Turbo Spec Miata

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Category: Projects

Published on Thursday, 27 October 2011 19:15

Written by Jeff Lyon

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I have come up with a pretty simple straight forward recipe for converting a 1.6 Spec Miata into a Turbo Spec Miata.  The goal is to keep the conversion cost around $5k and to up rear wheel HP from 120 to somewhere in the 180 to 200 HP range while maintaining reliability and policability.

To do this I am keeping the stock, unmodified ECU. 

Details of the parts used, costs and pictures will be added as the build progresses.

Doug Gale, owner of TrackMaster Racing, has a Spec Miata built by Justin Hall.  It is a proven car and will be the mule for this project.

Here is the current Build Sheet, this may change slightly as the project progresses.

Here are some comparisons of Brakes and Tires for the TurboSM car build.

First a comparison of the front brake rotors.
The original rotors are 9.09" in diameter and the 1.8 rotors are 10.03" in diameter and they both have 5.5" of hub in the middle so some quick math shows that:
1.6 rotor has 41.14in² disc area
1.8 rotor has 55.24in² disc area
So the 1.8 disc has 14.10in² or 34.27% more area.

The pads are so straight forward due to their shape.  They are both about 4" wide but the 1.8 pads are just over .25" taller so some more quick math found that they had approx. 1.08in² or 15.6% more surface area than the 1.6 pads. 

Here is a pic for comparison.

The rear brake pads are about the same size between the 1.6 and the 1.8 but the rotor is larger moving the pads out further.  I believe this is done to improve braking efficiency by moving the leverage arm further out.  It would also improve heat dissipation somewhat.

Next is the tires. 

Currently we run Toyo RA-1's in NASA and SCCA Regional and SCCA National announced that they will be moving to the Hoosier R6 next year but all in the 205/50-15 size.  I figured we needed a little more rubber on the ground so I went to 225/45-15 Hankook Z214's for this project.  The Hankook is proven, popular and affordable.  So how much benefit do you get from going from 205/50's on a 7" wheel to 225/45's on an 8" wheel?  Would you believe just over a full inch of tread width?  That is about a 13.5% increase in rubber on the ground! For this comparison I compared apples to apples.  Doug's car just happed to have 205/50-15 Hankooks on it.

December 15, 2011 Update

I drove Doug's car at Infineon on Nov 19th and was amazed at the difference the brakes made.  The car felt great.  The Turbo mods will take place in January. 

Unitl then here are a couple of pics.  The kit from CX Racing is very nice.  The kit comes with a T28-2 Turbo

A very nice exhaust manifold

A nice larger Intercooler

and a whole bunch of tubing and brackets!

We will have lots and lots of pics of the install as it progresses.

January 8, 2012 Update

 In addition to pulling the engine, ECU and wiring harness out of the '95 Miata for the Ginetta G20 Project, I also managed to pull the differential out of Doug's Spec Miata in preparation for the '95 Torsen install.  For those of you who haven't tackled the diff swap it is pretty straight forward.  Here are a few pics to help.

First you want to disconnect the two half-shafts.  There are 4 nuts on each side.  It is easier to actually remove the half-shaft off of the studs after the diff is loose so there is more play.  If you try and force the half-shaft by the studs while the diff is firmly in place watch the cover on the end of the half-shaft as it will try and pop out and large ball bearings will fall out.  Ask me how I know this.  The second tip is to loosen the two large vertical bolts that go through the torque tube and diff but do not take them out.  Leave about 3/4" of thread showing at the bottom and use a mallot to pound them back up.  This will release the nuts on the top that are pressed into the torque tube and slightly protrude into the diff.  This will make removing the diff easier.

Next remove the 4 bolts that connect the driveshaft to the pinion flange on the diff.  If you have the car up on a lift you might want to suppor the back of the transmission as well.  This brace or torque tube as I have been referring to it acts like the rear engine mount so nothing is supporting the rear of the engine/transmission once the diff is removed.

There are two smaller nuts that keep the mount in place and then one large nut in the middle of each mount.  Remove the two small nuts first on each side.  This is a two person job, have something supporting the diff then remove the last two nuts.  It usually takes some manuvering to get the diff out.  Try and tilt the diff so the bottom comes out of the torque tube first then the diff will slide straight down.

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Here is a side-by-side comparison of the two differentials.  As you can see the '95 Torsen Diff is quite a bit beefier.  And for those that are out picking through the recyclers, here is what you are looking for.

If there is a large Pin or obstruction when you attempt to look through the half-shaft holes then it is an OPEN DIFF.  If you can see through it like this pic, then it is a TORSEN.

With the differential out, this is a perfect time to inpect the rear wheel bearings for play.  If either wheel bearing has excessive play check out this How-To Replace Rear Wheel Bearing to replace the bearing before continuing.

Here is a picture with the Torsen diff in place and one half-shaft installed.  After 1995 Mazda went to 1 piece hafl-shafts so I installed the right side half-shalft first then the differential.  Since I have the left side torn apart replacing the wheel bearing, I can install the left side half-shaft before reassembling the left side hub assembly. 

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