Diff Ratio And Speeds

[graham1948]

I am interested to find out which diff ratio would be the best to allow me to maintain a cruising speed on the highway of about 65mph or 110kph. I have a single drive B model with a 250 turbo C motor coupled to a quad box 7220. Currently I have a 7.48 radio and it is suggested that a 6.36 ratio will allow me to do 65mph but a friend of mine has a 6.36 ratio and he seems only able to get it up to 60mph. I am revving out to about 1900rpm. Operating range seems to be 1200 to 1900rpm. Mack Trucks in Australia told me that the old Mack engines like mine usually operated at that range. I told them I was expecting to rev it out to 2100 and they said that was too high. Any thoughts? Graham

[fjh]

I am interested to find out which diff ratio would be the best to allow me to maintain a cruising speed on the highway of about 65mph or 110kph. I have a single drive B model with a 250 turbo C motor coupled to a quad box 7220. Currently I have a 7.48 radio and it is suggested that a 6.36 ratio will allow me to do 65mph but a friend of mine has a 6.36 ratio and he seems only able to get it up to 60mph. I am revving out to about 1900rpm. Operating range seems to be 1200 to 1900rpm. Mack Trucks in Australia told me that the old Mack engines like mine usually operated at that range. I told them I was expecting to rev it out to 2100 and they said that was too high. Any thoughts? Graham

5:02 or 4:64 would get you in the ball park!

Problem might be you have a front loaded diff! ????

[graham1948]

Yes it is a front loaded diff. Does this present a probem? Graham

[fjh]

For me it does !I can't give you exsact ratios offered in that style of diff!

the road speed you want is in the range I previously posted thou!

[Rob]

For me it does !I can't give you exsact ratios offered in that style of diff!

the road speed you want is in the range I previously posted thou!

Is it a front loaded single, or double reduction carrier? It would make a significant difference in driveshaft length.

Rob

[Freightrain]

My double reduction front load is 4.62 and runs 65 with single over triplex. You could get away with low 5. ratio with double over box and get 65 mph. I"ve heard of 4.00 to 9.20 ratios available.

You need to get the govenor up to 2100 first, then deal with gear ratio. I run my 673 on the govenor all day without a problem. Pretty much hold it to the floor and go. Very rarely do I get it off the floor or I'll be doing 50 mph in no time and be run over by traffic

How heavy is your friend? maybe that is why he can't get road speed, not enough power to pull it? At 26,000 lbs I'm able to hold 60-62 on level ground(on the floor).

[Bollweevil]

If you can find a data plate on the axle housing, We might be able to reference the numbers and find a gear set. Look for a model number.

[HK Trucking]

1900rpm. Mack Trucks in Australia told me that the old Mack engines like mine usually operated at that range. I told them I was expecting to rev it out to 2100 and they said that was too high.

Tell them they're full of sh*t.

As I stated previously, the ENDT 673 C 250 hp has an operating range of 1500 to 2100 rpm's, and a no load high idle speed of 2250 to 2310 RPM.

These figures are from Mack tech literature of that era, I'm not making them up.

Exhibit A:

Get the darn thing running at the right RPM, then change the rear diff ratio.

A ratio between 5.50 and 6.00 should get you where you want to be for top speed.

I'm not familiar with what ratios are available for the front loaded rear, but if I remember correctly there were 5.88's and 6.34's available for the top loaded tandems (CRDP92 & CRD93 and CRDP112 & CRD113)

[graham1948]

I reckon they are full of the brown sticky stuff too. Too many desk jockeys who have not have the practicle experience. This week we are going to get the revs right. We have a lot of loose play in the linkages and we are not getting the throtttle open enough. Looks like I'll be able to get a 6.36 ratio diff and with the improved revs and the faster diff I should be able to get to 65 mph. An old guy down here tells me my air cleaner is too small for a C motor and maybe we are not sucking in enough air for the big turbo. Mine is an air cleaner from a 711 and he says I should either get a C motor air cleaner or install a second one on the other side of the cab to get more air on board. How does that sound? Regards Graham

Forgot to mention there is a couple of photos of my B model posted under Vibrations in B model that show the air cleaner if you want to have a look at it. Thanks Graham.

[HK Trucking]

Graham, I looked at the photos and your air cleaner appears to be the type that was factory installed on the B613, for use with the ENDT673 @205 HP, or the ENDT673B @ 225 HP, both are turbo engines, not much different from your 250.

I had a 1962 B613 with a ENDT673B and that is the type of air cleaner that was on it

I also once installed a ENDT 675 Maxidyne into a truck which previously had a 711 and used the existing top inlet air cleaner without a problem.

In my opinion that type of air cleaner is OK for a ENDT673C 250 HP, provided it is clean internally and kept properly serviced.

