VX final drive swap: Who to believe?

[Cleft_Asunder]

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.

[Cleft_Asunder]

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

[MetroMPG]

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.)

[brucepick]

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.

[Cleft_Asunder]

Originally Posted by brucepick

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)."

[PaleMelanesian]

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.

[wagonman76]

Originally Posted by Big Dave

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.

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.

[some_other_dave]

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

[thebrad]

Originally Posted by PaleMelanesian

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.

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.

[PaleMelanesian]

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