the torque and fuel economy of my 440, that's all.

So then tell my if I'm out to lunch here on these numbers.

If a moderately pumped 6BT could put out 350-400/hp and 650-700/lbs of torque and run 200 or 300,000 miles or more and get 25-30/mpg in a 3/4 ton pickup then, by extrapolation, a similarly set up 4BT should produce roughly 66% of these numbers and be 2 to 300/lbs lighter to boot. Right?

So the 4BT numbers could be - 231 to 264/hp & 429 to 462/lbs.ft.
These kind of numbers and a 50% improvement in fuel economy would make me a happy camper. The stock 60/gal tank in the FMC would let us see 700-800 miles between fill-ups.

If I could see numbers like this from a 4BT and achieve the fuel economy I'm after along with more torque for hill-climbing, I'd be down for this and so would a lot of the other 800 or so FMC owners out there today.

Now, from you guys in the know, how skewed are my numbers?
Would a 4BT/6BT running at this level be running too hot or be a fuel guzzler?
Is my hypothetical HP to Torque ratio out to lunch or fairly close? I'd like to see 500/lbs.ft. or better but in the high 400's is still better than my current 440 set up.

Well, what do you Cummins Guru's think? Am I on track here or do you all still think I need to just bite-it and find a way to get the 6BT in there somehow?

Conversely, if you think this could be doable, what model year 4BT should I start looking for and what other equipment would you recommend I start collecting for the build? P-pump? What turbo? Intercooler-aftercooler-beercooler? What?

I'm pretty much set on an Ally 1000. It has awesome gearing for this application, with lock-up and a tail-shaft parking brake can be fitted. Also, I can leave the stock instrumentation as is on my coach.

Thanks again to all for helping me with this.
 

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For max power it would seem logical to find a p-pump version. The p-pump on the 6bt will produce 700-800? hp without 13mm plungers and such. The ve pump 6bt is capable of ~500. And a little history lesson, from 89-93 it was ve pumps. 94-98 were p-pump. I would assume the 4bt would share similar years. This would also mean somewhere around 98.5 the 4bts could have switched to the 4cyl equivalent of a vp44 which has a LOT of wiring compared to earlier years. From what ive seen and heard about 4bts, they can make 400ftlbs without much work. The newest one ive seen had 130hp and 355ftlbs.

Just saw your thread of 4bt forums, if our trucks can haul over 30k lbs without much thought and 4bt is 33% smaller. Then it should work. Considering the bread van life of a 4bt would probably have a driver thats not doing the maintenance himself and if hes anything like my old bus drivers he wasnt gentle. In short it can handle abuse.

And theres always the v8 diesel option. I havent personally torn down a PS or DM so I cant say to much about wiring. You could also throw a detroit 4-53 in there.
 

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After reading most of this thread, the only real concern I could see would be RPM's at cruising speed. Both Cummins engines like to run around 2,000 RPM + or - a little and if I guessed, your 440 is most likely turning up around 3,000 or so at cruising speed and a lot more if you run on up to 65 to 70 mph (not sure what that is in KMH). As far as HP/torque, I think either engine would do for you. Of course you would have to do more modding the 4bt than the 6bt. When you start comparing gas and diesel engines, the only engine Dodge has that comes close to comparison is the V10. The 440 is a good engine for building for a racing application for all out HP at high RPM's, but it leaves a little to be desired in a towing application. I realize there were a lot used for this, but that doesn't mean it was the best choice. Your transmission is a good transmission. The 727 is a very good transmission no matter what anyone says. When the lockup converter and OD were added to it, those became the weak links behind a modded diesel, but the core of the trans held fine which is a 727. I think your best bet would be a 47RH trans behind either Cummins. I don't know what connects your trans to your drive axle, but if it could be shortened, then I would make that switch. The RH is easy to adapt to a non-pcm controlled vehicle. It wouldn't take much in money to make the trans handle your torque. The combination of OD and lockup would drop RPM's significantly and increase mileage considerably. Now as far as mileage, I don't think your going to see 25-30, or even close. I believe realistically you will see in the 10-15 range which will most likely still be a drastic increase in the 5-10 mpg you now see. Towing my camper with a loaded truck, I see 12-14 mpg in a similar situation as your motorhome. My GVW is around 15,000lbs and I tow 65-70 mph on the interstate.

