How much horsepower will a bigger carb 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 | 22mm clone carb | stock Mikuni 18mm | diff |
|---|---|---|---|
| 1500 | 2.42 | 2.28 | -0.14 |
| 2000 | 3.33 | 3.14 | -0.19 |
| 2500 | 4.28 | 4.04 | -0.24 |
| 3000 | 5.27 | 4.97 | -0.29 |
| 3500 | 6.33 | 5.98 | -0.35 |
| 4000 | 7.44 | 7.03 | -0.42 |
| 4500 | 8.60 | 8.12 | -0.48 |
| 5000 | 9.81 | 9.26 | -0.55 |
| 5500 | 10.26 | 9.69 | -0.57 |
Horsepower at each rpm. Everything except the part named is identical between the two runs.
The stock carb is an 18mm unit sized for a governed engine that never sees 4,000 rpm. Once the governor is gone it becomes the restriction.
A 22mm clone carb is the cheap answer and it does the job at this level. The jump is real, and the main jet moves with it — the two figures at the top of this page are both outputs of the same calculation.
There is a limit to this. Past a certain build the 22mm becomes the restriction in its turn, which is a different page.
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 | Stock Mikuni clone 18mm — Hemi factory |
| Air filter | Pod / cone filter — GPS / OMB angled filter kit |
| Exhaust | GPS header + screw-on muffler kit |
| Fuel | Pump 87 octane |