Honda transmission switch

[Sparticle]

Hi folks. Honda CRX's from the 80's came with different manual transmissions. One with tall gears and one with short gears. The tall gear car gets amazing gas mileage, the short gear car gets good gas mileage. Can I switch one with short gears for one with tall gears? Thanks.

[Shoden]

Unfortunately, it's not as simple as just swapping the transmissions. The best transmission for good gas mileage was in the 1988 CRX HF, and it had a (going from memory here, my notes are at home) 2.90 final drive. The Si version of the same year had a 4.25 final drive, which made acceleration much more sporty, but hurt gas mileage (still, 35 to 40 mpg isn't bad).

The problem is with the axles. The HF used lighter weight axles and hubs, and the spline count is different on them than on the Si version, so Si axles won't work with the HF transmission or the HF hubs, and the HF axle won't work with the Si transmission or the Si hubs.

You can either order or build hybrid axles, which have the HF spline on the transmission end, and the Si spline on the hub end.

I had all the parts to do the swap, plus I was going to go even more extreme by using the engine from a Civic VX, which would have also required a wiring harness adapter to go from OBD0 to OBD1. Actually, I think I've still go most of the stuff in my barn, but we got rid of three of our CRXs, so down to just my B16 swapped toy now, and priorities have changed, so I'm not planning on the build anymore.

I can post some more from my notes when I get home.

Oh, and another option that has been used is to just swap 5th gear, and possibly the final drive (could be wrong on that), from the HF transmission to an Si transmission. This kept the good acceleration in lower gears, but made for more efficient highway cruising.

[Shoden]

Okay, I've got my old notes now. A few errors in my previous post, but that's what I get from going off memory of a document I haven't looked at for a while.

First mistake, the spline count difference isn't based on the model differences between HF/standard/Si, but on the model year. The 88 models are different than the 89-91 models, which is where my confusion came from. I was working with parts from an 88 HF going into a 91 Si.

Second, it wasn't the axle splines that are the problem. It's the transmission input shaft and clutch that are different on the 88 models. The 88s have a 21 spline input shaft and use a 190mm clutch disc, while 89-91 have a 20 spline input shaft and use a 200mm (89) or 212mm (90-91) clutch disc.

So, you should be able to swap an HF transmission into an Si of the same year with almost no problems.

Also, for some reason, the California CRX HFs had a different final drive ratio than the rest of the US cars. They have a 3.25 final drive instead of the taller 2.95, so you'll want to avoid getting a transmission from a CA car if you want the most efficient gearing.

Another thing to be aware of is that even if you swap an HF transmission into a standard(DX) or Si CRX, it won't result in gas mileage as good as an actual CRX HF. The HF model got its amazing gas mileage due to a combination of a lower power, higher efficiency engine, lighter weight, and the transmission.

This post over at EcoModder has some good info: http://ecomodder.com/forum/showthrea...ios-26279.html, and unlike me, a lot of those guys actually made it past the research stage.

There's also the 5th gear swap I mentioned. Since it's one of the better formatted parts of my notes, I'll just post it here (in quotes, since I copied it from some other poster on some other forum long ago):

5th gears

You can swap any of the 5th gearsets for cruising at a higher/lower
rpm. This is a simple swap, once your inside the tranny. The 0.771 from
the 4g Civic/CRX L3 trannys fit perfectly and is a nice swap to a
DX/EX/Si 0.702 5th. Make sure you keep the gearsets with each other. It is
very easy to get the 27 tooth gears mixed up. They are very close in size,
but will not interchange. You have the following choices.
0.878---??/??---CRX ZC JDM
0.771---35/27---Civic/CRX 88-91 DX/Si
0.750---36/27
0.702---37/26
0.694---36/25---Civic/CRX 88-91 HF

[Sparticle]

Thanks so much! You really know your stuff. I haven't decided what to do yet, get an original or change out the transmission on one. But, this help a lot. Thanks again.

[cobra]

thank you, great info.
i am looking to make the 5th gear swap at some point.
i drive 100 miles mostly highway so it would be nice to cruz easier.