3.9 and 4.11 with '77 5-speed

[texasz]

Ok, I've been through the calculator at http://www.geocities.com/z_design_studio/ and I have also read up on the charts on http://www.geocities.com/MotorCity/Boulevard/6117/datsuntransmittionratios.html

Now what I'm trying to determine is what is the noticable difference between an R180 3.9 and an R180 4.11 in a 240Z with an F54 block, N42 head, and a '77 5-speed? Will I notice any difference between the two diffs? I will be autoxing the car and also an occasional weekend/weekday drive for fun.

Please do not say that these ratios are not available in the R180, they are but they are just hard to find.

[2ManyZs]

Here's another good chart but it doesn't list the 4:11

http://www.geocities.com/zgarage2001/ratios.gif

For a street car with the 77 5 speed, I'd be tempted to go with the 3.90 for the simple reason your overall gear ration will be nearly the same as what it was stock. In other words, you will turn basically the same RPM's as you would with the stock 4 speed and the 3:36 rear on the highway.

If you are going to use it for mostly auto-X and don't care about a couple hundred more RPM's while cruising, go with the 4:11.

It's all up to which gear ratio you can find... neither is a bad choice with the 77 5 speed. I just saw a brand new 3:90 ring and pinion on Ebay last week going for 450, or about half price of what it sold for new......wish I'd had the money or I would have bought it myself.

[texasz]

2many, I saw that one last week also but that was out of my price range. FWIW the guy got no bids on it so I'm sure that he still wants to sell it and he may be negotiable on the price now.

[ZmeFly]

for an overall great driving car and autoxer i would go with the 3.90. you cant beat it with the 77 5 speed, its a great combination.

as like 2many said with the 4.11 you will have a few more on the tach with the 4.11 but the accelleration will be outstanding.

if it were me id have to go with the 3.90 just for the fact of it being an all around great gear ratio with the transmission you picked

[2ManyZs]

Well, 450 is a bit steep, but.... it's brand new and compared to spending a couple hundred on one of unknown condition used, I'd go with new if possible. Another 100 or so for bearings and seals, and you'd have a brand new rebuilt rear good to go for as long as you own the car...

Most of the R-180 3:90's you will find, have either been used in a race car which means you'd be rebuilding it anyways or at least changing out the center section if the spider gears are welded, or would be from a well used street car.....

I know where you could probably find an R-180 with the 3:90's, but the rear is welded and you don't want that on the street, and the price is nearly the same as the new ring and pinion for the complete (but used) diff. Which means you'd end up pulling it apart to swap out the spider gears and then you'd want to change the bearings... yadda, yadda, yadda... see what I mean?

Only other alternative is to find one in a junkyard from the front of the 720 4x4 truck, which might be in somewhat better condition as most probably weren't used that often in four wheel drive in your neck of the woods.

[texasz]

The dilema is that I could get a used R180 4.11 now for about $100 including shipping. Or I could wait and look for a 3.9 and the junkyard 720 4x4 is the route I was going to go. We all know what finding parts in a junkyard can be like :stupid: so I'm just trying to decide and needed some advice/info/help.

[N5572B]

Hey gang,

Am I understanding this correctly? The front diff out of a 720 4x4 will fit the Z housing (R180) and comes in 3.9 or 4.11? How do I tell the ratio? Just looking at what my options are for a lower ratio for when I get mine on the road and 5 speed found and installed.

TIA.

Dave

71 240, l28 project.

[1975yellowBSPZ]

2many, sooo got a lead on a welded 180 huh. I was wondering if it would be an option to run a welded rear in my auto-x car untill I could afford an lsd, but I think it might be more trouble then its worth. I just have a 180 running 4.11's and a 4speed currently. Non-adjustable sway bars front and rear, sys dynamics springs yada yada. thoughts from the zpeanut gallery?

bw

[texasz]

N5572B, see this thread where 2many has kindly provided us with a listing of what diff came in what vehicle.

http://www.classiczcars.com/forums/showthread.php?s=&threadid=1568&perpage=15&pagenumber=2

[2ManyZs]

I don't know that I would use a welded rear in an auto-X car, unless you were to use adjustable sway bars. The welded rear will tend to push the front end of the car (the heavier you are on the throttle, the harder it will push) and without adjustable sways bars to bring the handling back more towards neutral you will end up hurting your times while trying to wrestle with the understeer. Slow speed with a welded rear is a real work out just trying to turn the wheel.... it will make the steering even heavier than normal....

Since most auto-X's are fairly low speed, at least compared to road racing, I'd stick with an open rear until you can get some type of LSD. I ran a welded rear in my road race 280 for 3 years and never got the handling right.....

I think one of these Phantom Grips would be better than a welded rear for an auto-X car and I haven't heard too many good reports on the Phantom Grips....even a factory LSD is better than the Phantom Grips, of course, none of them would be as good as a Quaiffe...but even a Quaiffe would have it's drawbacks on an auto-X course where you go from off throttle to full throttle a lot.

[1975yellowBSPZ]

Thats kind of what I thought. It'd be a while before I can get the quaife and that seems like the way to go... I have more important things to worry about right now anyway - like the fact my engine has less then 40 psi on half the cyl :-( anyone with a solid running l28 in texas that can help me up - don't be shy :-)

BW

[hmsports]

I purchased my 4:11 from a local salvage yard a few years back for $150.00 -- from a 4x4. It is an exact bolt-in swap -- simply remove the rear cover and turn it 180 because on the 4x4 the diff is installed upside down compared to the 240. You also need to swap the flanges for the half shalfs. That's it -- the housing is the same.