RB25 Trans behind a KA24DE

Advanced discussion of improving KA-T components
AceInHole
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Post by AceInHole »

I saw the PSM car and found a steal of a deal on an RB tranny, so I went ahead and bought it. It should be here tomorrow and I'll get started on it next week or so.

What I found across the internets (rumors/ folklore/ etc):

The RB25DE transmission is similar to the RB20 transmission: it's just as weak as the stock SR/KA transmission. I've seen pics of it next to an RB25DET transmission and it's definitely different.

You can swap internals in the RB25DET transmission with a VG transmission, which should make rebuilds easier to do. Both gearsets are notably larger/ sturdier, which is indicated by the entire transmission looking bulkier than the SR/KA tranny. The easiest visiual difference is the RB25DET transmission has a slanted rear shifter plate, while the RB20/ RB25DE transmission has a flat plate similar to the SR/KA.

PPG makes gearsets for both the KA and RB transmissions. They recommend the RB transmission for cars with more than "400 engine horsepower". It's a pretty strong indicator that the RB tranny is going to be a major upgrade. The ultimate would be a PPG gearset in the RB transmission (or a G-Force T5).

The RB tranny is around 20lbs heavier than the KA tranny. Not a huge difference, but piece of mind that my car isn't going to eat 2 trannies a season is worth it, IMO.

The RB25DET transmission is around 3" shorter from the bellhousing to shifter than the KA transmission. It looks like I've got around 1.5" or so to push my engine back, so I'll be attempting to compensate for shifter position (as well as weight balance) by redoing my motor mounts.
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Post by Alonso »

Damn that's a lot of extra work just to get the shifter to come up thru the hole. With the z32 tranny I just shortened the shifter assembly...
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Post by AceInHole »

I could just make a new shifter that tilts back more, but I'm always looking to improve my car's performance, so setting everything back is a welcome change. I've already dropped the engine an inch to compensate for the Avid mounts, so another cut and weld should be easy.

One thing I never found an answer to with the Z32 tranny: Mazworks appears to clock it differently than Xcessive. Did that affect the shifter position laterally? Also, was the transmission mount parallel?

Transmission just came in:
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Notes:
- It's huge. Way bigger than the KA/SR tranny. Bigger is better, right?
- Slave cylinder is different. The bolts run in-line with the cylinder instead of on opposite sides. Already ordered a new slave and pivot to get that out of the way.
- This tranny didn't come with a shifter. Any recommendations?
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AceInHole
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Post by AceInHole »

Got a chance to compare the RB25DET tranny to the stocker today:

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Took a KA dust cover and lined it up:

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Pretty much what I expected. The starter bolts, and all bolts below the center are lined up pretty perfectly. The 2 bolts up top are slightly off, and there's one bolt that's just way off. Debating doing something about the bolt that's way off (weld a flange to it) and also might go so far as to fill in the two bolts up top before I redrill those, as well as the dowel pin between them. (If need be I can get it on a CNC milling machine)

Notes:
- The KA release bearing, sleeve, and fork all fit perfectly (so far).
- Looks like a KA shifter might work? Didn't get a chance to check but will confirm next time I'm working on it.
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Post by 91white_ka »

I'm so excited to see what happens here. Good job man.
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Post by k4rr »

tt
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Post by turbonola »

when i asked this damn question i got no answer. glad to see someone doing this though.
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Post by DjayS12 »

Interesting thread! keep us updated! :)
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Post by MattPowers »

i did it. i ran rb25 slave cylinder and also rb25 shifter. ka is different. custom mount, drilled the top holes out, always bolted the first 2 side bolt holes in first, starter i just drilled the threaded section out and run the bolts in from the engine side into the tranny where its threaded on the rb. ka/rb clutch is the same. i used an rb25 single plate on my ka tranny and just kept it when i did this. then i ran this setup with an os giken gear setup. then i just switched to a dog box. i never had an issues but i tihnk to drill out the wholes and stuff is kinda janky. i wouldnt do it again. also i had to remove all the dowell pins.
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Post by trk240sx »

Bump!! Update on this thread. Looks possible with alittle welding and redrilling. Anyone with first hand info?
AceInHole
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Post by AceInHole »

Micro update: Dropped the transmission off at 240sxMotoring to have the upper bolt holes welded up so I could re-drill it in the proper position, and it was recommended to just weld in a new upper section. This way, I can also include the third bolt hole that doesn't match up at all (2 o'clock position looking from the input shaft side) as well as retain the upper dowel pin (3 o'clock position dowel looks to already line up).

Luckily, I have a couple spare transmissions, so a donor bell housing was easy to come by:
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I'm thinking that I should have cut it further back and also replaced the starter bolt holes, although they're still beefy enough to hold onto the starter if I choose to do that. If not, I've got another transmission sitting in the garage with a blown 2nd gear.
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Post by mo_hish »

Are you just going to chop the same bit off of the RB25 trans and then weld the KA trans flange on?

