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First Post: Help on STI r180 ratio


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Hi! This is my first post on hybridz.


I'm 21 and have some experience as a home mechanic and have been tinkering with my 240z that I've had since high school.  The car is a 72' 240z with amazingly little rust.  However, the car has never ran reliably and had a rebuilt 'mystery' engine with e31 block and e88 head which I don't believe would be correct for the model year.  Long story short, Eiji at Datsun Spirit is building me a 3.0 liter l28 that should make around 300hp at the crank.  I've decided the best route would be the KA 5 speed swap and an sti r180 diff as this should be adequate for my power level and driving style and require the least amount of modification.  I'd like to keep this car street-able as it is my cruiser and I don't plan on racing.  I'd like some help from some more knowledgeable people on recommendations for the sti r180 diff.  I know that there are 3.9's available although the 3.54 is much more readily available on eBay.  I liked the feel of the stock 4 speed drive ratio so I wanted some recommendations on which ratio would pair best with the ka 5 speed




ps. I included some pics of the car and DatsunSpirit engine for anyone interested





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Using a calculator should be able to solve your problem pretty easily.




Using a 25" tire height, with the FS5W71C tranmission (91 240sx transmission), in second gear at 7000 rpms, with a 3.9 rear end, you'd be doing 70 mph.


In 5th gear, at 2600 rpms, you'd be doing 65.


It really depends on what you want to do, and the speeds you think you'll be traveling. 

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In the end there are usually two basic considerations - what RPM would you like to see on the highway, and how quickly would you like to accelerate.  The first is 5th and the second is 1st.


The transmission and diff ratios are shown in the various service manuals and can be found on the forum.  Around 12 is the typical 1st gear overall ratio.  4th is always 1:1, and 5th varies depending on the transmission.


280Z's - 3.321 x 3.54 = 11.76

ZX's - 3.062 x 3.9 = 11.94


Get the KA's first gear ratio to start.  You'll have a lot of torque in a light car so you might not really need a super low first gear.



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I have the 3.9 in mine with Datsunrestomods axles. I have driven it with both the stock 4 speed and the FS5W71C from Godzilla Raceworks. My build is not kicking a ton of HP since it is 2.818L and the CR is only around 9.5:1.  There is a noticeable difference between first and second gear  between the two. The 4 speed has a much better snap to it and is a lot more fun to drive. The FS5W71C is way better on the highway. I have a 3.2L that I'm slowly getting together and feel that the extra HP and torque will more than makeup for the lack of gearing. Ultimately after doing a ton of calculating with ratio calculators the CD009 with the 3.9 would be the best choice for my style of driving. That is a hill that is too high a climb for me at the moment. My gut feeling is if most of your driving is around town I would go with the 3.9.


Beware that used Subaru r180's can be a crap shoot. I bought mine from a reputable seller on ebay that supposedly had 36K miles on it and it howls at 50mph. And unfortunately 50 is my in town cruise speed so it's pretty crappy. They are getting really popular and the prices are no longer what I would consider cheap. If I had to do it again I'm not sure that would be the direction I would go. I paid someone who has done quite a few Datsun R180 diffs to try and get the wipe pattern dialed in and the Suretrac carrier gave him a ton of trouble trying to get it out of the diff housing.  The long and the short of it is I'm at $1200.00 for a diff that is noisy so make sure you choose wisely.  When they were $350.00 it was worth taking a chance. At $700-900 not so much. 

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