Honda DPFI vs MPFI...

[GasSavers_bobski]

Originally Posted by aalb1

my suggestion is swap the SI 5th gear to an hf 5th gear. You'll keep your first through fourth for acceleration purposes but have lower rpm's for cruising... best of both worlds!

Yeah, a little. The HF does (as do most '92-01 Civics) have a longer 5th gear ratio (0.694 HF vs. 0.771 DX/Si), but it's super long gearing is mostly a result of its final drive (4.25 Si, 3.888 DX, 2.95 HF fed, 3.25 HF CA/VX). A 5th gear from an HF or newer Civic would reduce your cruising RPMs by about 10%. A DX final drive would reduce the RPMs across the board by about 9%.

[aalb1]

Originally Posted by bobski

Yeah, a little. The HF does (as do most '92-01 Civics) have a longer 5th gear ratio (0.694 HF vs. 0.771 DX/Si), but it's super long gearing is mostly a result of its final drive (4.25 Si, 3.888 DX, 2.95 HF fed, 3.25 HF CA/VX). A 5th gear from an HF or newer Civic would reduce your cruising RPMs by about 10%. A DX final drive would reduce the RPMs across the board by about 9%.

This is exactly how it will translate so you can decide for yourself:

Straight SI Tranny in 5th Gear:
1500rpm - 30.86 MPH
1750rpm - 36 MPH
2000rpm - 41.14 MPH
2250rpm - 46.29 MPH
2500rpm - 51.43 MPH
2750rpm - 56.57 MPH
3000rpm - 61.72 MPH
3250rpm - 66.86 MPH
3500rpm - 72 MPH
3750rpm - 77.15 MPH
4000rpm - 82.29 MPH

SI Tranny W/HF 5th
1500rpm - 34.28 MPH
1750rpm - 40 MPH
2000rpm - 45.71 MPH
2250rpm - 51.42 MPH
2500rpm - 57.14 MPH
2750rpm - 62.85 MPH
3000rpm - 68.56 MPH
3250rpm - 74.28 MPH
3500rpm - 79.99 MPH
3750rpm - 85.71 MPH
4000rpm - 91.42 MPH

[dkjones96]

Wow, I'm getting tooooo used to my new gearing cuz those numbers look high. 0.69:1 and 3.55 rear end gives me 60mph at 1750 rpm. If the TC is locked I'm at around 1175 rpm at 40.

[aalb1]

yea displacement is a big factor... when I had a 2000 Civic SI my dad was simply amazed how that 1.6L would cruise @ 80mph @ 4500rpm and still get 30mpg

[GasSavers_bobski]

Originally Posted by dkjones96

0.69:1 and 3.55 rear end gives me 60mph at 1750 rpm. If the TC is locked I'm at around 1175 rpm at 40.

I assume you're talking about your Durango? Don't forget the last element between the drive shaft and road... Wheel and tire size can play a big role in gearing. '88-91 Civics and CRXs have an overall wheel/tire diameter of about 23", meaning around 875 revolutions per mile.

[dkjones96]

Yeah, the Durango tires are 31.5"

Now I've got a question about that. If I had the 8.25" ring gear instead of the 9.5" that I have and still had the same tires and 3.55s would it change that final ratio? Like, right now that's a 3.32:1 ratio, would the 8.25 make it a 3.82:1?

I'm pretty sure it does. Just verifying.

[GasSavers_bobski]

The ratio depends on the number of teeth on meshing gears, not their physical dimensions. So, you're going to have to tell me how many teeth are on each gear, assuming the pitch of the teeth, physical location and such allow them to mesh properly.

[theholycow]

Wouldn't the tooth size have to be the same, therefore they would increase proportionally with the larger dimension of gear?

[dkjones96]

That's what I was thinking. 8.25:31.5 vs 9.5:31.5.

The 3.55 ring and pinion ratio stays the same.

[theholycow]

That's the opposite of what I was saying...the ring mates to the pinion, and if you change the size of the ring it will have more teeth, so it changes the ring-pinion ratio.

The end result is the same as comparing the ring to the tire as long as the pinion is staying the same.