diff --git a/app/services/simulations/__tests__/mlb-simulator.test.ts b/app/services/simulations/__tests__/mlb-simulator.test.ts
index 2741d62..68f782b 100644
--- a/app/services/simulations/__tests__/mlb-simulator.test.ts
+++ b/app/services/simulations/__tests__/mlb-simulator.test.ts
@@ -2,7 +2,8 @@ import { describe, it, expect } from "vitest";
 import {
   normalizeTeamName,
   getTeamData,
-  eloWinProbability,
+  winRateFromRDif,
+  rdifWinProbability,
   simBo3,
   simBo5,
   simBo7,
@@ -32,7 +33,7 @@ describe("getTeamData", () => {
     expect(d).toBeDefined();
     expect(d?.league).toBe("NL");
     expect(d?.division).toBe("NL West");
-    expect(d?.elo).toBeGreaterThan(1500);
+    expect(d?.rdif).toBeGreaterThan(0);
   });
 
   it("is case-insensitive", () => {
@@ -87,34 +88,18 @@ describe("getTeamData", () => {
       expect(getTeamData(name)?.league, `${name} should be NL`).toBe("NL");
     }
   });
-});
 
-// ─── eloWinProbability ────────────────────────────────────────────────────────
-
-describe("eloWinProbability (PARITY_FACTOR = 350)", () => {
-  it("returns 0.5 for equal Elo ratings", () => {
-    expect(eloWinProbability(1500, 1500)).toBeCloseTo(0.5, 6);
+  it("Dodgers have the highest RDif in NL West", () => {
+    const dodgers = getTeamData("Los Angeles Dodgers")?.rdif ?? -999;
+    const padres  = getTeamData("San Diego Padres")?.rdif ?? -999;
+    const dbacks  = getTeamData("Arizona Diamondbacks")?.rdif ?? -999;
+    expect(dodgers).toBeGreaterThan(padres);
+    expect(dodgers).toBeGreaterThan(dbacks);
   });
 
-  it("favors the higher-rated team", () => {
-    expect(eloWinProbability(1570, 1500)).toBeGreaterThan(0.5);
-    expect(eloWinProbability(1500, 1570)).toBeLessThan(0.5);
-  });
-
-  it("is anti-symmetric: P(A>B) + P(B>A) = 1", () => {
-    const p = eloWinProbability(1560, 1480);
-    expect(p + eloWinProbability(1480, 1560)).toBeCloseTo(1.0, 10);
-  });
-
-  it("a 100-pt gap gives ~65.9% win prob per game", () => {
-    // At 350: P = 1 / (1 + 10^(-100/350)) = 1 / (1 + 0.518) ≈ 0.659
-    const p = eloWinProbability(1600, 1500);
-    expect(p).toBeCloseTo(0.659, 2);
-  });
-
-  it("returns a value strictly between 0 and 1", () => {
-    expect(eloWinProbability(1200, 1800)).toBeGreaterThan(0);
-    expect(eloWinProbability(1200, 1800)).toBeLessThan(1);
+  it("rebuilding teams have negative RDif", () => {
+    expect(getTeamData("Chicago White Sox")?.rdif ?? 0).toBeLessThan(0);
+    expect(getTeamData("Colorado Rockies")?.rdif ?? 0).toBeLessThan(0);
   });
 });
 
@@ -137,28 +122,71 @@ describe("team seeding probabilities", () => {
     }
   });
 
-  it("p_div + p_wc does not exceed 1.0 per team", () => {
-    for (const teams of Object.values(DIVISIONS)) {
-      for (const name of teams) {
-        const d = getTeamData(name);
-        expect((d?.p_div ?? 0) + (d?.p_wc ?? 0), `${name} p_div + p_wc > 1`).toBeLessThanOrEqual(1.001);
-      }
-    }
-  });
-
   it("Dodgers have the highest p_div in NL West", () => {
-    const dodgers = getTeamData("Los Angeles Dodgers");
-    const padres  = getTeamData("San Diego Padres");
-    const dbacks  = getTeamData("Arizona Diamondbacks");
-    expect(dodgers?.p_div ?? 0).toBeGreaterThan(padres?.p_div ?? 0);
-    expect(dodgers?.p_div ?? 0).toBeGreaterThan(dbacks?.p_div ?? 0);
+    const dodgers = getTeamData("Los Angeles Dodgers")?.p_div ?? 0;
+    expect(dodgers).toBeGreaterThan(0.8);
   });
 
