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Old 11-17-2018, 03:24 PM   #1
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5.9 vs. 6.7 vs. 8.3

I am finally approaching the $$ goal that I need to buy a pusher...will be looking for a 36 to 40 ft bus with intention of making it full-time live in rv. The dream is to do cross country driving during the summer. I am a teacher, so that is more realistic. I am hoping to find a bus with an Allison 3xxx or switch out to one and be able to actively use the 6th gear without blowing up the engine. There is one model that has a granny first gear, of 6 to 1, which makes towing something more agreeable.



Survey says that the 5.9 would be a little underpowered. From what I have read, using an underpowered engine might result in lower mpg than using a bigger engine that is more suitable for the job, and would reduce the life expectancy of the engine.



Like all of you, I am hoping to maximize fuel efficiency, while having a drive train that will give me reliable service for more than 250K miles. I am also hoping to find an engine that will not break the bank every time I need a new injector or some other part. With those requirements in mind, how do the 6.7 and 8.3 rate? I am guessing that parts for the 6.7 will be higher because it is a newer engine and likely to have more electronic stuff on it. What about use in this application?


My neighbors used to do long haul with an 18 wheeler pulling 50K pounds and got 11 mpg. They did have a manual transmission. Greyhounds cruise up and down I5 in California get get 11 mpg doing highway speeds (per a bus technician). I am hoping to breach the 10 mph limit. Any comments or insights to my random thoughts would be appreciated. I am not committed to a Cummins...but it is the brand name that keeps coming up in my circle. I would like to have an auto tranny with the specs of the Allison 3xxx - I haven't seen another one like it.
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Old 11-17-2018, 05:47 PM   #2
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It's been said (and with much truth) that a small engine running at full throttle will get less economy than a larger engine running at half power. In a larger vehicle, the small engine will strain/work harder to ascend hills, while the larger engine will do it more easily.


I have no long term experience with the 6.7, but the 5.9 and 8.3 both have excellent reputations. Parts are widely available and they're said to be pretty easy to work on. The 8.3 will definitely provide plenty of torque.


You may find some IH offerings. The 6.0 (Ford) / VT365 (IH) has a bad reputation from the factory. The deficiencies in these engines can be corrected (at no small cost) but if the engine fails, it will be completely ruined. More than one forum member has had this happen, and replacement runs $25K or more. The 7.3 (Ford) /T444 (IH) has an excellent reputation but is not known as a powerful engine (I have one of these in my shorty). The DT466 also has an excellent reputation, wide parts availability, and good power. Avoid the '05 and up "MaxxForce" versions - the emissions hardware is a nightmare. IH abandoned this engine design because of this.
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Old 11-17-2018, 09:45 PM   #3
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opinion

dt466 or 8.3

um i know of one guy selling a pusher 8.3 with 3060 on craigslist for about $7000 says has less than 100,000 miles it is a pusher rv chassis. it is work by no means just bolt up and go... but it is all there. I think he is in one of the western states, CA AZ UT

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Old 11-24-2018, 04:02 PM   #4
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I have an 8.3 with an MD3060. 6th gear is locked out. I wouldn't need it anyway, the ring and pinion have much more bearing on top speed. We travel at 70 mph @ 1800 rpm and 8 mpg on flat highways. There's enough left over to get up to 80 mph.

Previous experience on other trucks with larger power packages tells me that unless the transmission and engine are "stepped up" significantly, for example Cummins L10 mated to a 4000 series Allison, you won't get enough reliable headroom in power and torque.

Even then, with that much power you really need to pay attention because the vehicle dynamics of a giant steel bread loaf lacking a tag axle are not the same as a tractor trailer combination.
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Old 11-24-2018, 09:39 PM   #5
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I have the DT530 300hp. with the 3060 and 4:10 gears, 6 speed it gets 14mpg empty and hauling my 5th wheel 17K lbs I get 11.5 mpg it pulls a 7% grade from a dead stop @ 45mph. I got real lucky when i found this truck for sale@$22K in mint condition I mean this truck is like bran new.
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Old 11-24-2018, 11:05 PM   #6
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Quote:
Originally Posted by aaronsb View Post
I have an 8.3 with an MD3060. 6th gear is locked out. I wouldn't need it anyway, the ring and pinion have much more bearing on top speed. We travel at 70 mph @ 1800 rpm and 8 mpg on flat highways. There's enough left over to get up to 80 mph.

