modifications > OTL 3" Catted Downpipe

Why did you install an OTL 3" Catted Downpipe on your GRC?

The factory GRC exhaust is good for the stock car. It has a fancy electronically controlled valve that opens at certain times to make the exhaust louder and flow a bit better. Up to this point, my GRC was a car with minor bolt-ons and a tune with 93 octane + E30 maps. In 1Q2026 I will be installing a larger turbocharger. The OTL catted downpipe is equipped with a high-flow catalytic converter, but the OTL aftermarket mid-pipe does not have a catalytic converter like the OE midpipe contains.

A brief internet search states the OTL catted downpipe is the ONLY aftermarket downpipe that doesn't throw a check engine light (CEL). A catless downpipe will definitely throw a CEL. However, other vendors that sell catted downpipes still throw a CEL. My GRC is a daily driver, so I need it to be CEL free.

Date Completed:

12/14/25

Materials:

- OTL Performance 3" catted downpipe (Offthelineperformance.com)
- Liquid Wrench Penetrating Oil
- Shop towels
- Anti-Seize
- Paint pen

Tools:

- 1/4" drive ratchet with multiple extensions and sockets (10mm, 12mm)
- 3/8" drive ratchet with multiple extensions, swivel, and sockets (12mm, 14mm, 15mm deep, 7/8" O2 sensor)
- Combination wrenches (10mm, 14mm)
- 3/8" drive torque wrench (10-75 lb-ft)
- 1/4" drive torque wrench (40-200 in-lb)
- 1/2" breaker bar + cheater pipe
- 9,000 lb 4-post lift (optional, but highly recommended!)
- Rechargeable headlamp w/soft strap = Coast XPH30R 1200 Lumen (Amazon)
- Flat-blade screwdriver
- Gloves (both leather and nitrile)
- BFH for smacking things
- Exhaust hanger removal pliers
- Needle nose pliers
- Scissors

Notes:

1. Several wiring connectors/plugs need to be temporarily disconnected (mass air sensor, upper/lower O2 sensors, etc.).
2. Note this installation is slightly different than a stock GRC because I already have an Eventuri intake installed. Your mileage may vary.
3. Unlike what others have done, the following items do NOT need to be removed: turbo inlet, wiper cowl, driveshaft, or underbody exhaust heat shields.
4. Weights = OE downpipe + midpipe = 17.4 lb. OTL catted downpipe + midpipe = 22.2 lb. Added weight = 4.8 lb.
5. Reversible back to stock (if needed).

Removal:

1. Put GRC up on a 2 or 4-post lift.
2. Loosen center tunnel brace. (x8 12mm).
3. Remove various plastic clips and (2) bolts to loose both tunnel undertrays. (x2 10mm).
4. Remove center tunnel brace.
5. Remove OE catback to midpipe bolts. (x2 14mm)
6. Remove front tunnel brace behind K member. (x4 12mm)
7. Remove rear O2 sensor and bracket. (7/8" O2, 10mm)
8. Remove midpipe bolts to downpipe. (x2 14mm)
9. Remove midpipe exhaust hangers and midpipe.
10. Remove Eventuri intake scoop & filter housing. (x2 10mm wrenches, 7mm, 10mm)
11. Remove OE downpipe bracket (14mm wrench, 14mm)
12. Disconnect upper O2 sensor and harness from wire hanger.
13. Remove OE downpipe v-band clamp to turbo. (12mm)
14. Remove transfer case heat shield. (x2 12mm)
15. Remove OE downpipe clamshell heat shield (2 pieces). (x5 10mm)
16. Remove OE downpipe. Rotate it as-needed for removal.

Installation:

Note: I torque 33%, 66%, 100%, then ring fasteners again.
1. Install upper O2 sensor in new downpipe. Apply anti-seize first.
2. Install new downpipe to turbo v-band flange. Hand tight for now.
3. Install new midpipe to new downpipe. Keep band clamp loose. Expand slip-fit connection with crescent wrench as-needed.
4. Remove OE midpipe donut gasket. Install on new midpipe.
5. Install new midpipe using new bolts, but re-use (2) existing springs from the original hardware.
6. Remove existing exhaust hangers. Lube them up. Reinstall on new curved midpipe hanger.
7. Reinstall both exhaust hangers on their respective underbody bracket.
8. Tighten new midpipe exhaust band clamp.
9. Torque new downpipe v-band bolt. (52, 104, 156 in-lb)
10. Reinstall transfer case heat shield. (x2 18 lb-ft)
11. Reinstall front/rear tunnel braces. (17, 34, 52 lb-ft)
12. Install rear O2 sensor in new downpipe.
13. Reinstall rear O2 sensor wiring bracket. (7 lb-ft)
14. Reconnect rear O2 sensor wiring.
15. Reinstall undertray clips and (2) bolts.
16. Install new downpipe bracket from up top. (x2 larger bolts = 11, 21, 32 lb-ft, x1 in downpipe = snug).
17. Reconnect upper O2 sensor wiring. Put wiring into factory wire hanger.
18. Reinstall Eventuri intake.
19. Mark all fasteners with paint pen to detect future movement.