An End-of-build Retrospective on my Rx7 Evo GT by gmonsen

11 minute(s) of a 280 minute read

CHASSIS, STEERING, AND BRAKES

My Rx7 Evo GT20b is all about balance; making sure no one element in any area overwhelms the others or 

does not fit and work well with all of the others. The brakes shouldn't overwhelm the wheels and tires. The engine 

shouldn't overwhelm the chassis or the brakes.

When we talk about upgrading the FD's handling, we have to remember that the stock car was and remains one 

of the finest handling, more extreme sports cars. The suspension on the car from the factory used squeeze-cast 

A-arms and Rose-Heim joints. Nearly every tester at the time commented on how extreme the handling was. 

How light and how stiff to the point of being bone jarring. So, you can upgrade the handling, often notably, but 

it was already head-of-the-class to start with... 

My biggest issue was deciding on what approach to take on the subframe needed to mount my 20b motor, so 

that I didn't lose any of the handling the car came with originally. The problem is that the 20b doesn't clear the steer-

ing rack locating the motor on the existing mounts and there's not a lot of room. If you sit the motor up and in the 

same fore-aft location as the 13b, the front end weight increases and the center of gravity moves up. You get worse 

handling and often bump steer. 

I had watched several people build custom front sub-frames to mount their 20b motors in a way that didnt affect 

the handling or the steering. These custom subframes move the motor down and back a bit so the motor clears 

the steering rack. No bump steer, the CG may be lower, and the weight distribution is stock-to-slightly-more-

rearward than stock. 

While those are great objectives and they can be met this way, in practice, the sub-frames are difficult to make, ex- 

pensive, require changes to the firewall, transmission, and sometimes the driveshaft. I decided to go with an after-

market subframe that doesnt require any alterations. There may well be some minor disadvantages 

to this solution -- especially, perhaps, as regards the center of gravity -- but that is debatable at best. 

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RxSpecialties Subframe

So, I could have made a custom subframe made that would lower and move back the engine or one that posi-

tions the engine less than an inch higher and doesnt require moving the engine back into the firewall. I chose the 

latter from RxSpecialties in Canada. Theyve been supplying these subframes for years, they are extremely 

well engineered and made, and have no bump-steer. So, while its possible to debate whether this solution is 

optimal, I am fairly certain it results in as good weight balance and center of gravity as most FDs and was more 

of a factory install with no effect on the rest of the car structurally or cosmetically.

(Since I made this decision in 2007, Defined Autoworks in Ohio has come out with a kit that relocates the mounts 

and let's you use the stock subframe.) 

RxSpecialties sub-frame next to stock subframe:

Subframe mounted on chassis:

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Zeal Function XS Coil Over Shocks, plus springs and sway bars

When I first upgraded my old FD, Peter Farrell Stage II had Koni yellow and some progressive rate springs. I later 

went to Koni 2812 gas nitrogen race shocks paired with some 600-700 pound springs, harder bushings, M2 toe-links, 

and whatever the PFS sway bars were. It rattled my teeth and just rattled. I didnt want that again and decided to up-

grade to more adjustable shocks that have lower pressure valving to suit the softer springs I preferred. 

Zeal (Endless) makes several very nice lines of coil over shocks (as well as great brake products). Since my car is 

not intended for more than street and occasional track days, I do not need endless (no pun intended) adjustments, 

so I chose the Zeal Function XS series, with 6 way adjustable damping and a valving system that provides a very 

smooth ride over a wide variety of speeds. I am using the front-adjustable Racing Beat sway bars and have quite a 

few sets of springs.

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4.44:1 Ring and Pinion Gear

The stock 4.10:1 ring and pinion is well-suited to the stock motor and trans-mission ratios. Since my 20b NA mo-

tor revs to 9,000+ rpm and makes its best power from 5,000-9,000 rpm, I can really benefit from the higher torque 

multiplication of the 4.44:1 ring and pinion, and lose little or no top end speed. (I seldom do 180 even on track days.) 

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Atomic Rex manual rack

I did the looped line manual steering conversion on my first FD and tasted the feel that manual steering provid-

ed. I never tried the Maval manual conversion. When Atomic Rex came out with their new billet aluminum CNC-

machined manual rack, I bought the first one shipped here. Not only is there great feel, but it is also quicker at 

2.4 turns lock-to-lock (versus 2.9). It is also much more precise due to the different rack design and better ma-

chining of the rack and gear, as well as the much more substantial solid aluminum mounting blocks to the sub-

frame. This makes a tremendous change in the way the car feels, the way it turns in; the way it handles 

overall. [I am close to buying Atomic Rexs Electronic Power Assisted Steering (EPAS) kit that provides 

assistance at parking speeds, since its a bit much on occasion.] 

