Overdrive Swap - Transformation

We changed the idle-feed restrictor in the Demon 750 carb from a fat 0.040 inch to 0.036 inch, and now the Chevelle runs down the road at a 14:1 air/fuel ratio in Overdrive. That's the reason it was able to squeak out almost 15 mpg on pure freeway driving at 70 mph. It cruises very smoothly at 2,000 rpm at 70 mph on our new Mickey Thompson radials.

We changed the idle-feed restrictor in the Demon 750 carb from a fat 0.040 inch to 0.036 i

Road and Track Comparo
The goal for this story was to compare the two transmissions on the road and on the strip to evaluate the 200-4R's several advantages. The most obvious is the 0.67:1 overdrive that reduces engine speed by a solid 33 percent. Right away, we gained a tremendous driveability advantage on the freeway, generating 2,200 rpm at 72 mph with our 3.55 rear gear and a set of 28-inch-tall Mickey Thompson S/R tires. The combination of the overdrive trans and the new M/T radials just seemed to calm the Chevelle on the freeway compared with an older set of skins that had long since passed their prime. Now between 70 and 80 mph, the car is smooth and you can actually talk inside with the windows rolled up. Before, we had to yell to be heard.

We also wanted to test the fuel mileage, so we kept track of the miles driven using our GPS system all the way through the CC Anti-Tour to Phoenix and back, a total of more than 900 miles. With some simple idle-feed restriction changes to the Demon carburetor, we were able to generate an overall average for the trip of 14.3 mpg. The car usually achieved more than 15 mpg on the freeway, but in town, the loose converter and big carb really conspire to kill mileage. This is where EFI would really benefit the car if mileage were a big concern. But hey, we're getting 15 mpg with a carb, so we're thrilled.

The 200-4R is also lighter than the TH400. This is almost all in the spinning mass of the transmission and we thought we might see an improvement in dragstrip e.t., but it didn't show up on the timeslip. We did see an immediate improvement in our 60-foot times. This was clearly due to the converter. The Chevelle had run 1.83- and 1.84-second 60-foot times with the TH400. While not stellar, the times were consistent. The very first run with the 200-4R immediately resulted in a 1.70-second 60-foot time, and the Chevelle used that quicker time to run a best of 7.57 at 89.07 mph. After the first pass, the 60-foot times suffered because of tire spin. With the added torque multiplication from the CPT 10-inch converter and the deeper gear ratio, this increased torque has exceeded the car's ability to hook consistently. Now we have to work on the chassis to make it plant the rear tires better. Once we do that, the car will be even quicker.

Dragstrip Comparison
All tests were conducted at Irwindale Speedway on Thursday nights. The weather was considerably warmer on the night we tested the 200-4R, which might explain the slower mph. The engine was slightly lean for both tests, but we didn't want to change jetting since that would add a variable that was not part of the trans swap.

By Doug Roe

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