How to mount a secondary transmission backwards for added gears?

[KrautBurner]

[QUOTE=dtmerritt;417160]The vehicle I want economy from is a 1955 Pontiac Star Chief ( almost 4000 lbs and square as a brick. It has the very old dual range Hydro Matic without a park position. Economy is one factor, another is, who knows how to fix it id it breaks?

Van get a $800 adapter and put a GM overdrive trans behind the engine for about $2500 Total, will pay for it,self in 40,000 miles.At 69 and with various heath issues, driving 5,000 miles a year, that is a chancey bet.

I could put a Gear Venders OD behind the hydromatic for about $3,500, then I still have the worry of the original trans.[/QUOTE]

Gear Vendors under/overdrive transmissions the most awarded auxiliary transmissions.

[Frank Lee]

I'd convert to 5-speed stick or modern a/t with OD and locking converter before adding a GV or another trans. Multiple gearboxes = more driveline efficiency losses.

[cRiPpLe_rOoStEr]

If you would be getting something like a TH400/3L80 backed by a GV overdrive it wouldn't be so bad, but would be more cost-effective to just get a self-contained 4-speed OD transmission (or even a 6-speed if you can deal with the controllers). Working around a secondary transmission would definitely be too much of a PITA for eventually getting a slight mileage improvement if you would manage to cross-shift properly...

[Big Dave]

As Post #9 showed this is an old idea. It works, within limitations. all you really have to do is deal with the mounting and sealing issues, and it will downspeed the engine (to use the term in vogue in automotive engineering circles) and probably improve MPGs.

Post #11 brings up a valid point. Automotive automatic transmissions rely on the torque converter to pump ATF to the cooler. If you downspeed the TC, you reduce oil flow to the cooler. You'd risk cooking the transmission and getting spectacular transmission failures.

I have the GV second-stage overdrive, and like in Post #12, I view it as sub-optimal but I am already down on the lowest ratios commercially available. I suppose I could substitute a Tremec 6060 with its 0.50:1 top gear for my setup (net 0.56:1 in double overdrive) but it seems like a big project to gain not much more overdrive.

[racprops]

Post #11 brings up a valid point. Automotive automatic transmissions rely on the torque converter to pump ATF to the cooler. If you downspeed the TC, you reduce oil flow to the cooler. You'd risk cooking the transmission and getting spectacular transmission failures.


Close, the front pump of a automatic is driven by the spinning engine, via the TC. And the lower RPMs can lower it pumping action, so coming off a big mountain by dopping into natural can with only idle to lower the pumps action and can harm you auto, BUT at 1000RPMs I believe things are good.


I have the GV second-stage overdrive, and like in Post #12, I view it as sub-optimal but I am already down on the lowest ratios commercially available. I suppose I could substitute a Tremec 6060 with its 0.50:1 top gear for my setup (net 0.56:1 in double overdrive) but it seems like a big project to gain not much more overdrive.


You did not tell Make, model, year of you car, nor is it stock or hot rodded.

You need to have an engine that can make use of lower RPMs, you need to map out you engines torque peek, and find where it loses power and starts lugging.

Example: SBC 350 1980/90s, torque peek at 2800/3000 RPMspms, drop that engine to
1500 and it will be lugging. the lowest most likely will be 2200 RPMs.

But take a 350/383 where the torque peek is 2000, then dropping it to 1500 can work.

Rich