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Does anyone here have any experience or possibly have a car with this overdrive unit installed ?
When my mind wanders off into La La land I think how cool it would be to swap my 3:23 rear for a 3:91 and then add the overdrive unit. The end result would get me all the fun around town and with a .78 final drive ratio, a negligible rpm change on the highway.
I ran one for years in my 70 with 3.90's & 28" tall tires. (Runs 10.59@124)
It was great for humming around town keeping the RPM's below 2500.
Ultimately I took it out though and will be using it in the restore I'm doing with 3.55's. (2.76 w/ OD)
I want that to be a fun highway cruiser.
They are pricey and will add weight, rob a little horsepower and require a new prop shaft and possibly some massaging of your floor boards.
But I never had a problem with it...except with their electronics...they are crap.
Best to leave it manual.
I run one only as OD for highway with 3:73 rear end.
Ditto electronics suck. I run old school with main power switch and an activate dimmer. A little tunnel massage is needed.
Overall very happy, no power loss I can point to.
Pushing 800+ thru it.
4,000 # car 9.97 1/4 135mph
Thanks for the input fellas. Sounds like I need to start putting some cash in the cookie jar for this upgrade. A toggle switch for highway driving is all I need.
tc
From my install it is a bit disappointing. It appears my 71 convertible with a 4 speed will need tunnel mods. The 2 man transmission shop that has a very good reputation and is doing the install wants a second opinion from a body guy that is supposed to come look at it and give his advise. With the unit installed and raised up to the tunnel the crossmember with stock tranny mount will not fit. We have a low mount but that may present fan to shroud issues along with drive line angle issues. I should know more next week. I prefer modifying the tunnel and may pull it out and do it myself.
The body guy is as it turns out a very well know guy in the industry, SEMA builds and such. I did some searching (he has a very shall I say unusual first name) I am being patient but am more than willing to start forming and cutting myself if necessary. I replaced the floor pan previous so I have the tools and mig welder if need be. I have talked to quite a few 68-72 a body folks that did the install and minimal hammer work was all that I anticipated.
I have a born with M20. I’m not sure if the boxed frame has anything to do with crossmember location compared to an open frame.
The fore/aft location of the crossmember is solely dependent on the trans. The frame has no impact. The only effect that the boxed frame has on the crossmember is the width of the crossmember (between the bolt holes to the frame rails). Obviously the boxed frame requires a crossmember that is narrower than the one for an open frame.
A few years ago I installed a low mile used GV OD in my '67 Vista Cruiser and I have since put over 7000 miles on it. I have a 220/220 @ .050 455, variable pitch TH400, and .342 gears. The combination is great and allows highway cruising at reasonable RPM and plenty of fun for pushing through the gears.
I installed the GV supplied control box (hidden behind kick panel), as well as the GV auto/manual selector switch unit. I did their recommended additional foot switch down by the high/low beam switch. The GV engages by putting 12V to a solenoid so as others have mentioned you can go full manual on that solenoid but I find the controls useful as they will always force the unit to disengage at a low mph setpoint. This feature protects the unit from engaging when running dry (or in reverse) as its internal ATF pump in driven off the drive shaft and only gets lube flow when the car is moving. With the supplied GV control unit in auto mode, it shifts in and out of OD at fixed road speed setpoints (in at 47, out at 20 mph). I prefer to use the manual mode that you engage and disengage with foot switch. In manual mode the GV control box will protect the unit and pull power off the solenoid and disengage OD whenever you drop below 5 mph. In either Auto or Manual mode the GV is locked out and can't be engaged below the min mph set points in the control box. This gives some piece of mind as I will let my kids and wife drive the Vista Cruiser.
I called the support number at GV when I first got it installed and the old timer there said that most muscle car users will eventually migrate to manual-only mode as you can do more things with the GV that way but he highly recommended sticking with their control unit for the above reasons. I run 300 mile vintage car road rallies every month or two that involve 100 to 200 miles of highway driving and another 100 miles of twisty mountain and backcountry touring. The auto control mode works great for the highway drive and turns the car into a 4 speed automatic but I just use manual mode and the foot switch all the time now. On the mountain climbs I will drop the TH400 into 2nd gear and run the OD on the manual switch depending on the grade. I can use the GV to put the engine right in the perfect power band for spirited hill climbing through the switchbacks. You could do all this without the GV control box but with a bit of additional wiring work you can have the benefit of the low speed shut off protection.
