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Old 01-28-2019, 08:19 PM   #30
d9m13n
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Okay. I want to revive this thread as I've decided to go ahead with doing this on my own build. Im adding some fresh information and will update with my own progress when I can.

First I'd like to make clear that for those of us in the US, our options on transmissions are limited. We really only have access to E46 M3 transmissions. The PMC motorsport and ABC Clutch adapter kits (seem to be the most popular overseas) are compatible with the Getrag 420g (m3 6 speed trans), and the zf s5-39dz (E46/e39 diesel 5 speed gearbox).
The 5 speed gearboxes have very short ratios, making it undesirable for higher hp sr20's as well as street cars (not suited to highway driving). Again, these are geared for low revving, high torque diesel cars. However, they are extremely cheap ($100-$200) and some european sellers will ship to the US.

Lets focus then on the E46 M3 gearbox. It is a 6 speed transmission with gear ratios suited for a low torque, high revving inline 6. That makes it very well suited to the sr20, which shares those attributes. This transmission has 2 versions, manual and SMG. SMG is a semi-automatic, single clutch transmission with hydraulically actuated gear shifting via the use of a sequential style shifter or steering wheel paddles. The SMG gearbox is the same exact Getrag 420g transmission as the traditional 6 speed manual. They key differences are the lack of shift detents and centering spring in the bellhousing of the SMG gearboxes.
This thread illustrates the conversion that specialist shops will be happy to perform for around $500-$600.

I picked up and SMG gearbox for $300, this is an especially low price for these transmissions, but smg boxes are pretty readily available around the 600$ mark. Manual trans' can range anywhere from $1000-$1600. It's not a bad idea to pick up an smg box and have it converted, as there is much less chance it was abused, and the synchros will most likely be in better shape (computer shifting rather than a human banging gears). The synchros in mine are immaculate, even though the previous owner said the mileage on the gearbox is unknown.

Heres the meat and potatoes

This is my transmission on a bathroom scale. 98.8lbs, filled with fluid but minus the slave cylinder.
FOR COMPARISON. The sr20/ka24 5 speed is around 100lbs with fluid.
CD009 is around 130-140.
T56 is around 130-140 as well.



Here are some (poor) photos of a test fit in my s13 trans tunnel. Unfortunately my phone died just as I tried to take a photo from directly underneath. I was incredibly surprised to find almost zero fitment issues. This is a 6 speed gearbox thats been known to hold 1000nm of torque, and it essentially slid right in.
The cd009, which is the closest comparison for our purposes, needs the 'U' mount cut and widened, as well as a good bit of tunnel-bashing to fit properly. Even still, it sits below the frame rails and is a major ground clearance issue for most applications.
The 420g here sits comfortably above the frame rails, with plenty of room on either side even at the tunnels skinniest point.
The only real clearance issue I found is that the trans tunnel might need a bit of massaging to get the transmission to sit a little higher. I do not have my sr20 (currently at the machine shop) to test fit and see where the transmission will sit with the engine/adapter kit in the proper place. I dont foresee much interfering with fitment at all. Im amazed at how simple it seems to be.





Lastly, I want to talk about how simple fitting a shifter should be (in theory). Ill try to get my hands on a stock m3 shifter to see where it would land in relation to the actual shifter opening on the trans tunnel at some point, but for now all I can do is speculate.
There are quite a few options for chassis mounted shifters for the e46 m3. The way bmw transmissions and shift linkages are set up, you can mount the shifter straight to the body, with the only physical connection to the transmission being the selector rod. Heres an image that displays this a bit better. The former is the oem style (selector rod on bottom, connected to the pivot point of the shifter. The top part is what connects to the body and introduces some play or 'slop' into the shifter via the use of bushings) The bottom photo is the chassis mount style, as you can see its hardmounted to the trans tunnel and connected to the transmission by just the rod. This allows very short, as well as direct and precise shift throws. Many of these shifters incorporate their own lockouts and secondary centering springs making them great and safe (via the decreased possibility of money shifting) for track use. This makes a retrofit into another chassis fairly simple.





One company, CAE (known for their incredible chassis mount shifters) offers a lineup of adjustable length selector rods. This makes a swap like this perfect, as it will allow you to set the shifter where you want and still be able to set the linkage to the perfect length. This will allow us to utilize our trans tunnel hole without modification (besides drilling holes to mount the shifter).
If anyone wants to try mounting up the stock BMW shifter that should be fairly simple as well.

Hopefully this helps open up a new possibility to some of us. ABC Clutch adapter kits are used pretty commonly in Europe by drifters/road racers. They make quality parts, as does PMC motorsport. Ill be using an abc clutch kit, however I have yet to source one. Ill be speaking with them directly soon, as I dont think they have any US distributors. Ill also try to see where I can find their kits the cheapest. PMC is definitely more budget oriented, but still utilizes good proven parts. They both make adapters for these gearboxes to mate up to JZ's, RB's, LS's, and lots of other oddball engines.

Lets see where this goes
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