One of the most common misconceptions about the IS300 (and other cars that share the 5 speed Lexus automatic) is its ability to handle some real power. In this article, I will outline what it takes to make it handle good power on the cheap, mods that almost anyone can do. Everyone’s mileage will vary, no doubt, and part of its longevity rests with you, the driver, but this will be the best thing you can do without spending a ton of money.
Part 1: Transmission coolers – Keeping transmission fluid cool is one of the most important keys to transmission longevity.
The stock cooling system for the transmission lives in the lower portion of the factory radiator. Fluid is simply pumped out of the transmission, through the radiator (you’ll notice two 5/16” hoses along the lower edge) and back into the transmission. There is very little fluid pressure here – the return essentially falls into the transmission pan. There is a factory temperature sender on the outlet of the transmission. This can be monitored by obd2 scan tools with live data ability.
A wide variety of external transmission coolers exist on the market place. You can’t have one too large, with regards to cooling, however, there isn’t a whole lot of space for large coolers without some ingenuity. Many will simply attach to the front of the condenser, behind the intercooler (on a forced induction car). Behind the drivers rear tire, there is some room for auxiliary coolers as well. Some have fans as part of their design, some will add external fans. No matter which you choose, it is a good idea to plumb the cooling system in such a way so that the fluid flows through the radiator based cooler first, then through the additional coolers. This provides for the best chance for the fluid to cool before returning to the transmission pan.
My personal setup has transmission fluid routed through the radiator, then to the rear of the car. Behind the drivers side rear tire, there is an unused space, away from anything else that produces heat. Here, I have two transmission coolers stacked, with an 8” cooling fan on them. Ideal transmission fluid temps are 175-225 degrees. I monitor transmission output temp through the AEM Infinity, and when transmission temps reach 175 degrees F, 80 degrees C, the auxiliary cooling fan is turned on. Sustained operation above 225 degrees will shorten transmission life, and any operation above 240 degrees is extremely detrimental, as the fluid itself begins breaking down.
A few key things that everyone needs to know with regards to this. First off, make sure any fluid lines used are rated for transmission fluid. More commonly available at parts stores is fuel injection hose, which will break down over time when used with transmission fluid. It is a good practice to use high quality fuel injection hose clamps instead of the more traditional worm gear clamps. I can’t say this is all I’ve ever used, but these are stronger, provide more consistent clamping pressure and once you use them, you will understand why I advocate for them. Next on the list is fluid type. Toyota T-IV fluid is the only transmission fluid to be used in most Toyota transmissions. It is relatively inexpensive at the dealer and it works great. Many manufacturers do make generic import transmission fluids that will work, but in general, they are the same price or even more expensive than the genuine fluid. Last but not least, no amount of cooling will help if you are are hard on the transmission and do high powered pulls and races back to back. Monitor temperature and allow the fluid to cool if it is getting unnecessarily high.