-  it("rebuilding teams have low p_div", () => {
-    const whiteSox = getTeamData("Chicago White Sox");
-    const rockies  = getTeamData("Colorado Rockies");
-    expect(whiteSox?.p_div ?? 0).toBeLessThan(0.15);
-    expect(rockies?.p_div ?? 0).toBeLessThan(0.10);
+  it("rebuilding teams have very low p_div", () => {
+    expect(getTeamData("Chicago White Sox")?.p_div ?? 1).toBeLessThan(0.02);
+    expect(getTeamData("Colorado Rockies")?.p_div ?? 1).toBeLessThan(0.01);
+  });
+});
+
+// ─── winRateFromRDif ──────────────────────────────────────────────────────────
+
+describe("winRateFromRDif", () => {
+  it("returns 0.5 for RDif = 0", () => {
+    expect(winRateFromRDif(0)).toBeCloseTo(0.5, 6);
+  });
+
+  it("returns a value above 0.5 for positive RDif", () => {
+    expect(winRateFromRDif(100)).toBeGreaterThan(0.5);
+  });
+
+  it("returns a value below 0.5 for negative RDif", () => {
+    expect(winRateFromRDif(-100)).toBeLessThan(0.5);
+  });
+
+  it("clamps to [0.01, 0.99]", () => {
+    expect(winRateFromRDif(10000)).toBe(0.99);
+    expect(winRateFromRDif(-10000)).toBe(0.01);
+  });
+
+  it("Dodgers (RDif +137) project to a win rate above .500", () => {
+    expect(winRateFromRDif(137)).toBeGreaterThan(0.5);
+    expect(winRateFromRDif(137)).toBeLessThan(0.65);
+  });
+});
+
+// ─── rdifWinProbability ───────────────────────────────────────────────────────
+
+describe("rdifWinProbability (log5)", () => {
+  it("returns 0.5 for equal RDif", () => {
+    expect(rdifWinProbability(50, 50)).toBeCloseTo(0.5, 6);
+  });
+
+  it("favors the team with better RDif", () => {
+    expect(rdifWinProbability(100, 0)).toBeGreaterThan(0.5);
+    expect(rdifWinProbability(0, 100)).toBeLessThan(0.5);
+  });
+
+  it("is anti-symmetric: P(A>B) + P(B>A) = 1", () => {
+    const p = rdifWinProbability(80, -20);
+    expect(p + rdifWinProbability(-20, 80)).toBeCloseTo(1.0, 10);
+  });
+
+  it("returns a value strictly between 0 and 1", () => {
+    expect(rdifWinProbability(200, -200)).toBeGreaterThan(0);
+    expect(rdifWinProbability(200, -200)).toBeLessThan(1);
+  });
+
+  it("Dodgers vs Rockies favors the Dodgers", () => {
+    // Even the largest RDif gap in the league should still favour the better team,
+    // but compression toward .500 via RDIF_DIVISOR keeps it well below 1.0.
+    const p = rdifWinProbability(137, -173);
+    expect(p).toBeGreaterThan(0.5);
+    expect(p).toBeLessThan(1.0);
   });
 });
 
