It was a good idea in 1984; you could order your new Corvette with a four-speed manual transmission that had an overdrive on the top three gears, and suddenly had a seven-speed transmission in your Corvette. This 4+3 transmission was really nothing more than two separate transmissions mated together to form one unit. By combining these two transmissions, Chevrolet was able to give us seven forward gears. We have the EPA to thank for this marvelous combination, but more on that later.
This poor 4+3 transmission was put down from day one. Everyone complained about the way the transmission shifted, and the way the overdrive unit worked. Most of this wasn't justified. A lot of the people who complained about the 4+3 had never driven one that was properly adjusted. The fact that the average Chevrolet dealership didn't know, or didn't care, to properly adjust the transmission, didn't help its reputation one single bit. This unit also suffered because the German six-speed that replaced it in 1988 was really much better. Nonetheless, if you have a 4+3 in your Corvette don't get all worked up. This is really a good transmission.
The 4+3 manual transmission was the old Borg-Warner T-10 that had been around for decades. Doug Nash, who is no longer in business, bought the rights to the transmission from Warner Gear in June of 1982, and built the transmission to Chevrolet specifications. The case was strengthened, and several flanges were added to the rear for the mounting of the overdrive unit. This basic four-speed unit has proven to be bullet proof, and very few people have ever had a problem with the manual section of the transmission.
One of the critical engineering tests Chevrolet was using at the time was called a "spike test." The transmission was run at a specific rpm and the clutch was engaged for both low and reverse gear. The test was done three times at 2,000 rpm, three times at 3,000 rpm, and so forth until 5,000 rpm was reached. This was one major reason for the strengthening of the case.
Another reason for the different case design was the due to the fact the fourth generation Corvette was designed so that the transmission and rear axle operate as a single rigid unit. The channel beam that you see under your Corvette is put there to tie the transmission and rear differential case together as a single unit. In earlier Corvettes this load was taken up with the frame crossmembers. The rigidity of this assembly is a real benefit to the Corvette, and the same design was retained when the six-speed was introduced in 1989.
The overdrive unit was hung off the back of the four-speed transmission. In a great many ways the overdive unit is very similar to the old two-speed Powerglide. This particular unit was originally designed for the Jeep CJ-7 and CJ-5. Chevrolet was working on such a tight deadline for the fourth generation Corvette that there was no time to develop a new unit specifically for the Corvette. The early thoughts were that the unit from the 1934 Chrysler Airflow could be used. The engineering team also looked at the GKN/Laycock de Normanville units that were used in a lot of European cars.
The problem with these units was that they couldn't handle the torque of the small block Chevrolet engine. In the end Chevrolet went back to the unit that Doug Nash had setting on his shelf. Delco Electronics solved the computer problems and made sure that the manual section and the OD were truly integrated units. All of this was really done to meet the EPA fuel mileage regulations, but at the same time the overdrive fourth gear provided a high-speed cruising capability that no previous Corvette ever had. The top speed of the C4 Corvette is tremendous. At the same time you get the great acceleration in the lower gears with the overdrive disengaged.
The EPA was the major reason that there was never a thought of putting a five-speed in the Corvette. There was no way that a five-speed could have had a 0.68:1 top gear. This was the secret to the 4+3; a tremendous ratio for the top end. Keep in mind that there were no six-speeds available at the time. The irony of this whole situation was that a wide-ratio three-speed transmission would have easily met the EPA mileage figures.
As Dave McLellan explained in 1983:
The electronics necessary to create this dual transmission weren't all that complicated but it did make for some interesting driving. There are really three unique, algorithms programmed into overdrive ECM, one each for second, third and fourth gears. The overdrive ECM logic is incorporated into the main ECM, or the chip as it's commonly known.
A program that monitors miles per hour, engine temperature, and throttle position engages the overdrive. The program will normally not engage at less than 184°, although my '85 lkes 176°;. I guess all computers are not created equal. The real trick though is to get rid of all these complicated electronics, and turn this transmission into an eight-speed.
Richard F. Newton
October, 2001
The Super T-10 transmission with the Doug Nash Overdrive attached.
The Doug Nash Overdrive unit.
".. the best possible manual transmission is a wide-ratio three-speed. I'm not saying it's much good for a Corvette driving down the road, but it's best for EPA numbers because of the way the EPA test is structured. EPA rules say you can't shift into second below 15 mph, into third gear below 25, or fourth below 40. And if you've got a fifth gear you can't get into fifth until you reach 45 mph. lf you run a five-speed transmission with a top gear for cruising fuel economy, you'll benefit the customer all right, but your advantage in the EPA highway cycle is only about a half a mile per gallon. If on the other hand, you've got only three speeds you can get into top gear a lot sooner, and that'll save mileage considerably, more than half a mile per gallon. Anyway, in a Corvette, what you need for acceleration are four normal ratios, lower than that will simply light up the tires. Any higher and the engine tends to bog. Then you need close enough spacing for the rest of the gears so you get satisfactory performance in terms of overlapping the torque curve at each shift point.
