axle ratio change

[kotflb]

Hi guys and gals, I'm new here on posting, but have been enjoying your site
for awhile now. I finally got myself a 92 FC International w/ a Ward 14
window. It has a dt466 and a MT643r trans. The air ride axle is a 4:44 on
295x75-22.5 tires. I drove it 900 miles to get her home at 60 mph.
It may have run a little faster but I didn't like pushing her over 2250 rpm. And THAT
may have been pushing too hard, I'm not sure.

Anyhow, I been doing some figuring? My idea is to replace the 4:44 w/
a rebuilt ratio of 3:42. This will give me a 65 mph cruise speed of 1886 rpm.
Has anyone tried a ratio this fast? And IF I ever find myself needing to
REALLY GET SOMEWHERE FAST, I can go 70 mph at 2031 RPM. Though
most of my driving will be at 60 mph which is 1741 rpm. I'm thinking that
dt466 at 210 hp shouldn't have any trouble working this axle ratio. Your
thoughts, please. [/b]

[oldog12]

How do you think it will handle a long uphill pull?

[the_experience03]

http://www.internationaldelivers.com...NGLE.pdf?src=1

Ignoring the fact that that is a new DT466 versus your older one (I don't htink the specs are all the different other than power output), you will see that maximum governed speed is 2600 RPM, the same as my 6.6L Ford. Please remember that is a governed top speed, not a mechanical redline. Sure, semis might be turning 1500-1700 RPM going down the highway, but they have well in excess of 1500 ft lbs available to them right at about that speed and enough gears to always have the "correct" ratio. You're limited by the fact that you have just over 500 ft lbs of torque and 4 gears (5 if you factor in torque converter lock up).

Basically, I think that you have a good deal going on paper, but in reality, you'd be MIGHTY sorry with that ratio. If you can even find a ring and pinion set in that ratio, you will have an amazingly difficult time achieving those speeds you have listed, especially with any sort of load like a hill. If and when your tranny shifts into 4th gear, you will be cooking that poor converter I think. Your tranny is going to think it's pulling twice as much weight around due to the lack of mechanical advantage.

Basically, my suggestion is this...keep the ratio you have. You aren't spinning the engine too fast as it is. If you really want to run it slower or go faster on the highway, swap to a true class 8 manual tranny. I know an RT6613 Road Ranger 13 speed was an option in my bus and might be something to look at if it's really that important to you.

[lapeer20m]

redline for the DT360 is 3,100 rpm, i would assume it is the same for the 466

[the_experience03]

If the redline varies like the governor does, the higher the output, the lower the rated redline for obvious reasons. I think it would be safe to assume his DT466 is in the lower output tier and thus would have one of the higher redlines. 3000 doesn't sound out of the question.

[kotflb]

Thank you gentlemen for the feedback. I gauss I assumed wrong, I was thinking the redline for the dt466 was 2600 rpm, but maybe that is the standard governor setting.

Therefore are you saying I can run this engine at 2250 rpm all day and not hurt the longevity of this engine. I had read on an earlier post that 1800 rpm is the ideal cruising rpm for this motor, and looking at the link you posted, it is at a good torque point, though at a slightly lower then max horsepower point.

Isn't the best fuel ecomony acheived somewhere between max hp and max torque, and if so, wouldn't that put my cruise rpm somewhere around 1800 and 1900 rpm? Since max torque is 1500, and max hp is 2300 rpm, after which increasing rpm just reduces the hp, where is the SWEET SPOT, on these engines?

As far as mountain driving goes, maybe 1 or 2 percent may be mountain driving, therefore my main concern is the other 98% of mainly flat and slight rolling hills. If I can get over the moutains in 2nd or 3rd gear without overheating, I'll be happy. Of couse I plan to install a radiator mister to help keep things cool. My goal is 12+ mpg in "no wind" highway driving between 60 and 65 mph. You thoughts, please.

[the_experience03]

Here is my way of looking at things...torque is most important as it is what gets things moving...but if you want them to move fast, you need the horsepower. The horsepower is what's going to give you that 70 mph top speed. Ok...so cruising you want top horsepower then because horsepoewr is a unit derived from torque. Torque can be altered via gearing, but not horsepower. Cruising obviously requires less torque than accleration so......gear for top horsepower at cruising RPM. You will then have extra torque on reserve for when you get loaded down such as a hill. In a bus, hill doesn't mean a mountain...it means any hill. I can feel so little as a 3% grade in the bus. Length, not steepness, is what seems to affect my bus more.

Long story short....I would just leave your gearing alone. If you can find a thirdmember with 4.10's in the boneyard, more power to you, but I wouldn't pay someone to replace them with new. It would take a LONG time to recover that cost at the benefit of a just a few miles per gallon. You're well withing the design tolerances of the motor with your foot on the floor all the time so no worries about longevity. That really just leaves you with speed. Even a 10 mph increase in top end isn't going to get you all that much farther in an 8 hour driving day. What's 80 miles? I'd rather just sit back and enjoy the view.

