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Vintage Cars and Overdrive

Matt Harwood

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Seeing as I've had more than a handful of PMs and E-mails asking about the final turn of the overdrive in my '29 Cadillac, I thought I'd post here so it can help anyone who has some interest.

To summarize the story up to this point, I bought a 1929 Cadillac a few months ago and had Lloyd Young in Columbus, OH (614-837-7832) install a Borg-Warner overdrive unit in the center of the torque tube (photos of the installation are in my online photo gallery at http://forums.aaca.org/members/matt-harwood/albums/ and I’ll add to it once I take more photos of the completed installation). The overdrive provides a 30% reduction in engine speed, and I figure that my car with 4.75 gears and 32-inch tires will be turning about 2000 RPM at 60 MPH, which is all I wanted. I don't care about more top speed, I care about not being a rolling speed bump and keeping the engine alive and healthy. 55-60 MPH is plenty, believe me.

The system consists of the overdrive unit and on-board solenoid, a second frame-mounted constant duty solenoid that acts as a relay to fire the overdrive solenoid, a pushbutton switch inside the passenger compartment to activate it (it also has a light that indicates when the OD is active), and a micro-switch on the clutch pedal to kick it out of OD when I depress the clutch. There's also a cable that controls the freewheeling feature, which is mandatory with the OD. Some guys put theirs on the dash, but I hid mine down low just ahead of the front seat so that I can throw a hat or something over it to hide it completely at shows. Not that difficult to reach it, and once it's engaged, you leave it alone until you park or get into the hills so it's not a big deal.

I had some teething problems that amounted to a refusal to engage under certain conditions. It would either engage halfway or not at all, and sometimes would completely disconnect the engine from the rear end as if I were in neutral. It was frustrating.

I first addressed the ground. Essentially, grounding the overdrive solenoid is what engages it, so I made sure the grounds were all clean and solid, and upgraded to a 10 gauge wire to connect the overdrive solenoid to the constant duty solenoid, since it needs about 30 amps to fire (it maintains OD with about 1.5 amps, however). No change. Dang.

My next thought was that the freewheeling cable was too long. Not only did internal friction make it very difficult to push or pull because of the long, circuitous route it took to the OD unit, but it was perhaps not fully engaging or disengaging the freewheeling because of all the play in the cable. So I shortened it from about 10 feet to less than 3 and ran it straight to the overdrive. It's slightly more visible under the car now that it’s not hidden inside the frame, but works effortlessly and seems to have cured many of the problems. I should have done it this way from the start.

The final issue is heat. The way the Cadillac is currently set up, the solenoid on the OD is very close to the muffler. The OD works flawlessly when it is cool, say when I first start driving in the morning, but after sitting in stop-and-go traffic or on hot days or in hilly country, it would refuse to engage about 30% of the time. Definitely heat-related. So I wrapped the muffler and exhaust pipes with Thermo-Tec insulation, which seems to have alleviated some of the heat-soak issues, but heat from the OD itself still sometimes causes the solenoid to struggle. However, I’m down to about 10% of the time it refuses to engage, and if I keep trying, it eventually relents and gives me OD. Since the exhaust system currently on the car is pretty well rotted out anyway, I'm going to have a new system made over the winter that moves the muffler away from the OD.

Lloyd is going to send me a new solenoid, and there are two sizes—a 4.5-inch one and a 3.25 inch one. I have the longer one now, but the shorter one will give me more clearance and hopefully stay cooler. I may also fabricate some cooling fins and tack weld them to the outside of the solenoid, but we'll see how the shorty unit works first. He also suggested that there are three different solenoids available, and I probably got one that works on both 6 and 12 volts, which is the known to be the weakest of the bunch. The 6V only, 3.25-inch replacement (I’ll have it this week) should completely cure the problem.

Now, with all that said, the overdrive absolutely TRANSFORMS the car. Like all old cars, it gets kind of busy-sounding at, say, 45 MPH without OD. And while I've seen 50 MPH or so without OD, that's about as fast as I'd like to push it, and certainly not for any extended period of time.

With the overdrive engaged, however, the car is whisper quiet, and happily hums along at 55 MPH without breaking a sweat. The speedometer says 35 MPH when I'm closer to 50, so imagine how well your vintage car runs at 35 MPH (35 is probably the sweet spot, no?). The Cadillac stays cool, the fan isn't roaring, the generator is still charging at 10 amps, the bearings aren't being pounded to mush by those long rods, and it keeps up with traffic easily. There's plenty of power once it's up to speed, and I rarely find myself taking it out of overdrive for hills. Usually only a stop sign or red light will force me to drop out of OD. In overdrive, freewheeling is engaged, but it does not freewheel in OD, eliminating one of my biggest issues—that feeling of rolling out of control in neutral. In OD, it coasts down with engine braking as it should. It actually pulls pretty cleanly from about 15-20 MPH in OD, although it's leisurely until you hit about 30-35 MPH (actual, not indicated).