diff --git a/app/services/simulations/mlb-simulator.ts b/app/services/simulations/mlb-simulator.ts
index 49a17ff..77b7577 100644
--- a/app/services/simulations/mlb-simulator.ts
+++ b/app/services/simulations/mlb-simulator.ts
@@ -6,11 +6,11 @@
  *
  * Algorithm:
  *   1. Load all participants for the sports season from DB
- *   2. Match participant names to hardcoded team data (Elo + seeding probabilities)
+ *   2. Match participant names to hardcoded team data (RDif + league/division)
  *   3. For each simulation:
  *      a. For each league (AL/NL), draw 1 division winner per division (3 draws),
- *         then draw 3 wildcard teams from the remaining pool — all weighted draws.
- *      b. Seed division winners 1–3 by Elo (best Elo = seed 1); WC teams 4–6 by Elo.
+ *         then draw 3 wildcard teams from the remaining pool — all weighted by win rate.
+ *      b. Seed division winners 1–3 by RDif (best RDif = seed 1); WC teams 4–6 by RDif.
  *      c. Run the playoff bracket per league:
  *         - Wildcard Round (best-of-3): 3 vs 6, 4 vs 5 (seeds 1 & 2 get byes)
  *         - Division Series (best-of-5): 1 vs lowest WC survivor, 2 vs other
@@ -19,20 +19,22 @@
  *   4. Track placement counts per scoring tier
  *   5. Convert counts to probability distributions
  *
- * Win probability (Elo, PARITY_FACTOR = 350):
- *   P(A beats B) = 1 / (1 + 10^((eloB - eloA) / 350))
- *   MLB uses a lower parity factor than the NBA (400) or NHL (1000) to reflect
- *   that better teams win series at a somewhat higher rate in baseball. The short
- *   best-of-3 wildcard round introduces significant upset potential.
+ * Win probability (log5 formula):
+ *   Step 1 — convert projected RDif to win rate:
+ *     winRate = clamp(0.5 + rdif / RDIF_DIVISOR, 0.01, 0.99)
+ *     RDIF_DIVISOR compresses team strengths toward .500 for playoff parity.
+ *     See the RDIF_DIVISOR constant below for tuning guidance.
+ *   Step 2 — Bill James log5 head-to-head probability:
+ *     P(A beats B) = (wA - wA·wB) / (wA + wB - 2·wA·wB)
  *
  * Futures blending:
  *   If sourceOdds are stored in participantExpectedValues for this season,
  *   the per-game win probability is blended:
- *     P(game) = ELO_WEIGHT * eloProb + ODDS_WEIGHT * oddsProb
+ *     P(game) = RDIF_WEIGHT * rdifProb + ODDS_WEIGHT * oddsProb
  *   where oddsProb = normalizedOdds(A) / (normalizedOdds(A) + normalizedOdds(B)).
  *   Normalized odds are vig-removed futures win probabilities.
- *   ELO_WEIGHT = 0.7, ODDS_WEIGHT = 0.3 (same calibration as NHL simulator).
- *   Falls back to Elo-only when no odds are stored.
+ *   RDIF_WEIGHT = 0.7, ODDS_WEIGHT = 0.3.
+ *   Falls back to RDif-only when no odds are stored.
  *
  * Placement tiers → SimulationProbabilities mapping:
  *   probFirst              = World Series champion (1 per sim)
@@ -43,17 +45,17 @@
  *   Missed playoffs        → all 0
  *
  * Team data keys:
- *   elo:    Elo rating derived from FanGraphs projected wins (ExpW) via:
- *             elo = 1500 + 350 * log10(winRate / (1 - winRate))
- *             where winRate = ExpW / 162
- *   p_div:  Probability of winning own division (FanGraphs divTitle).
- *             Must sum to ~1.0 within each 5-team division.
- *   p_wc:   Probability of claiming one of 3 wildcard slots (FanGraphs wcTitle).
- *             Used as relative weights among non-division-winners in each league.
+ *   rdif:  FanGraphs Depth Charts projected run differential for 2026.
+ *          Used for per-game win probability via log5.
+ *   p_div: Probability of winning own division (FanGraphs Depth Charts divTitle).
+ *          Must sum to ~1.0 within each 5-team division.
+ *   p_wc:  Probability of claiming one of 3 wildcard slots (FanGraphs Depth Charts wcTitle).
+ *          Used as relative weights among non-division-winners in each league.
  *
- * Elo ratings and seeding probabilities are hardcoded below (2026 pre-season).
- * Source: https://www.fangraphs.com/standings/playoff-odds/fg/div
- * Update at the start of each season using the formula above.
+ * Sources:
+ *   rdif:       https://www.fangraphs.com/standings/projected-standings
+ *   p_div/p_wc: https://www.fangraphs.com/standings/playoff-odds/dc/div
+ * Update at the start of each season.
  *
  * Divisions:
  *   AL East:    Yankees, Orioles, Red Sox, Rays, Blue Jays
@@ -79,177 +81,86 @@ import {
 const NUM_SIMULATIONS = 50_000;
 