Heres the Atomic Rex manual rack next to the stock rack. Talk about hidden Bling!

Here is a picture showing the mounting blocks to affix the rack to the sub-frame. 

The stiffer mounting increases feel and precision:

Heres the rack installed in the subframe:

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New Spirit R Brakes

When I started upgrading, I was still under the sway of Peter Farrell, so I bought his big brake kit, which was a Wilwood kit and a very nice upgrade. Later, I upgraded that to M2's 

BBK, which had 13+ inch rotors and huge AP calipers and pistons. They really stopped the car right then. Quick enough that I briefly wore my seat belts. 

The stock FD brakes are 11.6 inches (295mm) with 4-piston calipers in front and 

11.6 inch rotors with 2-piston calipers in the rear and they were the quickest and shortest stopping brakes around. I think they held the record at the time or were second to Porsche. 

You can buy 13 inch (330mm) front rotors and up to 6-piston calipers. I chose Mazda's own later upgrade to the Spirit R 12.37 inch (314mm) front and rear rotors with larger, diff-

erent, but the same number of, pistons. They are not the "best" or most expensive upgrade, but are great for my purposes, and I wanted this to be as much Mazda as possible, given 

that its meant to be an evolution. One of the guys at Mazda USA got them for me and I m very grateful.

The new OEM Spirit R brakes, which have 314 x 32mm front rotors with stronger calipers with bigger pistons (~40mm) and the rears, while the same size rotors as stock, have the 

rotors differently shaped and dynamically balanced with upgraded calipers as well. I like them, because they are larger and better designed and made, but still OEM Mazda parts. The 

Brembo-like red calipers with the silver-white Mazda logo also fit with my color scheme. 

The 4 colors I used on the car: black, red, white, and silver...

Carbotech Brake Pads

I chose the new Carbotech AX6 brake pads, which are an intermediate pad focused on autocross and acceptable for street, but not pure or serious race applications. Carbotech pads 

are one of the best pads today and are made in America. The Endless (Zeal) pads are competitive, but are priced quite a bit higher for what is likely for me an unnoticeable difference 

in useful performance. Stainless steel brake lines and a 929 master cylinder complete the braking system.

1-18-2011

First, I'm holding off about a week before starting on the engine section in order to get some good pictures. I'm going to cover why I did what I did. What it was when I started and what is is now. I'll go cover the intake, rotors, porting, headers, and exhaust. Then, I'll post several dyno graphs showing how the power corresponds to the changes made. The dyno testing won't be done for several weeks, so don't hold your breath.

Supernaut... Yes. I have drawn and even modeled several ducktail spoilers over the years and still think its the best looking in many ways. However, I cannot find any design I like better than the stock hatch without any spoiler. Arguably, the best lines on the car out of so many good ones.

Chudsoncoupe... Agree. The lip is part of a perfect curve that extends across the entire tail light group wiuth there being a branching "S" curve outward that defines the turn signals. Its also interesting that the designers seem to have somewhat purposely created a Kamm-effect tail, though the effects are not fully incorporated at the sides. The top 3 inches of tail light goes up and back (actually resembling a ducks tail from this perspective) and the bumper goes back quite some ways.

Dual Bumper-Exit Exhausts

I did decide to go through with my own exhaust treatment, though, perhaps in place of the spoiler. I guess I'll just show the drawings and describe it at least cosmetically. I have wanted a dual exhaust for a long time. That is, I have wanted an exhaust that has a pipe on either side of the back of the car. Not simply 2 pipes or tips coming out under the passenger side. However, until recently, I couldn't figure out how to do it without completely redoing the rear wheel well, gas tank and gas tank filler. I finally did figure out how to do it and here's how its going to look. (Before anyone says how ugly it is, wait until I describe it fully... )

First, here's how it would look basically from directly behind the car. The tail pipes exit on either side of the license plate holder indentation and are at least for now, shaped like a rotor. They could end up being round, if I think the rotor thing is overdone back there, what with the new emblem. 

Here's another set of drawings that show how they are made and how it works. The exhaust outlets around the rotor shaped tips are aluminum and will be powder coated gloss black (or might be anodized black). The tips will be polished aluminum, but they won't extend much or at all beyond the black aluminum exhaust outlets... The exhaust tips come out of a 25x6x3 inch aluminum box mounted in the bumper skin in place of the foam and other hardware there. (It will be a structural part of the bumper.)

And, here's a quickly drawn overview of the exhaust system and how the exhaust gets into the aluminum box. There are 2 exhaust pipes and a side laker style pipe. The primary pipe works up to about 4000 rpm, then the secondary opens, and finally over 7500, the laker opens to exit to air under the passenger door. I do not think the secondary pipe will be able to exit from this box. Not big enough, I think. It might, but it will probably just end earlier somewhere after the Borla.