How does upgrading to a 3.9 feel

[mdbrandy]

OK, I've read the tech article:

http://www.classiczcars.com/forums/showthread.php?s=&threadid=1957

and I've messed around with the transmission calculator at:

http://www.geocities.com/z_design_studio/

and I've put my own spreadsheet together comparing my '78 5 speed with the stock R200 3.545 versus the R200 3.90 that I have recently come into (see the attached pic for a table of my calcs). Essentially, at 70 MPH, I'm running less than 300 RPM higher (3211 vs 2919 RPM), and in that range, I'm losing around 7 MPH in 5th gear. Lower speeds have even less difference.

So, I know all the data. What I don't know, is how it feels to make this change? I do most of my driving around town, and/or around the local rural highways. I don't do more than 70 (or so) as a max speed anywhere anymore, and I'm really looking to pep up the acceleration a bit on the low end if this is a cheap way to do it. If I did this, I gather I would essentially have to shift more often (at lower speed), and the idea is that the engine would be kept closer to it's power band more of the time, and thus, as long as I shifted correctly, it would be able to, say, do zero to 60 a bit faster. So, for racers, or anyone who has done this specific '78 5 speed change to a 3.9 rear end, how did it feel? Were you happy with the result?

Thanks.

[mdbrandy]

Bueller? Bueller?

[2ManyZs]

I can't tell you what it feels like to go from a 3:54 to a 3:90, but I can tell you there's a very noticeable difference going from a 3:54 to a 4:11. At least there was in my ITS 280, with a basically stock engine.

Much better drive out of the corners, and it will put you back in the seat in the first 3 gears, especially in 1st and 2nd. Enough of a difference that you'd really appreciate an LSD especially on wet roads.

FWIW, you might find a 4:11 somewhat easier to find than a 3:90, as it seems to be the more popular ratio. There's such a small difference between the 3:9 and 4:1 that most people go ahead and opt for the 4:11.

[mdbrandy]

Originally posted by 2ManyZs

FWIW, you might find a 4:11 somewhat easier to find than a 3:90, as it seems to be the more popular ratio. There's such a small difference between the 3:9 and 4:1 that most people go ahead and opt for the 4:11.

Interesting. I've actually seen several 3.9's recently, bought one for $75, and I can't remember seeing a 4.11 (at least on e-bay). Other than e-bay and this site, I have very limited sources for used Z parts (there are always the dealers like z-barn and the like). Since the 3.9 is about 2/3 of the way between the 3.545 and the 4.11, I expect that I'd get a "pretty noticiable difference" versus your characterization of "very noticable". Know of any 4.11's around that would be easy to get and not terribly expensive? One reason I'm even considering doing it is that this summer I will be removing and refurbing much of the 280's suspension and drivetrain, so now is an easy time.

Oh, and thanks for the info! :classic:

[Bleach]

Yes, there will be a good noticeable difference.

In any given gear, at a certain speed with your 3.9 you'll not only be a few rpm higher but the gear ratio will be different between the back of the transmission and the ground. So its not just the added 300rpm that gives it more power but its actually changing the torque to the wheels. I've had a 280zx with the stock 3.9 and gone to a 4.11 and there was a noticeable difference between those two ratios!

Go for the 3.9 you have. You would be very lucky to find a 4.11 cheap. $75 is nothing. The cheapest I've EVER seen an R200 4.11 is $150. Most go for $200-300.

[2ManyZs]

Mark, the main reason I say that the smaller spread in ratio from the 3:54 to the 3:90 wouldn't be as noticeable is because you have to factor in the extra weight a 280 is carrying. In a lighter 240 the difference would be much more noticeable with a change of .36 in your diff than it will in a 280. Now, if you also change to a lower profile tire when you do the 3:90 swap, it will negate a lot of what I just said as the lower profile tires will help your overall ratio change.

Going from a 70 series to a 60/55/50 series tire when you swap the diffs and you'll notice a huge improvement, especially if you go to a 50 series with an overall smaller diameter.