 /**
- * Elo parity factor. MLB uses 350 (below NHL's 1000 and NBA's 400) to reflect
- * that teams of different quality win series at a higher rate in baseball
- * compared to hockey, where single-game variance is very high.
+ * Controls how much projected run differential spreads teams away from .500.
+ *
+ * Baseline: 1620 (≈ 10 runs per win × 162 games). This gives the raw
+ * Pythagorean-derived win rate, which tends to overstate dominance in
+ * playoff matchups where you face elite pitching. Increasing this constant
+ * compresses all team strengths toward .500 — reducing every strong team's
+ * win probability in each series and yielding more upset potential across
+ * the entire bracket. Decrease to amplify differences.
+ *
+ * Effect on Dodgers (RDif +137):
+ *   1620 → win rate 0.585 (raw Pythagorean projection — too dominant)
+ *   2000 → win rate 0.568
+ *   4000 → win rate 0.534
+ *   8000 → win rate 0.517 (current — near coin-flip vs any playoff team)
  */
-const PARITY_FACTOR = 350;
+const RDIF_DIVISOR = 8000;
 
 /**
- * Blend weights for Elo vs. Vegas futures odds when sourceOdds are available.
- * Same calibration as the NHL simulator (0.7 / 0.3).
+ * Blend weights for RDif vs. Vegas futures odds when sourceOdds are available.
  */
-const ELO_WEIGHT  = 0.7;
-const ODDS_WEIGHT = 1 - ELO_WEIGHT;
+const RDIF_WEIGHT  = 0.7;
+const ODDS_WEIGHT = 1 - RDIF_WEIGHT;
 
-// ─── Team data (2026 pre-season estimates) ────────────────────────────────────
+// ─── Team data (2026 pre-season — FanGraphs Depth Charts) ────────────────────
 //
-// elo:    Derived from FanGraphs projected wins (ExpW) via:
-//           elo = 1500 + 350 * log10(winRate / (1 - winRate))
-//           where winRate = ExpW / 162
-// p_div:  FanGraphs divTitle — probability of winning own division.
-//           Sum within each 5-team division should equal ~1.0.
-// p_wc:   FanGraphs wcTitle — probability of claiming a wildcard slot.
-//           Used as relative weights among non-division-winners per league.
-//           Does not need to sum to any specific value.
+// rdif: Projected run differential from FanGraphs Depth Charts.
+//       Source: https://www.fangraphs.com/standings/projected-standings
 //
-// Update these at the start of each season using the FanGraphs playoff odds page:
-//   https://www.fangraphs.com/standings/playoff-odds/fg/div
-//
-// Quick update formula (JavaScript):
-//   const winRate = ExpW / 162;
-//   const elo = Math.round(1500 + 350 * Math.log10(winRate / (1 - winRate)));
+// Update all three values at the start of each season from the FanGraphs pages above.
 
 interface MlbTeamData {
   league: "AL" | "NL";
   division: "AL East" | "AL Central" | "AL West" | "NL East" | "NL Central" | "NL West";
-  elo: number;
-  p_div: number;
-  p_wc: number;
+  rdif: number;  // 2026 FanGraphs Depth Charts projected run differential
+  p_div: number; // 2026 FanGraphs Depth Charts divTitle — probability of winning own division
+  p_wc: number;  // 2026 FanGraphs Depth Charts wcTitle — probability of claiming a wild card slot
 }
 
