dbarton291
Senior Member
RE: Allison Transmission Questions
Most air to oil coolers are used as auxiliaries in addition to the water to oil cooler the OEM typically engineers into their vehicle. My opinion is, the choice of which one to use is not normally based on a sound definition of additional cooling requirements, rather it's what will fit and not cost a bundle.
The performance of a water to oil cooler is predictable and can be tested. A water to oil cooler will also dissipate much more heat in a manageable size than an air to oil cooler. That's why Allison works with OEMs on either cooling calculations or testing requirements for water to oil anytime a new application is designed by an OEM. You can take the engine water flow and the transmission oil flow, using the cooler curve for the cooler the OEM is using, and compare the heat the cooler is capable of dissipating, to the transmission heat rejection at the selected test point, which is usually 80 or 85% torque converter efficiency depending on vehicle type. Torque converter efficiency is the speed ratio times the torque ratio.
If the cooler can dissipate more BTU/min than the transmission is producing, it will keep the transmission cool at that design point. If the cooler dissipates less BTU/min than the transmission is producing at that point, the transmission temp can't stabilize and the transmission will keep getting hotter.
An air to oil cooler depends completely on airflow vs oil flow. What you need to know is:
1. The cooler curve for the air to oil cooler you want to use which shows at a given air flow and oil flow how much heat the cooler will dissipate. Good luck finding this for most air to oil coolers.
2. The exact air flow you will be producing in whatever unit of measure the cooler manufacturer uses in his published data on the cooler....that is if they even have and publish cooling capacity data on the cooler.
3. Run an iscaan on your vehicle to find out the transmission heat rejection at 80% converter efficiency. An Allison distributor should be able to run you an iscaan for a fee.
4. Take the heat rejection being produced by the transmission at 80% converter efficiency and find that point on a cooler curve for an air to oil cooler. That will tell you what air flow the cooler needs to see to dissipate that much heat.
I would be willing to bet money, it will take an air to oil cooler of enormous size to dissipate the heat produced by an Allison at 80% torque converter efficiency and full throttle.
In my experience, vehicles that use an air to oil cooler as a primary method of cooling the transmission oil, and there are only a few, have cooling capacity that is highly suspect. As you are finding out, the information needed to predict the performance of these types of coolers is not easy to find, and because they are normally used as an auxiliary cooler rather than a primary, few people will commit to what they are capable of.
I don't know what reference to remote coolers on the Allison site you are referring to. I know for a fact, Allison does not endorse or sell specific air to oil coolers. Any reference to a remote cooler by Allison normally refers to a remote MOUNTED oil to water tube and shell type cooler, as opposed to a cooler mounted in or on the radiator.
The only cooler Allison currently will sell with their transmissions is an integral cooler for retarder equipped transmissions. It is water to oil, and it's not cheap. All other coolers are sourced by the vehicle OEM.
That's probably more than you wanted to know. But, if you want to proceed with trying to re-engineer your vehicle to use an air to oil cooler, whatever cooler you choose to use, I would suggest mounting oil temperature gauges on both the to and from cooler lines (at the transmission) to monitor what's going on in that cooler circuit.
If you can stand another one of my opinions, I wouldn't do it. I'd stick with oil to water. If you don't trust your radiator to stay together in your 1998 vehicle, put one of the remote mounted tube and shell oil to water coolers on the vehicle and bypass the radiator. I think Rocore in Indianapolis is a good source for those.
Most air to oil coolers are used as auxiliaries in addition to the water to oil cooler the OEM typically engineers into their vehicle. My opinion is, the choice of which one to use is not normally based on a sound definition of additional cooling requirements, rather it's what will fit and not cost a bundle.
The performance of a water to oil cooler is predictable and can be tested. A water to oil cooler will also dissipate much more heat in a manageable size than an air to oil cooler. That's why Allison works with OEMs on either cooling calculations or testing requirements for water to oil anytime a new application is designed by an OEM. You can take the engine water flow and the transmission oil flow, using the cooler curve for the cooler the OEM is using, and compare the heat the cooler is capable of dissipating, to the transmission heat rejection at the selected test point, which is usually 80 or 85% torque converter efficiency depending on vehicle type. Torque converter efficiency is the speed ratio times the torque ratio.
If the cooler can dissipate more BTU/min than the transmission is producing, it will keep the transmission cool at that design point. If the cooler dissipates less BTU/min than the transmission is producing at that point, the transmission temp can't stabilize and the transmission will keep getting hotter.
An air to oil cooler depends completely on airflow vs oil flow. What you need to know is:
1. The cooler curve for the air to oil cooler you want to use which shows at a given air flow and oil flow how much heat the cooler will dissipate. Good luck finding this for most air to oil coolers.
2. The exact air flow you will be producing in whatever unit of measure the cooler manufacturer uses in his published data on the cooler....that is if they even have and publish cooling capacity data on the cooler.
3. Run an iscaan on your vehicle to find out the transmission heat rejection at 80% converter efficiency. An Allison distributor should be able to run you an iscaan for a fee.
4. Take the heat rejection being produced by the transmission at 80% converter efficiency and find that point on a cooler curve for an air to oil cooler. That will tell you what air flow the cooler needs to see to dissipate that much heat.
I would be willing to bet money, it will take an air to oil cooler of enormous size to dissipate the heat produced by an Allison at 80% torque converter efficiency and full throttle.
In my experience, vehicles that use an air to oil cooler as a primary method of cooling the transmission oil, and there are only a few, have cooling capacity that is highly suspect. As you are finding out, the information needed to predict the performance of these types of coolers is not easy to find, and because they are normally used as an auxiliary cooler rather than a primary, few people will commit to what they are capable of.
I don't know what reference to remote coolers on the Allison site you are referring to. I know for a fact, Allison does not endorse or sell specific air to oil coolers. Any reference to a remote cooler by Allison normally refers to a remote MOUNTED oil to water tube and shell type cooler, as opposed to a cooler mounted in or on the radiator.
The only cooler Allison currently will sell with their transmissions is an integral cooler for retarder equipped transmissions. It is water to oil, and it's not cheap. All other coolers are sourced by the vehicle OEM.
That's probably more than you wanted to know. But, if you want to proceed with trying to re-engineer your vehicle to use an air to oil cooler, whatever cooler you choose to use, I would suggest mounting oil temperature gauges on both the to and from cooler lines (at the transmission) to monitor what's going on in that cooler circuit.
If you can stand another one of my opinions, I wouldn't do it. I'd stick with oil to water. If you don't trust your radiator to stay together in your 1998 vehicle, put one of the remote mounted tube and shell oil to water coolers on the vehicle and bypass the radiator. I think Rocore in Indianapolis is a good source for those.
