Diff. Gears? Cummins NHH 220 + RT-910. Crown or Gillig Pros?

[jasonfarrington]

Solved.

I have different numbers.

toyo is listing 471 revolutions per mile for the m610 11r24.5

at 60 mph 471 x 4.11 gives 1935 engine revs I use 60 mph because the math is simple at this point.

471 revs in one minute for one mile..... 60 mph is a mile a minute...

that is 1935.86 I will round up 1936 engine revs at 60mph

65 over 60 gives 1.083 that is the ratio of 65 mph over 60 mph

1.083 x 1935 gives engine rpm at 65 mph which is 2097 engine revs.

I say it is time to jack up one set of wheels in the drive axle and verify gear ratio...... otherwise all calculations are guesses. also hand held photo tachometer or mechanical tachometer - like a stewart warner... to cross check rpm readings.

is your bus a mechanical tachometer? all kinds of ways to be "off" and out of adjustment.

william

Boom. Thanks William.

I was relying on the referenced calculator and Toyo's 44" diameter spec - which looks to be within 2 or so mph of your calculations. Using your now-simple math, the numbers jive with my 60mph max cruise speed - things feel best at 1850-2000RPM, which when indicated by the tach is GPS 58-60mph.

Governed max rpm according to the Cummins Operator's Manual is 2100. At that engine speed I would indeed be at the 65mph, not ideal for covering miles. The manual calls for 1750-1850rpm for continuous duty/cruising applications.

Crown's suggestion for a change to 3.7x ratio and the RTO transmission seems spot-on. Maybe keeping the 1.1 RT transmission and dropping to 3.3x-3.5x would be best, definitely more economical. 3.70 would be 2000rpm at 70mph. 3.50 would allow for 75mph at 2000rpm. 3.30 would be 1940rpm at 75mph & reserve 10th for 65+ only. Hmm... I will see what gears are available for the Rockwell differential, not having much luck with the Q145PX24 part number listed on the tag...

Thanks!

Jason

 

[flattracker]

When I was determining what speed my "new Crown" reached redline I used

https://spicerparts.com/calculators/transmission-ratio-rpm-calculator to figure it out. I came up with 63 mph with 4.11 ratio and Michelin 22.5" tires. Since the bus would easily go 70 mph, I concluded that the engine was over-revved. Even if the original engine was still in the bus, I would have changed the gearing to accommodate 70 mph without exceeding redline. If a GPS shows I am going a certain speed I believe it.

 

[s2mikon]

How much horsepower does it take to push one of these boxes 80 mph?

 

[magnakansas]

oh man

you can figure this out your self but it will take some work.... two ways... pure math... and doing some testing on flat ground with head wind and tail wind. stop watch coast down testing

do some reading for relevant equations... probably need to know what your power level is to your rear wheels... there are some heavy truck dynamometers out there.

coast down with no wind from known speeds .. start slow like 15 mph, 10mph and 5mph use those to find trend and plug numbers in should get you rolling friction numbers....

or.... you find a place like kansas in summer time with a wind out of the south on a summer day of 30 mph and go into the wind and see if you can sustain a 50 mph speed at a certain throttle position that will tell you too

you can test in real time or go through ministrations and use math equations...

if you have good torque and hp at 80 mph and hit a head wind you see much speed change.... if you are at a crappy part of the torque curve and hit a head wind, the speed will drop off faster. just like a hill, if you are at a high enough torque your speed wont drop that much going up the hill, the more you are deficient of torque the more engine rpm you will lose.

