Spicer 4 speed versus MT644 trade offs

[Fredward]

I'm pretty close to getting an MT644 to replace my spicer. (thanks Clifford!) I know people have talked about worse mileage and other trade offs going from a manual to an automatic. But in this case, what would the trade offs really be? The spicer is 1:1 in fourth. The MT644 is 1:1 in fourth with lockup. Once its locked up in fourth, shouldn't it be pretty much the same as the spicer or is there a lot of horsepower sucked up by the hydraulic pump in the transmission? We cruise 72 mph all day no problem with the current setup but wouldn't want to try doing that with much less horsepower.

I know I have to install an oil cooler and adapt the transmission to the 871 bell housing and run a cable up front for the shifter.

Fred

[TomC]

One of the main reasons most get worse fuel mileage with an automatic is they drive the bus with their foot on the floor because it is so much fun feeling the bus accelerate like a car. You can combat that by accelerating at a more gentle rate-you know how fast it takes to get up to say-40mph. If you do it the same way with the automatic, it won't be that much different. Also, automatics, even with the lockup torque converter, will eat up close to 20hp in friction between all the clutch packs just with the fluid turning inside. This is why the trans cooler is needed. Heat rejection is rated at BTU's per minute-not per hour. Going down the road with the torque converter locked up and in direct 4th, your heat rejection can be low in the hundreds of BTU's per minute. But slow down to the point that the torque converter falls out of lockup into converter in a lower gear, and you can be putting out well over 10,000BTU's per minute. The MT643 has a shift pattern of 1C-2C-3C-3L-4L. I believe the MT644 is closer in performance to its big brother the HT740 with a shift pattern of 1C-2C-2L-3L-4L.
On my HT740 I had a manual lockup solenoid installed so if I am in 1st gear (like on a mountain road) I can manually lockup the converter. I think you can have that done with the MT also. You just have to remember to unlock it before stopping or else you'll stall the engine.
Besides a bit of a fuel mileage hit, you'll be having a big old smile on your face the first time you have to back up a hill or maneuver around the campsite. The Allison automatic takes all the anxiety out of driving. You made a VERY smart choice!. Good Luck, TomC

[wg4t50]

Tom is so right about the smile, but for me, another biggie is the ability to start moving on a grade, this is exactly why I changed from the 4 speed Spicer to the 10 Speed RTO910, starting on a hill went from a problem to easy and with the auto would be easier. More smiles per mile.
Dave M

[Ed Hackenbruch]

Fred, my 5A has the 644 in it and i really like it with one small exception.  55 mph speed limits are tough to run at because it wants to shift into 4th at 57. If i get into 4th i need to be doing at least 63or more to keep it from lugging. If i stay in 3rd the rpms are high and my fuel mileage suffers. We tow a jeep and over the 80,000 mlles we have traveled we have gotten from 5.9 to 8.4 mpg. During that time we have averaged 7.24 mpg. I manually upshift the tranny about 75% of the time and downshift it about 95% of the time. Downshifting is more important to keep it from lugging in the lower gear. Most important gauges for me are the temp gauge, then the tach, and then the speedo.  The PO put in 2 tranny coolers, mounted them to the outside of the radiators. When i first got the bus i thought that the gauge for the tranny oil temp was broken because it never moved......until i went up the hill from Laughlin NV. to Vegas in 107 degree heat. Looking at the setup i was a little concerned at first that the coolers would be affecting the radiators but it doesn't seem to be the case. Water temp usually runs at about 175-180 most of the time.  We are usually in the western half of the country so spend a lot of time going up and down and shifting a lot. This winter in Florida i only downshifted when slowing down for stop lights and traffic......most miles of flat land driving i have ever done!!!!

[Fredward]

Thanks guys - very helpful. I was planning on installing a cooler with a 24volt thermostatically controlled fan. And mounting it behind a big grill I have on the passenger side of the engine compartment. We are flatlanders so I'm not too concerned about heat, but might as well set it up so it can handle step grades, etc.

Any ideas on where to find a 24volt fan and a transmission cooler? How do I know what size cooler to get?
Fred

[Lin]

Haden makes large coolers, possibly the fans also.

We did the same cooling setup as Ed-- the biggest cooler that would fit in front of each of the radiators without using the engine cooling system since I did not want to add any more load to that.  However, it is important to note that the engine cooling system could be of value in warming the transmission fluid in really cold weather.  This has not caused us any problem yet since we don't go where it is very cold.  However, had I thought of this to begin with, I might have done it differently.

[luvrbus]

 Fred www.utxchange.com let those guys size it for you don't have to buy from United but their prices are not bad,very seldom do I see a add on cooler the lines are large enough.To me cold oil operating temperatures on a Allison is worse than heat 

[TomC]

With my Vdrive with the V730, when I turbo'd the engine, I needed more cooling. I installed an oil to air cooler with a thermostatically controlled 12v fan on the outlet side of the cooling circuit. My idea-cool the oil before it enters the shell cooler to take some of the heat strain off the engine cooling system. I kept the water to oil shell cooler since the shell cooler will heat up the fluid in the morning. It has worked well. Only real weird thing-I have the direction of the fan on the cooler going from the inside of the engine compartment to the outside-the big powerful engine fan creates too much pressure to have the trans cooler pulling in air from the outside. Good Luck, TomC

[uncle ned]

I also have the oil cooler mounted on the passenger side of HUGGY.

I tried different fans and different directions.  found out the best thing was to let the monster fan of the engine just push the air out.

It warms the fluid on cold days and lets the air out another hole in the bus

uncle ned

[Fredward]

I have a quote from Global in AZ for all the parts.
Does an 871 require an SAE#1 bellhousing adapter?
How hard is is to remove/replace the flywheel?
Any suggestions or tricks from those who have done this swap in the past would be welcome at this point

[luvrbus]

A 600 series does require the drop ring on a SAE 1 bell housing the flywheel is easy to remove,before you pull the trigger give me a few days maybe a week

I know where there is a Eagle the guy is starting to part out you can get the whole package from I think at a buy There were a lot of 1980-81 model 10 Eagles with the 8v71 that used the 600 series Allison that have all the parts you would need fwiw

[Fredward]

Clifford that sounds like a plan. I've got the dolly lined up ready to go. Let me know here on the board, call or text me at 612-801-4826 whatever works for you. I'm interested.

I'd take it and get it dyno'd up here before putting it in.

Fred