71 Jav 401 SST Trans swap

Before I start..... I did do a search on this subject but the outcome sucked, I'm sorry if you think me an idiot.... but please indulge me.  

I really need to do something about my trans. I have a BW.... I think cast iron body/alumn bell and tail. I would like to go with the Chrysler unit but I don't know what bits to get for the install, or for that matter if all the 727's will bolt up. In addition to the rest... I want an overdrive. So here are my questions, complete w/bullets.

Edited by rsrguy3 - Jun/25/2012 at 8:33am

Have used gm 700r4 overdrive tranny in my cars with a NOVAK adapter and have had good results.
                                                                                  Goops

The only other way to get OD is to use a Gear Vendors add-on unit. Over $1500. Might as well get the 700R4 and adapter. The only thing you will have to modify is the trans crossmember for the mount and the driveshaft. You will need a GM yoke that will fit the trans. Once the trans in installed you can take a measurement for the driveshaft and have a driveline shop make you a shaft with the correct yoke on each end that fits. This should be no more than $200 if you have both ends to give them. Talk to the shop about how they want you to measure for the shaft. It's usually done with weight on the rear wheels from the center of the cross on the rear axle to the end of the trans seal, but they might want you to do it differently.

Fuel economy is going to depend more on how you drive than anything else. With an OD trans you need around a 3.50 or lower (bigger number) rear axle. The engine needs to run at least 2000 rpm at cruise speed so it's inside it's torque band. A high torque/low rpm engine will get better gas mileage all the time. If you have a cam that has a 2500-6000 rpm torque band you need to be running at least 2500 rpm at your desired cruise speed, preferably 100-200 rpm over the start of the power band so you have some flexibility. After that the more rpm you run the more fuel you will burn. In effect an OD trans gives you a lower first gear rather than a higher top gear. The spread of the gears is better so that the engine stays in its best torque range more.

You can play around with tire size and gearing to get in your desired rpm range and speed. I usually shoot for 70 mph at 100-200 rpm over the low cam torque range number. The calculators at www.4lo.com are good (easy to remember site name!). I can then cruise in the 65-75 range without a big effect on economy. Note that aerodynamic drag increases exponentially over about 65 mph -- cruising over 75 mph in an old car, even one as aerodynamic looking as a Javelin (it still has a large flat frontal area, which is the killer!), really sucks gas!! (takes a lot of power)


Edited by farna - Jun/26/2012 at 11:35am

No, you'll still have the acceleration even if you don't change the rear axle ratio. Well, it depends on the first gear ratios of the transmissions then. I don't know how first compares between the TF 727 and 700R4 -- you'll have to look up the gear ratios.

Just because you built for more power doesn't necessarily mean you'll lose gas mileage. It might increase -- IF you take it easy. Usually when you build for more power you tend to stick your foot in it so you can feel that power at work more often -- hence a drop in mileage. Baby-foot it and drive it like your grandma may have (I had a lead-footed grandma...) and mileage may increase because you increased the efficiency of the engine to get more power. Depends on how radical you went to get more power. A mild increase is more likely to show no change or an improvement in mileage than a wilder build with a "rumpty" cam.

The 700R4 uses a TV cable that has to be adjusted right. The TV (throttle valve) cable controls internal pressure according to throttle position and kick-down -- just like the linkage does on the 727. Lokar has a cable kit that should be adaptable to any Holley or Edelbrock carb. They have them for the 700R4 and to replace the linkage on a 727.

The floor shifter should work. You will likely have to manipulate the length of the shift arm on the trans or the shifter to get it synchronized just right though. I'd make an arm with a long slot in it so I could make some minor adjustments until everything lined up right. Then take that and make a new arm with just one position so it won't get out of adjustment while driving. Will take a bit of time, but shouldn't be too difficult.