3.70 vs 3.90 R200 open diff

[logan1]

Hi guys,

I am currently in the process of swaping a BW T5 and a R200 into my 240z. Currently my car is running the stock L24 motor with a racing cam but i would like to replace it soon with a L28et with approx 300 whp. The car is going to be a daily driver for the most.

What is the real world performance difference between the 3.70 and the 3.90 diff's and if it enough to warrant holding out for a 3.90 rather than going with the cheaper and easily accessible 3.70 diff? Also how much do most 3.90 R200 cost?

[ezzzzzzz]

There is about a 500 rpm difference. It doesn't sound like much but you'll notice it immediately if you can compare it side by side. You'll be shifting quicker too with the 3.90. Now, that 2.4 liter likes upper rpms because that's where it's power lies. The turbo engine would do well with the 3.90 because it allows faster spooling. Now, let's talk about that T5. First gear is 3.50 and you'll find yourself going for second gear really really fast with either r&p but especially the 3.90. If I were going to stick with the stock T5 I'd go with the 3.70. The 83 280ZX turbo used a 3.54 r&p with the T5 while the N/A used a 3.90 r&p and 3.06 first gear (12.39:1 vs 11.93:1). I'm building a WC T5 with a 2.95 first gear and .8 fifth for my SC'd stroked L28 (LD block and crank/ P90A head) 240Z project using the Nissan T5 bellhousing, top cover and tailhousing. It is fitted with a R200 Quaife LSD 3.90 (11.5:1). Just food for thought....

Edited December 16, 2010 by ezzzzzzz

[tlorber]

ezzzzzz, are you sure about that 500 rpm delta? It seems that the difference between the two is 3.9/3.7 or 5.4%. Assuming one can use straight linear math on this, you'd have to be at 9,260 rpm for that percentage difference to be equal rpm.

I just put a 3.7 R180 in (I don't ever plan on getting this motor above say 170 hp) and I really like it with the '82 5 speed. I'm at about 2,800 rpm at 80 mph. When I had calc'd it out I figured that with a 3.9 I'd be at 2,950 rpm at the same speed.

[ezzzzzzz]

I didn't breakout my scientific calculator and I was suffering from insomnia last night while replying to Logan1's post =). When I replaced my 4.10 with a 3.90 I got noticable rpm drop at even given speed. Assuming that the .2 difference there also equates to the .2 difference of 3.70 to 3.90 I assessed the drop to be comparable. Using a gear ratio calculator you'd see about 2 mph differences between the two r&p's across the board. While my rpm may be off there is a noticeable change between those r&p choices. It becomes more important when you determine what engine characteristics you're placing out front. That is why I'm building that close ratio WC T5 to compliment the 3.90 R200 behind a SC'd L28 (lots of lowend torque). My response was not so much for accuracy ( I can see how it could easily be equated to that though) but to impart a simple lesson in matching driveline parts to suit the desired end result.

[tlorber]

This is a pretty cool calculator if anyone is interested. I probably got it off this site somewhere. I'm sorry to say that I too was suffering from insomnia last night. For no particular reason that I'm aware of.

http://webspace.webring.com/people/cz/z_design_studio/

[Z train]

Todd not to "one-up" you but......http://www.wallaceracing.com/Calculators.htm

I take it you like the gears?

Edited December 16, 2010 by Z train

[tlorber]

Holy crap! A calculator for every occasion.

I do indeed like the tranny/diff combo. Totally different car. This is how they should have come off the [assembly] line. In fact, I'm not sure why they didn't. My '69 311 has a 5 speed w/o drive and a 3.7 rear end, so its not like they didn't have the parts laying around.

[Z train]

Cool.Glad they worked out.I just picked up a set of 3.88s to replace the 4.37s in the truck.SHould be about perfect.Don't forget they had mileage & emissions & warrantee crap to worry about.

If your OD is .864,then here is your RPM scale:

Engine Speed (RPM) Speed (MPH)

8000 RPM 186.12 MPH

7500 RPM 174.49 MPH

7000 RPM 162.86 MPH

6500 RPM 151.23 MPH

6000 RPM 139.59 MPH

5500 RPM 127.96 MPH

5000 RPM 116.33 MPH

4500 RPM 104.69 MPH

4000 RPM 93.06 MPH

3500 RPM 81.43 MPH

3000 RPM 69.80 MPH

2500 RPM 58.16 MPH

2000 RPM 46.53 MPH

1500 RPM 34.90 MPH

1000 RPM 23.27 MPH

And back to topic,this is the scale with the 3.90s:

Engine Speed (RPM) Speed (MPH)

8000 RPM 176.58 MPH

7500 RPM 165.54 MPH

7000 RPM 154.51 MPH

6500 RPM 143.47 MPH

6000 RPM 132.43 MPH

5500 RPM 121.40 MPH

5000 RPM 110.36 MPH

4500 RPM 99.33 MPH

4000 RPM 88.29 MPH

3500 RPM 77.25 MPH

3000 RPM 66.22 MPH

2500 RPM 55.18 MPH

2000 RPM 44.14 MPH

1500 RPM 33.11 MPH

1000 RPM 22.07 MPH

Edited December 16, 2010 by Z train

[jmortensen]

Not a turbo guy, but I understand that to build boost faster you want a taller gear ratio. More load = more boost. I think that's why the stock ZXT came with 3.54s and the NA ZX came with 3.90s.

[ezzzzzzz]

You may be right, Jon. I always heard that turbo's needed rpms to get working where SC's liked tall gears since they build boost offline. As for the 3.54 vs the 3.90 you need to look at the transmissions. The T5 has a 3.50 first gear where the FS5 has a 3.062 first gear. 12.39 vs 11.93 combining first gear and the applicable diff ratio. The ZXT still has lower gearing offline (abeit only a wee bit).

[Unkle]

^And there's a difference between gearing for gas and diesel engines with theories for each . My Crown turbo kit runs out of steam at around 6000 rpm for the Z , where the Cummins bags out at 4-6000 depending on load . Size of housing matters too for faster spool up . Exaust diameter , air filter, etc etc.

Edited December 17, 2010 by Unkle