Upgrading the Drivetrain

Making a Fast car QUICKER with some RARE and Expensive Parts

Drivetrain goodies: "Built" transmission w/short RZ fifth gear, "Built" 4:30 Differential w/GReddy cover and powerFlex poly bushings, MazdaSpeed Power Plant Frame, Banzai transmission brace, Exedy Carbon - Carbon-D - twin plate clutch, HPP Differential brace.

Since day one of my ownership I haven't touched the drivetrain on my car except for a light weight 9lb SR Motorsports flywheel and an upgraded ACT clutch. It has served me well so why mess with it. Why? Because that's why we modify! To make it better, faster, stronger. But there are some specific reasons also: #1 my car is making substantially more power than it was 10 years ago. And although the ACT was holding up ok something a little bit more substantial was needed. #2 the gearing of the stock FD isn't good for the track as 5th gear is essentially useless. It's an overdrive gear designed for fuel efficiency on the highway. At my current power levels I'd find myself cruising at redline in 4th at only 3/4 of the way down most of the long main straights with a useless 5th gear the only thing left. #3 power levels that challenge the attachment points need help so stronger hardware and bushings are in order.


Gearing: The Key to Acceleration

Everything needed for a complete rear end assembly

In the beginning I was simply going to swap my existing 4:10 stock ring and pinion gear (The automatic cars came equipped with a 3:90) for a 4:30 ring and pinion found in the RZ and other '99-'02 models. But in order to minimize the down time of my car and the fact that I was able to buy at a significant discount a BRAND NEW IN THE BOX torsen unit I figured I'd just build another rear end. Consider that in order to swap out the gears it's good practice to replace bearings and seals I had what was necessary for a new ground up build with the exception of of the housing itself. So I hunted down and sourced a blown up differential on the cheap, emptied out all the chewed up guts, scraped, wire wheeled, and blasted all the rust off it and coated it with some heavy duty etching primer and some ceramic based black paint. Looks like new! I also removed all the hardware and gave them a good cleanup and reinstalled them. With that done there comes a time where one asks; "do I want to study up on how to shim a diff and buy the necessary tools, or just pay the couple hundred and have a pro do it properly." I'm sad to say I chose the latter. Primarily because the cost of a mistake was too high, and I didn't fancy the idea of having to remove the diff multiple times from the car if something was amiss.


A nice paper weight. One very "lunched" torsen. Emptying out all the guts to prep the housing for re-use. None of the gears were any good due to metal shrapnel being thrown around inside the diff when it let go.


Stripped and ready for paint. Followed by a nice coat of HD etching primer. Dummy studs installed to keep the piant out of the threaded holes.


Damn, that looks good!


My original 4:10 diff and the replacement with 4:30 gears. Not sure I like the purple but PowerFlex have a decent reputation. Nicely refinished bracket coated red.

A couple other items to add to the performance list include PowerFlex polyurethane differential bushings and a GReddy high capacity differential cover. The bushings for obvious reasons. Mine were old-ish, and the added durability of the poly bushings will help with reliability. The bushings pressed into a refinished and painted diff bracket. The GReddy diff cover increases the oil capacity by about a quart....better for cooling. And I filled it with about 2.5 quarts of NEO synthetic oil 75W90RHD


What Could You Do To Build a Better Tranmission?


SUPERNOW transmission cooling plate and the RZ fifth gear set along with some sample new synchros and a bearing...I ended up replacing them all.

What could you do to make a better FD transmission? Well...replace it with something else. The FD transmission is a small box built for lower torque and an engine that only produced 255hp at the crank in stock form. Try pumping 400ft lbs of torque through it and things are going to break if it isn't treated tenderly. One can cryo treat the gears or simply replace them with dog gears (very expensive). Weld up some of the small internals but not much more. It's just not made for transmitting a lot of power. But there are some other mods to be made. The first and most common performance mod, as mentioned above, is to replace the very tall US-spec overdrive 5th gear (0.719) with a short ratio fifth gear (0.762/0.806) from the JDM RZ Series 8 model. Since the R-type transmission that is used in the FD was also used in everything from the FC-TII, MPV minivan, 929 sedan, Mazda B series trucks, Ford Aerostar minivans, and Ford Ranger pick-up trucks finding gear sets is now not a problem. I recall that in the early days (late 1990's) prior to the JDM Series 8 cars becoming available the short gears were all obtained from the aforementioned MPV minivans and B series pick-up trucks. The short fifth gear creates a real and usable gear for spirited driving. Although highway cruising and gas mileage suffers due to the higher cruising RPM. My argument...I don't "cruise" in my car.