So looking at the seven-speed overdrive package, we've essentially got two four-speeds. We've got one four speed for performance driving with the proper ratios for performance. Then, to satisfy the EPA's city driving cycle, we really have another four-speed made up of low gear plus the three overdrive ratios."
Excerpt from Dave McLellan's book
CORVETTE FROM THE INSIDE
Soon after the 1973—1974 oil crisis, the Environmental Protection Agency (EPA) realized that their emission tests, because they measured the carbon-containing products of combustion, generated data that could also infer fuel economy. With the authorization of Congress, the EPA began to regulate fuel economy, creating the Corporate Average Fuel Economy (CAFE) requirement, and later the Gas Guzzler Tax, as well. Being under the huge corporate umbrella of GM, we didn’t have to worry about CAFE, since it was based on the average rating for all of a manufacturer’s vehicles taken as a whole. But the Gas Guzzler tax was a different story. The Corvette had a very low fuel economy rating, and this was a penalty that affected individual cars. The Gas Guzzler Tax assessed a fine of $500 for each car sold that missed the threshold, and it kicked in at ratings below 19 miles per gallon. The tax then increased with each mile per gallon missed.
The four-speed automatic transmission scheduled for introduction in 1982 was expected to improve Corvette fuel economy dramatically. This transmission had a 0.7:1 fourth gear and a 3.07:1 first gear. When compared with its three-speed predecessor, the new transmission had 74 percent more ratio range that could be used—with the appropriate selection of axle ratio—to improve performance or economy or both. Yet with our unit injectors combined into a single plane manifold concept, even the four-speed automatic could barely pass the Gas Guzzler minimums. The manual version was expected to be 2 mpg lower, in large part because of the EPA-imposed shift schedule.
The Porsche 928, the 330 GT Ferrari, and the Lamborghini Muira all had five-speed manual transmissions. Corvettes, since 1957, had used four-speed manuals. With their emphasis on power and their high numerical axles, these Corvettes had the first four gears of a five-speed. Driving them, I was always reaching for a fifth gear that wasn’t there. In any case, the four-speed was ill-suited to our fuel economy needs, and we were about to lose our production source as well.
Achieving fuel economy with a manual transmission should have been easy: Just shift to keep the car in the highest gear that would move the car forward. Unfortunately, quite early in the EPA fuel economy test program, several car makers had figured this out and pushed it to unrealistic limits, to get fuel economy numbers they could tout in their advertising. When the EPA realized what these manufacturers were doing, it adopted new rules regulating shift speeds, removing this loophole for all of us. With the new rules, we weren’t allowed to skip-shift, and there were minimum speeds set for each gear. However, the EPA gave us a loophole in the rule that plugged the loophole: Using an extensive driver survey, if we could prove that drivers were consistently shifting at speeds below 15, 25, 40, and 45 mph then the EPA would let us modify the shift rules. But, because this was quite an unlikely prospect with a performance car, we never even tried.
The obvious “play by the rules” transmission would be a wide-ratio three-speed, with ratios the same as the five-speed’s first, third and fifth gears. With a big, powerful V-8 engine, shifting into top gear at 25 mph was the best thing you could do for fuel economy, but a three-speed was a lousy transmission for a performance car.
The 4 + 3 transmission became our response to the EPA’s rule. A shift from first to second at 15 mph would engage the second overdrive, and subsequent up-shifts would engage the third and fourth overdrives. With the conservative cam timing of the V-8 engine, this strategy provided adequate torque for normal heavy-traffic driving, and improved our fuel economy by about 1.5 mpg— sufficient to meet the Gas Guzzler threshold. Flooring the accelerator pedal, when in any overdrive ratio, induced a computer controlled downshift out of overdrive.
When we first came up with this strategy, even our own people were in disbelief that it would work, and that anybody—much less the EPA— would buy into it. But, not having a better idea and wanting to maximize power, we persisted. So, we secured the backing of the Chevrolet organization and ultimately, the approval of the EPA, to use this as a shift strategy. While using the 4 + 3 transmission worked well as a fuel economy strategy, the car was always confusing to drive and the strategy came to be looked on as an inferior solution, by those who simply wanted a five- speed and didn’t care about fuel economy.
The manual overdrive required a planetary gear set, clutches, a hydraulic pump, and an electrohydraulic control system, all the elements of an automatic transmission, which was essentially what it was. Fortunately, Doug Nash Inc. was trying to sell the industry such an overdrive at the time. We ultimately contracted with them to produce the four-speed manual transmission, using Borg Warner’s tooling along with a highly- modified version of their original overdrive. Unfortunately, Doug Nash Inc. wasn’t very far along with their development of the overdrive, and the project was so complicated, that we were not ready for production until the 1984 model year was half over. Fortunately, time gave us the opportunity to work together to produce a far better transmission.
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| THE DOUG NASH 4+3 TRANSMISSION
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