The 4.78 Prettysik-diff
Introduction
An OEM 4.3 Torsen diff is the obvious choice Stage 1 Atmocharger for the 1.8 MX5 as explained in this other dodgy White Paper called Driver Controlled Atmocharging.
However when top speed and power is limited, even more atmocharging can be had by embracing another world of hand built 4.44, 4.78 (and beyond) Prettysik-diffs.
A 4.78 ratio identical spec Franken-diff was found here, but now rebranded for no useful purpose but to relate to its User rather than its Specification.
This excellence in dodgy research on the MX5, published in Australia's leading source of MX5 purist peer review, focusses on a Torsen 2 (T2) embodiment.
However it also covers other embodiments such as T1 LSDs, and clutch type LSDs like early RX7, Mazdaspeed, KAAZ, Cusco as well as fully open and no slip CIG Lockers.
The 4.78 T2 Prettysik-diff uses standard MX5 and Kia 7" diff parts, (no custom parts) mostly from private collections hand assembled with loving care by a hand picked skilled artisan. This is not some standard off the shelf number, this is relatively rare and unique with its rare beauty all on the inside.
This example features Torsen 2 and 4.78 gears and housing suit NA8, NB8A or NB8B. It will also fit an NA6 using later drive and axle shafts. It requires a speedo drive change to keep an accurate speedo.
FAQ....What are the speedo options?
Marua Motors in Japan have 4.6 ratio sender units and gears. These are 4% optimistic. (ie read 100kph at 96kph) so possibly a points saver. These are a replacement yellow dríven Gear for existing cable style Gear Units on NAs.
That gives 2.3 kph extra margin from going over the 60 kph.
Not much, but Every K Under is a Lifesaver logically follows from Every K Over is a Killer dogma of the Deep North.
(Sales of gilt edged number plate pics of a few k's over is the best little earner States have had since the Sydney Opera House Lotteries).
For NBs (5 and 6 speed) they have no such solution for their OEM electronic sensor. However for an electronic dríven speedo, a Digitech Corrector Module is available as a generic (not plug and play) kit from Jaycar. Also there is is the Yellow Box recalibration module .
A 99-05 Miata specific kit is available from Track Dog Racing.
Note that for a 4.44 Prettysik-diff the OEM 4.3 ratio sender units are 3.3% optimistic. That gives 2.0 kph extra margin from going over the 60 kph.
This diff will plug and play into an NA8, NB8 or NB8B using existing drive and axle shafts.
However it will only plug and play into an NA6 if 1.8 drive and axle shafts, are added to the collection.
The later single piece 1.8 axles and earlier 2 piece 1.8 axles (the ones with the stub shafts) all plug and play.
FAQ....What does it take to create a 4.78 Prettysik-diff ?
It may appear an adhoc combination of OEM diff parts, but when it comes to creation, the Prettysik-diff is a whole new creation with connections, albeit fanciful, to French Government recognised, rare exquisite beauty they call Haute Couture. That sounds like this in English and this in Japanese !
So a Prettysik-diff (PrettySik = Pretty + Sick = Beauty + Insanity = Beauty plus insane rev-ability factor), and rareability means its a boy's version of Haute Couture because priceless pieces come from private collections with each hand picked piece meticulously inspected before painstaking assembly into a rare treasure at the hands of a skilled local artisan.
The pricelessness depends on the blood sweat and tears getting the collection together and how much a specialist diff artisan guru charges to "sew it all together".
This research has established a link from the exclusivity of a Prettysik-diff collection of rare beauty on the inside, to an expression of rare beauty on the outside, seen in high French fashion, in a new awareness which has to impress SHMBO.
And speaking of Ooto Kuchuuru meeting Jinba-Ittai in a "PrettySik-diff" excited by insane revs, check out the ratios from these Samurai road warriors .
The recipe for a 4.78 Prettysik-diff includes a secret herb/spice Part Number (which by definition is no fun if anyone publicly discloses it).
