Jerico's Dan Cordier puts the finishing touches on a DR-4 drag racing box.
Big horsepower numbers are easy to come by. We no longer have to dream of someday building a 500hp engine. The reality is you can build one easily and relatively cheaply. But as the old saw goes, what good is all that power if you can't put it to the ground? Fortunately, the rest of the industry has kept up with the engine builders. Clutch material and design have improved, and tire compounds are way better today than even 10 years ago. Transmissions have seen great improvements, too. This article will illustrate some of the ways manual transmission builders have kept up with ever-increasing horsepower numbers.
How It Works
We should begin with the basics just to get everyone up to speed. Virtually every modern manual transmission consists of three shafts inside its case: the input shaft, the mainshaft, and the countershaft. The input shaft extends through the front of the case and engages the clutch hub splines, transmitting power from the engine into the transmission. The mainshaft fits into a hub in the back of the input shaft and extends out through the tailhousing of the transmission; it holds the driven or speed gears. The countershaft is located beneath the input and mainshaft assemblies; it holds the cluster gear.
The gear on the end of the input shaft meshes with an opposing gear on the cluster gear. These two gears usually are the same size and tooth count and turn at a 1:1 ratio. Because the cluster gear is usually all one forging, the whole thing turns as a unit. So when the engine is running and the clutch is engaged (foot off the pedal), these two gears turn at engine speed.
The cluster gear meshes with the driven speed gears on the mainshaft. Their differing sizes and tooth counts form the different ratios of the transmission. These driven gears are not mechanically connected to the mainshaft, however. They spin on bearings that allow them to freewheel. To understand this, consider what's happening when the engine is running with the transmission in Neutral. With the clutch disengaged, the engine is turning the input shaft and cluster gear. The cluster gear is turning all the driven gears, but the mainshaft--which is ultimately connected to the driveshaft--does not turn.
So how does engine power get to the mainshaft? The answer is through the synchronizer assemblies. Between each pair of gears is a synchronizer hub that is splined to the mainshaft and turns whenever the wheels turn. The synchronizer hub looks like a toothed wheel. It is encircled by a sliding sleeve or collar. This slider has internal teeth that match the external teeth on the hub. The slider's teeth also match the beveled dog teeth machined into the face of each driven gear. When you move the shift lever, the shifter forks move the slider forward or backward, causing it to engage the dogs on the gear face and the teeth of the synchronizer hub. When this happens, that gear is locked to the mainshaft, causing it to turn.
For the slider to engage the dogs on the driven gear, the two must be turning at almost the same speed. Consider a simple two-speed transmission with a 2:1 low gear and a 1:1 high gear. At an engine speed of 6,000 rpm, the input shaft and cluster gear are also turning at 6,000 rpm--so is our freewheeling high gear on the mainshaft. However, the mainshaft, synchronizer hub, slider, and driveshaft are turning at half that speed--3,000 rpm--because of the reduction of our 2:1 low gear. Attempting the shift would be very difficult. There are too many parts traveling at different speeds, making it nearly impossible to execute a smooth, grind-free shift.