biglake1

I'm preparing to tear into my 89 CRX Si restoration and was thinking of putting in a DX 5th gear to drop the top-end RPMs. The hope is that this will improve gas mileage as well as decrease the 3600rpm scream at highway speeds (75 or so). Looking at tables it appears I will get an 800 rpm drop. Has anyone tried this? Will the DX 5th fit in the Si trans housing? Are the mainshafts the same?

Thanks in advance for the help. Dave.

rustbucketrex

The DX and Si gears 1-5 are the same, it's the final drive(on the diff) that is different. The HF 5th is different and will lower your RPMs though.

nikitad

I second that.
And yes, either DX or HF 5th gears would fit (DX being the same as SI).

crxguy406

If you don't want to change out your final drive, (which would mean changing your main shaft, countershaft and everything else), your best bet would be to swap out your 5th with an HF 5th.

Assuming you are running stock 175/70 R13 tires, your RPM at 75MPH will drop from 3650 to 3300

(I'm not sure which charts you were looking at, but it's nowhere near an 800 rpm drop unless you swap in a full HF tranny.)

biglake1

Thanks all, you are correct. I was going by memory and a feeble one at that. I wonder if there are other transmissions out there which would fit and have more favorable gearing/final drive overall? The transmission in my car is likely tired anyway and was hammered by the PO so this would be ideal if something else would fit. Thanks, Dave.

**in thinking about this perhaps there's a later swap (engine/trans) which might be better suited for my MPG increase goal?

crxguy406

If you wanted to keep your stock si engine, you could always pick up a DX tranny (direct-fit and are dime a dozen) and swap an HF 5th into that.

Gears 1-4 would still be friendly to your engine for city efficiency, and at 75 you'd be cruising at 3100 rpm which isn't bad at all. If you throw on some slightly oversize tires you might even knock it down to 3000.

I wouldn't go any lower than that though or your mileage would drop like a rock.

GeoMetry

I love this idea and I have a source for the HF transmission and a spare Si transmission that needs bearings anyway. Can anyone here give me any additional guidance? What am I getting into here? What exactly do I need from the HF, is it literally just a single gear?

Red_ED8

iTrader: (1)

You need 2 gears, 5th from the main shaft and 5th from the counter shaft. Super easy if you find a hf tranny to steal it from, the 5th is the top 2 gears so don't even have to pull the whole gear set out of the case.

GeoMetry

You make it sound so easy... I have the Genuine Honda Service manual. I opened up the case, that was pretty straight forward following the instructions in the manual but that is as far as I got. I guess I need to search for a good transmission rebuild thread.

Dave K.

I found an HF transmission so will be looking at the gearing swap when I get around to that part of the restoration. Hope to have it back on the road this fall and when running my Civic Si down at 3100 I have to admit the change in RPM does make a difference in terms of noise. Will be an interesting test......

Thanks for the input guys. If I have the chance I will take pics of the mod and post it here....

89ED

i have a 1990 lx ed3 and i do 75mph @ 3500 exactly on 190 60 r14's

what D series tranny has the smoothest shifts?

biglake1

Question to those that have done this. How will this affect things at highway speeds and the powerband? I was discussing this with a local Honda shop which is very knowledgable about the EF having raced them and done a lot of mods over the years. The only thing they felt would happen is that the car would be sluggish if you stomped on it while cruising at 3100rpm or below in 5th with the HF gear. This makes sense since the gearing is optimized to the powerband of the D16A6 but I wanted to field it and see what everyone thought as there's a lot of knowledge here....

crxguy406

This is true, but unless you have a pretty ballsy car, most people downshift anyway when they want to pass.

You could also consider swapping out 4th gear from the HF as well. It would give you a less aggressive downshift, while at the same time giving you acceptable RPMs in town speeds. (1900rpm @ 35mph)