So what does having a wide or close ratio manual shift transmission mean anyway? First of all it has nothing to do with how close or wide your shifter moves within it's pattern. That term is actually called "long or short throw" shifter. I'll discuss here a simple method that visually will show what close and wide ratio is.
There is no particular formula as for why a close or wide ratio gearbox are what they are. I say this because what was considered wide ratio in the 1960's is considered close ratio today. Confused yet? Assuming that one reading this article knows what a gear ratio is, the difference between two gear ratios in a transmission can be called a ratio spread or drop. That drop is a percentage of the previous ratio. Figure 1 shows the formula. Let's say we have a close ratio 4 speed Super T10. The ratios are: 2.64 first, 1.61 second, 1.23 third, and 1.00 fourth. The ratio change between the gears is : 39%, 24%, and 19%.
Borg Warner's extra low ratio Super T10 had a 2.88 first, 1.75 second, 1.33 third and 1:00 fourth. Notice if you plug these gear ratios into the above formula you will end up with the following drops; 39%, 24%, and 25%.
Think of gear ratios as a distance between 2 points. In a typical muscle car 4 speed application fourth gear is direct or 1 to 1. Again thinking in terms of distance, consider fourth gear or direct as your final destination. The further you get away from 1:1 ( direct ) you in a sense get wider. The Muncie M21 and M22 Close ratio 4 Speed was the closest ratio 4 speed ever put into a production car. It's ratios were 2:20 first, 1.64 second, 1.28 third, 1:00 fourth. If you plug those ratios into the formula in figure 1 you will end up with 25%, 22%, and 22%. Compare these percentage drops to the previous drops found using the Super T10. Notice that although the 2-3 and 3-4 drops of the Super T10 and Muncie are pretty close percentage wise, the 1-2 drops vary quite a bit. The reason is that the Super T10's have a lower ( higher numerical ) first gear. The 2.88 and 2.64 ratios are further away from direct than the Muncie's 2.20 first gear ratio.
makes close - close?
What you have learned is that you can't gain distance without loosing closeness. The lower the first gear, the wider the 4 speed will be. If one were to look historically on what a close ratio box was, it usually meant a gearbox that had 25% or lower drops across all gear spreads. The problem is that when the Muncie close ratio box was designed in the 1960's we really didn't worry much about gas mileage. In order for you to get a "close ratio" Muncie your car had to be equipped with at least a 3.70 rear end gear. You needed at least that low of a final drive to get your car moving. The "wide ratio" M20 had a 2.52 1st gear. You could get a 3.31 or 3.55 rear with that, gain a little more economy, but it wasn't cool to have what they considered a wide ratio box in your Corvette. By late 1974, fuel economy and air pollution were considerations. Catalytic converters were mandatory by 1975. Axle ratios had to drop to get the better gas mileage the EPA demanded. So in order to get cars moving with 3.08 or 2.88 rear end gears, cars that had engines with decent amounts of low end torque had "close ratio" transmissions with 2.64 first gears, and with smaller cubic inch engines first gear ratios in the 3.0 range. So the newer close ratio transmissions are wider than the older wide and close ratio transmissions!
works and what doesn't?
Doug Nash Corporation had an interesting solution to this dilemma. Make a 5 speed box with a close ratio spread like a Muncie M21 and make 5th gear direct. Again think of what I said about distance between two points. The more stops you make in your travel to get to your final destination the closer the distance between your stops. By adding a lower first speed gear and keeping a direct 5th, you gained more distance, but added an extra stop. The Doug Nash Street 5 speed came with a 3.27 first gear. This allowed drag enthusiants to use a 3.08 final drive and still get good close ratio acceleration. This worked great.
Newer cars today produce a great deal of low end torque. Most peak power is made in the 4500 RPM range. The average 5 speed such as a T5 is actually extremely wide ratio by 1960's standards. The close ratio T5's used by Ford Motorsport use a 2.95 first, 1.94 second, 1.34 third, 1.00 fourth, and .80 fifth. You do the math with the formula. You can also see more results with my RPM calculator. These new five and six speeds in a sense are really wide ratio 4 speeds with additional overdrive gears for fifth and sixth speed. Cars can now cruise at 1800 RPM at 70 MPH because the engine's torque curve can handle the load. If you are still thinking in terms of distance, not only are you getting further away from our final destination of direct drive, but you are now going past your final destination into two levels of overdrive ( such as the T56 6 speed).
Chrysler's new PT Cruiser has a 2.4L 150 horspower 4 cylinder engine. The engine produces 162 lb-ft @ 4000 rpm. The 5 speed transaxle Cruiser has a final drive of 3.94! The first ratio is 3.50, second is 1.96, third is 1.36, fourth is .97 and fifth is .81. I hope you can actually see this is really is a 3 speed with two overdrive ratios. The engine really doen't produce power until it hits 4000 RPM. So the gearing sort of works, but the car can be boggy at times.
Wide and Close Ratio
I have enclosed a chart with shift points of some popular transmissions. Cars really are no longer being offered with close or wide ratio transmission options any more. If you look at the chart you will see how by plotting stops of a distance traveled you can mentally get what close or wide ratio may mean. In one sense if you compare stops some transmissions have similar distances between one or two stops. Others are drastically different. It is no longer an issue as to what is close or wide any more, but more of an issue of how much percentage drop your engine can handle.
Road racing events such as Nascar, or SCCA type races still require transmissions with a close ratio spread. It is not uncommon for some of these cars to have 1.86 first gear ratios. Plot a gearbox on the above chart with a 1.86 first, 1.59 second, 1.17 3rd and 1.00 fourth. How do those results look in comparison to the others? What percentage drops does that same box create using the formula in this article? Since road race cars tend to be operating at the higher range of RPM and speed,a low first gear is not needed. A ultra close ratio gearbox has other advantages as well. Since the load changes are not as severe, drivetrain parts tend to live longer. High shock loads of wide ratio transmissions usually cause gearbox failure and rear axle failure. Less heat is generated in a close ratio box. A car having a final drive of 2.98 and a direct 4th gear has the same overall ratio as a car with a .80 overdive 5th and 3.73 final drive. The car with the overdrive will use more horspower and generate more heat thru the transmission, then the direct drive box. However maybe the 3.73 rear may offer more low speed punch on turns. My point here is that when it comes to what is right for your car, it is still a matter of trial and error.
Getting your street or race car geared properly can produce fantastic results. Jericho, G-Force and Liberty's all make excellent 4 and 5 speed gearboxes for drag race applications. If you have a track only car then sticking a 4.11 or 4.56 rear end gear and using one of these will produce better results that any street 5 speed. When it comes to road racing try and find other people using similar engine and rear combinations as yours and compare how they do against one another especially if they use a different transmission.
Hope you enjoyed this TechZone article. We always like feedback as well.
Articles | Bulletin
Board | Home
© 2001 Medatronics. All Rights Reserved