GPS .308 vs GPS .356
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 | GPS .308 | GPS .356 | diff |
|---|---|---|---|
| 1500 | 2.74 | 2.86 | +0.12 |
| 2000 | 3.77 | 3.93 | +0.16 |
| 2500 | 4.84 | 5.04 | +0.21 |
| 3000 | 5.95 | 6.20 | +0.25 |
| 3500 | 7.16 | 7.42 | +0.26 |
| 4000 | 8.51 | 8.80 | +0.30 |
| 4500 | 9.93 | 10.26 | +0.33 |
| 5000 | 11.43 | 11.80 | +0.37 |
| 5500 | 12.91 | 13.35 | +0.44 |
| 6000 | 13.35 | 14.51 | +1.16 |
Horsepower at each rpm. Everything except the part named is identical between the two runs.
Hot street against strip. Same engine, same everything, two grinds a tier apart.
Read the delta table rather than the peaks. The bigger grind wins at the top and gives ground lower down, and on a machine that spends its life below the crossover the smaller cam is the faster one in practice.
Note the spring difference in the two parts lists. More lift needs more spring, and a spring that cannot follow the lobe turns a expensive cam into a misfire.
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 | GPS .308 cam — .308" lift · hot street |
| Valve springs | 26 lb performance (ARC DJ-1056 / GPS KD26lb) |
| 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 | GPS .356 cam — .356" lift · strip/race |
| Valve springs | 32 lb hot modified (NR Racing) |
| 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 |