The bottom inlet type air cleaner, as shown in the photos of Hot Rod's truck is the type I would be concerned about not supplying enough air for a turbo engine. I have never seen the bottom inlet type on a B model with a factory installed turbo engine.

If you weren't getting enough air you would see constant black smoke, high exhaust temperatures, and lack of power, but lack of enough air would not keep your engine from achieving the proper high RPM setting at no load.

As a side note, Watts Mack does have available a dry type air cleaner that is a direct bolt on replacement for your top inlet oil bath, no more messy oil changes & screen cleaning, and the appearance is darn near identical.

Herb.

[mudslut28]

I think my 58 rears would be technically called the 112 and 113. They have 5.73 ratio. Tops speed is 68 mph at 2000 rpm's...but this is with an 14708LL which has a pretty good overdrive in 8th gear. The 800 has 5.88's with a 6 speed and might top out at 50 on a good day..LOL

[graham1948]

Best I seem to be able to do down here is to get a 6.36 ratio diff and the general consensis of opinion is that I should be able to maintain at least 64mph on the highway. If that is so I will be happy the added momentum will have the effect of powering me up the other side of the hills. At the moment I don't get enough run up to power me up the inclines. Herb, thanks again for the great help in relation to the air cleaner. Today we'll work on the linkages and I'll make sure that the strainer in the air clearner is clean. We are not blowing black smoke all the time, in fact she is a pretty clean runner, just black smoke on take off and when she drops the revs a bit. Many thanks Graham

[HeyCharger]

Hi Graham and Herb,

Yes, Mack Australia is my last port of call these days. Unfortunately the old B's do not appear on their computers, so they do not exist.

Mack advised me it was alright to put their latest Gearbox oil in a quadbox, whereas I've been told that these should not be filled with oil greater than GL1, due to the later additives not being good for an older box.

Herb, regarding my aircleaner - I always presumed (probably wrongly) that the glass bowl on top was just a pre-cleaner. Is there a major difference between what Graham has and what I have.

I have made enquiries re replacing mine.

I presume that they both have 4" ducting to the engine.

Comments appreciated.

Thanks. Rod.

[HK Trucking]

Hi Rod.

The bottom inlet air cleaner I was referring to as not being used on turbo engine B models is shown in the following photo:

http://bigmacktrucks.invisionzone.com/uplo...52_61_74818.jpg

The top inlet air cleaner which was used on the factory turbo engine equipped B models is shown in this photo which I also found in the gallery:

http://bigmacktrucks.invisionzone.com/uplo...6_74_462247.jpg

Although I have seen some 711 & 673 non turbo B models with the top inlet type, I have never seen the bottom inlet type on a factory turbo B model.

Herb.

[Rob]

Hi Rod.

The bottom inlet air cleaner I was referring to as not being used on turbo engine B models is shown in the following photo:

http://bigmacktrucks.invisionzone.com/uplo...52_61_74818.jpg

The top inlet air cleaner which was used on the factory turbo engine equipped B models is shown in this photo which I also found in the gallery:

http://bigmacktrucks.invisionzone.com/uplo...6_74_462247.jpg

Although I have seen some 711 & 673 non turbo B models with the top inlet type, I have never seen the bottom inlet type on a factory turbo B model.

Herb.

I concur with that observation. My non turbo B-61's have the bottom inlet air cleaner, my turbocharged B-61 has the top inlet. The inner air tube is bigger on the top inlet type.

Rob

[graham1948]

Herb, I've put my B model into a workshop to get them to replace the throwout bearing (the current one is not dis-engaging and is making a hell of a noise) and whislt they are at it I thought they may as well replace the clutch plate and the pressure plate. Like all shops today they claim they know everything but in essence they only know modern trucks. They tell me that because they can't buy a new clutch and pressure plate for my truck they will have the old one rebuilt and fitted to the flywheel, which sounds OK to me. While they have the quad box out I asked them to lift the top off it to see if the bearings etc are OK. Do you have any advice as to what else they should be looking at, and are there any recomendations you would make in relation to this operation. I have also asked them to look at the issue I have with low RPM's and I have given them the print out you gave me from the manual re this topic. I still can't find a faster diff down here than the 7.48 currently in the truck. I might have to find one over there and ship it over here, any clues? I want to be able to cruise at better than 60mph. Regards Graham

[fuzzy buzzard]

To determine speed

engine rpm divided by transmission ratio divided by axle ratio divided by tire revs per mile times 60 = MPH

Think you said TRQ 7220 (should have checked again before I started typing this due to CRS) is 0.70 in 5th high split. Tire revs per mile for 20" or 22.5" is 505 (ball park), 22" or 24.5 " is 470 (ball park)

Happy figuring!!! but don't stop at 65, go for 100 mph - you can always slow down some.