The main reason I said a 4:11 might be better is for the weight difference of the 280 with all else being equal. That extra 500+ pounds makes a big difference.

I raced against 240's with 3:90's when I had the 4:11 in the 280(motors were pretty close to the same output), and with the same sized tires they could still out accelerate me out of the corners because of the weight difference. Now, if I had had a 4:37 in the 280, I might have been pretty close.

[mdbrandy]

Thanks for all the impressions and knowledge transfer - that's what I was looking for :classic: . Bottom line is that if I ever get the 280 out on the track, it will be fun, but I have never done anything like that before, and it isn't in the short term plans right now. Mostly, I'm just looking for a little more "kick" in my daily ride. Now, this summer, I am doing the poly bushing/tokico illumina + spring upgrade, plus new tie rods and ball joints, so if I get some track time, I'll be ready ( or at least the car will!) .

I am currently running 205/60/15's on the car. The transmission calculator indicated that with those, my speedo should be close to dead-on compared to stock tires, so I was assuming that the actual wheel/tire diameters are about the same as stock. 2manyZ's - is your guidance related to lower profile 14" tires, which would have a smaller overall diameter?

And I wish my daily driver was a 240, but it's not, and I'm happy with the 280...for now. I'll keep working on #215, but I will probably never use it as a daily driver if I get it back to where I want it to be.

[2ManyZs]

It all depends on what aspect ratio you are using when comparing overall diameters of 14 inch and 15 inch.

I was speaking in general terms relating to stock 14 inch tires and wheels, but the same principle applies to all the wheel diameters as well. It would be pretty easy to go to a 15 inch wheel and end up with a shorter overall tire, depending on the aspect ratio of the tire and tire size availability.

[ROOPZ]

I recently upgraded (?) my rear diff to a 3.9 and I have to say I really did NOT see any difference. Maybe the smallest amount but not enough to go WOW! If you are looking for that you will probably be happier with the 4:11. I also have the 4 speed so that could make a difference but I wouldnt think so. Atleast not in the seat of the pants feeling. Im just happy to have a rear end that doesnt whine and make noise! (Dont say it!)

[mdbrandy]

Originally posted by ROOPZ

I recently upgraded (?) my rear diff to a 3.9 and I have to say I really did NOT see any difference. Maybe the smallest amount but not enough to go WOW! If you are looking for that you will probably be happier with the 4:11. I also have the 4 speed so that could make a difference but I wouldnt think so. Atleast not in the seat of the pants feeling. Im just happy to have a rear end that doesnt whine and make noise! (Dont say it!)

Interesting. From your gallery pics it looks like you have a 77 or 78 280Z. How about gas mileage? Did you see a change?

[ROOPZ]

Yea, I have a 78! I honestly havent checked the gas mileage. It definitely gets better than my Toyota 4x4! The last time I did check it was at 25 mpg. Sorry I am not much help there. I will check it the next time! So what do you think you are going to do?

[mdbrandy]

Originally posted by ROOPZ

Yea, I have a 78! I honestly havent checked the gas mileage. It definitely gets better than my Toyota 4x4! The last time I did check it was at 25 mpg. Sorry I am not much help there. I will check it the next time! So what do you think you are going to do?

Since I already have the 3.9 coming, and I am going to drop the diff this summer while I am replacing bushings and the diff and trans mounts, then I will probably install it. I can't really afford to go out and find a 4.11 for what BleachZee indicates would probably be at least a couple hundred $$, especially since I already have the 3.9 on the way. It's purely a matter of fun anyway, since I don't race, and unless I find more time and money probably won't. The 3.9 has to do something for the car, and the extra 0.21 on top of the 0.36 would only be incremental. If I was trying to race and be competative, I'd definately be taking this thread's advice and go for the 4.11.

And I only get 17 - 18 mpg in the city now, so 25 would be amazing. The original EPA rating on the '78 280Z was 18 mpg, though, so I dont' think I'm doing too bad.