1. Add a 1.8 Diff inc housing as used in poverty pack ADM NA8, NB8 MX5 (7" size)
2. Separate the Open Centre and the CW&P (Crown Wheel & Pinion or Ring & Pinion)
3. Add an appropriate Centre to the collection:
-- Reuse the Open Centre as it is or
-- Rebuild the Open Centre to CIG Locker or
-- Replace open with LSD Centre: MX5 T1 or T2 or clutch type Mazdaspeed, early RX7, KAAZ, Cusco etc.
4. Add a secret ingredient, a 4.78 CW&P, the allure and excitement of which would be lost by public disclosure.
A small clue to its identity was found by careful scrutiny of a code red: 048 075 048 049 052 050 055 049 049 048 seen here etched on the crown wheel
5. Add Diff Specialist Parts including seals as instructed by diff guru. (Honda S2000 tragics also see Haute Couture in their rear ends)
6. Add a diff guru/artizan to carefully inspect, assemble (sew together), test and adjust (test fitting), show and charge.
7. Add fun & games finding & shipping all the priceless pieces of the puzzle to the diff guru, fingers crossed that any S/H bits selected are up to the guru's spec.
FAQ...How does 4.78 compare with the benchmark 4.3 as far as overall gearing with both 5 and 6 speed transmissions ?
Ratios----------------- 1st ---------- 2nd ----- 3rd ------ 4th ----- 5th ----- 6th
6-spd ----------------- 3.76 -------- 2.269 --- 1.645 ----1.257 ----- 1 ------- 0.843
5-spd ----------------- 3.136 ------ 1.888 ---- 1.33 ------- 1 ------ 0.814
In Top Gear, -----------------Overall Ratio------------ Comparative Revs and HP in Top
6 speed with 4.3 diff -------------3.65---------------------------- 1.0
6 speed with 4.78 diff ------------4.03---------------------------- 1.10 (10% more)
5 speed with 4.78 diff ------------3.89---------------------------- 1.07 (7% more)
Top gear overall releases more revs and thus horses available on demand, in comparison to a much loved and admired 6 speed/4.3 combination.
As a point of comparison, it only seemed like yesterday that 4 cylinder cars were non overdrive and typically ran similar 3.9 to 4.1 overall gearing with smaller wheels behind Pommie built single carby, pushrod 8V head and unbalanced cast iron crankshaft engines over long distances before cheap air fares.
It is nonsense to now imagine a Jap built fuel injected, twin cam 16V head and finely balanced forged steel crankshaft engine shouldn't be given the same opportunity to sing along at similar if not more revs across Australia.
This is particularly the case since speed cameras actually slow down the music, and oil and coolant water consumption no longer increase exponentially with revs.
In 1st Gear, -----------------Overall Ratio------------ Comparative Speed in 1st
6 speed with 4.3 diff -------------16.2---------------------------- 1.0
6 speed with 4.78 diff ------------18.0---------------------------- 1.10 (10% slower)
5 speed with 4.78 diff ------------15.0---------------------------- 0.93 (7% faster)
6 Speed 1st Gear is even more of a low range ratio designed for hill start dream/traffic crawler/clutch not to be downshifted to.
(Low range 1st gears are provided by some OEMs as a really useful low speed quasi torque converter drive to prevent clutch slipping and stalling off the line.)
5 Speed 1st Gear could still be downshifted to, but circumstances justifying that would be getting rarer.
It is becoming more of a hill start dream/traffic crawler/clutch saver low range gear when coupled with a 4.78 diff.
Overall it ticks all the boxes. (Except for its strength as a stump puller)
It means close ratio gears, more low range torque, more high range power and more revs liberated.
Gear ratios are closed up the same in both boxes by a 4.78. Power on demand in the top two gears around legal limits is increased by around 10%.
