B Model

[TandemB]

Have a B model tandem dump. Was converted from gas to diesel years ago.(by prior owner) Truck has trans problem so cant be driven at speed. Was wondering if years ago the rear ratios were the same for gas or diesel and what would happen if i replaced duplex with quad box or other(know someone with duplex,triplex and quad trans) Any thoughts or info appreciated thanks

[vanscottbuilders]

TandemB,

There is a stamping on the rear end carrier housings that should indicate the rear end ratio.

I am not certain as to the exact location, depending on the model of rear end.

Someone will chime in here with the exact location. But, being an impatient person, I have

found that if I look for a flat machined surface on the carrier and clean it off good, the ratio numbers

have always shown up. Usually with initials showing the carrier type, such as "CRD".

Be a little careful though, the numbers you want will be stamped, not cast, in the housing - made that mistake once already.

The subject of changing the ratio for more speed is pretty controversial around these forums.

If you are using the 673, 711 or 673T diesel engines, there isn't an overbundance of horsepower or torque

to allow a high road speed along with good pulling power. If you are using the truck as a dump truck, that might be a real

consideration.

As it was recently explained to me, Mack used the heavy duty rear end differential assembly, with all of it's big gears, to deliver the bulk of the torque multiplication, rather than using the relatively weaker transmission assembly like other manufacturers may have done. Consequently, the old Macks were extremely durable, albeit slow. The Macks could also use slower turning, lower horsepower engines. Again, for dependability, and also for fuel efficiency. Monkeying with the Mack formula now, fifty years later, probably has some consequences that we haven't planned on.

There isn't any one answer - but, I think that using a different transmission with an overdrive (or two) for better cruise speed is a logical solution, especially if you are going to work the truck with the original diesel engine.

Paul Van Scott

[terry]

If it was a gas job originally it will be pretty slow with a diesel in it if the rears have not ever been changed,also depends if the duplex is a direct or overdrive.Also the gas tranny is a smaller version of the diesel trans. Gas rear ends will be around a 7.00 ratio and a diesel can be mid 4.00 to down to high 5.00's or low 6.00's terry

[b61fred]

Hi,

I would like to recommend that you look on the top of the diff and slightly to the left of center looking down, there will be a 1 inch square pad with all the info you will need. The only problem I have seen is that if the diffs have been changed that sometimes a tag would be rivited to the housing there and somtimes the tag gets removed.

If you are using this truck with any load at all with a 673, I would not go over 5.13 to 1, If you could up the horse power you may be able to go faster, but not loaded.

I do have differental carriers and transmissions for sale if interrested

Talk later

Fred

[mackmixer]

Here is what the stamped pad looks like on my tandem axle. Mine is a 8.24 to 1 ratio!! Hard to read but is located upper center of pad.

[HK Trucking]

A lot also depends on what size rubber is on the truck.

With 11:00 24's 8.24 was a common ratio used in conjunction with a TRQ7220 quad box, and would give about 52 to 54 MPH in 5th hi at 2100 RPM if I remember correctly.

On 11:00 22's 7.49 was common, with a quad box it would also give 52 - 54 MPH at 2100 RPM.

On 10:00 20's, 7.00 or 6.34 was commonly used with a quad box.

As was mentioned earlier, if the truck previously had a gas engine, then the rear axle ratios will be much lower than what you want for a 673 or 711 diesel.

[Red Horse]

A lot also depends on what size rubber is on the truck.

With 11:00 24's 8.24 was a common ratio used in conjunction with a TRQ7220 quad box, and would give about 52 to 54 MPH in 5th hi at 2100 RPM if I remember correctly.

On 11:00 22's 7.49 was common, with a quad box it would also give 52 - 54 MPH at 2100 RPM.

On 10:00 20's, 7.00 or 6.34 was commonly used with a quad box.

As was mentioned earlier, if the truck previously had a gas engine, then the rear axle ratios will be much lower than what you want for a 673 or 711 diesel.

24" rubber with 8.24 rears and an OD quad box (.74 ) wiill give you

(2100 X 60)/442X.74 X 8.24= 46.75mph

This makes the assumption that 5th hi is .74 If it wasn't an OD and was direct, same combination would give 34.59 mph

I am going through same exercise.. I have what was a B-68X. It has a 9.05 rear and when built was a gas job with a quad box. somewhere in its 47 years, someone dropped a 673 with a duplex in it. I have not had it on the road but was told by the last guy who drove it on the road (he plowed with it) he said it would do 35! bottom line when yu give away 400 RPM's going from gas to diesel you pay the price.

[Red Horse]

Hi,

I would like to recommend that you look on the top of the diff and slightly to the left of center looking down, there will be a 1 inch square pad with all the info you will need. The only problem I have seen is that if the diffs have been changed that sometimes a tag would be rivited to the housing there and somtimes the tag gets removed.

If you are using this truck with any load at all with a 673, I would not go over 5.13 to 1, If you could up the horse power you may be able to go faster, but not loaded.

I do have differental carriers and transmissions for sale if interrested

Talk later

Fred

So Fred, would you have a CRD 118 in say a 5.52?