THE MODS ENCYCLOPEDIA

A ZRR0 IX reference — every category of modification, taught as lessons.

The goal of this document is deep literacy. For each mod you get five things, not just a name:

1. FUNCTION — what it physically does to the airflow / forces / mechanics.
2. POWER / HANDLING IMPACT — what it actually adds (and where it does nothing).
3. SOUND — the note it creates and what that note means culturally.
4. VISUAL / DISTINCTION — what it signals to anyone who knows.
5. AERO — where relevant, the air's behavior and the trade-off.

It is organized fundamentals → advanced, and it's built so you can choose mods by the effect you want: more power, better handling, a specific sound, or a visual identity. There's a chooser index at the very bottom.

A recurring theme for ZRR0 IX, given the drift focus: a drift car is not a grip car. The hierarchy of needs is angle + control + recoverability + a locked rear axle, and only then power. Many "fast car" mods are actively wrong for drift, and this doc flags those. The brand lines map naturally onto effect categories — HER0 (zero-to-hero) is the power/forced-induction story, MUTT (cross-breed, e.g. JDM chassis + American V8) is the drivetrain-and-engine-swap story, and ZRR0 IX (import) is the chassis/aero/fitment-identity story.

PART 1 — POWER

The mental model that ties this entire section together: an engine is an air pump. Horsepower is the product of how much air+fuel you can burn per cycle and how many cycles you run per minute. Every "power" mod is doing one of four jobs:

• Get more air IN (intake, turbo/supercharger, intercooler, ported heads, cams)
• Get exhaust OUT faster (headers, downpipe, cat-back)
• Match fuel to that air (injectors, pump, methanol)
• Tell the engine what to do with it all (ECU tune / standalone)

You cannot skip the fuel and the tune. A turbo with stock fueling and stock software doesn't make 400hp — it makes a hole in a piston. Keep that hierarchy in mind for the whole section.

1.1 — Cold-Air Intake (CAI)

Sound: This is the single biggest reason people buy one. An open-element filter unmasks intake noise — on NA cars a deeper induction growl under throttle; on turbo cars an audible intake "whoosh" and, with the right setup, the turbo's spool whistle becoming clearly audible. Culturally it reads as "this car breathes."

Visual / distinction: A polished or colored intake tube and an exposed cone filter under the hood. Low-key, but it's the first thing another enthusiast looks for when the hood is up — a "yes, it's touched" signal.

Heat soak (key concept): when the car sits in traffic, the engine bay bakes the filter and intake air temps (IAT) climb, killing the density advantage. The whole point of "cold" air is fighting heat soak. (Mishimoto, Car Performance Boss)

1.2 — Exhaust (the system, and how each section changes the NOTE)

Exhaust is where sound identity is decided, so this gets the most depth. Working from the engine backward, here is each section and what it owns.

Headers (a.k.a. extractors / manifold)

Downpipe (turbo cars)

Cat-back

Axle-back

The sound logic, summarized: Headers shape the engine's voice. The mid-pipe (X-pipe vs H-pipe) shapes the timbre — an X-pipe gives a higher, raspier, more exotic/European scream; an H-pipe gives a lumpier, classic American muscle burble. The muffler sets the volume and droop. Tip diameter is mostly cosmetic but affects the very end of the note.

Drift note: drift judges and culture do not care about exhaust note for points, but a loud, distinctive exhaust is part of a car's identity on the door. For a turbo drift car the downpipe matters for power; the cat-back is identity.

1.3 — Turbocharger Kits

Sound: the iconic turbo signatures — a rising spool whistle as it spins up, the blow-off "whoosh" on lift, and (love-it-or-hate-it) flutter (see §6). To a tuner ear, turbo sound = "boost."

Visual / distinction: intercooler showing through the front bumper, a fat charge pipe, an external wastegate screamer, a front-mount that says "this is a build."

Drift context: turbos are extremely popular in drift for the brute power to break traction and hold a slide, but the lag has to be managed; immediate, predictable power is what lets you modulate angle with the throttle. (Drifted, Chariotz)

1.4 — Supercharger Kits

Sound: the supercharger whine — that rising mechanical siren audible inside the cabin, varying by type (Roots/twin-screw/centrifugal each whine differently). Culturally it is unmistakable: it reads as raw, mechanical, "muscle/Hellcat" menace, and it's an instant identity signature.

