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.