Allison 750

[Mike in GA]

I currently have the Allison 740 behind my venerable 8v92 TA, in an MC96a3.
     Since owning it I have been concerned that there seems to be a big jump in the ratio when upshifting from 3rd to 4th, iespecially going uphill and in certain suburban driving: 3rd seems too low and 4th too lhigh, so there is a lot of either high revving or lugging. I also have been having to manually upshift to 4th due to a relatively minor problem internallywith the 740 - broken spring or something.
     A friend has an Allison 750 close ratio, where the extra gear is between 3rd and final. Note - this is not the dump truck five speed transmission with the granny low for the extra gear. It's out of an MC8.
     It comes with the shortened drive shaft to fit my MCI and an electric shifter. Asking price, about $1900-$2000.
     Question:  is this a sound idea?  What are the drawbacks if any? In pulling the 8v92 to do this work, are there any special precautions or hazards, aside from the obvious (oops!) ?
     Thanks in advance for the feedback!
Mike in GA

[kyle4501]

If you don't want it, I'll take it!

I think that since it already has the shortened driveshaft, you'd be in business in no time.



On second thought, you don't need that old thing. You'd be backing up if you don't upgrade to a DDEC or 4 stroke.    
Just let me have it to save you all the trouble . . . 

[luvrbus]

Here is the ratio for both transmissions you are not going to gain much where you would like

740                       
1st 3:69
2nd 2:02
3rd 1:38
4th 1:00   

750
1st 3:69
2nd 2;00
3rd 1:53
4th 1;25 
5th 1:00   reverse iis a killer in those 9:65 the 740 is 6:04 I would just have the shift point changed on the 740


good luck

[TomC]

Using 2100rpm as the shift point and the fact that either transmission locks up in 2nd gear, here are the RPMS at the end of the shifts.  740: 2-3 is 1434rpm.  3-4 is 1521rpm.  On the 750 2-3 is 1590rpm. 3-4 is 1715rpm. 4-5 is 1680.
Personally don't think it is worth it.  If you need more space, hold the transmission in 3rd to 2300rpm then shift-your landing rpm will be 1,666rpm.  Plus the 754CR is not as robust a transmission as the 740 because of the extra gear added on. Good Luck, TomC

[HighTechRedneck]

Post regarding 754CR for sale moved to Spare Tire section.

[kyle4501]

According to my math using the ratios listed & the axle ratio & tires on my bus:
Top speed (mph) in each gear (shifting at 2100 rpm) -
740/ 750  
1st = 19.1 / 19.1
2nd = 35.0 / 35.3
3rd = 51.2 / 46.2
4th = 70.65 / 56.5
5th = na / 70.65

So, for me, having the ability to downshift 5 mph sooner is a nice option to have. Especially if you are a little under powered.

[luvrbus]

Kyle, the older 750 had different ratio's the one I posted were for serial # 2800 and above the 1400's had 2 different ratio's that didn't work that great, so if you buy 1 buy it with the serial number of 20004 and above 

good luck

[TomC]

Typically you want to look for the HT754CR-which is the close ratio 5 spd.  The HT750 or HT750DR has the deep 1st gear that is not supposed to be shifted to second and uses 2-5 under most conditions.  Good Luck, TomC