 const TEAMS_DATA: Record<string, MlbTeamData> = {
   // ── American League East ───────────────────────────────────────────────────
-  "New York Yankees": {
-    league: "AL", division: "AL East", elo: 1545,
-    p_div: 0.30, p_wc: 0.42,
-  },
-  "Baltimore Orioles": {
-    league: "AL", division: "AL East", elo: 1535,
-    p_div: 0.28, p_wc: 0.41,
-  },
-  "Boston Red Sox": {
-    league: "AL", division: "AL East", elo: 1520,
-    p_div: 0.22, p_wc: 0.35,
-  },
-  "Tampa Bay Rays": {
-    league: "AL", division: "AL East", elo: 1513,
-    p_div: 0.12, p_wc: 0.25,
-  },
-  "Toronto Blue Jays": {
-    league: "AL", division: "AL East", elo: 1506,
-    p_div: 0.08, p_wc: 0.18,
-  },
+  "New York Yankees":   { league: "AL", division: "AL East",    rdif:  67, p_div: 0.374, p_wc: 0.358 },
+  "Baltimore Orioles":  { league: "AL", division: "AL East",    rdif:  23, p_div: 0.128, p_wc: 0.310 },
+  "Boston Red Sox":     { league: "AL", division: "AL East",    rdif:  48, p_div: 0.247, p_wc: 0.373 },
+  "Tampa Bay Rays":     { league: "AL", division: "AL East",    rdif:   2, p_div: 0.066, p_wc: 0.229 },
+  "Toronto Blue Jays":  { league: "AL", division: "AL East",    rdif:  37, p_div: 0.184, p_wc: 0.349 },
 
   // ── American League Central ────────────────────────────────────────────────
-  "Kansas City Royals": {
-    league: "AL", division: "AL Central", elo: 1519,
-    p_div: 0.28, p_wc: 0.30,
-  },
-  "Cleveland Guardians": {
-    league: "AL", division: "AL Central", elo: 1516,
-    p_div: 0.27, p_wc: 0.29,
-  },
-  "Minnesota Twins": {
-    league: "AL", division: "AL Central", elo: 1514,
-    p_div: 0.23, p_wc: 0.25,
-  },
-  "Detroit Tigers": {
-    league: "AL", division: "AL Central", elo: 1510,
-    p_div: 0.15, p_wc: 0.18,
-  },
-  "Chicago White Sox": {
-    league: "AL", division: "AL Central", elo: 1440,
-    p_div: 0.07, p_wc: 0.03,
-  },
+  "Kansas City Royals":    { league: "AL", division: "AL Central", rdif:   8, p_div: 0.297, p_wc: 0.157 },
+  "Cleveland Guardians":   { league: "AL", division: "AL Central", rdif: -38, p_div: 0.084, p_wc: 0.075 },
+  "Minnesota Twins":       { league: "AL", division: "AL Central", rdif: -15, p_div: 0.166, p_wc: 0.120 },
+  "Detroit Tigers":        { league: "AL", division: "AL Central", rdif:  26, p_div: 0.449, p_wc: 0.155 },
+  "Chicago White Sox":     { league: "AL", division: "AL Central", rdif: -112, p_div: 0.005, p_wc: 0.004 },
 
   // ── American League West ───────────────────────────────────────────────────
-  "Houston Astros": {
-    league: "AL", division: "AL West", elo: 1537,
-    p_div: 0.35, p_wc: 0.40,
-  },
-  "Seattle Mariners": {
-    league: "AL", division: "AL West", elo: 1528,
-    p_div: 0.28, p_wc: 0.35,
-  },
-  "Texas Rangers": {
-    league: "AL", division: "AL West", elo: 1522,
-    p_div: 0.22, p_wc: 0.28,
-  },
-  "Los Angeles Angels": {
-    league: "AL", division: "AL West", elo: 1475,
-    p_div: 0.09, p_wc: 0.08,
-  },
-  "Athletics": {
-    league: "AL", division: "AL West", elo: 1455,
-    p_div: 0.06, p_wc: 0.04,
-  },
+  "Houston Astros":      { league: "AL", division: "AL West",    rdif:  -1, p_div: 0.126, p_wc: 0.213 },
+  "Seattle Mariners":    { league: "AL", division: "AL West",    rdif:  66, p_div: 0.595, p_wc: 0.205 },
+  "Texas Rangers":       { league: "AL", division: "AL West",    rdif:  20, p_div: 0.201, p_wc: 0.270 },
+  "Los Angeles Angels":  { league: "AL", division: "AL West",    rdif: -65, p_div: 0.011, p_wc: 0.035 },
+  "Athletics":           { league: "AL", division: "AL West",    rdif: -17, p_div: 0.068, p_wc: 0.146 },
 
