I can't seem to find any in-depth write-ups on this.

I'm looking to use my ZC/Si transmission for peppy city driving, however I don't want to be stuck with 4400 RPMs on the highway.

Would there be any major problems with swapping out the 5th final drive with one from an HF?

For example.. would the mainshaft or any other internals be incompatible with the new gear? Would a 16 valve D16 lug or have trouble with hills as opposed to the 8 valve D15? What are the potential operation problems I would run into if I attempted this?

 

You can put in an HF 5th gear, but NOT a final drive. HF 5th gear should have you turning several hundred RPM lower on the highway.

Btw - I have an Si tranny and MPFI swap myself. Before my swap I could average 40 mpg all day. I listened to the ricers on here who claimed my mileage wouldn't suffer with the swap and now I can barely average 31-32 mpg. Thinking of doing this myself.

 

^Out of curiousity why can't you put an HF final drive set into an Si transmission?

By the way, I'm running ZC 1st-4th, and an Si/DX 5th with Si final drive set and I'm good for 35-36mpg and have gone as high as 41mpg. This is with a 1.6 block though, are you still using a 1.5block?

 

Well "final drive" isn't the same as "5th gear". Final drive would affect all the gear ratios whereas 5th gear itself can be changed out independently

*edit - I should clarify that I don't know that you "can't" change out the final drive just that it's not the same as changing out 5th gear itself

And yes, I have a D15B2

 

As to your edit - you need to match the gear teeth of the FD up with the mating teeth on the counter shaft.
So, you need to change out the FD and counter shaft together.

The 5th gear just needs to be replaced on the counter and main shaft.
Should work fine.

EDIT:
Found a write-up with pictures!
(it's a PDF download)

ZC/Si Hybrid PDF
Step 12 and 16.

 

Ok that would've been better to say with respect to 5th gear not being the same as a final drive SET because I've swapped both without any issues.

 

i did that write-up originally, and here is one for just 5th gear swap (also PDF)

 

Dude you are a genius lol

And btw when I had my mpfi dx motor/trans I was getting 45-47 mpg hwy. The best mpg I got was when I drove home from Utah. Got 498 miles and still had about 1/16 of a tank left when I filled up.

 

Awsome! thank you much!

 

idk how but a friend of mine routinely got 350+ to a tank on his bone stock 91 si hatch.

i'm lucky if i see 250 to a tank, with bolt ons.

supposedly you won't see much of a mpg change by using the hf fifth, however not having your engine screaming at 4400 rpms is nice. I made my self a hybrid trans a few years back from my old si transmission (the diff was worn out) using a crx hf case and diff but i swapped the final drive, counter shaft, and main shaft from the si trans into it, and kept the hf 5th.

the biggest draw back to the 5th gear swap is that at high speeds our engines don't have enough power (with out boost) to easily accelerate so if you're passing some one or climbing hills you're going to be down shifting a lot.