Manual transmission gearing 4>5?
 

Hi, as I am building an 89 Civic hatch, I came across a question that I could not find an answer to.

Basically, I have two transmissions available to me. The original manual 4-speed or the Sedan/CRX 5-speed.
I assumed that the 5-speed would give me better milage as the engine would operate at lower RPM's at highway speeds, but the fueleconomy.gov website begs to differ. It lists the 4-speed as 29/35 MPG while the 5 speed is at 27/33 MPG. I am wondering if the readings are skewed as the 4-speed came only on the STD Hatchbacks and never on the Sedans. The CRX with the 5-speed (which is slightly lighter than the civic hb) is listed as 29/37, but then again, it is a more aerodynamic shape.

Any idea if I should keep the 4-speed or go with the 5?
Thanks

ke5aux -

Welcome to EM! I don't think the number of speeds matter. What's important is the gear ratio of the top gear. Also, in your case I don't know if the "final drive" ratio is part of the tranny. I am guessing it is part of the tranny because you have a FWD. Here is what I did on my car :

http://ecomodder.com/forum/showthrea...swap-2779.html

I could do this because the trannies were otherwise identical. My car didn't come with the economical tranny, so I just swapped in the 5th gear and was ready to go.

CarloSW2

I do not remember how the sedan gear ratios compare to the CRX ones, but I do know that there are three different five-speed transmissions that were used in the 88-91 US-spec CRXes.

The transmissions from the "DX" (or "standard") model and those from the Si model had the same individual gear ratios, but the final drive (the ring and pinion ratio) was shorter in the Si, so effectively all of the gears are shorter.

The HF (I think I remember that there were two different versions with slightly different gearing) used taller gears and a taller final drive, so all of the gearing was much taller.

Here is a web page that lists out the different ratios. I believe you divide the gear ratio by the final drive to get the overall effective gear.

-soD

So should I look for a bigger or smaller final drive ratio?

Go for the 5, the STD transmission stinks :p

Smaller ratio (in terms of number) is best.

You want a taller FD ratio if you want to slow the engine down for the same vehicle speed.

The HF transmissions for the D-series are all the same gearing, through all 4 years. The 84-87 HF Carb'd transmissions won't fit, and have different ratios.

The available gear ratios for USDM Civic/CRX are as follows, from shortest overall to longest:

Si
EX (90-91 only)
DX/LX (Not the same exactly, but so close that they're interchangeable.)
HF

I'm not sure where the Std falls in there, but IIRC, the Std transmission is just a 5 speed with one gear removed. Honda used the 4 spd on base models because no other car at the time had a base model 5 spd (comparable vehicle), so there was no demand for it. They saved a buck, but the transaxle was designed around the then-available 5 speed gear box. I'm not sure what's necessary to change it to a 5 speed, but I don't believe any of the gears will mesh up.

There are several hybrid options to make longer/shorter gear ratios for your transaxle, if you have two or more available.

For instance, if you intend to run a turbo, HF transmissions keep the engine loaded as long as possible in each gear to spool the turbo and utilize the torque available, but they're extremely long in the gears... a good swap is to use the HF's 5th gear in the Si transmission, so you get faster acceleration when N/A, and you still get a longer 5th gear for cruising. You can also swap final drive assemblies between transaxles, making what would essentially be custom ratios.

If you're going all out on modding, it's a great idea to figure out exactly what you want, and where your "sweet spot" is, and build your own transaxle using available parts to suit your needs.

If you're just looking for a tranny swap, use the HF tranny if you can find one. (They're usually able to be gotten fairly cheap, as they lack in performance if you're not boosted.)

Other options include swapping tranny from 92-00 Civics, but this also requires that you either swap to hydraulic clutch assembly, or use an adapter to keep your cable clutch. You can also sometimes swap guts from one case into another, allowing you to use your cable trans with the hydro gears... Not sure if you can do this with the D-series trans though. I know the B-series can do it.e

Thanks for the info. I'm simply choosing between what is already in my garage - which is an 89 STD 4-speed or an 89 CRX DX 5-speed. Looks like I'll go with the 5-speed.

No problem. You should look into what it will take to make the 4 a 5, and possibly rebuild it with an HF final drive and 5th gear swap. It should definitely make for an interesting project, if nothing else, but should also net you much lower RPMs on the freeway and around town alike, which, depending on your driving style, should net you better mippigs.

ke5aux -

Ibid, this always mixes me up.

CarloSW2

Check the link I provided. In all cases, the Si transmission has the more performance-oriented ratios, while the HF has the economy-minded ones.

I have read that the gearsets from the manual-transmission D-series cars will swap freely and easily around. The 5th gear from a later D-series is almost but not quite as tall as that from the HF, so it makes for a setup that is easier to find. The HF ones are more scarce than they used to be...

"Mista Bone" on several Honda BBSes (like The CRX Community BBS) is a D-series transmission guru; he has built more of them than most of us have driven cars.

-soD