The entire GT4 class just got its biggest rework since the cars arrived in iRacing. iRacing rebuilt the class around modern physics primitives, a fully updated tire model (dry and wet), and a rules/BoP pass that aligns the sim with today’s IMSA Michelin Pilot Challenge (IMPC). The result: higher top speeds, lower minimum corner speeds, trickier cold tires, punchier powertrains—and faster lap times once you adapt.
What changed under the hood
Physics & Tires
- Latest tire model on all GT4s (slicks + wets). Lower overall peak grip, shorter warm-up than GT3/GTP but you still need an out-lap that builds temp progressively. Tire warmer temps are reduced.
- Chassis/vehicle systems repass across the class: revised inertia/mass distribution, updated aero balance & sideforce behavior, wider and clearer wing trim and ride-height ranges, refreshed damper/bump-rubber profiles for better curb compliance.
Powertrain & Brakes
- Driveline drag reductions and engine model tweaks deliver more drive at the wheels.
- Brake systems now hit realistic max line pressures and include two pad definitions—Sprint and Endurance—with different temp/µ profiles (no wear delta yet).
Electronics & QoL
- TC/ABS/ESC logic updated (where fitted).
- User-adjustable low-fuel warning (plus clear) added to most cars.
- Finer 0.25 mm spring-perch steps.
- Improved garage tech tips/layout.
- Opponent dashboards display status more accurately; windshield reflections updated.
Rules/Series Alignment
- Starting masses, camber/pressure mins, ride-height rules (compound-specific), fuel tank sizes and pit times now map to IMPC standards for more credible multiclass racing and BoP outcomes.
Pace & drivability: what to expect
- Top speeds are up ~10 km/h while minimum corner speeds are lower—cars feel livelier on entry/exit and demand better throttle/balance management. Despite reduced peak grip, lap times typically improve by 0.5–1.0s once you adapt (track-dependent).
- Cold tire phase matters: expect a shorter warm-up than GT3/GTP but you can’t attack max braking/rotation immediately—work pressures/temps in gradually.
- Setups: all pre-S4 GT4 setups are obsolete due to platform height resets and physics changes—start from the new baselines.
Balance of Performance & IMSA grounding
The class-wide BoP tools were reworked to produce more consistent competitiveness across track types (powertrain modeling, fuel capacities, aero windows). On the real-world side, IMSA’s 2025 documents outline how GT4 sets capacities and other technical limits, which iRacing used as the anchor for this re-alignment. Expect season-to-season BoP nudges in both sim and series.
Car-specific highlights
- Mercedes-AMG GT4: diff friction faces locked to 8; revised wheel-rate mapping; torque curve update; anti-dive/anti-lift re-tuned.
- McLaren 570S GT4: e-diff adoption; engine/steering feel updates; brake-bias adjustment removed; suspension/wheel-rate accuracy pass.
- Porsche 718 Cayman GT4 CS MR: front spring now adjustable; ESC/TC off buttons added; mass/inertia redistribution; ARB ranges corrected.
(Many others received matching system updates per the global pass.)
How it feels on track
Early testers are noticing braking points moving earlier than pre-patch (less peak grip) but straights arriving quicker (more top-end), which broadly mirrors the stated intent. As always, take anecdotal feedback with a grain of salt while you build your own reference laps.
Day-one checklist (do this before your first GT4 race)
- Bin the old sets: load the new iRacing GT4 baselines; re-establish rake/ride heights within the now-tighter rules windows.
- Tire plan: expect a measurable but shorter warm-up; open the stint at 8–9/10ths and chase pressures/temps to their new sweet spot.
- Pads choice: try Sprint for shorter stints/qualy, Endurance if you need a calmer temp window; wear rate is the same (for now).
- Electronics: revisit TC/ABS levels—the revised logic + lower grip will change where your “nanny” thresholds should live.
- Aero & dampers: use the expanded wing/ride-height notes and updated damper ranges to stabilize curb strikes (Watkins Bus Stop, COTA esses, etc.).
- Fueling: check the smaller IMPC-style tanks and fill times when planning stint lengths and undercut windows.
Where to race them right now
Season 4 spotlights the refreshed GT4s across multiple official series, with the new Mustang GT4 joining the grid and rain readiness across the class. Check the Season 4 hub and schedules for your favorite combos (and remember many tracks also changed this season).
TL;DR — the five biggest takeaways
- Latest tire model on all GT4s; reduced peak grip but a shorter warm-up than GT3/GTP.
- Higher V-max, lower min-corner speeds; once adapted, laps ~0.5–1.0s faster.
- Setups reset: start from new baselines; geometry/heights/aero windows changed.
- Rules alignment to IMPC: fuel capacities, pit times, camber/pressures, ride-heights, etc., for better real-world correlation and BoP.
- Electronics & braking updated; choice of Sprint/Endurance pad definitions; driveline and engine models give more usable drive.