Visual / distinction: a Roots blower sticking through the hood is the loudest visual statement in all of car culture — pure MUTT/HER0 theater. A discreet twin-screw under a hood with a "supercharged" badge is the subtle version.

Drift context: the instant, lag-free delivery is genuinely better for the precise throttle control drift demands — you can meter wheelspin to the degree. The downside is fewer off-the-shelf kits than turbos for oddball chassis. (Drifted)

1.5 — Intercoolers

Sound: none of its own (it's a radiator for air). But a bigger intercooler with new piping often comes with louder BOV/spool because the pressurized volume changes.

Visual / distinction: a big front-mount core grinning through the front bumper is one of the most recognizable "tuner build" visuals — instant turbo-car identity. Aero footnote: a large FMIC partially blocks the radiator and disrupts front airflow, a real trade on track cars.

1.6 — Camshafts

Sound: the lopey/choppy idle. A tight LSA (≈106–110°) increases overlap and produces that signature "buh-buh-bup-bup" hunting idle. That sound is a direct readout of cam aggressiveness — to anyone who knows, a lopey idle screams "this thing has a big cam in it." (DT Racing)

Visual / distinction: invisible, but the sound IS the visual — the idle is the calling card. It is one of the purest "tell" mods in the hobby.

1.7 — Ported / Ported-and-Polished Heads

Sound: indirect — more flow generally means a freer, fuller exhaust note at high RPM.

Visual / distinction: invisible (it's inside the head), but it's a "serious builder" credential — it signals the engine was opened up and worked, not just bolt-ons.

1.8 — Forged Internals

Sound: none directly (slight cold piston-slap tick on some builds).

Visual / distinction: invisible, but it's the credibility mod — "built motor" / "fully forged" is the phrase that separates a reliable big-power car from a grenade. For HER0 zero-to-hero builds, this is the backbone that makes the headline horsepower trustworthy. (NASA Speed News)

1.9 — Fueling (Injectors, Pumps, Rails, Lines)

Sound / visual: none meaningful. This is pure enabling hardware — but skipping it is the #1 way builds blow up.

1.10 — ECU Tunes & Standalone ECUs

Sound: indirect but real — tuning enables/disables anti-lag, launch control (bang/pop), pops-and-bangs on overrun, BOV behavior, and boost level (which dictates spool/whistle). The "tune" is what makes the car talk.

Visual / distinction: invisible, but it's the difference between "has parts" and "is dialed in." For drift, the standalone is also what enables drivability features (idle control with a big cam, anti-lag to kill the boost gap mid-transition).

1.11 — Nitrous (NOS)

Sound: none of its own — but the purge (venting nitrous lines before a run, a sharp hiss + visible vapor cloud) is pure drag-strip theater.

Visual / distinction: the purge mist and the bottle in the trunk/cabin. Old-school, drag-rooted "spray" identity. More drag than drift, but it shows up on grudge and street cars.

1.12 — Methanol / Water-Methanol Injection

Sound / visual: essentially none — this is a reliability and consistency mod, the quiet pro move on a high-boost street/drift car, not a show piece.

PART 2 — DRIVETRAIN

Power is useless if you can't put it to the ground — or, for drift, if you can't break both rear wheels loose predictably. This section is where a drift car is really made.

2.1 — Clutch & Flywheel

Sound: the light flywheel's gear rattle/chatter at idle is an audible tell, and the faster rev blip is its own signature. Aggressive clutches can add a chatter on engagement.

Drift note: the clutch kick (stab the clutch to shock-load the rear and break traction) is a core drift technique — so clutch durability and a responsive flywheel directly serve the driving style.

2.2 — Limited-Slip Differential (LSD) / Welded Diff

This is the single most important drift mod, period. An open diff sends power to the wheel with least grip — so when you initiate a slide, the unloaded inside wheel spins uselessly and the slide dies. You need the rear axle to act as one.

Function detail: "preload" is how locked the diff is even at zero torque; "ramp angle" sets how aggressively it locks as torque rises. A high-preload, aggressive-ramp 2-way clutch LSD gives the near-analog locking drift wants. (KAAZ)

Sound: a welded diff chirps/clunks going slowly around tight corners (the locked wheels skip) — an unmistakable "this is a drift/missile car" audio tell in a parking lot. Clutch LSDs are quiet with the right fluid.

