sr20 and rb20 box compatability?

[antus]

Long time member, first time poster... I have a track s13, decent power, after market suspension / brakes / skyline drivetrain behind the transmission. Manual 5 speed SR20 box of some description.

Im having some trouble with the trans - it doesnt want to select a gear, then it will, then it wont. If the car is stopped its the same deal, clutch and declutch, try again it'll generally go in. 50% of the time the car wants to creep forward while the transmission in is N.

This sounds like a synchro problem. Perhaps its easy to fix, but I know transmissions get really expensive really quickly and I have the skills to remove and replace the box but not to investigate the problem.

So I decided to just buy another SR box, swap it in, and if that breaks too then look at an RB25 trans conversion. I got messed around on the first attempt to buy one, then had an option to buy an RB20 box, from an S13. I held back for a while but the seller said it'd fit, and my mates said it'd fit, and the info I could find about it on the net said the belhousings needed to be swapped, but that it'd fit. So I took the plunge.

Now I find I had a bum steer and the box most definitely doesnt fit AND the bellhosing length is different between the two boxes, so even if I was to swap the bellhousing and if the parts inside were swappable to make a functional box it'd still end up longer and not fit in my car. The seller looked at the pictures of my box and thought that maybe I already have a RB25 box. Im pretty sure I dont because the box has CB71C on the side which lines up with it being an SR20 box, and it fits the SR20 engine with no adapter plate or visible mods.

So. Im currently thinking sell the RB20 box and call it a fail, and buy another SR box.

1) What box is the original one in the pics - SR20? I know there is a stronger triple synchro S14 box - Is it one of those? From the fault description, is it worth opening it or getting a shop to look at the synchros rather than replace?

2) Is there any way the RB20 box could be modified to fit? If it could would it be substantially weaker and a bad idea?

 

[GTSBoy]

RB25DET boxes are physically huge. Basically the same length as RB20DET & RB25DE, but seemingly twice as fat and heavy. You would know if you had a 25DET box there.

Here's an RB20DET box. The RB25DE box is basically the same. Does not look different.

Here's an RB25DET box. Note how bloody huge it is. The bellhousing barely tapers to the body!

Here's an S14 SR20DET box. You can see it's clearly not the same (in the rear housing) as the RB20DET box.

[antus]

Ok great. So I do have a standard SR box, and the RB box is incompatible, even if I swap the bellhousing. So Im revisiting my options, and considering the RB25 gearbox conversion.

Is the RB25 box different to the RB25DET box? In size?

Will I need to modify the trans tunnel to make it fit?

For an s13 with serious rubber and suspension and about 250rwkw should I be looking at an RB25 NA box or RB25DET box?

Im doing drive days at the bend motorsport park in the car, hitting about 220kph at the end of the straight and driving it hard the whole time (as you do)

Edited October 30, 2019 by antus

[GTSBoy]

12 minutes ago, antus said:

Is the RB25 box different to the RB25DET box? In size?

Yes, as I said above, the NA RB25 box is physically the same thing as the RB20DET box. Slightly different ratios. All else same.

12 minutes ago, antus said:

Will I need to modify the trans tunnel to make it fit?

I'm not expert on S13s, but I can't imagine that you will. There is very little difference between S13 and R32 and the big box goes into R32 no problem and thousands of people have put the big box into S chassis cars, and I've not heard anyone complain of even needing to clearance with a hammer.

12 minutes ago, antus said:

For an s13 with serious rubber and suspension and about 250rwkw should I be looking at an RB25 NA box or RB25DET box?

Big box all the way. The little boxes don't like lots of torque and you are going to be running at the bottom end of the range where they just have a short life. Which is no fun.

 

Edited October 30, 2019 by GTSBoy

[antus]

Great - thats what I needed to know. thanks!

Edited October 30, 2019 by antus

[antus]

After considering the above I've decided to go the RB25DET box. Im looking at buying an R31 RB25DET package with box, shifter, mount, stock GTR tailshaft with the adjustable length so that should all bolt in. But its out of an S15 so Im trying to understand what that'll mean for putting it in an S13. According to a post I found about the stock S15 box in to an S13 it seems the S15 flywheel is 20mm wider than the S13 flywheel, so generally you'd put an S15 flywheel in the S13 then everything else lines up. As this package is out of an S15 it looks like im going to run in to this and thus need to find a flywheel too. Is this correct?

 

I believe the S15 has the speed sensor in the diff, but that wont be a problem as the RB25DET box has a speed sensor in it. Will connecting that up to the S13 computer cause any issues? Will I need to change the pulses per kilometer setting or similar? I have nistune so that wont be a problem. And the dash speedo doesnt work anyway, might install a GPS unit in the cabin.

[GTSBoy]

Speed sensor does not connect direct to ECU. Search for the thousands of posts I have made on this topic.

[antus]

Ok, I can figure that out. Any recommendation about the flywheel? Would the S13 flywheel and clutch work with the R32 box previously installed in an S15 or do I need to be looking for an S15 flywheel too?

The car it came from has the twin plate setup, which I think fills the gap in an S15 but I am happy to keep my s13 flywheel and clutch as there was no problems with that before the trans failed if I can.

Edited November 4, 2019 by antus

[GTSBoy]

I have zero idea about SR flywheels and clutches. Something about not f**king with tractor engines.