I've done work like this to find drag coeficient numbers for racing cars. rolling friction, aerodynamic drag, then work on a chassis dyno to find engine numbers, then cross check with actual testing to see if dyno numbers match to results given... often they dont.... but after doing all the work. You can then go to a new track and make some pretty good guesses at what gears you want in the transmission and final drive.

most of the work I did was between sea level and 2,000 ft above sea level. once you get in the 4,000 to 5,000 ft above sea level then air density also becomes a more significant factor. gasoline engines are more sensitive to intake air temperature than the diesels. You can cool the diesel intake air too much.... I have gotten into one case where I had to block off the intercooler because the intake air temps were getting so low to the point of misfire in the combustion chamber, this was in -20 degree F weather. I think it might be possible at 125 degrees F or more - death valley - to over heat air intake in a diesel to the point of causing damage to pistons from detonation.

I am pretty sure in the 1954 24' 11,000lbs wayne with 350 hp and 800 ft lbs 3.73 rear gears, 245/70r19.5 tires 618 revs per mile, 5th gear at a direct drive, will just about touch 80mph at 3,000 rpm

the tires are not rated for those speeds... rated for 75 mph but at 11,000 lbs I would be under the max load rating of the tire.

my 1994 blue bird the front tires are near max load I would in no way ever want to run 75 mph on a hot summer day for any length of time.... that would be a life or death emergency kind of run...... My tires are not new and holy moly the heat build up in the tread would be terrible.... I have some pretty deep tread and tires like that dont do heat well. A tire that is slick and very short rubber thickness is better suited to high temp, high load. and that is not how a heavy truck tire is built.. Hey tire engineer guys how far off do you think I am... I think I have a pretty good grip - pun intended - on tire dynamics.

this is fun, something I know about.

william

 

[flattracker]

Another pertinent question about driving 80 mph in a skoolie, how much diesel or gas does it take to move one at 80 mph. Also in states like Idaho that have speed limits of 80 mph on the interstate, they limit trucks to 70 mph. I think the reasons for a truck to be limited to 70 apply some to skoolies. The listed weight on my "new Crown" is just under 24,000 lbs. Even with the more than adequate brakes Crown used, physics dictate a longer stopping distance than a car or pickup. From experience, I know of significant winds in Idaho and wyoming. A Skoolie has a large cross-section to sidewinds. Even though I am pretty sure the "new Crown" will be capable of exceeding 80 mph, I don't plan to drive it that fast. I realize others do not necessarily see things my way and that is OK. Just adding my two cents



My brothers and I will be traveling around the country later this year and we plan to go about 300 miles per day average. Being able to go faster through more hp and taller gearing will hopefully give me better fuel economy. With the price of fuel inching up to $5 a gallon that could push fuel costs to $250 a day. We will not be in a hurry.

 

[Sagebrush-SKO]

You list a NHH 220 turbo. A 220 was naturally aspirated. On the PT fuel pump there should be a tag near the throttle shaft. It should read 220 or maybe 262. A lot of 220,s were turned into 262,s with a turbo and a pump recalibration. If the pump is still set at 220 the "turbo" is a "compensator" to compensate for the thinner air at high altitude. Not as much output volume as a turbo.

The Rockwell drive axle is probably a SQR. Ratios are 3.70, 3.90, 4.11, 4.33, 4.44, 4.63, 5.29.

It will take a lot of HP to overcome the frontal area of a bus at 80 mph.
Not too sure a 220 or 262 has enough beans to do it. Most didn't have piston oilers so it is easy to score a liner if you are pushing them too hard (don't ask how I know this).
In the 1960's your setup was pretty high speed. 65 was about the limit.

Changing your transmission from rt to rto will give you a first gear ratio the same as 2nd gear now, and a higher reverse gear. Could be problematic.
I'd consider RTO 9513. Give you a .87 overdrive and a low 1st gear and reverse.

Just my 2 cents worth , I earned a little bit in almost 50 years around trucks.

 

[s2mikon]

Just to add to what Sagebrush posted Cummins did a lot of retrofits on 220 buses to reduce smoke in the 70's. It did add a little hp but not much. My sons Crown has a 4.63 axle and a RTO 9513 and the retro fit turbo. Those series of engines did not like to be overloaded. I learned to drive on a nh 220 many years ago. Keep it under 1000 degrees on a pull and about 17-18 hundred at about 80% load on the flat and it was happy. Just not fast. If you want to run 80 mph you should be looking for another rig.

