How much horsepower will a header add?
Engine output on a stand — there is no drivetrain and no chassis in these numbers, which is the same basis Harbor Freight rates the engine on. Simulated at 800 ft and 75°F; thinner air makes less power and wants less fuel, so both the horsepower and the jet move with them. Governor removed. Compression 9.06:1.
Where each one wins
| RPM | header + muffler | stock muffler | diff |
|---|---|---|---|
| 1500 | 2.42 | 2.25 | -0.17 |
| 2000 | 3.33 | 3.10 | -0.23 |
| 2500 | 4.28 | 3.99 | -0.29 |
| 3000 | 5.27 | 4.90 | -0.36 |
| 3500 | 6.33 | 5.88 | -0.45 |
| 4000 | 7.44 | 6.91 | -0.54 |
| 4500 | 8.60 | 7.96 | -0.64 |
| 5000 | 9.81 | 9.06 | -0.75 |
| 5500 | 10.26 | 9.48 | -0.79 |
Horsepower at each rpm. Everything except the part named is identical between the two runs.
The factory muffler is a restriction and a silencer in one part. Swapping it for a header and a screw-on muffler is one of the better returns per dollar on this engine.
The gain lands where you would expect — up top, past where the governor used to sit. Down low the two curves are close, which is why an exhaust alone rarely feels as different as it measures.
Open pipes measure a little better again and are unpleasant to sit behind for an afternoon. The calculator carries both if you want to see the trade.
The build
| Engine | Predator 212 Hemi (Gen 2) — 27/25mm valves · 5.5mm stems |
|---|---|
| Governor | Governor removed (full rev range) |
| Piston | Stock flat top — Hemi OEM (ARC DJ-1285PH · .550" CH) |
| Head | Stock Hemi head — as-cast factory |
| Head gasket | Stock HEMI — fiber .045" (ARC DJ-1310TP) |
| Camshaft | Stock cam — Hemi & Non-Hemi factory grind |
| Valve springs | 18 lb Hemi specific (OMB/GPS) |
| Retainers | Stock retainers — OEM (ARC DJ-1040) |
| Valves | Stock valves — OEM |
| Rocker arms | Stock rocker arm — 1:1 ratio |
| Connecting rod | ARC 6256 Billet — Hemi (+.020" raises CR) |
| Flywheel | ARC 6696 Billet — Hemi Speedway (32° BTDC) |
| Ignition coil | Stock coil — factory (no rev limiter) |
| Carburetor | 22mm clone carb — GPS / OMB Stage kit carb |
| Air filter | Pod / cone filter — GPS / OMB angled filter kit |
| Exhaust | GPS header + screw-on muffler kit |
| Fuel | Pump 87 octane |
The other one
| Engine | Predator 212 Hemi (Gen 2) — 27/25mm valves · 5.5mm stems |
|---|---|
| Governor | Governor removed (full rev range) |
| Piston | Stock flat top — Hemi OEM (ARC DJ-1285PH · .550" CH) |
| Head | Stock Hemi head — as-cast factory |
| Head gasket | Stock HEMI — fiber .045" (ARC DJ-1310TP) |
| Camshaft | Stock cam — Hemi & Non-Hemi factory grind |
| Valve springs | 18 lb Hemi specific (OMB/GPS) |
| Retainers | Stock retainers — OEM (ARC DJ-1040) |
| Valves | Stock valves — OEM |
| Rocker arms | Stock rocker arm — 1:1 ratio |
| Connecting rod | ARC 6256 Billet — Hemi (+.020" raises CR) |
| Flywheel | ARC 6696 Billet — Hemi Speedway (32° BTDC) |
| Ignition coil | Stock coil — factory (no rev limiter) |
| Carburetor | 22mm clone carb — GPS / OMB Stage kit carb |
| Air filter | Pod / cone filter — GPS / OMB angled filter kit |
| Exhaust | Stock muffler — factory |
| Fuel | Pump 87 octane |