How much horsepower will porting the head 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 8.23:1.
Where each one wins
| RPM | ported (bowl blend) | as-cast head | diff |
|---|---|---|---|
| 1500 | 2.98 | 2.83 | -0.15 |
| 2000 | 4.09 | 3.88 | -0.21 |
| 2500 | 5.25 | 4.99 | -0.26 |
| 3000 | 6.45 | 6.13 | -0.32 |
| 3500 | 7.75 | 7.36 | -0.39 |
| 4000 | 9.20 | 8.73 | -0.46 |
| 4500 | 10.72 | 10.19 | -0.54 |
| 5000 | 12.22 | 11.72 | -0.50 |
| 5500 | 13.67 | 13.29 | -0.39 |
| 6000 | 14.73 | 14.17 | -0.56 |
Horsepower at each rpm. Everything except the part named is identical between the two runs.
A bowl blend on the same engine, everything else held identical. This is hand porting — cleaning up the short turn and the valve pocket — not a CNC programme.
Read the compression figures on the two builds as well as the horsepower. Porting removes material, which enlarges the chamber and drops compression, so a bowl blend on its own gives back part of what its airflow earns. That is not an argument against porting; it is an argument for decking or milling at the same time, and the combination beats either change alone.
This is also why a big carb and a big cam disappoint on an untouched head. The head is the part every other airflow part has to work through.
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 | Hemi head — ported (bowl blend) |
| Head gasket | Stock HEMI — fiber .045" (ARC DJ-1310TP) |
| Camshaft | DynoCams 335 — .334" · 259.8/264.7 @.050 · 36 lb · 8,500-9,000 rpm |
| Valve springs | 34-36 lb open/race |
| Retainers | Billet w/ keepers, 36 lb — ARC DJ-1046 ($11.00) |
| Valves | Stainless kit, Hemi 27/25mm — GPS KDVALKIT27-25 · ARC DJ-SSVALVE HKT |
| 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 | ARC rev limiter coil — 9,000 rpm |
| Carburetor | Nibbi PWK24 — 24mm flat slide |
| Air filter | Pod / cone filter — GPS / OMB angled filter kit |
| Exhaust | DynoCams race exhaust pipe |
| 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 | DynoCams 335 — .334" · 259.8/264.7 @.050 · 36 lb · 8,500-9,000 rpm |
| Valve springs | 34-36 lb open/race |
| Retainers | Billet w/ keepers, 36 lb — ARC DJ-1046 ($11.00) |
| Valves | Stainless kit, Hemi 27/25mm — GPS KDVALKIT27-25 · ARC DJ-SSVALVE HKT |
| 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 | ARC rev limiter coil — 9,000 rpm |
| Carburetor | Nibbi PWK24 — 24mm flat slide |
| Air filter | Pod / cone filter — GPS / OMB angled filter kit |
| Exhaust | DynoCams race exhaust pipe |
| Fuel | Pump 87 octane |