Need rear axle drive ratio for 1979 Eagle Model 5 bus

[JimGnitecki]

I have a 1979 Model 5 Eagle bus and am doing an engineering analysis that requires among other things the rear axle ratio.

I have been unable to find it reliably stated anywhere in the Eagle "specifications", although I have noted that in the PARTS MANUAL for the Eagle model 10 (not model 5), the rear axle ring and pinion set is shown as 3.7 ratio, with no other choices shown in the parts manual.

Is the ratio in my Model 5 the 3.7, or something different than the Model 10?

For what it's worth, I have no tachometer in my bus, so don't know the rpm at any hgihway speed, but the bus loves to cruise at 73 mph, and if I daydream a bit or get distracted by traffic and then check the speedo, often the bus has climbed up to over 80 mph, so it apaprently likes it, and must still be below governor speed.

I have checked the speedo against Interstate mileposts repeatedly over 5 mile sections when there has been no traffic, and it is accurate.

Does anyone know what the rear axle ratio in a Model 5 Eagle is?Or, is there a tag or other identifier on the axle that would state it?

Jim Gnitecki

[luvrbus]

Jim, Eagles had 3.73 or 3.36 most 8 v71 were 3.73 a few had 4.11 but very few, check your RPM it may be set at 2500 rpm      good luck

[JimGnitecki]

lovrbus: Thanks for that informaiton, BUT, it still is not quite definitive.

My bus has Toyo M147 tires in the 11R24.5 size. Per Toyo, these tires turn 478 revolutions per mile.

My bus also has the Allison HT740 automatic transmission, which has a 1:1 top gear drive ratio (no overdrive ratios).

Combining the above data with my previously stated road observations (the bus likes to exceed 80 mph when unwatched), and assuming the governor on my 8V71 2-stroke engine is set at a tpyical 2100 rpm, we can make some ROUGH determinations.

First, I THINK I can rule out the 4.11, as 80 mph would require 4.11 x 478 x 80/60 = 2619 engine rpm!

The 3.73 is also marginal, as 80 mph would require 3.73 x 478 x 80/60 = 2377 rpm. Even if someone tweaked the governor somewhere along the way for more rpm and power, I doubt they would tweak it that far.

The 3.36 sounds reasonable, as 80 mph would require 3.36 x 478 x 80/60 = 2141 rpm. That is consistent with a governor set at or near 2100 rpm - maybe raised just a bit.

I have never tried to go faster, but I confess that i HAVE seen 84 mph indicated on the speedo a number of times when I did not pay strict attentiuon on the interstate.

All this SUGGESTS I have a 3.36 axle, but is there any tag or other identified on the axle that would verify that?

Jim Gnitecki

[luvrbus]

Jim, there is a tag on the housing on the drivers side on the flat part of the axle housing but bet you a cold drink that engine is in the 2400 to 2500 rpm range
 I have a 79 model Eagle at my house now that is set on 2400rpm you can go 2500 without problems from the 8v71 the 3.73 was the standard for the 05 with a 4 speed or auto and so were 24.5 tires       good luck

[JimGnitecki]

You can really spin an 8V71 at 2400 or 2500 rpm? Is it ok to do it for long periods, or only for limited bursts?

Jim G

[buswarrior]

For busnut use, nothing wrong with spinning the engine a little faster.

Some fire equipment has been reported to spin way up at 2800!!!!

Setting it somewhere below 2500, the few miles less engine life to re-build won't be our problem... unless....

How close is that engine to re-build right now....?

Lots of transit 6V71 engines were set to 2300 so they could at least do something approaching highway speed with the over 5:1 diff ratio.

FWIW, whenever it is that I run out of better things to do, I'll open my 8V71 up to 2300 from the 2100 it is set at now. I want a little more overlap for drivability between 3rd and 4th on the ups and downs of the secondary highways, not really more top speed.

happy coaching!
buswarrior

[RJ]

Jim -

From my filing cabinet, info from two Eagle sales brochures:


1979 Model 05

Engine: Detroit 8V71N, C60 injectors, 280 hp at 2100 rpm.

Rear Axle:  Rockwell Standard R162, ratio 3.7:1.

Transmission:  Spicer 8844A.  Ratios First 4.30:1, Second 2.28:1, Third 1.44:1, Fourth 1.00:1.

Wheels/Tires: 12R22.5.



1984 Model 10

Engine:

8V71N, C60 injectors, 280 hp @ 2100 rpm.

6V92TA, C75 injectors, 270 hp @ 2100 rpm.

Transmission:

W/ the 8V71: Fuller T11905M five-speed, ratios 5:47, 2.67, 1.76, 1.32, 1.00.

