VX final drive swap: Who to believe?
 

I have a VX differential assembly (3.25 FD) that I want to add to my D16Y7 transmission which has a 3.7 FD. The reason I'm swapping is due to a nearly broken differential, and I want to increase MPG. The problem is that I'm getting mixed opinions on weather a VX FD will actually increase mileage due to engine efficiency at low RPM. Maybe I will receive no gain, or a decrease in MPG at "x" highway speed.

Well, here's the thread where we're discussing it. Most everyone agrees that high RPM seems to produce better MPG.

Cool B-series differential swap vid. - Page 2 - D-series.org

My guess as to what's going on is, the other board is mainly a performance / tuner environment. They'll be all about every possible way to get more air/fuel mixture to go through the engine. Including gearing towards higher rpm's, not lower. Intuitively form them, lower rpms is just plain counterproductive and wrong. But I'm sure it will help you get better mpg.

I'm driving a '97 Civic HX. As the general successor to the VX, I expect it has a similar gear ratio setup. I know from the owner's manual that it's geared higher (i.e. lower rpm's) than the DX, HX, and the rest of the Civics of the same year.

The '97 Civic gets out of its own way just fine, thank you. And turns something like 2300 rpm at 60 mph. If I really do need/want to accelerate quickly from 50 or 60 mph I can always downshift into 4th or 3rd - but so far I've only needed to do that for fun, not for transportation.

The CX had the same gearing as the VX IIRC, so maybe look at that for your mileage increase?

who to believe
 

Without a BSFC map for your engine,it'd a shot in the dark as to the effect the gearing might affect.Your engine has a "sweet-spot" and Honda should have selected a gear set which took advantage of the particular merits of your engines behavior.You might succeed in wearing your clutch out prematurely.You might gain,although around town you already should have a good selection of gears and control of your speed to optimize efficiency.--------------- Had you lowered your road load,either by reducing rolling resistance or aero drag,you probably could use taller gearing ( a 6th-gear would be best here ),otherwise you suffer acceleration,hill-climbing,and lugging perhaps.It's a very hard call.

Your HX has the same gear ratio's as my 97 DX, therefore your FD is 3.7. Honda did something different with your engine but I don't know the details right now. Check the chart at the bottom for gear ratio comparisons:

crxMPG - Gas mileage never looked so good » Uncategorized

Hey there -

I'm actually the owner of the crxMPG website, :)

Go for the swap. It'll help. You can look at metrompg's post on his transmission swap for other data. The d-series guys think they know a lot about how engines work but none of them have actually hooked up an mpguino to measure instantaneous fuel economy, so they're not fully informed.

Best of luck (PS: I'm running an HF tranny on my VX engine in a crx)

Also, the answer you quoted in the link shows that the person doesn't understand how the VX engine works, in its most basic sense. I wouldn't think too much about what he says. I used to get great mileage at high rpms in my auto DX, but that was because it was the only time I took it out on the highway. He could have fill error, not know how to calculate mpg, or something else, but I imagine it was just a bit of fluke fill during the summer and there were all sorts of factors involved, because he wasn't trying to do any sort of decent experimentation.

As for what else Honda did to the engine:
The HX has Lean Burn technology, which offers a lean fuel mix under certain favorable conditions and up to about 1/3 throttle. And, it uses variable valve timing (VTEC) but it's a different setup from what's used on the EX and Si versions. I'm not sure but I believe it kicks in at lower rpm's and is a less extreme change to valve operation than the EX / Si setup.

Hmmm. Sorry - I'm going just a bit off the original topic on this but what the heck.

Gearing:

(Edit)
My HX at 60 mph:
2350 using ScanGauge data
2400 using instrument gauges
(/Edit)

My '97 Civic owner's manual doesn't give gear ratios but it does give top speeds for each gear, up to a max of 118 mph. I'm assuming the speed stated is at the red line max, which I'm also assuming is the same for all models covered (rpm's computer-limited by intermittent fuel cut).

For 3rd gear, which is the highest gear where it offers a comparison of all models it mentions, here's the data:
DX: 92 mph
EX: 82 mph
HX: 100 mph

for 4th gear:
DX: 118 mph
EX: 113 mph
HX" "Top Speed" is what it states. Apparently more than 113 mph.

So if red line is the same for all and top speed is limited by max rpms, then the gearing for each of these models is different. Sorry, the CX and LX aren't mentioned in my manual.

Cleft Asunder: I've often seen people (usually people who don't care about fuel economy) claim that "my car gets better MPG at higher RPM", yet I've never seen any attempt to provide credible evidence.