Distinction: invisible, but it's the dividing line between "looks like a drift car" and "is one."

2.3 — Gearing (Final Drive & Gear Ratios)

Drift note: drift cars often run gearing that keeps them in the powerband through the higher entry speeds and minimizes shifting mid-run — you want torque on tap, not to be hunting gears at full lock.

Sound/visual: none directly (higher cruise RPM is a faint tell).

2.4 — Shifter (Short-Shifter / Bushings)

Sound: a mechanical, metallic "snick" into gear, plus more drivetrain noise in the cabin. Visual: an aftermarket shift knob — small but a constant tactile/visible identity touch.

PART 3 — HANDLING

For a grip car, handling mods maximize mechanical grip. For a drift car, the goals shift: you want steering angle, weight-transfer control, a planted-but-adjustable rear, and predictable breakaway. Same parts, different tuning philosophy.

3.1 — Coilovers

Drift use: drift coilovers favor high spring rates (to control the violent weight transfer of initiation) and ride height set for angle clearance, not minimum height. Damping is tuned so the rear steps out predictably.

Sound/visual: the stance — a properly dropped car on coilovers is the foundational visual of the whole modified-car aesthetic. No real sound (cheap ones can clunk).

3.2 — Sway Bars (Anti-Roll Bars)

Drift use: a stiffer rear sway bar helps the car rotate and break loose more willingly. Often run with adjustable end links.

Sound/visual: none — pure dynamics.

3.3 — Bushings (Poly / Spherical)

Drift use: reinforced/spherical bushings (and chassis reinforcement) keep the geometry honest under the violent loads of curb-hopping and full-lock transitions.

Sound: more road/suspension noise and clunks transmitted into the cabin — a "race car" texture. Visual: none (telltale colored poly inside the arms).

3.4 — Alignment (Camber, Caster, Toe)

Drift use (critical): drift cars run huge positive caster (so the wheels gain camber and the steering self-centers/recovers at full lock) and lots of steering angle — this is what an "angle kit" (modified knuckles/arms) provides: more lock so you can hold bigger slides and recover. This is the alignment-and-geometry heart of a drift build.

Sound/visual: the aggressive static camber stance (wheels tucked in at the top) is a defining visual identity — though on a drift car it's functional, not just for show.

3.5 — Wheel Spacers

Visual / distinction: the cheapest way to get the wheels out to the fender for that flush or poke look (see Part 6).

Sound: none.

3.6 — Big Brake Kits (BBK)

Drift note: ironically, drift cars care most about a strong front brake (you brake with the fronts to initiate weight transfer) and often run a separate hydraulic handbrake for the rear (locks the rears on demand to kick the tail out / link sections). The hydro e-brake is itself a signature drift mod.

Sound: track pads can squeal cold — a "race pad" tell. Visual: big colored calipers behind open-spoke wheels = a clean performance statement.

PART 4 — AERO / EXTERIOR

The big idea: at speed, air is a fluid you can push against. Aero either creates downforce (presses tires into the road for grip), reduces drag, or manages flow (keeps high-pressure air out from under the car). Below a certain speed aero does little — most street "aero" is form. The honesty test for every part below is: function, or form? Both are legitimate for ZRR0 IX, but you should always know which you're buying.

Drift caveat: drift happens at high yaw (the car sideways), where conventional aero (designed for the car pointing forward) is partly stalled. So in drift, aero leans heavily toward visual identity and durability (replaceable, cheap-to-fix bumpers/lips for door-to-door contact) more than measured downforce — with rear wings doing real work at entry speed.

4.1 — Front Lip / Splitter

Visual / distinction: an aggressive splitter (often with rods) = "track car." A subtle lip = clean, OEM+ aggression. Aero: real, but only meaningful at speed. (Grassroots Motorsports — diffusers/aero)

4.2 — Rear Wings vs Ducktails/Ducklips

Function detail: wings and diffusers work together — a wing energizes the underbody flow so the diffuser works better, and the wing pulls real downforce up top. A ducktail is a low-drag, low-downforce clean-up device that needs the whole rear bodywork designed around it (e.g., Singer reshaped a 964 roof to feed a ducktail). (Rennlist, Revozport)

Sound: none. Visual / distinction: the wing is one of the loudest statements a car can make about its intent — GT wing = "I do track speed"; ducktail = "I have taste." Choosing between them is choosing an identity.

