Some thoughts on choosing the transmission that’s best for your vehicle.
More and more customers are looking for overdrive transmissions for their muscle cars and hot rods.
High gas prices drive the demand for better gas mileage, and customers want late model transmission performance for drivability.
Customers are tired of new Camaros and Mustangs beating them on the street. New cars have six-speed transmissions; a three-speed and sometimes even a four-speed automatic won’t be able to keep up with a six-speed on the street.
The transmission rarely gets much more than a single thought. Yet, how the engine puts the power to the ground is just as important as the engine itself. In fact, an incorrect transmission or converter can make the difference between a good hot rod and a great one.
As a builder or shop owner, you need to determine not only what transmission will best fit the vehicle, but also if it’s strong enough to withstand the power level and stresses caused by that particular application.
DO THE CALCULATIONS
Don’t let your customers get too wrapped up in having the latest technology and having as many transmission gears as possible. Do the calculations and ensure that the car is going to perform properly. Too often a transmission is picked before the rear gear ratio or tire size is decided.
For example, say your customer wants to go with the latest GM six-speed automatic, the 6L80E. It’s a six-speed so I will get better gas mileage right? Maybe, but let’s look at the first gear ratio.
The 6L80E has a 4.03:1 first gear. If you don’t use the correct rear gear ratio combination, first gear may be completely useless and you might as well start in second. The 6L80E has a gear ratio spread of 4.03, 2.36, 1.53, 1.15, .85 and .67. The advantage is that GM can build a vehicle with a higher rear gear ratio, while maintaining great acceleration due to the transmission’s first gear ratio and achieve close ratios between gears for maximum performance while gaining a good cruising RPM.
Keep in mind that most popular four-speed automatics such as the 4L60E, 4L80E and 4R70W transmissions have .70-.78 overdrive ratios, so you are not looking at a huge difference in final ratios – only approximately 80- 300 RPM difference while cruising at 60- 65 mph.
AOD FIT AND FINISH
When installing modern automatic overdrive (AOD) transmissions in older performance vehicles there are many things to consider. Some of the most important points are below:
• Overall Dimensions – Will it fit in the tunnel correctly without modifications? Sometimes you have to massage the tunnel if you are going with a big transmission such as a 4L80E or similar. Transmission size charts are readily available from most manufactures to help with this. Many of the modern transmissions are much larger than the old three-speed automatics and three- and four-speed manual transmissions.
• Driveshaft – Most overdrive swaps require you to swap or modify your original driveshaft. For example, if you swap a TH350 to a 700R4 transmission, you will need a shorter driveshaft. However, the same yoke will work. If you swap the TH350 for a 200-4R though, the same driveshaft can be used.
-¢ Crossmember – Most AOD transmission swaps will require a modified crossmember or a different crossmember. There are many good aftermarket crossmembers available today. You need to decide which one will fit your customer’s application. Also keep in mind that the crossmember needs to allow proper room for the exhaust system to be routed. Many times exhaust is another item that may need to be modified or changed when upgrading to a new transmission.
• Driveline Angle – The wrong combination will cause poor driving conditions such as vibrations and harmonics that will drive you crazy. Don’t end up trying to install it with too much of an angle to clear the tunnel. This results in the transmission being too low and bottoming on a bump or driveway approach. Just because you can make it fit without cutting the tunnel doesn’t mean that you should.
• Rear Gears – The proper rear gears will depend on the use it will see. Most vehicles with an overdrive transmission will see at least some amount of highway time; you need to give thought to matching the rear gear ratio to the application. This means that cruising RPM should be considered. It’s a simple calculation that can be seen below.
RPM = ((Rearend Ratio*MPH*336.136*Trans Ratio*(1+(Converter Slip%/100)))/Tire Height)
Using this equation, you can calculate your cruise RPM. An example of using this calculation would be*: Rear Gears: 4.10 MPH: 65 Transmission (overdrive) ratio: .70 Converter Slip %: 0% Tire Height: 27.106 (205/75-R15 tires) Using this formula, the example above would yield an RPM of 2314 at 65 MPH cruising speed. *example makes no accommodation for power loss. Your Mileage May Vary – Literally.
• Wiring/Computers – Most overdrive transmissions will cause unique wiring challenges as well. Even some of the non-electronic transmissions (700R4 and 200-4R) require the torque converter clutch (TCC) to be wired. Those who aren’t familiar with computer calibrations sometimes get confused with the whole transmission controller idea. This is a must though, if you are going with a late model electronic overdrive transmission. It’s common to use an aftermarket stand-alone engine and transmission controller. This option allows you to make the necessary changes with a push of a button or a few clicks on the laptop. You can’t just pull a late model LS engine and 4L60E transmission with the factory wiring and computer, install it in your ’69 Firebird, and expect it to run, shift and perform the same. The factory computer was calibrated to work with the original vehicle. You must have the computer reprogrammed to match your combination if you want decent performance.
