Jetting a Predator 212 for elevation
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 | 800 ft | 5,000 ft | diff |
|---|---|---|---|
| 1500 | 2.42 | 2.21 | -0.20 |
| 2000 | 3.33 | 3.05 | -0.28 |
| 2500 | 4.28 | 3.92 | -0.36 |
| 3000 | 5.27 | 4.82 | -0.45 |
| 3500 | 6.33 | 5.80 | -0.54 |
| 4000 | 7.44 | 6.82 | -0.63 |
| 4500 | 8.60 | 7.88 | -0.73 |
| 5000 | 9.81 | 8.98 | -0.83 |
| 5500 | 10.26 | 9.40 | -0.87 |
Horsepower at each rpm. Everything except the part named is identical between the two runs.
One engine, one build, two altitudes. Nothing mechanical changes between these runs — only the air.
Thinner air carries less oxygen, so the engine makes less power and needs less fuel to stay at the same mixture. Both the horsepower and the main jet move together, and the jet figure at the top of each column is what the calculator recommends for that air.
The rule of thumb is that you jet down as you go up. If you built and jetted at sea level and then trailered to the mountains, the engine is now rich and lazy — and if you did the reverse, it is lean, which is the expensive direction to be wrong in.
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 | GPS header + screw-on muffler kit |
| Fuel | Pump 87 octane |