The FD, 5th gear, rear end ratios, and you
 

Hey guys -

One of the local guys here (Jimmy) just upgraded his rear end to 4.44 gears from an RX-8. We were talking about how it would effect interstate cruising and such. So, I put some research and some science into the mix :).

OK, let's start up front with the 5th gear in the transmission. The stock 5th gear on US FD's is a .719 ratio gear, which is actually pretty good. The lower the number, the lower your cruising RPM, the better your highway fuel economy will be. This also means your theoretical top speed is better than one with a higher gear number.

Well, the JDM transmissions have different 5th gear ratios. I went through my stack of JDM dealer brochures (I have every one I've ever found!) and noted the 5th gear ratios. The Japanese cars went for a higher 5th gear, which makes the RPM difference between 4th and 5th closer for better acceleration in 5th. This does bring down the theoretical top speed and will also increase cruising RPM.

So, next off in the chain is the rear end. The stock 5-speed rear end is 4.10, whereas the automatic has a 3.90 gear. The higher the number, the more mechanical advantage you get in acceleration through all the gears, but it does also increase cruising RPM. It's like riding a 10-speed bike, it's much easier to get the bike moving in 1st gear than 10th gear. Some JDM FD's had 4.30 rear ends (like the Type-RZ) and there are other gear sets out there. The RX-8 manual trans has a stock 4.44 ratio, and the newer RX-8's are even higher at a 4.7 or something.

So, I'm thinking about doing the same to my car. Problem is, I have a JDM trans in my car. Since I don't know which model the trans came from, I did some homework to figure it out.

First, check out this site -

http://www.bokchoys.com/differential...calculator.htm

Yes, it's for BMW's, but it works perfectly for this purpose. You can put in your tire size, what 5th gear you have, what rear end you have, and what your interstate cruising speed is and find out what RPM you'd be driving at. So, using this I made a little spreadsheet and solved for various 5th gears and rear end ratios.

NOTE: This spreadsheet was done with my rear tire size, which is 265/40/17. A different tire size will give different results. But, my tire size is within a few RPM of a stock tire size so this is close enough to give you the idea.

70mph 4.1 4.3 4.44
0.697 2653 2782 2873
0.719 2737 2870 2964
0.762 2900 3042 3141
0.806 3068 3217 3322

80mph 4.1 4.3 4.44
0.697 3032 3180 3283
0.719 3128 3280 3387
0.762 3315 3476 3589
0.806 3506 3677 3797


On the 5th gear ratios -

.697 - this was offered in some of the non-turbo 2nd gen RX-7s Whether or not this gear would work in an FD is unknown as the non-turbo and Type R turbo gear box do have some differences.

.719 - Stock US FD 5th gear. Dealer brochure rounds it to .72.

.762 - Some JDM cars had this, seems kind of uncommon. The Type-RZ had this ratio.

.806 - Most JDM transmissions will have this 5th gear ratio.

I got out on the interstate with my GPS, set the cruise control to 70 as indicated by the GPS (my speedo is off just a bit) and checked my RPM. Sure enough, I have a .806 5th gear.

I am planning on upgrading my rear end ratio, and I'd like to try and swap the US 5th gear into my transmission. Don't know how hard that will be, I've done a few 5th gear synchros, so I've gotten at least to that neighborhood in the transmission. I'd love to know if the NA 2nd gen 5th gear would work since that would help cruising RPM for those long trips up to Deal's Gap :).

Anyhow, figured I'd share my little project :).

Dale

Nice compilation.

I've done the switch from the stock US .719:1 to JDM .806:1. It's no different than the 5th synchro job, since 5th gear has to slide off to get to the synchro. The synchro, shift sleeve, and 5/R hub are the same. So for any other ratio that fits in an FD the install process will be the same.

Dave

The 87-88 T2 had a .762 5th gear:

https://www.rx7club.com/attachment.p...1&d=1327373039

later FC turbo models had the .719 5th gear

I would be very surprised if the n/a FC 5th gear worked. That is more like a Miata box. I have pulled them apart before. When I had my T2 transmission rebuilt, the 5th gear syncros were different between the 87 and 88 model year... a lot of the 5th gear stuff isn't even interchangeable among turbo models.

Good to know on the NA 5th, I figured that was the case. I imagine the shaft size in the transmission is likely smaller as well.

Also, good to know that it isn't too bad to swap the 5th gear. I may need to find someone with a bad transmission I can buy the 5th gear out of :). Sad part is I hauled at least 3 transmissions to the scrapyard about a year ago, should have grabbed a 5th!