Operation is simple. Push in the plunger to activate freewheeling, accelerate normally to about 30 MPH, press the button (and make sure the light comes on), then lift off the throttle. You‘ll feel it drop into OD like an automatic transmission shift, the engine gets really quiet, and you keep accelerating. That’s all there is to it. To drop out of OD, just depress the clutch and it clicks off. Interestingly, you can make the 1-2 and 2-3 and even 3-2 shifts without the clutch when freewheeling is activated, which is nice for clutch wear, and it shifts a lot more smoothly without any driveline lash. But as I said, the downside is that when you’re out of OD and have freewheeling activated, there’s no engine braking. If you need engine braking, just lightly apply some throttle and pull the plunger out and it returns to fully engaged with plenty of engine braking. If any of this is confusing, let me know because it was confusing to me until I actually started using it, then it made perfect sense.

To make a long story longer, the overdrive makes the car vastly more user-friendly on the road. It's easy to use, and the difference it makes in comfort, performance, and longevity cannot be overstated. I was on 225-mile tour two weeks ago, and it easily kept up with a 1948 Cadillac running at 50-55 MPH through some rolling hills here in Ohio, never went above 165 degrees on a 93-degree day, the battery stayed charged, and it was comfortable rather than tiring. Fuel economy is still awful, but not quite as awful as it was before (perhaps 15 MPG on the open road with the overdrive engaged).

I spent about $2300 on the overdrive from Lloyd, and probably another $200 or so in miscellaneous wires, brackets, screws, and fittings, plus about $150 for the exhaust wrap. I'm not counting the money I paid to remove and install the differential/torque tube/transmission since all that stuff was out anyway for a transmission rebuild, but that was probably a truly massive PITA job.

In summary, it was WORTH EVERY PENNY, even with the frustration and early teething problems.

I hope this helps someone else considering an overdrive. I wouldn't hesitate to do it again, and am seriously considering sending my '41 Buick's torque tube down to Lloyd to get one while I can (he's about 90 years old). Imagine what it could do with 200 horsepower behind it and a set of 3.90 gears in the Buick!

Edited by Matt Harwood (see edit history)
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I did not go the overdrive route, but installe a ring and pinion to change the ratio. My 1940 Buick came with a 4.4 axle ratio, and I installed a 3,4 in its place. The engine has more than enough torque to handle the taller gear, and the car now cruises effortlessly at 65-70 MPH

Overdrives are really the way to go as they leave you with the lower starting gears.

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If you are down to only 10% of the time not engaging, try this trick:

Instead of just pushing the button to engage OD and seeing ther light go off again, Hold your finger on the button and let off of the gas - the OD will kick-in, and once engaged, will stay in until you step on the clutch. Guess how I know? Been there, done that, love Lloyd's ODs. He has done at least three for me. When you are doing an actual 60mph, your engine thinks it is doing 42 mph --- a 30% reduction.


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I read with interest your OD treatise. Thanks for the info."

I am not at all familiar with 20's era cars. You did state that "If any of this is confusing, let me know because it was confusing to me until I actually started using it, then it made perfect sense," I am letting you know.

How does one "make the 1-2 and 2-3 and even 3-2 shifts without the clutch when freewheeling is activated?" In modern-day cars, even with the engine off, one would need to depress the clutch to shift gears.

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Shifting gears is simply a matter of matching the speed of the gears on the driven [engine] side of the transmission with the gears on the output side [wheels]. In the early days, double-clutching did this, decoupling the gears from the engine, then re-engaging the clutch in neutral to speed them up to the speed of the wheels, then clutching again to put them into contact. As long as all the parts are spinning at the same speed, it all fits together nicely.

Synchronizers do this automatically, speeding up the gearset to match the speed of the wheels, hence no clashing (in fact, my 1929 Cadillac is the first car to have synchronizers in the transmission and they work rather well). Skilled drivers are able to do this by practice, and even with synchronizers, it's important to match revs to avoid wear and tear. Today, most heavy truck drivers do not use the clutch to do anything other than get the truck rolling from a stop--after that, all shifts are made by matching revs and without the clutch.

Next time you drive a manual transmission car, try shifting without the clutch. The key is not allowing either the engine to drive the wheels nor the wheels to drive the engine; you can't simply lift off the throttle entirely, but find a speed at which there's no load on the transmission. You'll have to go slowly and don't force it, but eventually you'll find an engine speed that matches road speed and the shifter slips effortlessly into gear with no clutch necessary.

Freewheeling is essentially like a clutch, but instead of decoupling the engine from the transmission, it decouples the wheels from the transmission. The net effect is the same, and it's easier to match revs and make a clean shift.

In short, you're simply moving the clutch from in front of the transmission to behind it, but it all works the same when it comes to shifting.

Hope this helps.

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It is not necessary to use the clutch when shifting gears, even on today's cars. Downshifting is a little more difficult, but it can be done.

The freewheeling makes it much easier because you don't have to match the engine speed with the transmission.

Are you coming up for CoA? Maybe you could teach me to shift without a clutch. I'm not having any luck at all.:(

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