Either way you go, 4 or 6, I think you'll be happy. If you can squeeze a 6 in there, then I believe you would probably be happier with a 6 since parts are so much easier to come by.

Just my personal opinion.
 

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Discussion Starter · #19 ·
Thanks guys!
I would choose the 6BT over the 4-53 Detroit hands down. They are about the same size and weight: Diesel Engine Specs

I'm sorta' feeling a little better about this idea the more I learn about it. Maybe I'm not trying to fit a round-peg in a square-hole after all. Frankly, I'm excited about the prospect of being perhaps the first person ever - in the near 40/yr. history of the FMC Motorcoach - to put a 4BT in one of these babies.

RP: The FMC needs a trans-mounted parking brake and I'm not sure you can get one or retro-fit one to the 47RH. But who knows, like you said, the 47RH is an ancestor of the venerable old 727 that I have now and it has a trans mounted park-brake. I know the Ally (AT545) is about 27" long without the park brake, not sure bout' the 1000/2000 series but I've heard that they can be configured to stand-alone behind a mechanical engine (non-electric).

Anyhow, space is KING and every inch counts with the FMC. If you look here you can see what I mean:
http://www.fmcmotorcoach.com/files/Parts_manual/18_Differential_Assembly.pdf

If there were more room with the stock set-up I'd simply throw in a Gear Vendors and be done with it because like you said, the 727 is among the best 3-speeds ever built. But a transmission guy told me a year or two ago that you need about 18" extra room to get a GV in there. That would have been the way to go; about $3/K and boom - 11 to 12 mpg with awesome engine braking to boot.

I learned from a fellow FMC owner - who is an x-Chrysler mechanic and huge Mopar nut - that a guy did a 6BT conversion on his FMC and was forced to leave out the slip-yoke assembly entirely (like in the pic from the manual) and bolt the trans U-joint directly to differential U-joint in order to fit the thing in there. So, apparently it is doable but I'm not sure about he implications of a hard coupling (no slip-yoke) between the trans and the diff? Supposedly, it's working out okay. I've a good transmission shop here in Victoria so there's no shortage of advice there.

RP, if all I could get in the end after the swap was 10/mpg at best, I wouldn't do it. Me thinks that would be too much work, money & grief for a 25% increase in fuel economy.
Even if I got great deals on parts for the swap and did all the work I could myself (I'd still have to pay a good mechanic sooner or later during the project) I imagine $10/k to do this could be way conservative. What do you think? What would you guys estimate, best case, worst case?

So if I decide to pursue this I should be looking for a P-pumped 4bt (94' to 98')?
What other mod's would a guy do to build a real fuel-efficient mini torque-monster?
What turbo? What exhaust and how would I set up to monitor EGT's so I know I'm not burning her up on long hills? What other gauges would/should I look at besides what I have now? Bearing in mind of course that when all is said and done you'll give yourself the best crack at 12/mpg.

Thanks for your time and trouble.
 

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Ive heard hy35, tho restrictive for the 6 it has been used a lot on 4bts from what ive seen.
10 mpg MIGHT be on the low side. If the new trucks with dpf see ~10 when towing the same weight a non emissions diesel will see 15. This is me hauling hay with my 09 6.7 vs my 97 12v.
Also 90% of the mods carry oven from the 6bt to the 4bt. Almost everything will cross over between the 2.

Gauges are a necessity for turning up the power. Egt boost trans temp would be starters. For egts the best place is in the exhaust manifold about 1 inch before the flange. For boost there are several ways. I dont know it the 4bts have it but there is a spot on the intake manifold that has a small bolt in it. If not the just tap the intake horn. A 3 inch exhaust should work. 4in would be overkill but would work just the same.
1250 is safe on the pyro but more than that can melt pistons. Spikes from 1500 and 1600 wont hurt unless you stay there. Sled pullers and racers can hit 1800 all the time.

And yes the 727 is a good tranny but a 3 speed leaves something to be desired.
 
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