Dale

How much more of an increase in mileage will you get?
Total fuel cost per year?
Seems like alot of work (trouble), for the couple tenth's
you will save. :scratch:

mmm keep us posed dale! id love a nice 5th gear dedicated to cruising to deals gap.... ahhaha

have the 4.30 gears and accerlation is noticed but 5th gear highway @80 mph went from 3000 rpms(4.11 stock) to 3275 rpms @ 80 mph . G

I did the 4.44 RX-8 rear end swap myself two weeks ago. I didnt imagine that going from 4.1 to 4.4 diff ratio would make such a difference in acceleration. I honestly feel like its a different car.

neat chart! iirc on the stock ECU the staging/coming out of closed loop is right @3200rpm, so best mileage occurs right @3199rpm. a power FC car might be different.

um secondly the NA gears will not fit. the M type trans and the R type trans have very few parts in common. however since the R type was used in everything from the T2, FD, MPV, 929, B trucks, aerostars and rangers, there may be a taller 5th than the .719.

thirdly, the JDM cars either got the 17" wheel (255/40/17 tyre)/.762 5th and the 4.3 rear OR the same 225/50/16 and 4.1 as the US cars, but with an .806 5th.

tire diameter seems to be the same between the two tyres...

Should anyone want a perl script that does these calculations, I have one.

Big problem is I have the JDM .806 5th gear, so going to 4.44's means I go to the bottom right corner of the spreadsheet. Compared to a stock FD, cruising RPM will be 500-600 RPM higher which is significant.

I don't think there will be any 5th gear ratios available lower than the .719, and if it is available it would be harder to source. Finding the FD 5ths should be far easier.

Dale

The JDM RX8 5 speed gearbox (yes, they had a 5 speed version in Japan...yet kinda rare) might have a lower 5th gear.. (lower than .719) in it...

Trans looks Identical to FD 5 speed with the exception of a different bell housing and tail housing..

Might be able to source the gear specs for you...

I currently have an S8 box with the .806 5th while I wait for my OS Giken trans to be finished (round 2, anyway) and 4.30 RZ gears. Cruising in 5th gear down to DGRR is kinda rough, both on fuel consumption and my ears :lol:

id like to have a little lower rpms in just 5th.. when im cruising around 70-90,... i know around 80 im at about 3k

I have a US trans so I guess .719 5th and 4.30 rear gears.

I really noticed a difference autoxing. Switching from 1st to 2nd takes times and upsets the car. I noticed I had more power and was in a better rpm range than with stock gears.

Another bene is that with my street tires, 275/40/17 rear and the 4.30 rear the hwy rpms are almost the same as stock.

One does not simply cruise to deals gap
http://iheartbacon1376.tripod.com/si...es/boromir.jpg

I'd love a 6 speed box where the 6th gear is an "overdrive" gear for low RPM highway cruising. None of the bolt on options have the right gearing though.

Ugh party foul...

Are you autcrossing in a Prepared or SSM class?

Dave

You should try 4.77 gears. Single biggest performance benefit for autox in an FD.

I'm wanting the same thing for my 20b that's why I will eventually have to upgrade to a t56. The stock fd 5th gear at highway cruising leaves the rpms to high to get proper fuel economy with a larger displacement engine. With the 20b's added bottom end torque, I hope to be able to get my highway cruise rpm's down to 2,200 at 70mph. That rpm range should put the engine in a better position to make better hwy fuel economy.

First gear for me does not have enough leg; with that said, traction is an issue in both first and second anyway. I would like to step to a higher rear ratio and with that a lower 5th gear to keep highway RPMs very slightly above (100-200 rpm) stock. The last solo event the car was ran hit the rev limiter on two of the straights in second; a slightly taller rear gear would have shaved a tenth or so. Unfortunately, the automatic rear ratio is a little too tall....

so the japanese spec rear end gear comes with a 4.30 usually , is there a rare model that has a 4.90?? im guessing your highway rpm would be a bit crazy and a low top speed but would it be quick as ever all the way to 5th

A little scary that the week my JDM transmission is going in to my car a thread about it starts. It's like the forum knew. :ponder:
To get back to my question. Is any one running the JDM transmission with the 4:10 rear end? What RPM are you at 80MPH and how much did your MPG drop? Are we talking a couple hundred RPM, a couple MPH, a couple MPG or larger? Should I be worried?
I know I have to give up something for better performance. I just loved the way my FD cruised at 80MPH @ 3000RPM. The sound, feel and MPG where great. I was getting mid twenty MPG on long trips.

the non turbo transmissions are completely different than the turbo transmissions, so rule out that as a higher gearing possibility.