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Installing an Si FD into a ZC Transmission

by Dave P

So, you finally got your ZC in. You have a header and an intake and you are happy…for a few weeks. Like every one else you want to go faster. It is hard to get good cams for the ZC, you could go turbo, or NOS but what else could you do? Well, there is something you can do that won't give you any more HP but you will able to go faster. What is it you ask? How about a hybrid transmission to go along with your engine.
The idea is to take a JDM ZC transmission and swap out the final drive to a US Si final drive. What will this do for you? Well, here are the numbers and what you will get:

USDM Si TrannyJDM ZC TrannyDave’s Hybrid Tranny
Final Drive4.253.884.25

As you can see in the numbers, when you swap out a 3.88 and put in a 4.25 you are going to have a noticable difference in transmission performance. And you can tell, when you drive with this tranny, it is for acceleration. One thing that it is bad for though, is highway driving. At 80 mph you are at about 4300 rpm, but if you don't want that you could always swap out 5th gear for the Si. The thing that I like most about it is that my car is much quicker, but I still only have two bolt on parts.

Parts that you will need to make this work

ZC transmission
Si ring gear and counter shaft
ZC or 1990-93 Integra axles
ZC or 1986-89 Integra intermediate shaft

The Parts to Swap

There are two parts that need to be removed from the ZC trans and replaced with Si parts in the swap. One is the ring gear, and the other is the countershaft. If you get the ZC and Si parts mixed you can tell which one is which by counting the number of teeth on the ring gear and dividing that by the number of teeth on the countershaft. The Si final drive has 68 teeth on the ring gear and 16 on the countershaft (68/16 = 4.25), while the ZC has 70 teeth on the ring gear and 18 on the countershaft (70/18 = 3.88).

Once you have your transmission out of your car, open it up. Remove the counter and main shaft assemblies from your Si trans. You are now going to remove the lock nut at the top of the countershaft and press off the bearings at the top. Once you have those off, carefully remove your gears and synchros in a stack. Now you have your first part that you need to put in the ZC trans, the countershaft. Next, you can move on to your second part that you need to remove, the ring gear. Remove the differential from your Si trans and take off the ring gear, the bolts are left hand thread so watch out. Now that you have the two parts you need, move the Si parts out of the way. Now, you can get your ZC trans and crack it open. Once you have your countershaft out and the lock nut and bearing pressed off, you can care fully remove the gears from the ZC countershaft and slide them on the Si shaft. Put the bearing back on and torque the lock nut back on the shaft. Next, you need to remove the differential from the ZC trans and replace the ring gear with the one from the Si trans. Now, carefully reassemble the ZC trans. Make sure that you clean both parts of the trans case well before you reassemble it with Hondabond. After you have the transmission back together, put a screwdriver in the linkage and make sure that you can find 5 distinct positions. Make sure that you have all the positions in the trans before you install it. I put mine in and put everything back together and found out that I did not have reverse… check the positions.
If this sounds easy, it's not. I had my friend, John, who is a tech at Honda, do mine, and he didn't have fun doing it. If you do try it, take your time and have a good manual, like Helms' handy so if something falls apart you know how it goes back together.

This is John taking apart my Si transmission
at Performance Honda (Cincinnati)

Here is the countershaft with the
lock nut and bearings installed.

Shot of transmission with the countershaft
and mainshaft removed.

Here is my ZC transmission with the
Si countershaft and ring gear installed.

article, pictures and text by
Dave P.

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