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The A604: Improvements

Since you unlucky 3.3/3.8 owners can only use the A604, being able to build a performance A604 is essential. People get rebuilt trans all the time for these cars, so why not have a racing one built? Well, lets take a look at A604 mods- big & small alike.

Here are some improvements to do with your A604...

Adding a drain plug to the A604 oil pan
External Tranny Fluid Cooler
External Tranny Filter
TCM Upgrade
Torque Converters
Final Drive changes
Limited Slip Differential
Rebuilding the Trans Completely



Drain Plug

The OEM oil pan on the A604 does not have a drain plug. Since you should be replacing the A604 ATF3/ATF4 fluid and filter every year or two (2 MAX!), this is the only way to prevent an anual bathing of the warm A604 fluid.

In the OEM setup, you had to unbolt the oil pan on the A604, then use a hammer and flathead screw driver to pry off the pan- thus allowing all the fluid to "fall out" at once. You need a BIG container to catch it, and if you're careful you can pry off just a corner- and let it drain out slowly...

Summit Racing (and prolly others) carries a kit for adding a drain plug. Basically you drill a hole and put on a bolt, washer, etc. While you do this- I also advise getting the drain plug bolt that summit sells which has a magnet on the end of it (to catch metal debris in your fluid). The kit is ~$5-10 USD and the magnetic bolt is abt $5 USD.

Note that due to the low clearance of the A604 oil pan, and the type of trans filter used, it is possible that you may run into some trouble finding a proper location for a drain plug.


External Tranny Fluid Cooler

The A604's typically used the radiator for external cooling (external minicooler on some minivans). This is bad since that cooling system will often fill with dirt and eventually clog up- thus cutting off the flow of fluid inside the A604 and resulting in massive part failure. Just about every after market parts retailer offers kits to add an external cooler, ranging in price from $30-200+ USD. The limit is what you want, need, and can afford.

A Cheap unit is worth while, I advise mounting it in front of the OEM radiator and disconnect the A604 from the radiator and use only the external cooler. I also advise getting (if the kit does not already include) a fan to put behind this external cooler.

The cooler you keep your A604- the better.

If you plan on modifying your car that has an A604, you may want to get a tranny temperate gauge. See what the temperate gets to OEM- and make sure it doesnt ever get any hotter as you play with stuff.

There is some debate as to rather or not you can get the A604's fluid "too cold" with an external cooler. If you're that paranoid about it, grab an external cooler from a minivan to use since its OEM spec for the A604. The newer models even use a temp sensor controlled valve to control when the external cooler is used. In any case, you never wanna be racing a cold trans anyway, get your car up to operating temps before putting it under stress- your engine, and your a604, will last longer.


External Tranny Filter

An external filter (if you have room in your engine bay) can be worth while, as the cleaner your fluid stays, the better your A604 will react. This is not as mission critical as some of the other improvements mentioned on this page, but if you want to go all out...

Again, price will range but a place like JC Whitney or Summit Racing will have a cheap kit that will do the job.

If you do this- be sure to replace the filter regularly, you do NOT want to wait until it gets too dirty, clogges up- and cuts off the fluid from reaching the A604.


TCM Upgrade

If you read my page on A604 Basics, you might have noticed that I mentioned how the TCM changed over the years. Newer models actually allow the dealer to "flash" the TCM's memory with a newer version of the software code.

These newer TCMs can actually be used in place of older ones.

You can go to a junk yard, and look in newer minivans for them. They typically are the TCM's with fins & sometimes fans on them for cooling the computer. A call to your local Mopar dealer may get you a part # or info on which models used this. I believe they started the flashiable TCMs in 1992.


Torque Converters

Some other engines and cars got the A604 (like the Neon/Stratus, Avenger, etc) and it may be possible to obtain a torque converter for a smaller engine as a mod to get more out of your A604 in performance applications.

Basically the idea is to get a torque converter with a higher stall rpm. The stall rpm is the point where, at a dead stop with the brakes on and the gas pedal pressed- that the engine overcomes the power of the brakes and moves the car. The idea is to get the stall rpm about where the start of the engine's power curve is. That way, in racing the car- you need only press the brake, press the gas until a rpm or two away from the stall rpm- and release the brakes and go to WOT.

Sadly I do not know if the stall rpm varried from year to year, or from engine combo to engine combo (3.3 with a A604 may have a diff stall rpm from a 2.0 stratus with an A604, etc).

However some places do sell high stall torque converters for the A604, I have seen 2300rpm, 2500rpm, and 2800rpm TC's for this tranny. Prices range from about $250 USD and up depending on the specs. Here are some sources for them:
Exile Racing
Power Demon


Final Drive Mod

The drive gears are different in different engine-A604 combos of later years. It is possible to take say, the drive gears out of a 99 2.0 stratus and put them in a older A604- this should basically give you better acceleration due to the gear ratio differences.

How the final drive mod works, is you change the gear ratio (actually decrease it) between the differential gear and the pinion gear. This in return, increases accelration.

However it does slightly harm mpgs on highway use, and will make your speedo lose some of its acuracy. In city driving, if you use the increase in acceleration to press the gas pedal less often- it could, in theory, increase city-use mpgs.

There is a guy who has done this mod to an other A604 car+engine combo who did a write up on his website, which can be found here.


Limited Slip Differential

The purpose of the differntial is to allow the wheels to turn at dfferent speeds, to keep you from skidding out of control. The purpose of the limited slip, is it controls/limits how much the speed can varry- which translates to better traction at take off.

This is not something the average A604 owner will need, but once you start getting into the higher hp & torque figures, it might be worth looking into.

Its gonna cost you, figure about $300 just for the parts.

Right now, you can get a limited slip directly from Phantom Grip, which can be found here.


Rebuilding the trans completely

One of the best -but most expensive things you can do with your A604, is a complete rebuild built to your specs. This will cost you. Most shops you're looking at about $1,500-2,500 USD for a basic, no frills, stock rebuild. A peformance or racing build would be a bit more, due to parts costs. Lastly, a few places sell performance built A604s online- but now we're talking core deposits & shipping costs.

At the bare min you should get a decent set of differential safety pins, the A604's diff pin is known to leave the case - and thats without doing burnouts and the like, this is what killed my A604 at about 150k. You should also do a performance overhaul kit, like Transtar's K7200H.

From what I can gather online, the ultimate A604 build list would include:
-Limited Slip Differential (Phanton Grip- part #904-GOLD)- $300 USD
-Transtar High Energy Rebuild/Overhaul kit - this is the kit with kelvar based clutches (Transtar- part#K7200H)- $193 USD
-Differenal Safety Pins (Transtar part#72984), need 2- $10 USD total
-Shift Kit (Transtar part#K72908J)- $26 USD
-Booster Valve Upgrade (transtar part#7028V)- $15 USD
-Final Drive Gears to a 1999 2.0 Stratus A604 (see local junk yard)- $100 USD
2500rpm high stall torque converter (Exile Racing part#ERT2500TC)-$270 USD
-Rear Planetary Carrier- racing spec (Transtar part#48762T)- ? USD
-Racing Output Shaft Gear (Transtar part#72963)- ? USD
-Racing Kit for Transfer Shaft (Transtar part#K72965)- ? USD
-Racing Transfer Shaft Gear (Transtar part#72967)- ? USD
-Racing differential Bearing (Transtar part#K48761)- ? USD

*Note- not sure about items in italc.

Rebuilts are availble online, built to racing/performance spec, from Power Demon or Exile Racing.