Most atmocharged MX5 engines can achieve these advantages by:
The 6 Speed losing a downshift but liberating the other 5 gears to make a "super close ratio 5 speed box" with a stronger low range and more top end power with ample revs in store to regain top speed.
The 5 Speed losing a downshift but liberating the other 4 gears to make a "nice close ratio 4 speed box" gaining a low range and more top end power with ample revs in store to regain top speed.
All it requires is to realise that mid range driving habits do not have to change because of the choice of gear selection available.
Furthermore it can also be realised that overall, low range at the bottom end is actually a good idea and that revs and responsiveness at the top end of the speed range is also a good idea.
And saving the planet by holding up a fractionally lower Highway Cycle Fuel Number has no significance. That is to say when it comes to a Good Look, a Good MX5 Highway Cycle Satisfaction Look is pretty useless and certainly not as useful as a Good Pee in the Wetsuit Look.
FAQ....The 4.78 has a small looking pinion diameter....why is that ?
A 4.78 Prettysik-diff necessarily has a smaller diameter pinion gear (9 tooth) verses a 3.9 (11 tooth) or 4.3 (10 tooth), both of which have crown wheels of equal diameter (43 teeth).
Simply changing the pinion size enabled Mazda to maintain a range of interchangeable 7 inch rear ends for Mazda, Ford, Kia, Honda and possibly other branded products over a number of decades.
FAQ....Is the smaller pinion strong enough ?
Besides being smaller, the 4.78 pinion has different size/shape teeth and the smaller diameter also means larger loads and shorter life for the same size pinion bearing.
See the difference with this bespoke 3.3 ratio pinion.
Ignoring the effect of tooth size and shape, we can establish a relative load index for the 4.78 Prettysik-diff for the MX5 in a scaling exercise relating to known performance.
We will compare the 4.78 Prettysik-diff (9 tooth pinion) with a 4.3 MX5 (10 tooth pinion) in an NB8B SVT using both 5 Speed and 6 Speed gearboxes.
We will compare the 4.78 Prettysik-diff (9 tooth pinion) with a 3.9 MX5 (11 tooth pinion) in an Turbo NB8B SP using a 6 Speed gearbox.
The 6 Speed, having a traffic crawler 1st Gear will apply the higher load of the two.
Base Case for a 4.3.... common retrofit to MX5 NB8B 6 Speed Application
Engine Torque 168 Nm
1st gear ratio 3.76
Top Gear Ratio 0.843
Crown Wheel Teeth 43
Pinion Teeth 10
Diff Ratio 4.30
Axle Torque 1st Gear 2720 Nm (Nominal Rating)
Axle Torque Top Gear 609 Nm
Axle Torque Index set at 100
Pinion Size Index set at 100
Tooth Load Index = 100 x Axle Torque Index/Pinion Size Index =100
Tooth Life Index = 100 x the"mean" of cube of inverse of Tooth Load Index/100. (a rough approximation) =100
Case 1 ...a 4.78 Prettysik-diff in NB8B 6 Speed Application
Engine Torque 168 Nm
1st gear ratio 3.76
Top Gear Ratio 0.843
Crown Wheel Teeth 43
Pinion Teeth 9
Diff Ratio 4.78
Axle Torque 1st Gear 3020 Nm
Axle Torque Top Gear 677 Nm
Axle Torque Index 1st Gear 111
Axle Torque Index Top Gear 111
Pinion size index 90
Tooth Load Index 1st Gear 123 (23% above Base Case)
Tooth Load Index Top Gear 123 (23% above Base Case)
Tooth Life Index 54 (54% of Base Case)
Case 2 ...a 4.78 Prettysik-diff in NB8B 5 Speed Application
Engine Torque 168 Nm
1st gear ratio 3.14
Top Gear Ratio 0.814
Diff Ratio 4.78
Axle Torque 1st Gear 2520 Nm
Axle Torque Top Gear 654 Nm
Axle Torque Index 1st Gear 92.6
Axle Torque Index Top Gear 107
Pinion size index 90
Tooth Load Index 1st Gear 103 (3% above Base Case)
Tooth Load Index Top Gear 119 (10% above Base Case)
Tooth Life Index 73 (73% of Base Case)
Case 3....a 3.9 in NB8B SP Turbo 6 Speed Application
Engine Torque 289 Nm
1st gear ratio 3.76
Top Gear Ratio 0.834
Diff Ratio 3.91
Axle Torque 1st Gear 4250 Nm
Axle Torque Top Gear 940 Nm
Axle Torque Index 1st Gear 156
Axle Torque Index Top Gear 156
Pinion size index 110
Tooth Load Index 1st Gear 142 (42% above Base Case)
Tooth Load Index Top Gear 142 (42% above Base Case)
Tooth Life Index 35 (35% of Base Case)
These back of the bear coaster sums suggest a possibility of theoretical life reduction in this Prettysik-diff in a 6 speed atmocharged application and less so in a 5 speed atmocharged application.