Pretty much anyone who has spent time with fuel economy instrumentation will tell you that shifting to a higher gear whenever possible will reduce fuel consumption for a given road speed - right down to the point of lugging the engine. That's been my experience in every car I've driven, anyway.

If I were you, I'd go ahead and do the VX final drive swap.

FYI, I did a swap where I reduced my RPM by 25.6% compared to stock ( Project 'nerd gear': taller tranny transplant nets +5.2% MPG - MetroMPG.com ). That's a massive change. And it improved fuel economy.

I'm with Metro. I've never seen lower RPM cause reduced MPG. Claims that higher RPM gives more MPG are never substantiated,

I substantiate the opposite premise (that lower RPM promotes better MPG) every day.

This is all great info. I really gotta think about this. I've got an offer on a stock 3.77 FD, so it's a choice between the DX and VX differential assembly. And I feel like I'm between a rock and a hard place in terms of reaching a decision.

I'm leaning towards the VX FD, because in the back of my mind I will always be thinking, "hmmm, I wonder what MPG I'd be getting if I put in that VX FD."
And even if I break even in MPG, there is still reduced engine wear compared to a 3.77, because you are turning fewer rotations for the same speed.

So how will this effect the feal of my vehicle? Power should be the same, but the gears will feel different. What about torque?

It'll accelerate slower in all gears and run quieter in top gear. Throttle response will be muted. It'll feel more laid back.

I didn't actually measure the acceleration change in my car, because I don't really care. But the stock Metro had something like a 14 second 0-60 mph time flat out. Now it's slower.

You're not changing the engine, so its actual torque characteristics won't change.

If you really want to get a sense of what it'll be like, stick some big wheels/tires on the front end and try it out. (Ideally do the calcs and find a wheel/tire combo that's as close as you can get to your proposed F.D. change.)

I'm with you and with MetroMPG. Get the VX setup that will give you lower rpms. If you want improved mpg that's the way to go.

It's not as if you're putting in gearing that was not designed for your engine and car. That gearset WAS designed for your engine and car. Or at least, very close matches to what you're driving. Take advantage of what's available to get lower rpms and higher mpg.

Actually, my D16Y7 is a 16 valve engine, so I'm worried that might be a problem. Although my original gears are still there. Someone on the forum wrote:

"The D15B8(CX) and D15B6(HF) are 8-valve motors. The D15B7 is a 16-valve motor and makes like 30 more HP than the B8. The B6/B8 are not the same as the D15B7, or the D15B1/B2 for that matter.

The D15Z1 is a VTEC-E engine; it operates in "12 valve mode" in lower rpm and just cracks the other 4 intake valves to prevent fuel from puddling(essentially acting like a 8-valve engine except it has 2 exhaust valves per cylinder instead of 1). When VTEC is engaged, all it does is open up the rest of the intake valves.

The 8v/12v setup indefinitely helps maintain air velocity at lower rpm, which is why they're able to utilize the longer gears(and why Honda put them on those 8v/12v engines)."

I can confirm that the HX has taller gears than the DX. My DX runs 2750 rpm at 60 mph, while Bruce above recorded 2350 / 2400 at 60.

Me too. Only exception is if you gear to such a low rpm that you cannot keep going in a lot of your typical driving situations without downshifting, then youll lose mpg.

I only have one data point for you, and it's not exactly the same as your situation:

I killed the transmission on my previous 1990 CRX Si. I had it replaced with one from a wrecker's, and they sent me the DX transmission by mistake. I never realized it until I started figuring my MPG and my RPMs/speed. My FE went up between 5% and 10% (and this was back in my "bad old days", driving 10 MPH over the posted limit) because the DX final drive is "taller" than the Si one. Even though the rest of the gears are the same.

I also realized that the acceleration in gear was noticeably slower, once I paid attention to it.

-soD

Could be down to wheel/tire size.
CX/DX generally have 13" steelies, while the HX came with 14" Enkeis from the factory which weighed ~11 lb.

Here's a calculator application which some of you may find helpful.
http://www.fatboyraceworks.com/gears/

If anyone has the whole gamut of gear ratios for HF, VX, and HX trims I'd love to have that knowledge. I've glanced through the service manual for the 96-00 Civics and was unable to find it.

175/70-13 vs 185/65-14 is almost exactly the same diameter. Only the 1996 had the 13", the 97 and up had 14" steelies. Bruce, what size wheels and tires do you have? Brad, what do you have? And what is your rpm at 60 mph?

Here are a couple of sources. They have some errors, and disagree with each other in some places. That's why I posted 2, but I still take the real-world results from my DX and Bruce's HX with more authority.

