I'm looking at a 99 Diplomat 38' with the ISB 275HP. Am I barking up the wrong tree as far as any performance issues. I don't know much about diesels except for some conversation about HP not being a factor just the torque. This unit only has 22K miles on itand is in excellent condition. I see that Roadmaster increased the HP the following year. I sure could use some help to make the right decision. We're moving up from a gas rig but then again don't want to make the wrong move here and be sorry in the end. If it does make a difference we are thinking of fulltiming in the new unit. Thanks for any help.

The Cummins ISB is a strong engine. Not a ton of power, but it will get you where you're going. Expect to slow down going up grades, maybe to 35-45 mph, depending on the grade. On the level you'll travel with the best of them.

One thing to strongly consider is what tranny is attached to the ISB. If it's a 6-speed Allison, then there are some options here. For example, Banks makes an upgrade for the ISB which pumps up the torque and hp. But be forewarned -- you cannot boost the engine over what the tranny is rated for. So check this part out. You can call Allison and tell them what tranny is in this coach, and they will tell you it's maximum torque rating.

quote:

Originally posted by Pusherman:
The Cummins ISB is a strong engine. Not a ton of power, but it will get you where you're going. Expect to slow down going up grades, maybe to 35-45 mph, depending on the grade. On the level you'll travel with the best of them.

One thing to strongly consider is what tranny is attached to the ISB. If it's a 6-speed Allison, then there are some options here. For example, Banks makes an upgrade for the ISB which pumps up the torque and hp. But be forewarned -- you cannot boost the engine over what the tranny is rated for. So check this part out. You can call Allison and tell them what tranny is in this coach, and they will tell you it's maximum torque rating.

Steve and Sal,

Pusherman's reply is right on target.

The ISB is a strong medium duty engine. It has limited HP and torque though compared to bigger engines, including the Cat C7 (which is used in similar applications as the ISB quite often - The ISB will have somewhere around 600 +/- torque while the Cat has about 800 +/-).

I have the ISB in my 99 Dodge which would pull the 32' fifth wheel I used to have, with no problem (18-19K lbs).

I now have a 34' MH with the ISB. The combined weight of the MH and toad (Liberty diesel) is about 28000 lbs. The MH pulls fine on level and moderate grades. It does slow down considerably on steep grades. If you do not want to have to slow down then I would suggest a bigger engine (Cat or Cummins) if you are going to buy anything bigger than 36' (I personally will not get another ISB, although I love the engine, unless I were to upgrade to another 34' MH which is unlikely).

Also, (as Pusherman stated) if the MH you are looking at has the 5 speed Allison it will work fine except that you will not be able to upgrade the engine HP / torque as the 5 speed is close to or at it's max HP/torque rating with the stock ISB.

Ed

Thanks for the info from both of you. I guess I have some checking to do but I think it is the 6 speed trans. The MH is a 38' with single slide and only has 22K miles on it. That's the tempting part but then again maybe there is a reason it wasn't used much because the people weren't enjoying it's performance. I'll do some thinking here. Thanks again I'll keep this updated.

Steve and Sal:

I posted my $.02 over on the Monaco Forum FYI.

Ken

quote:

Originally posted by quikduk:
Steve and Sal:

I posted my $.02 over on the Monaco Forum FYI.

Ken

Ken,

Although your engine has 275 HP it is the 8.3 mechanical which has 800 ft/lbs of torque vs the 5.9L ISB that Steve N Sal is looking into. The extra torque from the larger 8.3 will make a difference in performance.

Ed

quote:

Originally posted by Steve N Sal:
Thanks for the info from both of you. I guess I have some checking to do but I think it is the 6 speed trans. The MH is a 38' with single slide and only has 22K miles on it. That's the tempting part but then again maybe there is a reason it wasn't used much because the people weren't enjoying it's performance. I'll do some thinking here. Thanks again I'll keep this updated.

I drove my ISB 275 over 80,000 miles pulling a Tracker and I lived at 7000+ft so was in mountains all the time. Yes, it slowed down on mountains to 35-45 on severe grades like Parley's Summit on I-80 in UT but otherwise it was a great engine. One thing no one has mentioned and is important in today's environment - it gets great fuel mileage. I got near 11 MPG constantly. I miss it with my newer higher HP where I get 7 or so.

Before you by a 1999 ISB you better be sure it does not have the "53" block. You can research the problems associated with this block. A fix could cost over $20K.

quote:

Originally posted by andyk:
Before you by a 1999 ISB you better be sure it does not have the "53" block. You can research the problems associated with this block. A fix could cost over $20K.

The "53" block is / was an issue. However, if the "killer dowel pin" has been replaced with the "fix" then it becomes a non-issue.

I have a "53" block in my 99 Dodge and have not done the "fix" and still no troubles at 100k+ miles.

Ed

we just upgraded from a gas motorhome to a 99 diplomat 1 slide isb 275 hp and we really like it. We tow a 99 tracker 4 door 4x4 and get over 11 mpg.Banks will upgrade it to 340 hp if you want more power.it does have the alinson 6 speed,so buy it and enjoy.