   // ── National League East ───────────────────────────────────────────────────
-  "Philadelphia Phillies": {
-    league: "NL", division: "NL East", elo: 1553,
-    p_div: 0.35, p_wc: 0.42,
-  },
-  "Atlanta Braves": {
-    league: "NL", division: "NL East", elo: 1550,
-    p_div: 0.32, p_wc: 0.40,
-  },
-  "New York Mets": {
-    league: "NL", division: "NL East", elo: 1534,
-    p_div: 0.22, p_wc: 0.33,
-  },
-  "Washington Nationals": {
-    league: "NL", division: "NL East", elo: 1483,
-    p_div: 0.07, p_wc: 0.07,
-  },
-  "Miami Marlins": {
-    league: "NL", division: "NL East", elo: 1468,
-    p_div: 0.04, p_wc: 0.03,
-  },
+  "Philadelphia Phillies": { league: "NL", division: "NL East",    rdif:  51, p_div: 0.243, p_wc: 0.444 },
+  "Atlanta Braves":        { league: "NL", division: "NL East",    rdif:  67, p_div: 0.357, p_wc: 0.425 },
+  "New York Mets":         { league: "NL", division: "NL East",    rdif:  71, p_div: 0.389, p_wc: 0.413 },
+  "Washington Nationals":  { league: "NL", division: "NL East",    rdif: -113, p_div: 0.001, p_wc: 0.006 },
+  "Miami Marlins":         { league: "NL", division: "NL East",    rdif: -48, p_div: 0.011, p_wc: 0.075 },
 
   // ── National League Central ────────────────────────────────────────────────
-  "Milwaukee Brewers": {
-    league: "NL", division: "NL Central", elo: 1529,
-    p_div: 0.35, p_wc: 0.38,
-  },
-  "Chicago Cubs": {
-    league: "NL", division: "NL Central", elo: 1515,
-    p_div: 0.27, p_wc: 0.28,
-  },
-  "St. Louis Cardinals": {
-    league: "NL", division: "NL Central", elo: 1507,
-    p_div: 0.20, p_wc: 0.20,
-  },
-  "Cincinnati Reds": {
-    league: "NL", division: "NL Central", elo: 1503,
-    p_div: 0.12, p_wc: 0.13,
-  },
-  "Pittsburgh Pirates": {
-    league: "NL", division: "NL Central", elo: 1498,
-    p_div: 0.06, p_wc: 0.07,
-  },
+  "Milwaukee Brewers":   { league: "NL", division: "NL Central", rdif:   9, p_div: 0.243, p_wc: 0.170 },
+  "Chicago Cubs":        { league: "NL", division: "NL Central", rdif:  23, p_div: 0.347, p_wc: 0.183 },
+  "St. Louis Cardinals": { league: "NL", division: "NL Central", rdif: -55, p_div: 0.038, p_wc: 0.049 },
+  "Cincinnati Reds":     { league: "NL", division: "NL Central", rdif: -31, p_div: 0.086, p_wc: 0.095 },
+  "Pittsburgh Pirates":  { league: "NL", division: "NL Central", rdif:  13, p_div: 0.285, p_wc: 0.178 },
 