Using my wheel and tire combo of 285/35 on an 18" rim I calculated the following speeds in each gear ratio.
Gear
Gear Ratio
Speed @ redline (8000rpm) Speed @ Shiftpoint (7200rpm)
1st
2nd
3rd
4th
5th
reverse
Final
3.483
2.015
1.391
1.000
0.719 [stock]
3.288
4.100 [Manual stock]
43.09mph
74.48mph
107.89mph
150.08mph
208.74mph
.
.
38.78mph
67.03mph
97.10mph
135.07mph
187.86mph
.
.
At 70 mph Cruising speed in fifth gear- Engine speed is 2683 rpm
1st
2nd
3rd
4th
5th
reverse
Final
3.483
2.015
1.391
1.000
0.762 [RZ 5th]
3.288
4.300 [RZ rear]
41.09mph
71.02mph
102.88mph
143.10mph
187.80mph
.
.
36.98mph
63.92mph
92.59mph
128.79mph
169.02mph
.
.
At 70 mph Cruising speed in 5th gear- Engine speed is 2982 rpm
1st
2nd
3rd
4th
5th
reverse
Final
3.483
2.015
1.391
1.000
0.806 [SpiritR/RZ 5th]
3.288
4.300 [RZ rear]
41.09mph
71.02mph
102.88mph
143.10mph
177.54mph
.
.
36.98mph
63.92mph
92.59mph
128.79mph
159.79mph
.
.
At 70 mph Cruising speed in fifth gear- Engine speed is 3154 rpm

Ok so lets look at where the new gear ratios put the car when on the track. Note that with the stock gears and 4:10 differnetial Ring and pinion gears (R&P), 4th gear at redline takes you up to about 150mph. My shiftlight is set for 7200rpms roughly 135mph. I'd typically run it out to redline at 8000rpms in 4th on the track verses shifting to 5th for a couple hundred yards. Why because to shift from 4th to 5th at 7200rpms drops the car down to 5000rpms...out the bottom of the power curve and which makes the 5th gear useless.

The new gearset and 4:30 R&P you hit redline (8000) in 4th at 143mph. My shiftlight comes on at 7200 which is about 129mph. At this speed, 129mph, this leaves the car in the power sweetspot in the lower 5th gear at 5500rpms with the shiftlight coming on at 170mph. So it should still be pulling hard at the end of a typical long straight on a track. For reference I've added the super short 5th gear 0.806 ratio for comparison. Perhaps for the really short time-attack tracks in Japan this might be preferred but for US tracks, some with longer straights, the 0.762 is the better choice.

The second modification, as is with any other component on the car is to improve cooling. In this case, higher grade oil and more of it is the key. To get more into the transmission a bigger pan is in order. I installed a VERY rare SuperNow cooling plate and a 1" pan billet aluminum spacer all bolted down with M8x1.25x55mm stainless hardware. This adds another quart of oil to the capacity.

For this particular transmission, I had every single synchro, every single seal,and the fifth gear and reverse slider assemblies, and every piece of consumable hardware replaced. It is a new as you can get without buying a new a factory transmission from Hiroshima. And it was all topped off with 4.5 quarts of NEO synthetic oil 75W90RHD at $(insert stupid amount here) per quart. Oh and I polished the bell housing because I just can't resist polished aluminum bling.


More Cooling: SuperNow cooling plate and large capacity spacer plate


The Pieces of the assembly. Note the standoffs for the windage tray. Stainless hardware for bolting it all up.


Not that I could tell but down in there is a short fifth gear ratio.


A Better Clutch

With more power and torque and the shorter gearing (torque multiplier) a robust clutch is needed. Although the trusty SR Motorsports flywheel/ACT clutch combo was working quite well, in a rare moment of fiscal weakness I purchased a lightly used EXEDY Hyper Carbon - Carbon D Series Stage 7 twin plate clutch. (Part Number: ZM022SDMC). This particular model is now quite rare and discontinued. Fingers crossed I don't run into problems if I need parts or replacement carbon discs. I've read replacement carbon discs are $1200!!! (Part number: DP03 (drvn plate)for those who need to know) and need to be replaced when the thickness reaches 3.3mm according to Exedy but folks have worn them down to as low as 2.5mm and still found the clutch functional. New thickness is 4.2mm for the record.