However practically it confirms absolutely that it will live in atmocharged applications much longer than a low risk diff conversion we already live quite comfortably with, namely a 3.9 in an SP.
FAQ....So do 4.78 pinion loads ever exceed their OEM (Kia) specifications. ?
The answer is No for atmocharged MX5s with either 5 or 6 Speed gearboxes.
OEM Engine Torque 175 Nm
4WD Torque split 50/50
1st gear ratio 3.72
Top Gear Ratio
Lo Range Transfer Case Ratio 1.98:1 (low range possible on sealed roads)
Crown Wheel Teeth 43
Pinion Teeth 9
Diff Ratio 4.78
Axle Torque 3080 Nm
Tooth Load Index 1st Gear 126
This compares with a 123 in 1st Gear for the 6 speed atmocharged MX5 and 103 in 1st Gear for the 5 speed atmocharged MX5.
FAQ....ATEOTD the 4.78 pinion is weaker, so can it be made any harder and thus stronger anyway ? And what about the pinion bearings?
Seems like some Honda S2000 tragics like to get their pinion hard using liquid nitrogen....ie having it cryo'd with similar results to rubbing snake oil on it.....hit and miss.
Even though MX5 pinion bearings are larger then Kia, still consult with your diff guru ensuring no chances being taken with cheap pinion bearings, old pinion bearings, contamination control, poor lubricant, pinion lubrication and applications combining extreme loading conditions with extreme heating conditions.
Whilst it might "smell great" playing with a small pinion in his mate's rear end, stump pulling could smoke the bearings holding on with dear life to that poor little pinion.
But wait, maybe there is a fix for stump pulling his pinion ? Also as a precaution, why not just Ban the Button !
FAQ....So is being endowed with a rear end of such inner beauty as a Prettysik-diff the creation of your twisted mind 97 MXV ?
No, there are others who sought ever more 人馬一体 long before this, tracked down right here in our community.
Hardcore rev-er-rants have collected these rare treasures and experienced the meaning of 人馬一体 Extreme.
人馬一体 Extreme is when you give the horse its head via this special ring and pinion in its rear end.
人馬一体 Extreme -- A Register of Prettysik-diffs (aka Franken-diffs)
Search of private collections has unearthed these extremely rare 人馬一体 pieces at some stage in the hands of the following:
Babalouie (4.44)
colster3 (4.78)
CT (4.44)
flycasta (4.44 & 4.78)
Garido (4.78)
greenMachine (4.78)
KIJIMA (4.78)
Magpie (4.78)
Madjak (4.78)
Matty (4.44)
MiGoreng (4.78)
mrpham (4.78)
plohl (4.78)
sailaholic (4.78)
Shiredhr (4.78)
slammer (4.44)
StanTheMan (4.78)
toppertee (4.78)
Waz man (4.78)
97 MXV (4.44 & 4.78)
Forum members share some experiences: here, here and here for example.