   // ── National League West ───────────────────────────────────────────────────
-  "Los Angeles Dodgers": {
-    league: "NL", division: "NL West", elo: 1570,
-    p_div: 0.65, p_wc: 0.28,
-  },
-  "San Diego Padres": {
-    league: "NL", division: "NL West", elo: 1525,
-    p_div: 0.15, p_wc: 0.30,
-  },
-  "Arizona Diamondbacks": {
-    league: "NL", division: "NL West", elo: 1520,
-    p_div: 0.12, p_wc: 0.28,
-  },
-  "San Francisco Giants": {
-    league: "NL", division: "NL West", elo: 1510,
-    p_div: 0.06, p_wc: 0.20,
-  },
-  "Colorado Rockies": {
-    league: "NL", division: "NL West", elo: 1442,
-    p_div: 0.02, p_wc: 0.02,
-  },
+  "Los Angeles Dodgers":    { league: "NL", division: "NL West",    rdif: 137, p_div: 0.900, p_wc: 0.082 },
+  "San Diego Padres":       { league: "NL", division: "NL West",    rdif:  -9, p_div: 0.023, p_wc: 0.240 },
+  "Arizona Diamondbacks":   { league: "NL", division: "NL West",    rdif:   5, p_div: 0.039, p_wc: 0.322 },
+  "San Francisco Giants":   { league: "NL", division: "NL West",    rdif:   3, p_div: 0.038, p_wc: 0.317 },
+  "Colorado Rockies":       { league: "NL", division: "NL West",    rdif: -173, p_div: 0.001, p_wc: 0.001 },
 };
 
 // ─── Public helpers (exported for unit testing) ───────────────────────────────
@@ -269,12 +180,26 @@ export function getTeamData(name: string): MlbTeamData | undefined {
 }
 
 /**
- * Elo win probability for team A over team B.
- *   P(A) = 1 / (1 + 10^((eloB - eloA) / PARITY_FACTOR))
+ * Convert projected run differential to a compressed win rate for matchup probability.
+ * Uses RDIF_DIVISOR to control how much team strength spreads away from .500.
+ * Clamped to [0.01, 0.99] to avoid degenerate log5 values.
  * Exported for unit testing.
  */
-export function eloWinProbability(eloA: number, eloB: number): number {
-  return 1 / (1 + Math.pow(10, (eloB - eloA) / PARITY_FACTOR));
+export function winRateFromRDif(rdif: number): number {
+  return Math.min(0.99, Math.max(0.01, 0.5 + rdif / RDIF_DIVISOR));
+}
+
+/**
+ * Bill James log5 head-to-head win probability for team A over team B,
+ * given their projected run differentials.
+ *   P(A beats B) = (wA - wA·wB) / (wA + wB - 2·wA·wB)
+ * Exported for unit testing.
+ */
+export function rdifWinProbability(rdifA: number, rdifB: number): number {
+  const wA = winRateFromRDif(rdifA);
+  const wB = winRateFromRDif(rdifB);
+  // Denominator is always > 0 when wA and wB are in (0,1).
+  return (wA - wA * wB) / (wA + wB - 2 * wA * wB);
 }
 
 // ─── Internal types ───────────────────────────────────────────────────────────
@@ -286,13 +211,13 @@ interface TeamEntry {
   originalSeed?: number;
 }
 
-/** Get Elo for a team entry. Fallback 1400 for unknown teams. */
-function elo(entry: TeamEntry): number {
-  return entry.data?.elo ?? 1400;
+/** Get projected RDif for a team entry. Fallback 0 (league-average) for unknown teams. */
+function getEntryRDif(entry: TeamEntry): number {
+  return entry.data?.rdif ?? 0;
 }
 
 /**
- * Weighted pick without replacement using the given weight key.
+ * Weighted pick without replacement using the given weight function.
  * Returns undefined if no eligible team has positive weight.
  */
 function weightedPickByKey(
@@ -370,8 +295,8 @@ export function simBo7(
  * Steps:
  *   1. For each of the 3 divisions, draw 1 division winner weighted by p_div.
  *   2. From the remaining non-division-winners, draw 3 WC teams weighted by p_wc.
- *   3. Rank division winners 1–3 by Elo (best Elo → seed 1).
- *   4. Rank WC teams 4–6 by Elo (best Elo → seed 4).
+ *   3. Rank division winners 1–3 by RDif descending (best RDif → seed 1).
+ *   4. Rank WC teams 4–6 by RDif descending (best RDif → seed 4).
  *
  * Returns an array of 6 TeamEntry objects in seed order [1..6], each annotated
  * with originalSeed, or undefined if any draw is degenerate (no eligible team
@@ -398,7 +323,7 @@ function drawLeaguePlayoffField(
     allDivisionWinnerSet.add(winner);
   }
 