Tom

[graham1948]

Tom it would be great to be able to tell people the old girl is capable of doing 100 mph, but stuffed if I'd like to be hanging onto the steering wheel at anything over 70 to 75mph. I drove one for 5 or 6 years in the late sixties and she was capable of 75 mph. It was a lot of fun chasing the steering wheel all over the cab, but then I was just 25 years old. 60 to 65 mph will do this old fella now that I'm 60. Thanks for the equasions, I'll have a few beers and then try to work it out. Regards Graham

[HeyCharger]

Happy figuring!!! but don't stop at 65, go for 100 mph - you can always slow down some.

Tom

Hi Tom, I've searched high and low in the old forum for your description of you travelling so fast, your 'dogs eyes were watering.

If you still have this article, would you please repost it (or email it to me) as I can remember laughing so much at your description.

Regards, Rod.

[HK Trucking]

Herb, I've put my B model into a workshop to get them to replace the throwout bearing (the current one is not dis-engaging and is making a hell of a noise) and whislt they are at it I thought they may as well replace the clutch plate and the pressure plate. Like all shops today they claim they know everything but in essence they only know modern trucks. They tell me that because they can't buy a new clutch and pressure plate for my truck they will have the old one rebuilt and fitted to the flywheel, which sounds OK to me. While they have the quad box out I asked them to lift the top off it to see if the bearings etc are OK. Do you have any advice as to what else they should be looking at, and are there any recomendations you would make in relation to this operation. I have also asked them to look at the issue I have with low RPM's and I have given them the print out you gave me from the manual re this topic. I still can't find a faster diff down here than the 7.48 currently in the truck. I might have to find one over there and ship it over here, any clues? I want to be able to cruise at better than 60mph. Regards Graham

Like you said, just check that the bearings arent loose, especially in the back half, since loose bearings there could contribute to that whine in 5th hi.

Also make sure the bottom end of the compound shift lever is not worn severely, because if it is, I have seen them jump out of one shift rail, resulting in the transmission being "jammed" in gear or if the lever gets into the other shift rail after it jumps out, it can cause the compound to lock up in 2 gears at once, and if the truck is moving when that happens, it's usually the end of the transmission.

As far as finding a different rear end, if you can't find one on your side of the pond, try Adelman's truck parts in Canton Ohio.

I've heard they have a lot of stuff like that down there.

Herb.

[Freightrain]

Adelmans is truck heaven. They're local to me(I'm in North Canton). They have fields of trucks and parts. Makes a nice day of junk'n if you like doing that.

[graham1948]

Herb, Gearbox out and top off. The workshop foreman reports nothing abnormal. Bearings OK, he reports some sign of minor rust on the gears but nothing to worry about so I told him to seal it up and just concentrate on the clutch and pressure plates and the throwout bearing. Hopefully the rattling noise I was reporting was to do with the throwout bearing not disengaging. I guess I'll know in a few days when they get her back on the road. Next trip is in 10 days, we're off on a 2000 mile promotional trip for our country music radio network. Regards Graham

[graham1948]

Herb or anyone who can respond as soon as possible. I have just spoken to Rick at Adelmans and he thinks he may be able to supply a diff for me that is either in the high 4's or low 5's. He wasn't sure about the serial number of the diff. He mentioned a CRS 84. Is that the part number I am looking for? Thanks Graham

[HK Trucking]

Herb or anyone who can respond as soon as possible. I have just spoken to Rick at Adelmans and he thinks he may be able to supply a diff for me that is either in the high 4's or low 5's. He wasn't sure about the serial number of the diff. He mentioned a CRS 84. Is that the part number I am looking for? Thanks Graham

Good Morning Graham.

I have not messed with the single axles in quite a while, so I can't remember the models offhand.

However I did some research in my old Mack books and here is what I came up with:

Previously you stated that your current ratio is 7.48. The book shows that ratio as being available in the double reduction front mounted carrier models CRD 116, CRD 117, CRD 1161, and CRD 1171. The entire list of available ratios in these models are 3.71, 4.00, 4.25, 4.62, 4.88, 5.43, 5.78, 6.36, 6.80, 7.48, 7.85, 8.39, 8.83, and 9.87.

This leads me to believe that you have a CRD 116 or CRD 117 series rear end in your truck, however the only way to find out for sure is to find the model # stamped in the differential carrier housing. This should be at the right front area.

According to the book, the CRS 84 is a single reduction front mounted carrier type with available ratios of 4.25, 4.63, 5.14, 5.57, 6.17, 6.83, and 7.14. The fact that a 7.48 ratio is not available in the CRS 84 leads me to believe that this is not what you have in your truck.

Again, find the model # on your existing differential and this will ensure that you get the proper unit.

Hope this helps.

Herb.

[graham1948]

Again many thanks Herb. I'll get the serial number and then I'll talk again to Adelmans and also to Watts who are both looking for a diff for me. Regards Graham