W/ the 6V92: Fuller T11905F five-speed, ratios 7:23, 3.92, 2.16, 1.45, 1.00.

Either engine:  Allison HT-740.

Rear Axle:

Rockwell Standard R162.  Stock ratios for the 8V71 is 3.7 and 3.36 for the 6V92.

Wheels/Tires:  12R22.5  (As a side note, the brochure I have for the Model 01 lists wheel/tires of 11R24.5.)



Where is this 80/60 ratio you're using coming from?

Plugging your tire size into Mallie's calculator (link in your other thread) gives 84.8 mph @ 2500 rpm w/ a stock axle, or 2250 with the 3.36.


So, you've still got some homework to do:  find out the rear axle ratio from the tag on the side, and find out what your engine rpm is set at.  Then you'll have real numbers to work with, not theory.

FWIW & HTH. . .

 

[pvcces]

Or block the front wheels, and jack up one rear wheel and block it. Then, carefully mark both the pinion to the differential and the wheel to the body.

Slowly rotate the wheel TWO full revolutions and see if the pinion turned 3.7 times. If it does, your rig is stock. Done right, there will be no doubt.

A good reason to do this is that many rigs have had ring and pinion switches, without the tag being changed to match.

Good luck!

Tom Caffrey

[JimGnitecki]

RJ: The 80/60 is simply reflecting 80 mph versus 60 mph. The Toyo tire turns 478 revolutions per mile at 60 mph, so you need to multiply by 80/60 to reflect 80 mph speed.

Note from the 1979 specs you provided that the bus originally apparently came with 12R22.5" wheels and tires, whereas mine now has the 11R24.5" wheels and tires on it. By using the published 478 revs/mile for the actual Toyo tires I have, we can avoid getting into comparing the revs per mile of 12R22.5 tires versus 11R24.5 tires, since different tire manufacturers have slightly different diameters even for the same nominal size!

Engine rpm = tire rpm @ 60 mph x actual mph/60 x axle ratio x top gear transmission ratio

With 3.73 gearing, my engine rpm at 80 mph = 478 x 80/60 x 3.73 x 1.0 = 2377 rpm

With 3.36 gearing, my engine rpm at 80 mph = 478 x 80/60 x 3.36  x 1.0 = 2141 rpm

So, the 3.36 gearing is more believable if we assume that the governor has not been altered, but the 3.7 is more believable because very few people actually change the rear axle ratio in a bus! It is of course also possible that BOTH the rear axle raito AND the governor have bene altered!

Guess I need to, just like you said, physically check both governor rpm and axle ratio.

I'm not tool or skill equipped to do either (Tom, I have no jack substantial enough to lift the bus - and I would be afraid of lifting incorrectly and twisiting something), so will seek out some help from a professional mechanic friend who has the tools and skills to do both properly!

Thanks for the assistance with the Eagle specifications data. I really appreciate it. I also pulled out my CD version of the Eagle "operator's Manual" last night, and yes, the horsepower rating is indeed the 280 versus the 318. Note the injector size in the specifications you provided.

Jim G

[JackConrad]

   When we purchased our MC-8, we found it would easily run 80+. I thought the speedometer was off. When we installed out tach, we found the governor was set at 2550. Speedometer accuracy was later confirmed with GPS. We have a 3.7 rear end and are running tires with a TRPM of 484. Allison 740 transmission with high gear drive ratio 1:1
   It is very difficult to find what was done to the governor settings down through the years. Although our bus started it's life as a Greyhound, the last owner was a Michigan based charter operator and who know who else used/worked on the bus in bewteen those 2 owners.  Jack

[cody]

Wow, this thread sure is complicated, I'm not sure where all this becomes relevent in the grand scheme of having fun with my bus, I'm just happy when I turn the key and it cranks and fires and I know that I'm going to head out lol. 

[TomC]

I have my engine turned up to 2400rpm no load.  I can get 75mph out of it if I want-but cruise at 58 which is 1800rpm.  Have done 65 mph which is 2100rpm-and the engine doesn't care either.  Good Luck, TomC

[luvrbus]

Jim, email me the vin number of your Eagle and I can tell you the way it was equipped when it left the factory at Brownsville TX.
 I have the work orders on most of the Eagles built not sales brochures and FWIW I can only find 2 1979 models that were equipped with 22.5 tires  
good luck

[JimGnitecki]

luvrbus: I am sending you my VIN via private message.

Thank-you for the offer to get me the data!!

Jim G

[JimGnitecki]

luvrbus: Your comment on there being only 2 1979 Eagles that had 22.5 inch wheels also reassures me a lot that my 24.5 inch wheels are the original equipment.