Tranny/Diff Ratio and RPM @ Speed

[Dave WM]

I would think with a peakier engine (if that is what happens) then it would be a good idea to make sure you can use all 5 gears for 90% of driving, so a 3.9 or even numerically higher diff would be a good idea. A stock engine with a heavy flywheel I think is fine with a 4 speed since the torque band is so wide, you don't need to keep the engine in a narrow rpm range.

[madkaw]

I’ve have the 3.90 . I had it with my 2.4 and my 3.2 , it just works well. I originally had the  T-5 which is a bit of a wide ratio, but it made for a great passing gear in third . If your engine made power all the way to 7k , then you had a hell of a passing gear . 
Anyone hear third ?

Im not much of a high speed guy, rather just be quick . Like said above , what meets your driving criteria . Don’t do a lot of highway cruising and wouldnt be afraid do hum  along at 3500 rpms either . 

[Terrapin Z]

When I bought '70 my car the PO had a B 4sp and 4:11. He auto-crossed it a little, and liked the quick off the line zip. Horrible for the highway though. I found a 3:54 from the automatic cars and that worked pretty well It just screamed a little still on the 70-80 mph highway runs. 

I then found an 83ZX 5 speed and used a calculator like you have here. I ended up with a 3:70 R180 open diff from a Maxima. That worked out really well for my driving style. 

[texasz]

16 hours ago, madkaw said:

I’ve have the 3.90 . I had it with my 2.4 and my 3.2 , it just works well. I originally had the  T-5 which is a bit of a wide ratio, but it made for a great passing gear in third . If your engine made power all the way to 7k , then you had a hell of a passing gear . 
Anyone hear third ?

Im not much of a high speed guy, rather just be quick . Like said above , what meets your driving criteria . Don’t do a lot of highway cruising and wouldnt be afraid do hum  along at 3500 rpms either .

Nice sounding Z...and I heard you get a 3rd gear scratch out of it.  🙂

Edited March 25, 2021 by texasz

[madkaw]

On 3/25/2021 at 12:55 PM, texasz said:

Nice sounding Z...and I heard you get a 3rd gear scratch out of it.  🙂

That’s a 2.4 with 3.90’s . Makes a huge difference in performance . I think biasing to a shorter gear is your best bet for overall fun , unless it’s just going to be a highway queen 

[Derek]

Not trying to plug my product but I'm working on a conversion bell housing for the Aisin TL70. The TL70 is a six speed out of the BRZ and FRS cars. The swap does not require cutting the car like my CD009 conversion does. I've had the CD009 in my car for a bit and I can say with certainty that a 6 speed with 1:1 5th matches my style of driving. Fast off the line and the overdrive in 6th helps a ton on the highway. I'm running a 3.54 rear.  Here are the TL70 ratios. You may want to plug then into your calculator. The TL70 is cheap and plentiful since young drivers have a habit of wadding them up:) 

TL70 Gear Ratios

1st 3.626

2nd 2.188

3rd 1.541

4th 1.213

5th 1.0

6th 0.767

[AZDatsun]

Now that would be awesome Derek


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[texasz]

Interesting option Derek presents, the numbers look nice with the 3.362 diff ratio.  Questions that this then raises are fitment, reliability, durability, and is there a short throw shifter option available?  Oh yes, and cost!?  😉

[inline6]

I created a spreadsheet many years ago that is nice for exploring transmission options.  Here are a couple of screenshots:

In the rows below the screenshot above, I have the speeds in gears return in formulas for each 1000 rpm  This sheet is nice because it accurately (I think) takes into consideration the tire size as well as the rear ratio and the gears.  You only change a few parameters... and, it allows you to plot speeds in gears and look at a graphical representation:

 

I used it recently to investigate using a 240SX transmission.  If you want to use it, I can upload it somewhere for you.  Let me know.  

[Patcon]

Put it in the resource section

[Derek]

9 hours ago, texasz said:

Questions that this then raises are fitment, reliability, durability, and is there a short throw shifter option available?  Oh yes, and cost!? 

So many questions:)

Durability and reliability are directly connected to HP, torque, tire width, driving style etc etc. Turbo car with wide tires and a propensity for hole shots....You are going to have a bad day. Under 275 to the wheels and the occasional hole shot when your brother in-law comes to visit....Probably going to be fine.

Here is the first non functional prototype. I took one of my extra CD009 prototype castings and sliced and diced it to see if the conversion was feasible. 

 

This is the final-ish shifter design. It uses the stock BRZ shifter. There are aftermarket short throw shifters available.

 

 

Costs are going to be in line with the CD009 conversion. The bell housing will be right around $1000.00. The shifter mount should come in under 250.00.  For the total package you will need a TL70 new or used, bell housing, shifter, new yet to be designed cross member and most likely a driveshaft.  Used transmissions are around 250.00 - 500.00. New ones are available for 2300.00 or so. Rebuild parts are Reilly available as well as aftermarket gear sets.  Also there is no provision for a speedo cable so you either need to use a GPS speedo or Speedhut makes a motorized conversion that takes GPS data and converts it to a motor drive.

The biggest reason for this conversion is for you to help fund my retirement but beyond that you don't need to modify your sheet metal so returning to stock is easy. Second would be bragging rights. "yes but mine has 6"  

Seriously though, rebuild parts are drying up for the 240sx transmissions and that is what originally got me started on the CD009 conversion. I wanted a new transmission in my car and both the CD009 and the TL70 are available new at what I think is a reasonable price.

I'm waiting of the first two castings to arrive from the foundry so I can finalize things. They are poured I'm just waiting on them to heat treat.  I'm keeping a list of people interested so if anyone is, email me at kick@datsunworks.com with the subject: TL70 6 speed.

 

Derek

 

 

 

[Captain Obvious]

2 hours ago, Derek said:

The biggest reason for this conversion is for you to help fund my retirement but beyond that you don't need to modify your sheet metal so returning to stock is easy. Second would be bragging rights. "yes but mine has 6"  

LOL.  I like it!