Previous experience on other trucks with larger power packages tells me that unless the transmission and engine are "stepped up" significantly, for example Cummins L10 mated to a 4000 series Allison, you won't get enough reliable headroom in power and torque.

Even then, with that much power you really need to pay attention because the vehicle dynamics of a giant steel bread loaf lacking a tag axle are not the same as a tractor trailer combination.
It wasn't you who posted the vid of their 6th unlocked?
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Old 11-25-2018, 03:20 AM   #7
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Quote:
Originally Posted by pengyou View Post
... ... ...

My neighbors used to do long haul with an 18 wheeler pulling 50K pounds and got 11 mpg.... ... ...
I drove 18-wheelers for 27 years and my fuel mileage was always in the top handful of drivers, among hundreds of drivers in the company.
And my best was around 7.6 or 7.7 mpg.

Just a friendly reminder that with statistics you can prove anything you want.
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Old 11-25-2018, 08:38 AM   #8
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Quote:
Originally Posted by Brad_SwiftFur View Post
It's been said (and with much truth) that a small engine running at full throttle will get less economy than a larger engine running at half power. In a larger vehicle, the small engine will strain/work harder to ascend hills, while the larger engine will do it more easily.


I have no long term experience with the 6.7, but the 5.9 and 8.3 both have excellent reputations. Parts are widely available and they're said to be pretty easy to work on. The 8.3 will definitely provide plenty of torque.


You may find some IH offerings. The 6.0 (Ford) / VT365 (IH) has a bad reputation from the factory. The deficiencies in these engines can be corrected (at no small cost) but if the engine fails, it will be completely ruined. More than one forum member has had this happen, and replacement runs $25K or more. The 7.3 (Ford) /T444 (IH) has an excellent reputation but is not known as a powerful engine (I have one of these in my shorty). The DT466 also has an excellent reputation, wide parts availability, and good power. Avoid the '05 and up "MaxxForce" versions - the emissions hardware is a nightmare. IH abandoned this engine design because of this.



this is prettyy much on.. in general the inline 6 engines will net you better torque than the V-8s.. I have a shorty with a T-444E and even 'turned up' doesnt seem to produce the torque the DT series engines do..



the V-8's youll find out there in school busses are generally the CAT-3208, Navistar T-444E(ford 7.3), Navistar VT-365(ford 6.0), Navistar MaxxForce 7(ford 6.4)..



The inline 6's you see most still are CAT-3126, Cummins 5.9 (2 variants.. the older 6BT or 12-valve, and the newer electronic 24-valve), Cummins 8.3 (I-6), Navistar DTA-360 (mechanical I-6), navistar DT-466 (mechanical I-6), navistar DT-466E(electronic I-6), navistar DT-530(E) (electronic I-6), Navistar MaxxForce DT(full emission control version of DT-466E)..


im sure i missed a couple.. I put out there the most common ones.. we all have varying opinions on which engines to stay away from and which to not.. alot having to do with one's mechanical abilities to perform Preventative measures or make repairs if you do break one.. All engines have some type of achille heal..



when looking at numbers.. pay attention to Torque.. Torque Moves Metal down the highway(which is what you want to do).. Horsepower wow's consumers and sells engines...

-Christopher
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Old 11-25-2018, 11:50 AM   #9
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....and some of us have to drive around with our old smokey 3116's


Just to add another to your good inline six list, Chris


They are all mighty machines, mostly finely built and designed, when you think about what's available.
It pays to study them to see if you are up to the task of upkeep.



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Old 11-28-2018, 07:21 PM   #10
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Not trying to make a fuss but.. Horsepower is work done, torque is only twisting force applied. Must have torque applied over time and have actual movement. This is horsepower.

Granted an engine with more torque at a given rpm will have more HP at the same rpm then and engine with less torque at the same rpm. Torque X time = HP
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