4.3 — Rear Diffuser

Visual / distinction: aggressive fins/strakes under the rear bumper = motorsport language. Often cosmetic on street cars, genuinely functional on built ones.

4.4 — Canards / Dive Planes

Visual / distinction: stacked canards = "aggressive time-attack" signature. On most street cars they are form, not function.

4.5 — Widebody (Function vs Form)

Drift note: drift widebodies are a blend — they're functional (clear the angle + wide rear tires) and the defining visual of competition drift (the riveted overfender + wing aesthetic). This is the MUTT / ZRR0 IX signature look.

Aero: wide bodywork changes frontal area and flow; serious builds integrate it with splitters/diffusers, but a bolt-on flare alone is neutral-to-slightly-worse on drag.

4.6 — Hoods (Vented / Cowl / Lightweight)

Visual / distinction: a vented carbon hood is a strong "built engine underneath" signal; a cowl hood reads classic muscle. The exposed-carbon-weave hood is a core tuner identity piece.

Sound: indirect — a vented hood lets more induction/turbo noise out.

4.7 — Wraps / Paint / Livery

Sound/aero: none. Distinction: this is the distinction layer.

4.8 — Lighting

Visual / distinction: a distinctive light signature is how a build is recognized at night — valuable for a brand fleet.

PART 5 — SOUND, AS ITS OWN DISCIPLINE

Sound is not a side effect — for a builder it's a deliverable. Here's the full vocabulary, what causes each note, and what it means to people who know. (Cross-references to the parts that create each.)

5.1 — Blow-Off Valve (BOV) / Diverter

5.2 — Turbo Flutter (Compressor Surge)

The cultural meaning is the whole point: flutter went viral as a "JDM/boosted" status sound, so plenty of builds run it intentionally and accept the turbo wear as a cost of identity. Knowing it's damage — and choosing it anyway — is exactly the kind of literate decision this doc is for.

PART 6 — WHEELS / STANCE / FITMENT

Fitment is its own language. The grammar is offset, width, and tire stretch — and the "sentence" you write places you in a subculture.

6.1 — The Core Vocabulary

(Velgen, ModBargains, Suspension Setups)

6.2 — The Fitment Styles (each is an identity)

(Velgen, StanceNation, Kipardo)

6.3 — Function vs Form (the honest split)

• Functional (drift/grip): correct offset to fill a widebody, square or staggered setup with real tire width, sane stretch (a little stretch helps bead seating and avoids tire roll on the rim — a legit drift trick), strong forged/flow-formed wheels (lighter = less unsprung mass = better response).
• Form (stance/show): max poke, heavy stretch, aggressive static camber — chosen for the look, accepting tire wear, ride harshness, and reduced grip.

Drift reality: drifters often run a stretched tire on a wide wheel for a deliberately firmer sidewall and secure bead under huge slip angles — so here the "aggressive" look and the function genuinely overlap.

Sound: none. Distinction: fitment is, after the wrap, the loudest static identity statement a car makes — it's read instantly in any parking lot.

PART 7 — WEIGHT REDUCTION

The principle: power-to-weight is what matters, and lighter helps everything at once — acceleration, braking, cornering, tire wear, and response. It's "free power" with no engine risk. And it's directional: where you remove weight matters as much as how much.

Trade-off: comfort, NVH, daily usability, and (for drift) you actually want enough mass and the right balance — a too-light tail can be twitchy, and competition rules often impose minimum weights.

Sound/visual: stripped interiors + exposed carbon panels = unmistakable "purpose-built" identity. The car sounds hollower and more resonant inside (less deadening).

Drift note: drift cares more about balance and momentum than minimum mass — many drift cars keep weight for stability and run the engine's torque to overcome it. Don't strip blindly.

PART 8 — INTERIOR

The cabin is where safety, control, and identity meet — and for a brand, it's a photographed, customer-facing space.

8.1 — Seats (Bucket / Racing)

Distinction: branded buckets (the famous Recaro/Bride/Sparco-style names) + a matching harness are a core JDM/motorsport interior identity.

8.2 — Roll Cage

Distinction: a clean, well-triangulated cage (often color-matched) is the strongest "this is a real build" interior statement there is.

