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Old 03-30-2016, 12:49 PM   #1
Mini-Skoolie
 
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Year: 1991
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Cool Allison 545 to ZF5 or ZF6 conversion

As the title states, I'm wondering if anyone here has done that conversion, and was it worth the effort? I have a 1991 Thomas dog nose short bus, that is built on a 3700 chassis. 7.3 idi non turbo. Hoping to get a little better mileage out of it. Thank you in advance!
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Old 03-30-2016, 12:56 PM   #2
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Depends on what your current gearing and transmission is. Most likely the OD would be worth it if you travel enough. Plus any satisfaction or enjoyment you would get out of the swap.
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Old 03-30-2016, 01:45 PM   #3
Mini-Skoolie
 
Join Date: Sep 2015
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Year: 1991
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Chassis: 3700
Engine: Navistar 7.3 idi
Rated Cap: 30-ish?
As mentioned in the title, I'm currently using the Allison 545, and if I remember correctly, my rear end gear is 5.41 or so.
Long highway trips are indeed the goal.
Have you done this conversion?
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Old 03-30-2016, 02:03 PM   #4
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With that rear gearing you should be able to cruise easily at 60 MPH all day long and not bog down on the hills too much.

Swapping out the automatic for a stick shift is not going to be cheap or easy. And when you are all done you most probably won't see much increase in the top speed.

The T444 that was used by IHC in their buses had various different power outputs from as low as 140 HP to as high as about 210 HP. It still takes a certain amount of HP to pull your "brick" through the air. Whether you are doing 60 MPH at 2250 RPM or 2600 RPM it isn't going to change your top speed significantly except for when you are going downhill. In order to take advantage of the higher potential road speed you are going to have to significantly increase the power output of your engine. Your engine will literally run out of HP trying to go much faster.

If your goal is to put significant numbers of miles at highway speed I would suggest it would be less expensive and a whole lot less of a headache to find another bus with a lot bigger engine, transmission, and highway gearing.

Retrofitting what you have is not that simple, easy, or inexpensive.
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Old 03-30-2016, 02:14 PM   #5
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i agree with Cowlizt, in that changing what you have is more expensive than starting over with what you want.

if speed is your goal, you're not even fixing the right part. if you spend money on it, fix the problem (the 5.4 ratio rear end). an over drive may be a solution, but does not fix the intrinsic problem (HP and the pumpkin).
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Old 03-30-2016, 03:04 PM   #6
Mini-Skoolie
 
Join Date: Sep 2015
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Chassis: 3700
Engine: Navistar 7.3 idi
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Thank you, cowlitzcoach and turf. Higher speed was not the goal, just a bit more efficiency. I was just dreaming, so it seems. ;)
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Old 04-12-2016, 12:31 AM   #7
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a dedicated over drive class seven transmission and sae 2 bell housing is just the start and your in over 2 easy grand, then you have to get the clutch assembly from dash bulkhead down to trans. best go change rear end gearing instead, so engine is near peak torque at cruze speed. my combo 4.44 @2500 =60mph w 210 h.p. i want a 3.55 ratio, but so does ever other truck on the road.
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Old 04-12-2016, 11:48 AM   #8
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have you considered a turbo and intercooler for your bus? should be way less than drive line mods.
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Old 05-17-2016, 09:56 PM   #9
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One of the problems you would run into most likely is the following.
On the back of you 7.3, there is an adaptor, it goes from the proprietary bolt pattern to sae2 and also houses the flywheel and clutch or torque converter, there is then a ring, that necks down to sae3 to mate to the Allison. That is all well and good if it were as simple as removing it. This clutch housing is also the location of the rear motor mounts. Meaning to use the zf, you would need custom motor mounts, ( ask me How I know..)
You're best bet would be to find a spicer 5 or 6 speed as they are the most similar to a normal stick shift and have sae bell housings. You will need a matching clutch disc that fits whatever flywheel pressure plate combo you use, You would need to fabricate a clutch linkage but a lot of people overestimate how hard that is, you just need a lever and a cable/rod. All the older truck transmissions are
Mechanical throwout clutches.
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