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Old 05-02-2006, 06:32 PM   #15
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1993 21' Sovereign
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Kent, my opinion is, If It Ain't Broke, Don't Fix It.
You have a good-running, virtually brand-new engine. As long as the current transmission continues to perform satisfactorily, leave it alone. If and when it starts acting up, then you can make a change, if you want to.
There is nothing wrong with gathering the information ahead of time, though.
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Old 05-02-2006, 06:55 PM   #16
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I can't add much info to this discussion, but I've read that the best economy for my Cummins diesel is BELOW peak torque. It's where the torque and horsepower curves cross.

For my truck, this is 1800 - 1900 rpm.

Chaplain Kent, your engine should have an associated "sweet spot" and that's were you want your cruising speed to be.

No more from the peanut gallery, however, Terry would know a lot more about this than I do, and I agree with not fixing it if ain't broken.

Lamar
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Old 05-02-2006, 07:03 PM   #17
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Quote:
Originally Posted by SafeHarbor
It's where the torque and horsepower curves cross.
Chaplain Kent, your engine should have an associated "sweet spot" and that's were you want your cruising speed to be.
yes lamar

that's what i was trying to suggest but i left out the 'curves cross' point....
which is of course the important point....
maybe the whole point....

for mine too it is 1900 rpms or so...
i also agree with retreating to the gallery

cheers
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Old 05-02-2006, 07:45 PM   #18
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So for my benefit, lets repeat.

You replaced a 454 V8 with a diesel?

The 454 would turn 3000 to 3500 rpm to haul a 15,000# moho 75 mph.

The diesel will not turn 3000, but will out pull anything at 1800 to 2200 rpm.

So it looks like you need half-again more rpm? To run at 75 mph.

New Higher speed rear end gears would fix it. Don't add more new
spinning things in the moho to wear out, if you don't need to?
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Old 05-03-2006, 09:17 AM   #19
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1985 34.5' Airstream 345
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No, CK didn't replace a v8 with a diesel. He always had a diesel.

CK, my engine governs out at about 3000 rpm's. That's high for a diesel. I used to drive Cummins that governed at 2200. Is your engine still set for break-in? Check the dealer and see if the governor can be increased. What is your rpm at 55 mph?
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Old 05-03-2006, 09:20 AM   #20
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Quote:
Originally Posted by guy99
Joe,
Good Point. There are times when the safest thing to do is SPEED UP. If you can't, it could be bad.

Kent,
What RPM are you at when you top out? Could it be that your new engine has a governor that could be adjusted? I have a cousin who is a truck driver and all of their trucks have top speed governors.
I thought of this to, so when I had it in to the Isuzu dealer for servicing i asked and they said no governor on the engine. The tach reads 30, steady all the time. If I hold the pedal hard down to the floor I can get it to 31. Driving down to Georgia the turbo kicked in only one time on the interstate on the way home it never kicked in.
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Old 05-03-2006, 09:33 AM   #21
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Okay CK. My tach reads 3000 and the speed is about 67 mph. Your rear end is set for too low a speed. Needs pinion and ring gear change. You will have plenty of power. Check with a bus garage. they whould work on plenty of P30 Chasis.
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Old 05-03-2006, 11:53 PM   #22
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Dont change the rearend gears ,thats a mistake,you have a big heavy rv that needs that gear ratio to get it all moving . these coaches are geared acertain way for a reason .By the way if the rear end gears are setup properly ,meaning with the correct tools,pinion depth gauge ,dial indicator etc, they wont be noisy at all ,noisy means they were setup wrong ,thats a fact ,incorrect backlash ,pinion not in the correct position.On a rare occasion I can say you may get a bad set of gears but not very likely .The gear vendors over/under or the allison 5 speed automatic truck transmission are the best solutions ,they both will cost to do it .With the allison there will be more involved .The gear vendors gives a variety of possible ratios you can utilize with your current transmission ,plus you should be able to buy what you need readily available .i would agree that 65mph is fast enough but i can see if it feels that that is all there is and the engine is straining at rpm then you can upgrade your powertrain if you desire.

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Old 05-04-2006, 06:05 AM   #23
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kinetic energy

CK and forum members The Gear Vendor gearbox is the clear choice to adapt the diesel torque/rpm/fuel consumption curves to higher cruising speed. However the brakes, suspension, steering, and mass of these M H makes it very dangerous to go fast. We set the speed control at 60mph on the flat and enjoy floating along while all the idiots try to kill each other. Our fuel burn is 10/11mpg and and I feel somewhat secure that in case of trouble I can control the kinetic energy that I am sitting on the front end of. Cheers Tom
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Old 05-04-2006, 07:23 AM   #24
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Lots of opinions and ideas. You have the same brakes, frame, suspension, steering that we all have in our rigs. The v8's will run 70mph won't they? I agree that 55 is not fast enough.

How much you got to spend? You have a newer engine with more power than mine now and a smaller lighter rig. All you really need is to change the rear end ratio. I don't give a ---- --- about power curves or any of that. I once had a diesel rig with 15 foreward gears. Used about 8 of them. Skipped the rest. Had another with 16. Same deal.

You will always get better mileage going slower. that's a given. Why don't we all drive 55? Because we want to GET WHERE WE'RE GOING!!!

Bowing out of the discussion now.
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Old 05-04-2006, 07:33 AM   #25
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One more thing. If you do go with the 5-spd transmission, you will probably have to change the rear end ratio anyway. I do agree that the two speed axle is nice. Have driven a lot of them. Just a lot more money than you need to spend.
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Old 05-05-2006, 11:36 AM   #26
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Your old engine

CH Kent: I'm just curious. At what RPM would your old engine give you 55mph?

Rob
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Old 05-05-2006, 04:54 PM   #27
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Hi All--I have never had any experience with them, but what ever happened to two speed rear ends? I would be interesting in hearing of any real life experience. Seems like the solution to CK's situation.--Frank S
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Old 05-06-2006, 03:03 AM   #28
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Gear Vendor-The real deal

I have an 86 Chev crewcab 4x4 with 3.73 gears. It had a 454 cu inch gasser with a turbo 400 tranny. I installed a gear vendor and created a more driveable vehicle due to the gear splitting and was able to increase speed at a lower rpm and it did increase mileage by 1 or 2 mpg.[depending on my right foot].
I later installed a 5.9 cummins turbo diesel in the same truck with the same trans and gear vendor setup and again because of the G.V. I was able to use the engine to better advantage when towing up hills etc.
200k miles later I installed a 4L80 trans with the lock up torque converter with over drive and also the Gear Vendor. Now I have 2 overdrives, and the truck is now as good as can be. I can drive the truck at speeds that are unsafe, like over 100 mph, which I choose not to do.[at about 2150 rpm in final drive or cruise at 75 +/- at about 1850 rpm. My truck weighs in at 8900 lbs and when towing total weight around 17000.

My experience is that the only way to go is with the Gear Vendor, It works, has never let me down and will solve your problem while being good for your engines longevity. IMHO.
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