It seems like this would be a much cheaper alternative to a trans adapter kit. Hmm
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Post by AceInHole »

Yes, a section of the KA trans flange will get cut off and grafted onto a similarly cut out section of the RB bellhousing in order to line 3 bolts and 1 dowel pin up, using the dust cover as a template. Also, I recently got my aluminum RB25DET to S14 driveshaft in, but I won't be starting the swap for a few weeks as I'm moving all my stuff to a new garage.

The total swap should cost less than a bolt-on conversion kit, although the fab work is definitely more time consuming. I also got a good deal on the transmission, so savings will vary. The other consideration is that VG transmissions are easy to find, so something like the Xcessive kit would be great for someone that blows transmissions up all the time. In this case, I'm doing this in order to not blow things up (knock on wood), and if this gear set doesn't hold up, I'll most likely upgrade again.
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Post by AceInHole »

Figured I'd keep everyone posted on progress, since there was some interest out there.

Since Jeff at 240sxMotoring has been busy with his race car (something I can relate to) and customer cars, I took it upon myself to do the cutting and fitting so that all he'll have to do is TIG it up. Started by marking off where I'd need to start my initial cuts:

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Took a deep breath and hacked the marked section out of the new tranny:
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Figured out where to cut on the previously cut bellhousing section:
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And ground everything down until it all fit decently. Some sections will need to be filled in with weld, but nothing more than an 1/8" gap:
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Hoping to get it welded at the end of this week, followed by a weekend of autocrossing. Hopefully it'll all get installed next week!
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Post by Jus14 »

sweet action cant wait to see it done
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Post by mo_hish »

This guy basically did the exact same thing for a CA
http://zilvia.net/f/s-chassis/281673-sh ... eme-2.html
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Post by sidwayz240 »

Well Done! Qaulity Work.
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Post by AceInHole »

Normally with cutting the entire bell housing flange, you've gotta jig everything up to ensure position and flatness. Since the KA and RB bell housings are so similar, you only need to cut out about 1/4 of the bell housing. This leaves you with the ability to jig everything up relative to existing holes rather than the tranny input shaft. It also allows you to keep things flat since you've already got a decent amount of surface area to clamp and measure relative to.

Update: Welding is done. Just need to grind down a couple spots near bolt holes:
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Hoping to test fit everything today or tomorrow. I've got an event on the 14th followed by a ProSolo. Hoping to have some confidence in the drivetrain when turning up the boost :)
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Post by mo_hish »

I'm curious to see how the whole repositioning of the motor will work out.

In theory, you could just run the driveshaft for rb25 swaps, right?
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Post by AceInHole »

Got around to popping the tranny in. Since my engine is lowered a bit it actually went in pretty straight. Only had to tilt the engine back by loosening the motor mounts and putting a ratchet strap across the front of the head, and the tranny went right up.

It's a pretty tight fit, but it clears with a little bit of room to breathe. I'd imagine with the stock engine position it'd require a bit of hammering of the tunnel, but in this case it went right up:
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Shifter position was pretty much as expected:
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I've got enough room to get a hand behind the head to the firewall, so I think I can move it back about an inch and a half. That should get the shifter closer to the right spot, and also help weight balance of the car. I'll probably cut and re-weld the shifter so the shift knob sits in the right position, though.
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Post by sdaigle240 »

haha your crazy bro, if u move the motor back your seriously addicted to autox. are you having a DS made/modified locally?
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Post by AceInHole »

Bought an aluminum RB25DET conversion driveshaft through 240sxMotoring. Still have to see if it has enough clearance to shift the engine back an inch and a half. If not, I'll have to see if I can swap it for an ABS driveshaft.

From what I've read online, the RB shifter sits 1.5" forward of center when attached to the RB. That indicates that I'll both have enough room to move the engine back (since the RB fits back there) and the driveshaft should work as it is. I'll know more this weekend when I finish things off.
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Post by AceInHole »

The RB transmission is in and working! Some notes on the remainder before I get more pics up:

- The driveshaft I ordered was made for the RB crossmemeber swap that places the shifter in the stock position, so it ended up not working. I ended up buying a steel driveshaft that was 1.75" longer, although I'd go with a 2" extension using stock motor mounts. That should leave you with about .25" of slop in case the drivetrain moves around. With mine shifted back about 1/4" and the new driveshaft it sits like this:
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(going to eventually redo it with a new oil pan to clear the cross member and shift it back another 2")

- I built a transmission mount with some urethane swaybar bushings I had lying around, since the Avid mount is a solid block of aluminum and has no "give". I needed just a small amount of play to get the custom mount to fit, and I didn't want the engine moving (Avid mounts up front with urethane inserts) with the transmission fixed as I think that would create unnecessary stresses in the transmission case.

Got some shaky video from the weekend showing the car at 14psi running without issues:
http://www.youtube.com/watch?v=yxbIG173N-U

Next project is going to be a more stable camera mount :P
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Re: RB25 Trans behind a KA24DE

Post by R34SR »

So i guess ill find out for sure soon....rb25det tranny on the way, hoping to simply notch the one bolt that does not align. Also hoping a CC white bunny setup will work.
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