• Lines – It’s important to ensure that you have proper clearance for cooler line routing and ensure that the proper cooler is installed. Transmission cooler lines routed too close to the exhaust or kinked or bent will result in damage to the transmission or could even cause a fire.
Classic Tube advises: “Remember to tell your supplier all about the vehicle and its modifications. If you order a set of lines for a ’69 Camaro, but you’re swapping to a modern tranny, chances are you will need to do some field engineering as lines designed for the original transmission will not fit properly.”
A BIT ABOUT STALL.
Kevin Winstead of TCI Auto says, “A high stall torque converter will allow you to quickly get the engine RPM up to the part of the power curve where your engine makes the most power. The entire combination needs to work well together. That’s why a good torque converter manufacturer will ask a lot of questions about your combination.”
“This really depends on the application, but most of your mild street cars will run a 2200-3200RPM stall. The bigger the cam, generally the bigger the stall you will need. Boosted applications don’t normally require a really high stall because the added torque generated by the boost will increase the stall speed. To spec a stall speed, I usually ask for cam specs, motor size, expected horsepower and torque, rear gear, weight of vehicle, power adder or n/a and type of driving – race, street or both.”
Mark Bowler, of Bowler Performance Transmissions adds, “Torque converter stall is the most commonly misunderstood item in the powertrain lineup. Stall is the speed at which the converter will limit the engine RPM if the transmission output is prohibited. By not allowing the transmission to move forward the increase in engine RPM ‘stalls’.”
“The key point to remember is that stall speed is a balancing act. The balance between the engine’s ability to produce power and the converter’s ability to hold it back. The speed at which stall occurs with a given converter is a function of the engine’s maximum torque. Correct stall speed on a given converter will not be the same when coupled to a mild small block engine as it would be when coupled to a high performance big block engine.”
WHY IS PROPER STALL SPEED IMPORTANT?
Proper selection of stall speed will make for quicker launch, better 60 ft. time, a better elapsed time (ET) in drag race applications, and will allow for the correct performance, acceleration, cruising, as well as daily driving.
Selection of the right stall speed for your vehicle should be matched to the engine peak torque, engine torque curve, and vehicle weight.
When selecting stall speed without having an accurate engine maximum torque rating; be conservative rather than overestimate it. If you overestimate the torque output, the resulting stall speed will be lower than intended and is likely to make the vehicle sluggish off the line.
It’s a common misconception that a stall converter will affect your in-town driving. Generally speaking, torque converters with stall speeds up to approximately 3000 RPM do not adversely affect normal driving.
A very high stall speed converter – above 3000 RPM – is not generally recommended for a vehicle whose primary purpose is street driving conditions.
How much of a performance gain can your customers expect to see when swapping to a modern AOD? That’s going to vary wildly depending on the exact setup of the vehicle. Every piece of the vehicle will affect the numbers. We asked TCI Auto for a couple of examples.
Don Mudrich, TCI Auto gives us: “A few years ago, when we first developed the 6X Six Speed auto, we ran a back-to-back comparison at the drag strip to a 4L80E transmission. The vehicle was a 3,650-lb. 1986 Buick Regal with a 375HP 355cid engine. It ran consistent 12.4s with the 4L80E transmission. The best ET was a 12.44. After switching to the 6X, it ran a best 12.29s. The engine was operating in a much smaller RPM range and resulted in a quicker ET.”
“A good example is a guy that has an early ’70s Nova that currently has a worn out TH350 and he wants to go overdrive. He has a 383 stroker with mild cam specs and no power adders; it is a street cruiser with occasional trips to the drag strip. Let’s say it has 500 HP and torque. I would recommend him a 700R4 Street Fighter package with a 2800 RPM stall. It should run high 11s no problem.”
Modern transmission performance is very achievable in older vehicles, especially with all the advances currently being made in the aftermarket. Today there are six-, seven- and eight-speed automatic transmissions, and nine-speed options are in the testing phase. Performing proper due diligence, and matching your transmission and components to the entire vehicle, will yield the proper performance your customers seek, and keep them coming back to you.
TCI Auto Kevin Winstead, Technical Consultant Don Mudrich, Sales & Technical Support Representative www.tciauto.com Quik-Link: 800-306-0133, ext. 16401
Bowler Performance Transmissions Mark Bowler, President www.bowlertransmissions.com Quik-Link: 800-306-0133, ext.
16402 Classic Tube www.classictube.com Quik-Link: 800-306-0133, ext. 16403