 

[jasonfarrington]

Gentlemen - thanks.

Thanks for the information - very much appreciate you sharing experience.

I've not been able to locate an engine tag - I've not really looked that hard, just assumed the horizontal 743 was the NHH-220 listed in the Operator's Manual. I will do that ASAP to confirm if this is the 220 with turbo added later, or the original NTO-6 262 turbo, the turbo looks just like the VT-50 pictured in the Cummins Shop Manual... I also need to check the fuel pump and turbo for ID tags. Thanks for this!

I don't want to run at 80mph, just looking to increase about 10mph to 65mph on interstate at 1700-1900rpm - not able to edit my original post...

Right now, max engine speed of 2100rpm translates to 63-65mph. I like to cruise on wide-open western interstate and secondary highways at 1700-1900rpm, this translates to about 58mph. Smooth, quiet, calm, and slooow. Wind noise is actually lower than my GMC Savana at 55mph.

Not lugging even on 10-11,000k passes. Current 4.11 gears pull right to 2100rpm in 10th on the flats - of course mountain grades are generally 25-45mph in 5th-7th gear keeping the exhaust temp below 1,000F and engine in the 2000-2100rpm range.

I would like to cruise in 10th gear at 65mph with an engine speed of 1700-1900rpm - topping out at 70-75mph at 2100rpm - some of the flats/gradual downgrades in my area can be 5-20 miles long (not talking steep grades, those are handled in lower gear with engine brakes).

It seems a swap from the 4.11 to 3.70 gear set would allow for this with minimal downside? As a result, perhaps in the wind or long steady slopes, I would drop into 9th at 55mph to keep everything cool and unstressed.

Same tires our company 80,000 fuel tankers run - 75mph rated H-Ply 11R24.5 tires are rated at Max 7,000lbs each for steers & Max 6,600lbs each for the 4x drives. Fully loaded I cross the scales at 11,000 front/16,000 rear axle. I've never felt the tires get warm even crossing NV in June at 100F degrees.

 

[s2mikon]

I think your idea of 3.70 with a direct 10th gear is right on. My son's Crown with 4.63 and .87 13th is about 4.0 final and a bit short even with 11r24.5 tires. But I have made 2 trips in it at 60 mph and got just a little over 10 mpg. Once fuel prices go up my son may think it is perfect as is.

 

[Sagebrush-SKO]

4.63 drive axles and a RTO is a nice setup.
That is what I ran till 1983. Then I went to 3.70 and a 15 direct.

 

[capnron-SKO]

Kudos on the Transmission. Manuals rock.

 

[LargeMarge-TC]

I think your little engine would be hard-pressed to bull-doze that shoe-box at the speed you desire.
.
A couple minutes research indicates your 220hp engine has a torque of around 545.
(These days, those specs are beat by a stock Cummins 359ci in a decade-old pick-up truck.
A new Ram pick-up can nearly double those numbers.)
.
First, I think no gearing can overcome the aerodynamic inefficiency of that square body.
.
Second, to overcome that aerodynamic inefficiency requires substantially more torque than your drive-train is capable of handling.
Twisted drive-shafts, spun carrier bearings, pretzeled U-joints.
.
In other words:
* Your vehicle inherent inefficiencies are cumulative and cascading.
.
Third, speaking of aerodynamics...
What happens to handling and braking at your proposed velocities?
You are moving 15-20? ton on a suspension engineered a half-century? ago.
.
'Yes', you can add an 18-speed through a 3-speed Brownie into a 2-speed axle.
You can gear so your speed is near the bottom of your power-band.
.
I am trying to talk you into not fussing with a proven design.
I am also probably trying to talk you into a vehicle engineered for your desired velocities... something Italian or German.