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 some power on the cheap, mods that almost anyone can do. Everyones 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.
There are a few theories on how to make automatic transmissions last. I knew, going in, that my stock transmission might not last too long, so I was prepared to do some experimenting. What I have ended up with is a transmission that was amazing at a 560 whp level (over 500 ft lb of torque also), and today, is still holding up to 650 whp, once in 3rd gear and above. I have not modified the valve body in any way and the transmissions guts have never been removed. The transmission pan and valve body will need to be removed, but that is it really. How, you say? Well, I did a bunch of research and found that it is a delicate balancing act to make these transmissions work right with some real power behind them.
The first problem I ran into was hitting the rev limiter under power. Stock rev limiter is around 6200-6300 and the transmission starts its shift around 6100. Doesn’t really give much time to do its thing, but it works just fine at the rather anemic stock power level. There are two ways around this – raise the rev limiter or make it shift faster.
The best way without a full transmission build, and even without a valve body replacement is to modify the accumulators. Accumulators are the small piston like devices that allow fluid to accumulate, in order to dampen shifts. The springs need to be stiffer to make the shifts firmer, but they still need to be in place to maintain a little bit of shift damping. There is no need to be chirping tires for a quarter throttle 2-3 shift. I was never able to find good information on stock spring rate, or even find a good source for stiffer springs that would fit properly. Instead, I simply shortened the space the spring could travel. Unfortunately, I do not have measurements, as I simply did not take them when I was performing this mod 3 years ago. Some day, I will pull her apart and get exact measurements. To shorten the travel, I simply made bushings for the springs. Anything metal that fits inside the accumulator will work. I shimmed all 4 of mine approximately 15 mm. That was attempt #2. Attempt #1 was about half of that and though it helped, shimming more helped more, but is still not harsh. There are small holes inside the accumulators that you do not want to block, but the a650 design prevents that to an extent. One more quick mod related to this is to simply turn line pressure up. On the side of the valve body, there is a pressure control – simply use a straight edge screwdriver to push in and rotate to the next detent.
here are the accumulators in an a340 circled – the a650 is extremely similar
The second part includes raising the rev limiter and a few other little tricks. Even with the above listed modifications, getting the trans to shift too quickly will not be good. At the power levels required for these types of modifications, hopefully you are running some sort of standalone already. The 2jz is very capable of spinning higher than the stock 6300 rpm redline. That redline is put there because the stock cams and naturally aspirated motor just simply don’t make much power past that. We can safely spin a stock head / cams / valvetrain car past 7500 rpm, and probably higher than that if necessary. I have my redline set up through the AEM Infinity I run at 7300 rpm. One other thing I do is retard timing during the shift. Since no standalone will control the auto trans, what I’ve done is drop timing to 0 at the shift point. This reduces power just slightly (low timing keeps the turbo spooled though) until the lower RPM after the shift point is realized and the timing is obviously much higher there. These mods are enough to allow my transmission to survive at 650 whp / 610 ft lb. It doesn’t do so well in 1st and 2nd, mainly because the power comes on so strong, but once in 3rd, it holds together extremely well, all the way to redline in 5th.
Bad ideas – One of the most popular “mods” to these transmissions is the rods that go in the accumulators. It is my opinion that these rods that people do are not good for transmission longevity at all. The fact is, they make the transmission slam into gear pretty much every single shift. This is not good on the internal components of the transmission. Things like this are what causes the infamous sprag gears to roll over. An instantaneous hit of power is much harder on the sprag than applying the power smoothly. (Drop a hammer on a piece of glass versus lay a hammer on a piece of glass).
Good ideas – Don’t let the transmission slam into gear. Use the transmission selector buttons to maintain the gear you want. Prevent the transmission from downshifting if at all possible under full throttle. If the engine is below 4000 RPM, applying full throttle will invoke a downshift, no matter what gear you have selected. There is one way around this – the snow button. If you have snow mode activated, you can go full throttle as low as 3000 rpm, but will be limited to 60% throttle until snow mode is turned off.