[oldog12]

Don't fix it if it ain't broke

[lapeer20m]

i think ia gree with old dog. A lot of us would kill for a 4.44 rear end.

not to mention you have the highly coveted dt466 coupled with the mt643.

[kotflb]

thanks guys for the replys.
I'll leave it as it is and enjoy the views since I can live with the current 10 mpg it now gets, and start working on the conversion and body painting. That by itself should keep me busy for the next six months.

[lapeer20m]

i'd kill a man if i could get 10 mpg out of my bus. That's nearly double the 6mpg i currently get. i do get as good as 7.5 at times

[busone]

Yikes 7.5 is all you get? My bus will give me 13 if I stay at 55MPH. I get 9-10 at 72MPH.

[Wolfman]

An option you may want to look for is a Gear Vendors overdrive unit. It installs after the transmission, and gives you an extra "gear" for steady flatland cruising. Get into the hills, kick out the OD, and let the engine pull up on revs. They make these things for medium duty applications, so you should be able to find something that will get you closer to your goals. Caveat is though, that they are NOT cheap.

[the_experience03]

As cool as I think the GV is (and it IS cool), I don't think they are rated for the GVWR a bus has to deal with. On top of that, finding a way to mate one to an Allison could be difficult at best. There are, however, similar auxilary transmissions that are somewhat common in MDT's.

[kotflb]

Busone, what kind od setup do you have to get 13 mpg? Is your powertrain stock or have you made a few mods?

Wolfman, I can get a rebuilt pumpkin from the International dealer for $1400.00 plus 4 hours of labor at $85/hour. That may be cheaper then the OD unit and modding the drive shaft.

My thoughts are, it takes horsepower to climb mountains, and with 210 hp, I'm only going to get 30 to 35 mph on a long 6 percent grade, be it in 2nd, 3rd, or 4th gear, depending on the axle ratio. Therefore I may as well cruise the flats at a comfortable ( and quiet ) RPM, plus get the extra MPG bonus by not taching out my engine. Perhalps a 3:42 ratio may cause too much strain. I may re-consider a 3:70 ratio, which may give me the best of both worlds. I will admit, the 4:44 ratio is great for city stop and go traffic, but I plan to cruise the big roads. What say Ye?

[the_experience03]

16 mpg is a VERY optimistic prediction of mileage with the higher gears versus my stock 8 mpg. You will get twice the mileage. That means that I have to spend twice as much to go the same distance...simple enough. At $2.25 per gallon for diesel, I will drive buy 800 miles on that $1700 you just spent on the conversion. That gets me 6400 miles. That might not seem like much, but that's what you would have to drive just to break even on the swap assuming that astronomically high 16 mpg. At a more realistic 12 mpg you will have to drive over 8000 miles to break even. Start factoring time, unforseen setbacks, etc and the swap starts to seem like less of a finacial savings. I will stand my ground that his ratio is already low enough. Since the difference in RPM is not important to the longevity of the engine, the only issue I see is mileage. A better investment of that $1700 would probably be a WVO system or other alternative energy source. It is just like the people trading in their SUV's for a Kia to save on fuel while taking an enormous hit in depreciation. As great as thing sound up front, you really need to look at the actual return on investment. Just my $.02 (jumps off soapbox)

[gmrubin]

I just bought a 1993 ten window Intl. 210 HP aftercooled DT466 (all mechanical) with an MT643. My only trip so far has been driving it home 200 miles from Seattle through rush hour traffic. I got an honest 10mpg on that miserable trip driving a bus I was not familiar with and constant slowing and speeding up. I'm pretty confident I will be able to squeeze 12MPG out of her on 55-60 mph highway cruises. I was turning 2200 @55mph and 2400@60mph. It seemed to find a sweet spot around 58mph which will be a good cruising speed for my travels in my retirement.

Based on my prior experience with boats and OTR trucks a mechanical diesel likes to be run at 3/4 to 7/8 of full power (think boat's cruise speed). It's different with electronic diesels. My last engine, a Cat C-7 with an Allison 3060 could be safely run anywhere from its 1340rpm torque peak up to its powered redline @2500rpm. That engine had an unpowered redline (downhill coasting) of 2950rpm. This DT466 went up to 67mph with a little bit left but I was driving a newly purchased vehicle so I wasn't going to push it too hard. I think it's safe to say the mechanical DT466 can run 2500rpm all day with occasional forays into the 2900rpm range (governor allowing).

I think anyone would be very hard pressed to get 16 MPG from a DT466 in a 20k lb. vehicle