-  // Draw 3 WC teams from non-division-winners
+  // Draw 3 WC teams from non-division-winners, weighted by p_wc
   const wcPool = leagueTeams.filter((t) => !allDivisionWinnerSet.has(t));
   const wcTaken = new Set<TeamEntry>();
   const wcTeams: TeamEntry[] = [];
@@ -410,11 +335,11 @@ function drawLeaguePlayoffField(
     wcTaken.add(wc);
   }
 
-  // Rank division winners 1–3 by Elo descending (best Elo = seed 1)
-  const sortedDivWinners = divisionWinners.toSorted((a, b) => elo(b) - elo(a));
+  // Rank division winners 1–3 by RDif descending (best RDif = seed 1)
+  const sortedDivWinners = divisionWinners.toSorted((a, b) => getEntryRDif(b) - getEntryRDif(a));
 
-  // Rank WC teams 4–6 by Elo descending (best Elo = seed 4)
-  const sortedWcTeams = wcTeams.toSorted((a, b) => elo(b) - elo(a));
+  // Rank WC teams 4–6 by RDif descending (best RDif = seed 4)
+  const sortedWcTeams = wcTeams.toSorted((a, b) => getEntryRDif(b) - getEntryRDif(a));
 
   const seeds = [...sortedDivWinners, ...sortedWcTeams];
   return seeds.map((t, i) => ({ ...t, originalSeed: i + 1 }));
@@ -503,9 +428,9 @@ export class MLBSimulator implements Simulator {
     const alTeams = teams.filter((t) => t.data?.league === "AL");
     const nlTeams = teams.filter((t) => t.data?.league === "NL");
 
-    if (alTeams.length < 5 || nlTeams.length < 5) {
+    if (alTeams.length < 6 || nlTeams.length < 6) {
       throw new Error(
-        `Each league needs at least 5 recognized participants ` +
+        `Each league needs at least 6 recognized participants (3 division winners + 3 wild cards) ` +
           `(got AL: ${alTeams.length}, NL: ${nlTeams.length}). ` +
           `Add MLB teams before running simulation.`
       );
@@ -542,18 +467,18 @@ export class MLBSimulator implements Simulator {
 
     /**
      * Blended per-game win probability for team A over team B.
-     * When odds are available: 70% Elo + 30% vig-removed futures head-to-head.
+     * When odds are available: 70% RDif log5 + 30% vig-removed futures head-to-head.
      */
     const gameWinProb = (a: TeamEntry, b: TeamEntry): number => {
-      const eloProb = eloWinProbability(elo(a), elo(b));
-      if (!hasOdds) return eloProb;
+      const rdifProb = rdifWinProbability(getEntryRDif(a), getEntryRDif(b));
+      if (!hasOdds) return rdifProb;
 
       const o1 = normalizedOddsMap.get(a.id);
       const o2 = normalizedOddsMap.get(b.id);
-      // Fall back to Elo if either team lacks odds — avoids inflating one side to 100%.
-      if (o1 === null || o1 === undefined || o2 === null || o2 === undefined) return eloProb;
+      // Fall back to RDif if either team lacks odds — avoids inflating one side to 100%.
+      if (o1 === null || o1 === undefined || o2 === null || o2 === undefined) return rdifProb;
       const oddsProb = o1 + o2 > 0 ? o1 / (o1 + o2) : 0.5;
-      return ELO_WEIGHT * eloProb + ODDS_WEIGHT * oddsProb;
+      return RDIF_WEIGHT * rdifProb + ODDS_WEIGHT * oddsProb;
     };
 
     // ─── Placement count maps ──────────────────────────────────────────────────
@@ -597,7 +522,7 @@ export class MLBSimulator implements Simulator {
     if (effectiveN === 0) {
       throw new Error(
         "All simulations produced degenerate brackets. " +
-          "Check that each division has teams with positive p_div values."
+          "Check that each division has teams with non-degenerate RDif values."
       );
     }