5th Gear Swap from other cars?
 

Damian and I have been discussing gear ratios for the FD.

I've done several searches, but I can't come up with anything relevant.

I know I've heard of putting the B2000 5th gear (from Howard Coleman) into the FD transmission to shorten 5th gear to make it usable on the track. In my car (case 'A' and case 'C' in the spreadsheet/image) with a 4.1 ring/pinion and stock wheels this would work well. That ratio (.859) would put the top end at about 170 and bog less from 4th->5th while giving much better acceleration in 5th.

However, if you switch to a 4.3 ring/pinion like Damian was contemplating the B2000 5th is too short. You can see this by switching the ratio for the 5th gear in case 'B' to .859. That puts top end just over 155 which isn't enough for a track like Brainerd, with nearly a 1 mile straight, and is probably close to too short for a place like Road America.

I was looking at some of the 5th gears from other cars from Mazda, for something that's in the middle. According to the Mazda Transmission chart, the Cosmo from 1977-78.5 has a .791 5th, the RX-7 GSL-SE from 84 has a .758, the RX-7 from 84 and 85 has a .807 and the RX-7 GTUs from 89 has a .758. Any of these would produce much better acceleration than stock 5th, while still giving sufficient top end (170-180) for the big tracks.

Has anyone tried rebuilding their transmission with any of these other gears? Is there a thread I missed somewhere that discussed this before?

Reference material I've mirrored here:

Mazda Transmission Chart

Gear Ratio to Speed excel spreadsheet

If you don't have excel here's a screenshot of the relevant graph:
Gear Ratio to Speed pic

This is why some race cars have quick change gears. Different tracks require different gears.

I believe the Japanese RX-7's came stock with shorter 5th gear ratios as well - I believe it's actually not *that* much shorter. Might be worth looking into.

Dale

Turbojeff - Yep, I've seen them do complete transmission changes at Road America imbetween runs to switch gears. I don't think Damian or I are that hard-core yet unfortunately.

dcfc3s - I think you're correct. This post backs you up:
https://www.rx7club.com/showpost.php...0&postcount=21
However he also mentions the 86/87 T2, which I have listed as .719. Hmmm...

I am either going to switch my 5th gear to the 86-88 Turbo II (.762) or the Bathurst Type R (.806). Wargasm has the Bathurst 5th and seems to like it. I have 4.33 gears, so I have to decide whether I want a 170 or 180 top speed.

I know the TurboII tranny is VERY similar to the FD's tranny. Also, I think the FC also had different 5th gears for Japan as well - shorter gears.

Dale

As pianoprodigy mentioned, I have the 0.806:1 5th and I love it. 5th is no longer a suck-fest like with the stock 0.719. I am not aware of the 0.859 option, but that seems like it would be pretty buzzy on the street at 80-90 mph (my cruising speed). The 0.806 give a nice bump up from stock, but still keep the revs right at 4000 at 90 mph highway cruise.

Yeah, the .806 looks to be the way to go for me on the stock 4.1 rear end. Plus I still drive my car on the street so noise is a factor.

Damian's car is full track and obnoxiously loud anyway, so noise/revs aren't as much of an issue. With a 4.3 and the .806 that puts him just over 170 top speed and I think that's all he'd need on any road course... Well I hope anyway. :) I'll point him to this thread and see what he thinks.

Best part is it sounds like the parts are readily available from Ray at Malloy. This is looking good!

Thanks all!

Where do you find this 0.806 gear for sale anymore? Sorry I didnt read the whole post. How much is it?

Bump.

Does anyone have a part number for the 0.806 5th gear? Wondering who/where sells it too!

For the 0.806 5th in an FD trans case (should work in a Turbo II trans also)

Counter Shaft 5th Gear - r50417308
Main Shaft 5th Gear - r50417610a

I think that's 27 teeth on one gear and 53 on the other gear. Make sure you confirm item description and check teeth to figure out ratio before you install just to be safe.

Brian

PS - Contact Ray Crowe at Malloy Mazda in Woodbridge, VA for the parts.

There is a guy in LA who has about 40 low mileage J-spec FD Transmissions. He'll sell them for $400 each. They have the .806 5th gear. I have one in my car and LOVE it. You can actually use it to accelerate on the freeway! Good gearing for really fast tracks too... no more 5th gear bog. The car hits 170 MUCH faster now.

I think I have his contact info... I'll look it up later and post it.

Is that Km/h or mph? :o)

Dave

I appreciate the info Wargasm and John. I have a bad 5th gear synchro and figured I'd throw in the 0.806 gear at the same time. I guess the cost will determine which route I take. Although, I think I'm leaning toward the used JDM transmission as suggested by John - especially since I'm in LA. Here's John's thread with the contact info. Thanks again!

https://www.rx7club.com/3rd-generation-specific-1993-2002-16/great-source-inexpensive-low-mileage-used-fd-transmissions-393397/

Hell yeah I wish someone in my area had used JDMs trannys easily available. It's a whole lot easier to work on fixing a tranny when you have a swapper already installed, and the car is out of the garage :o)

Dave