My regular race car does not have one of those trick gear boxes that allows for changing gears track side and for a while I thought this is what I have let me make the best of it. After learning the car and beginning to look for the limit I found it a bit lethargic coming out of the turns. A buddy, Randy, and I were watching some video the week after a race weekend and he made an off hand comment that the engine did not sound like it was on throttle coming out of the turns. This puzzled me as I knew in that particular turn I would go to full throttle at or around apex. The hunt was on; what was happening, was I not going to throttle when I thought I was, should I be grabbing a lower gear, is the engine not healthy? As I often do I turned to the data.
Ultimately the challenge was with the MR2 gear box. The initial concept with the WSR was to utilize as many stock MR2 power train components as possible to keep build and upkeep costs down. The MR2 gear box used in the WSR was designed for a street application with well spread out gears and a reasonable overdrive gear. Spreading out gears is great for smooth shifts and a comfortable ride, not so gear for putting power down. Overdrive gears are used for highway cruising where fuel efficiency trumps performance. Below was my analysis of potential gear box solutions to find a way to compete with the big power 914's that dominate my class (at the time of this effort the 914's had just been given a displacement bump which would make them all the harder to chase down).
We will start by looking at the engine output power and the gearing map.
Engine builder supplied dyno numbers as measured at the crank.
Standard WSR transmission; the website blocklayer.com was used to generate these gearing maps.
Engine output
Above is the dyno graph for my 4AGE motor in the WSR. These engines, much like the legendary Toyota 3G turbo monsters from the IMSA GTP class, can be built to output tremendous power. Think 162.5 Hp/liter! For the WSR the engines were built much more humbly to keep cost manageable. Peak power for this engine is 181 cHp at 7300 rpm and peak torque is 142 lbs-ft at 6000 rpm. We will therefore set the starting point of the engines operational window between 6000 and 7300 rpm. The horsepower is fairly flat after peak so we could consider the power band extends up to 8000 rpm.
Gearing map
Next up is a gearing map. This graph shows the relationship between engine speed and road speed for each gear ratio in a given transmission. Each gear and differential combination in a gear box will produce unique road speed for a given engine speed. The table above shows us if this vehicle is moving at 64 mph the engine speed will be 8k rpm in second, 5.5k rpm in third, and 4.1k rpm in fourth gear. Using the gearing map we can get an idea of the power we are putting down exiting each turn in a circuit with a simple data system.
Lets consider the third gear to fourth gear change as this is one of the most common used around a circuit. Starting with a 7.3k rpm red line, at peak power, after the shift to fourth the motor will be turning 5.4k rpm. Going back to the dyno curves the engine out put at 5.4k rpm is 136.2 lbs-ft and 140 cHp. If we delay shifting for 700 rpm the engine output stays at high for a short bit longer (albeit in dropping power curve) and when the shift to fourth does happen the the engine output is now up 3 lbs-ft and 16.2 cHp. Even though power beings to fall after 7300 rpm it makes sense to stay in the gear that extra 700 rpm. As long as the engine was built to take higher rpms the red line should be moved up to take advantage of the power being made on top.
The Ullman Straight, 3000 feet of runway! That's a long straight. Typically you would select your top gear so that you reach peak power just before hitting the brakes. With the C52 (the stock WSR transmission) we would arrive at the braking zone in fourth gear just bouncing off the red line at 8k rpm. In 3000 feet of endless runway we did not even need to use fifth gear. Generally we have two options for matching gear boxes to a track. Option one is changing the final drive which shifts each gear lower or higher equally. Option two is changing the individual gears for closer spacing which has the added benefit of keeping the engine speed in a narrow range.
Now consider turn 13 at Sebring, tower turn. Mid corner speed is say 58 mph. Using the C52 gear box second gear would have the engine turning 7,300k rpm and third gear has the motor turning 4,983 rpm. In second gear the motor is at peak power (181 cHp) but down in torque (130 lbs-ft). Further more at 7.3k rpm the engine is nearing red line, how much time will be spent in second before a shift is needed? In third gear torque (136.3 lbs-ft) fractionally higher but power (129.7 cHp) is low. This is where Randy's ear was telling us I had a problem, no pull out of T13. The spacing between these gears is just too far apart. For what its worth T5, right before the long straight to T7, is a about a 60 mph turn, T13 leads on to the Bishops Bend straight, and T16, which leads on to the aforementioned Ullman Straight, is a about a 65 mph turn. Three of the most important turns at Sebring and they all are flirting with the long spacing between second and third.