Reference: Transmission Specs - ClubCivic.com - Honda Civic Forum
6G Civic Trim Differences - Honda-Tech

The first link has at least some errors in the 88-91 section. I know that the HF transmission has taller gears and a taller final drive than the DX transmission does. That list shows the same final drive as the DX.

Here's another BBS post listing gear ratios for the 88-91 CRXes. I believe that the Civics had the same ratios:

The CRX Community Forum :: View topic - D-Series Clutch and Gear Ratio information

-soD

I took her out yesterday and did 60 momentarily, Scangauge was putting out 231x when I looked down. I'll try again tonight with a more steady MPH rating to confirm. My Civic has a 97 DX tranny according to the VIN on the housing.

Yeah it's definitely between 2290-2350 at 60 mph in fifth gear.

Do the VX gears...if you are wanting to improve fuel mileage. I used to get annoyed with people grabbing the 5th gear out of junkyard crx hf cars for their SI or DX cause that was one less spare transmission for me. I'd do around 1800 rpms at 60mph. 3rd, 4th, and 5th were overdrive gears!:eek:

As for the HX vtec...from what I've gathered is that the vtec-e doesn't allow the valves to all open all the way until the v-tec is engaged then they open all the valves all the way. At lower rpms you don't need the valves to open all the way if you are looking for economy.:eek:

Also, in rebuilding my HX transmission I found a number of sites (no linky at the moment) that showed the HX having lower gearing (numerically) than the other civics. Also, the VX was lower and my crx hf was the lowest of all:thumbup:

Good news guys! I found the gear ratios in the Honda service manuals under design specifications by sheer luck.

So here they are

D16Y5 (HX)
1 - 3.250
2 - 1.782
3 - 1.172
4 - 0.909
5 - 0.702
R - 3.153
Ratio: 3.722

D16Y7 (Civic CX/DX)
See above, all are the same except for the ratio unless you own a coupes or sedans, then it's 4.058

D16Y8 (Civic EX/LX, I think)
1 - 3.250
2 - 1.909
3 - 1.250
4 - .909
5 - .702
R - 3.153
Ratio: 4.250

I'll try to get HF/VX later for y'all, let's get this thread going again now that I have access to OEM data. :p

I'm pretty sure the LX is also D16Y7, leaving the EX alone with the Y8.

The CRX HF has two options for gearing.. the final drive was longer if it's a non-cali model, something like 2.95, and 3.25 for california models.

Awhile back, I did a thing w/ a gear calculator to make the longest possible honda transmission that could actually be built.. I can't remember exactly what gears it used, but the 2.95 FD from the CRX HF wasn't an option, because it's a small bearing case, and the gears from the large bearing cases in the hydro models won't fit with it.

I'll see if I can find it the next time I fire up the old computer, I might still have the text file from it.

Also - the 88-91 has more than 3 5 speed trannys..

There are:
DX, LX, Si, HF cali, and HF 49, and EX (90-91 Only) For USDM

Then there are EDM and JDM and UKDM and AuDM options, which include the VT and VTi, GT and GTi (I think) and ZC models, and the Si-R. Each of which may have had it's own transmission as well.

The DX and LX for years have been touted as being the same, but they're not. The gears almost match up when you add in the FD, but they're all different.

92-95 Civic transmission gears
As printed in a 1992 Civic service manual

D15B8/D15Z1
1 - 3.250
2 - 1.761
3 - 1.066
4 - 0.852
5 - 0.702
R - 3.153
GR: 3.250

D15B7
1 - 3.250
2 - 1.761
3 - 1.172
4 - 0.909
5 - 0.702
R - 3.153
GR: 3.888 (hatch)/ 4.058 (sedan)

D16Z6
1 - 3.250
2 - 1.900
3 - 1.250
4 - 0.909
5 - 0.702
R - 3.153
GR: 4.250

1988-1991 CRX Gear ratios
As posted in a 1991 service manual

HF
Primary reduction - 1.000
1 - 3.250
2 - 1.650
3 - 1.033
4 - 0.823
5 - 0.694
R - 3.153
Final reduction - 2.954 (49 st) / 3.250 (CA)

Std and Si
Primary reduction - 1.000
1 - 3.250
2 - 1.894
3 - 1.259
4 - 0.937
5 - 0.771
R - 3.153
Final reduction - 3.888 (Std) / 4.250 (Si)

2000 - 2006 Insight Gear ratios
As posted in a 2000 service manual

1 - 3.461
2 - 1.750
3 - 1.096
4 - 0.857
5 - 0.710
R - 3.230
Final reduction - 3.208

Anyone interested in automatic ratios? I'm assuming those gears can be used as well, they are all listed as single helical gears.

The Std and Si setups is wrong... STD is a 4spd tranny.