8.3 — Audio

Distinction: a show/lifestyle build leans into audio; a competition build deletes it. ZRR0 IX likely splits this by line — show/brand cars vs comp cars.

PART 9 — CHOOSE YOUR MODS BY THE EFFECT YOU WANT

Use this as the decision map. Find the effect, build in the order listed.

"I want MORE POWER" (HER0 / built-motor path)

1. Fueling + ECU tune/standalone first — the safety net (§1.9, §1.10).
2. Bolt-ons: downpipe (turbo) or headers (NA) + cat-back + intake (§1.1–1.2).
3. The adder: turbo (most peak power) or supercharger (instant, lag-free) (§1.3–1.4).
4. Support it: intercooler + forged internals + ported heads + cam (§1.5–1.8).
5. Push further safely: methanol injection; nitrous for on-demand spikes (§1.11–1.12).

"I want BETTER HANDLING / a real drift car" (ZRR0 IX / MUTT path)

1. LSD (2-way clutch) or welded — the #1 drift mod, do this first (§2.2).
2. Coilovers + alignment (huge caster + angle kit) (§3.1, §3.4).
3. Sway bars (stiffer rear) + bushings for balance + precision (§3.2–3.3).
4. Clutch/flywheel + hydraulic handbrake for initiation (§2.1, §3.6).
5. Big front brakes + tire/wheel fitment for control (§3.6, §6).

"I want a SPECIFIC SOUND"

• Deep muscle rumble → headers + H-pipe cat-back (§1.2).
• High exotic scream → X-pipe + high-revving engine (§1.2).
• Lopey aggressive idle → camshaft (§1.6).
• Whine → supercharger (§1.4).
• Boosted "pssh" / spool → turbo + BOV (§1.3, §5.1).
• "Stututu" flutter → turbo, no BOV (accept the wear) (§5.2).
• Pops & bangs → ECU tune (§1.10).

"I want a VISUAL IDENTITY / brand presence" (the ZRR0/HER0/MUTT livery layer)

1. Wrap/paint + livery — where the brand lines live (§4.7).
2. Fitment: wheels + offset + stance (§6).
3. Widebody — the biggest silhouette change (§4.5).
4. Wing/ducktail + lip/splitter + diffuser — pick form vs function deliberately (§4.1–4.4).
5. Vented carbon hood + lighting signature + interior (buckets/cage) (§4.6, §4.8, §8).