The Exedy Carbon clutch "sandwich." Note the sprung center hub and the floating plate in the middle of it all. (photo courtesy of: Exedy USA

Why a twin plate clutch? In laymans terms more friction surface means greater ability to hold torque. For this model the spec is 475 ft lb of torque. In addition if you have a single and a multiple disc clutch both rated for the same input torque, generally the multiple disc clutch will have a lower clutch pedal effort. Multiple disc clutches also allow the use of a smaller diameter clutch disc and pressure plate. In this case the clutch discs are only 225mm in diameter...pretty small. This lowers the inerta of the clutch setup freeing up some horsepower.


Carbon Fiber Goodness (Photo courtesy of: somewhere on the RX7 Forum)

The offical description: "Hyper Carbon Series D-Core D-Core is the new generation of clutch systems by Exedy. The revolutionary carbon clutch system is developed to achieve the ultimate goal of comfort and ease of operation. It is purpose built to absorb the noise and vibration emanating from the drivetrain components such as differential gear, transmission and engine. The D-Core system is designed to protect the drivetrain by absorbing and dissipating "shock torque" and is especially effective in vehicles fitted with 6 speed transmissions. Superior engineering enables the D-Core clutch system to possess an ideal clutch engagement position, increased clamp loads and lower pedal effort, while the unique carbon fibre friction material allows comfortable half-engaged clutch operation and responsive gear changes. In 1995 Exedy provided clutches to a top F1 team. One of the the primary features of Exedy F1 clutches, carbon clutch technology, has now been introduced into performance products for street use. The carbon clutch series enables an improved shift feel and easy clutch operation which are requirements of performance clutches for street use."

Kinda neat to know I have F1 technology in my car! After driving this clutch for a while the pedal weight is significantly lighter than my old setup, but the takeup although smooth varies on clutch temperature. Lots has been said about the longevity of a carbon clutch for a street vehicle as a result of the need for a warm up procedure to get the most performance out of the setup. Most speak of the issue with "daily drivers." I don't really see my car as a daily driver...I only put a couple thousand miles on y car a year so I figured I'll get some use out of this clutch for a while. Regarding the temperature issue Exedy states the following: "Carbon clutches are not recommended for street use due to the inconsistent friction coefficient from hot to cold. Carbon clutches require a warm up procedure before being driven aggressively. Due to this nature we only recommend carbon clutches for race only applications or to customers who completely understand the characteristics of a carbon clutch. A very big misconception of a carbon clutch is that the vehicle is warm so isn?t the clutch? These units work excellent on dedicated race cars due to the driver being able to do the warm up procedure prior to the start of the race and every time driver upshifts or downshifts at higher RPM he is continuing to add heat to the clutch disc. In a street car you often stop at lights, drive in a specific gear, or shift at a lower RPM not allowing adequate heat to be given to the clutch disc. Without heat in a carbon clutch the friction coefficient is significantly lower than when heated. It is at these points in a street car that the customer may decide to put the pedal to the metal causing the clutch to slip and wear out prematurely due to inadequate heat being in the clutch disc."

I think the long and short of it is **IF** you are going to launch the car hard, warm it up first! The proper warmup procedureaccordig to Exedy is: "Due to the inherent properties in the warm up procedures of the carbon material we must specify the proper way to get the discs up to operating temperature. This process will heat the discs so that they will hold the specified torque ratings. The correct method for heating up an Exedy Carbon Clutch is three, five second “slips” of the clutch within 30 seconds. This process may seem excessive but is necessary when producing such high power levels."

This hot cold performance characteristic takes some getting used to. Oh and the sound is quite different! The Japanese retail websites list the sound as "Disc Rattle" and having a design with a "Boss Drive with unique sports clutch sound." Unique Sports Sound....LOL!. The noise was unique enough that I thought there was a problem with my install. Turns out that the center plate that floats when the clutch pedal is depressed just kind of clangs around in there....sounds like a small bell ringing. Perfectly normal EXEDY says.


Exedy Stage 7 Hyper Carbon-D twin disc clutch installed. Pressure plate bolts torqued to the recommended 23lbs w/threadlock for added safety. The old flavor; ACT pressure plate and clutch with SR Motorsports 9lb aluminum flywheel.