I did the trans tunnel mods and install by myself on my back with the chassis high up on blocks. I did not do any cutting or welding just lots of pounding with the round end of a big hammer but I got it all in with the driveshaft angles within spec. All the pounding marks are covered by the GV once installed, and those marks are covered by the front seat above. There are some differences for the long roof wagons with its OEM long tail TH400, including a cross member that is further rearward I believe, but the trans tunnel messaging I had to do was totally consistent with what I read in install articles and forum posts by those with Olds A body sedans and convertibles through early 70s years as well. If you work methodically under the car there are only a few places where the GV unit is restricted from locating up within the tunnel. The length of the hammered area is only about 6 inches over about 120 degrees at the top of the tunnel. There are bumps and bolts on the GV casting that you need to bang out clearance for in the floor pan.
I started with a mock-up of the GV unit by hacking up a coffee can of the same general OD and duct taping it it to a spare TH400 tail shaft that had the mounting boss so I could mount it to the crossmember. Using the lower profile version of the trans mount significantly reduced the areas needing clearancing but still kept the driveline angles in range. I eventually was holding up the heavy GV unit into the tunnel for fine tuning the clearancing. I would guess about 2 to 3 hours total of hammer pounding spread over a weekend of setup and mocking things up.
I also noticed Rallye469 did some interesting mods to lower his crossmember which would have similar effect to using the low profile trans mount:
There are some differences for the long roof wagons with its OEM long tail TH400, including a cross member that is further rearward I believe,
Actually not. The trans mount on the long tail TH400 is in exactly the same place as on the short tail. The only difference is that the VC uses the convertible trans crossmember due to the boxed frame.
Well, Tranny man called me today, it is in with only some tunnel massaging and a low profile tranny mount shimmed up with 2 of the 4 supplied shims. He did a short drive, all systems working and then dropped it off at the muffler shop for the redo of a portion of the exhaust. I will put eyes on it tomorrow am. My preference would have been max massaging and use the stock tranny mount. I think the worst at this point would be adj. upper arms to adj. pinion angle.
It is home. Tranny man did a very clean install but used the low profile tranny mount with 2 shims totaling 1.25" in height which gives me an operating u joint angle of 4.2 and 4.6 degree. The shaft is within 1 degree of parallel which is good. I would have beat more of the tunnel and used a few more shims on the tranny mount, my transmission centerline is down 4.6 front to rear. The drive home was with very wet roads and wrecked cars. When I get back on it I will see if more shims can be used to raise the unit as is or drop the GV and do more tunnel work. Nothing in the tunnel or any bracing was cut. I also have adjustable upper trailing arms to work on the 4.6 angle at the pinion. Maybe those working angles are ok but I think 3 +/- would be ideal. 4 speed shifting and in and out of the GV was fine.The shifter boot was not installed and the roads were wet which led to considerable road noise.
So I have decided after spending some up close and personal time on my back to drop the unit and massage the tunnel a bit more to raise the tranny closer to stock height. I have the trans mount now at 1.5" high, stock is around 1.875 to 2". I have it set and will put a small notch in the floor brace to relieve a slight bend in the speedo cable. Speedo now comes in on the drivers side as opposed to passenger side. Also my angle between GV and pinion is not parallel. Tranny down and pinion facing down. I have adjustable upper arms to that will be the final adjustment. Everything except the trailing arms should be set this weekend.
Reworked and done. 2 drives totaling 50 miles. The tunnel took quite a bit of massaging with an air hammer. 70 in 4th no gv is about 3200 with gv rpm drops to 2500 and change. I installed Umi adjustable upper arms and brought the pinion nose up to within 1.5 degrees of tranny to allow for pinion climb. No vibrations.
March 3rd, 2023, 08:29 AM