Sources

• Holley — Axle-back vs Cat-back: https://www.holley.com/blog/post/axle-back_exhaust_vs_cat-back_which_is_best_/
• MagnaFlow — Cat-back vs Axle-back: https://www.magnaflow.com/blogs/magnaflow/cat-back-exhaust-vs-axle-back-exhaust-systems
• Drifted — Turbo vs Supercharger: https://www.drifted.com/turbocharger-vs-supercharger/
• Chariotz — Turbo vs Supercharger 2026: https://www.chariotz.com/blog/turbo-vs-supercharger-in-2026-a-decision-framework-that-is-not-just-about-power/
• UTI — Supercharged vs Turbocharged: https://www.uti.edu/blog/automotive/supercharger-vs-turbocharger
• Mishimoto — How an Intercooler Works: https://www.mishimoto.com/engineering/2025/01/how-does-an-intercooler-work
• Haltech — How Intercoolers Work: https://support.haltech.com/portal/en/kb/articles/how-intercoolers-work
• Car Performance Boss — Intercooler vs Cold Air Intake: https://carperformanceboss.com/intercooler-vs-cold-air-intake-power-costs-tuning/
• LSXMag — Camshaft Tech (Lift/Duration/LSA): https://www.lsxmag.com/tech-stories/camshaft-tech-with-howards-cams-lift-duration-and-lsa/
• DT Racing — Cam Specs (lopey idle/LSA): https://dtracing.com/camshaft-specs-explained-lift-duration-lsa-hemi/
• Jalopnik — Camshaft Specs Explained: https://www.jalopnik.com/1991029/camshaft-specs-explained/
• NASA Speed News — Lightweight Flywheels/Clutches: https://nasaspeed.news/tech/drivetrain/reducing-rotating-mass-lightweight-flywheels-and-clutches-unleash-more-power/
• Clutch Masters — Dual-Mass vs Single-Mass Flywheel: https://clutchmasters.com/n-150-dual-mass-single-mass-flywheel-whats-difference.html
• Haltech — ECU Buyer's Guide: https://www.haltech.com/ecu-buyers-guide/
• Project JDM — DIY ECU Tuning Comparison: https://projectjdm.org/blog/diy-ecu-tuning-comparison-guide
• Haltech — Nitrous, the Forgotten Power Adder: https://www.haltech.com/nitrous-the-forgotten-power-adder/
• Chevy Hardcore — Power Adders: https://www.chevyhardcore.com/tech-stories/power-adders-nitrous-blowers-and-turbos-for-serious-power/
• PistonHeads — Methanol vs Nitrous: https://www.pistonheads.com/gassing/topic.asp?h=0&f=66&t=1455554
• R&R Transmissions — 1-way/1.5-way/2-way LSD: https://www.rrtransmissions.com/blog/2023/9/23/the-great-differential-debate-demystifying-1-way-15-way-and-2-way-limited-slip-differentials
• DSPORT — Everything About LSDs: https://dsportmag.com/the-tech/speed-shop/everything-you-need-to-know-about-limited-slip-differentials/2/
• RacingDiffs — 1.5-way vs 2-way: https://racingdiffs.com/blogs/news/1-5-way-vs-2-0-way-lsd-pros-and-cons-which-is-better
• KAAZ — Clutch-Type LSD: https://kaazusa.com/tech-library/the-kaaz-advantage-kaazs-clutch-type-lsd/
• Drifted — Welded Differential Guide: https://www.drifted.com/welded-differential/
• Texas Drift Academy — Open vs LSD vs Welded: https://texasdriftacademy.com/what-differential-for-drifting-open-vs-lsds-vs-welded/
• A-Premium — Sway Bars vs Coilovers: https://a-premium.com/blogs/sway-bars-vs-coilovers
• ES Motor — Suspension Upgrades Explained: https://esmotor.co/2025/07/21/porsche-suspension-upgrades-coilovers-sway-bars-control-arms-explained/
• Verus Engineering — Canards/Dive Planes: https://www.verus-engineering.com/blog/informative-8/do-canards-or-dive-planes-really-improve-aerodynamics-130
• Occam's Racer — What Do Canards Do: https://occamsracers.com/2023/11/12/what-do-canards-do/
• Grassroots Motorsports — Diffusers Explained: https://grassrootsmotorsports.com/articles/downforce-sorcery-diffusers-explained/
• Revozport — What a Rear Diffuser Does: https://revozport.com/blogs/main/what-does-a-rear-diffuser-do
• Rennlist — GT2 Wing vs Ducktail: https://rennlist.com/forums/993-forum/1095464-rear-end-aerodynamics-discussion-gt2-wing-v-ducktail.html
• Velgen — Flush/Functional/Poke Fitment: https://www.velgenwheels.com/blogs/blog/flush-functional-poke-wheel-fitment-styles
• ModBargains — Wheel Offset: https://blog.modbargains.com/wheel-offset/
• Suspension Setups — Wheel Offset: https://suspensionsetups.com/garage/wheel-offset/
• StanceNation — Stretch & Poke: https://www.stancenation.com/car-features/slammed/stretch-poke/
• Kipardo — Fitment Styles Explained: https://www.kipardoracing.com/different-types-of-fitment-explained-flush-poke-stance-tucked/
• Wikipedia — Blowoff Valve: https://en.wikipedia.org/wiki/Blowoff_valve
• GFB — The Truth About Compressor Surge: https://gfb.com.au/tech/tech-articles/11-the-truth-about-compressor-surge/
• Drifted — Turbo Flutter: https://www.drifted.com/turbo-flutter/
• Low Offset — What Is Turbo Flutter: https://low-offset.com/workshop/what-is-turbo-flutter/
• Formula DRIFT — 2025 PRO Technical Regulations: https://www.formulad.com/storage/rulebooks/May2025/2025%20Formula%20Drift%20PRO%20Technical%20Regulations%20V1.5.pdf
• Formula DRIFT — 2025 PRO/AM Safety Regulations: https://www.formulad.com/storage/rulebooks/December2024/2025%20Formula%20Drift%20PROAM%20Safety%20Regulations%20V1.0.pdf
• Formula DRIFT — Rulebooks index: https://www.formulad.com/rulebook