Rigidity - MazdaSpeed


The real deal

First off, my car is a street car first and a track car second. It's already on the ragged edge of being streetable. It's loud, it's smelly, it clanks it bangs over road bumps...it's a hard car to drive comfortably. So anything I was considering in the way of a more robust drivetrain had to be taken very seriously. I'm a big fan of MazdaSpeed parts because of their rarity and origins. So I did manage to score a used MazdaSpeed power plant frame...a genuine one. Much has been debated about the merits of the MS PPF ...different steel, better welds, different heat treating, etc. etc. to make it 10% stronger as they advertised. The Quote from Mazda Motorsports; "The Mazdaspeed power plant frame is made from hot-rolled steel plates instead of the stock steel stamped piece and is 10% stiffer. The extra stiffness reduces drivetrain bending and flexing due to on and off acceleration." I like it because...it's red. So there.


A little scruffy but a rare find and nice addition for my car.


Rigidity - Banzai


Banzai transmission crossmember bushing/brace

It's simple and effective. A transmission crossmember brace produced by Banzai Racing that replaces a stock aluminum crossbrace but adds an additional attachment point for the rear of the transmission. This takes some of the load of the tail end of the transmission and PPF resulting in less chance of cracking the PPF and more accurate shifting since the transmission isn't moving around as much under load. NVH is taken care of with an Energy Suspension HYPER-FLEX polyurethane bushing. Given all the other additions this round of upgrades I haven't noticed any driveability drawbacks to this brace.


The location of the stock crossmember (removed). After installation highlightling the Energy suspension bushing arrangement.


Rigidity - HPP Differential Cradle


HPP Differential Cradle/Brace all nice and powdercoated red.

Drag racers and some hard core autocrossers are constantly snapping PPF's and cracking differential casings due to a combination of power loads and the RX-7's infamous wheel hop problems. The guys at HPP developed a cradle that combined many of the best of what other braces and cradles had to offer into a single piece. And one that would be compatible with the GReddy diff cover. It is a solid piece, heavy gauge plate material, cut and welded together. So this piece is a bit of a tradeoff: it's VERY heavy and it's a solid mount from differential to chassis so it's going to transmit quite a fair amount of NVH. On the plus side you sure as heck aren't going to break any more diff housings or crack any more PPF's. Well in my case, referring to what I said earlier about my needing to keep my car as streetable as possible perhaps it was for the best that it didnt' fit with my PFS exhaust. So after installation, it was removed and sold. Such is life.


Even with some tweaking and shimming there was simply no way I was going to get the clearance necessary to get the HPP cradle to work with the PFS exhaust


Installation Notes

So below are the now obligatory shop pics of the installation of all the above. It was all done in one weekend...about a day and a half in the garage of a good friend who happens to have a lift. Oh so very convenient. I simply couldn't have imagined doing this work by myself on my back in my own garage. But there was a hitch, after installing all these parts, and breaking my cardinal rule of "only one mod at a time," there was "funny noises" coming from the transmission and differential. Or one or the other...which is exacly why I only do one mod at a time. Well given the nature of the noise and the fact that *if* it was an internal issue I wasn't going to be able to diagnose and fix it anyway I decided to trailer it up to the guys at IRPerformance for them to figure it all out. After a second rebuild of the transmission, determining that the clutch noise was normal, shimming the differential for more clearance to the rear subframe, and installing new rear suspension pillow balls, all was well and right in the world. Gratitudes to Ihor Huk and Rich Farrell for all the support and help.


Ok, let's get started. A shame I had to remove the diffuser to get the exhaust out.


Pulling the differential. Not nearly as bad as I thought it would be. It's sitting there propped up before removing the halfshafts. You can see that the halfshafts have been popped out already. Just be sure to orient the C-clips properly before reinstallation.


Pulling the transmission. Where's waldo, I mean where's Dave? That would be Doctor Newbern (he really does look like a doctor doesn't he?), who graciously elected to help me versus work on his kitchen remodel that day. I think he secretly wanted an excuse to tinker with an RX7.


Only a minor clearance issue. A little fin shaved down to clear the wastegate dump tube and all fit up nicely.


All polished and sitting pretty after the install.

All that work and investment and you can't see a damn thing!!!


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This page last updated November 29, 2013


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