Once we identified the short coming of the gear box we set out to find a gear box that had more favorable ratios. Lucky for us Toyota used this gear box for several "performance applications". The above gear box is a C50 or a C52; C designating the type, the 5 for 5 speed, and the final digit indicated which evolution of the box it is. The Wikipedia entry for the type-C transmissions is pretty detailed. Lets see what happens when different gearing is available.
Changing the Final Drive
The C52 was equipped with a 4.312:1 final drive. There is only one lower OEM final drive (4.529:1) and it was available on the C60, C64, and C160 trans-axles. Also there is a shop that makes ring gears with a ratio of 4.8 & 5.0:1. That would put the top speed at 134 and 128 mph respectively at 8k rpm. There are options, good. The ring gear and output shaft alone are about 2k$; available but pricey.
Alternate Gear Boxes
Of the long list of type-C transmissions three stand out as interesting options; the c60, c64, and the c160.
The c60 and the c160 have the same ratios and are relatively common. These transmissions where used in the Celica, Vibe, Matrixs, and Corolla (the c160 was used in the Japaneses domestic market Corollas). The c64 was used throughout Europe, Australia, Japan, and in the US (for the US only in the Lotus Elise/Exige). The c64 is less readily available and therefor was not an option initially.
Of the three transmissions the c160 was the more natural choice. The bell housing was a match and the existing half shafts were interchangeable between the c5x and the c160s. The other two transmissions these parts would have to be changed and or modified. Below is the gearing map for the c160. Notice the ratios are closer together and the final drive ratio goes down to 4.529:1 from 4.312:1.
The gearing of the c160 transmission is much more favorable in the lower gears. At each gear shift the drop in engine speed is reduced and the lower final drive provides more torque. 2nd to 3rd the engine speed drops now drops to 5780K rpm vs 5504k rpm, for 3rd to 4th the engine speed drops to 6266k rpm vs 5918k rpm. Unfortunately the at fourth to fifth shift the engine speed drop increases 412 rpm; from 6729k rpm to 6317 rpm. In addition there is 6th gear which allows the top speed to increase from 142 mph to 159 mph. Although this does not seem to be a likely attainable speed as the gearing of 6th is so high.
The top gear is still a problem. At 0.725:1 this is an over-over drive gear. The top gear in the stock C52 is 0.815:1; still an over drive gear but not nearly as extreme. With this sixth speed our top speed would go down. The lower final drive combined with such a long sixth gear turns this gear box into a close ratio 5-speed. What would be ideal is the fifth gear from the 5-speed as the sixth gear for the 6-speed. The fifth gear from the 5-speed has a ratio of 0.815:1 with sits nicely between the 6-speed fifth gear (0.916:1 ) and its sixth gear (0.725:1).
It seems Lotus realized this too when they wrote the spec for the Elise. The C64, the Toyota transmission manufactured for Lotus, has this same configuration. See below for the gearing map for the C64.
Notice two interesting points of the above map; first top speed is reduced and second the engine speed drop from fifth gear to sixth gear. This sixth gear arrangement is supper short; there is less then 1000 rpms to red line. Also at 7118 rpm when the shift to sixth happens the engine is very near peak power!
We realized the first phase of this transmission swap before the 2017 racing season and put it to use for the first time in April. We chose to start with a C160 as they are readily available on eBay from JDM importers for less then the cost of a replacement C52. There was an initial question of fitment but this turned out to be a non-issue. Once on track the character of the WSR was changed! Though it seems like a small changing, where on the power band the engine is when exiting a turn makes a big difference in how the power can be used to rotate the car and its acceleration out of the turns. The lethargic nature was gone but as identified by the analysis sixth gear was useless.
Phase 2 will be to swap in the Lotus sixth gear. Stay tune!