Optimum Performance Combinations for Drivetrains - Hitting The Sweet Spot

"I would rather have a 2,200-2,400 cruising rpm and have the engine in its power band where it runs efficiently. The '32 with that high rear gear needs the deep 3.06 first gear of the 700-R4. Depending on how aggressively it'll be driven I would recommend our Level 2 4L60-700R4 with a 2,400 stall 12-inch lockup converter." With the 700-R4 the '32 is now going to be spinning 1,850 rpm at 75 mph. We figured this by multiplying 2,652 x .70 (the 700-R4's overdrive ratio). We're now looking at the possibility of making the engine work too hard. To get it back in the 2,200-2,400 rpm range we're looking for, we need to swap the rear gears for a set of 3.73:1s and we'll be cruising at 2,300 rpm (75 x 3.73 x 336 = 93,996 divided by 28.5 equals 3,298 rpm then multiplied by .70 (overdrive) equals 2,308 rpm. This combo will really make the '32 come alive as the 700-R4's lower first gear and the new rear gear will make the car come out of the hole a lot better than the Turbo 350's 2.52 first gear and the 3.00:1 rearend gear.

With a 300hp 302 Ford-powered '34 coupe with a C-4 trans, again running a 9-inch with 3.00:1 gears and with 32-inch-tall Firestone grooved rear tires it would be running at 2,362 rpm at 75 mph. The C-4's first gear is a pretty high 2.46 so it might not leave the stoplight too hard. If you'd like to get pushed back in the seat a little harder and still keep the engine in the sweet spot while cruising, Zack recommends a Level 2 four-speed AOD with 12-inch lockup converter and a lower rearend gear. The Ford AOD's first gear is even higher at 2.40, but with a 4.11:1 rear gear it'll throw your head back and still cruise 75 mph at 2,168 rpm thanks to the .67 overdrive. It may be useful for you to know the final drive ratio of the Ford AOD is 0.67:1 with converter dampener equivalent of lockup, or lockup 0.60:1 without the dampener which is how Gearstar builds their Level 3 and 4 transmissions.

Finally, I had to ask him about my own present car, that '46 Ford with a 305, TH350, 27-inch tires, and a 3.08:1 gear which calculates out at 2,875 rpm at 75 mph. "A Level 1 4L60-700R4 would work best here. I would not recommend a 200-4R for this application as it works best with a low rear gear ratio," Zack told me. This trans would put the '46 just a tick over 2,000 rpm at 75 mph. If I wanted to bump it up a little I could go with a 3.50:1 rear gear and bring the rpm up to just under 2,300.

But what if you have a TH350 or any other transmission for that matter that you're happy with, but would like overdrive? Well, there's another option, and it's a product that can not only increase performance but also reduce fuel consumption. Gear Vendors' Under/Overdrive bolts to the rear of a regular transmission and allows the driver to shift to a gear with more mechanical advantage than the next stock gear in automatic transmissions or wide ratio manuals. It also splits the gears so you end up with double the number you started with. For instance a three-speed auto gains first-over, second-over, and final overdrive as well as first, second, and third gears. If you also have factory overdrive you will now have three overdrive gear choices. All this means six or eight forward ratios that you can shift to sequentially. Heavy, high-horsepower cars will benefit most from a Gear Vendors unit, though a lower-horsepower motor in something like a Deuce roadster will also benefit from the close-ratios that the unit provides and make it more fun to drive, too.

With a ratio of 0.778:1 a Gear Vendors Under/Overdrive reduces engine rpm by the same rpm as fifth gear on a manual transmission and can be more fun to drive than a standard overdrive. Why? Because of that gear-splitting capability. As the unit is mounted on the back of the transmission it is behind the governor, so when it is engaged it reduces the governor speed by 28.6 percent, automatically providing kickdown to the second-over gear which gives headroom for acceleration, rather than second gear which might be too low. A common misconception also is that a low rpm stall converter is required with the Gear Vendors unit, but this is not the case. Even with a high-stall converter such as 3,500 rpm, when cruising, nowhere near peak horsepower is being made, so the converter will still be hooked up. More detailed information can be found at the Gear Vendors website if the idea of over and underdrive appeals to you.

It should be obvious now why John Beck's advice that everything has to work together is important. Once the final ratio at the driveshaft is determined, you can work out your differential ratio, remembering that rear tire diameter is also a major factor here. There are a number of websites that offer rearend ratio calculators which can be found by searching the Internet, requiring you to input data such as tire diameter, engine rpm, road speed, and such to determine the ideal ratio needed. Various ratio ring and pinion sets are available for all popular axles. For instance the 9-inch and 8-inch Ford can be fitted with everything from a 2.80:1 set right up to pretty much whatever you can think of, though it's unlikely any road-going car will need anything above 4.88:1! The range of ratios available between these two is amazing, enabling you to dial in your requirements perfectly. The GM 12-bolt and Dana 44 and 60 axles are also well represented with many ratios available for each, though Zack Farah recommends that the ideal gear ratio with overdrive transmissions is 3.55, 3.73, or 3.90:1, saying, "If you go with a taller differential ratio (lower numerically) with one of today's performance engines you risk dropping the engine rpm below the camshaft's powerband." Again though, tire diameter is a factor too, and that's where we come back to aesthetics again. Looks, economy, and performance, but nowadays you can have it all!