351C 4V open vs closed chamber heads
within 1% of peak across that whole range
Engine dyno, as configured — flywheel horsepower. Standard test config: stock manifolds through 18" of pipe, air cleaner lid flipped, no accessories driven. Factory SAE net ratings are measured as‑installed and read 15–25% lower. Rev ceiling for this combination: 6000 rpm.
Where each one wins
| RPM | open chamber | closed chamber | diff |
|---|---|---|---|
| 2000 | 268 | 279 | +11 |
| 2500 | 287 | 298 | +11 |
| 3000 | 309 | 321 | +12 |
| 3500 | 340 | 353 | +13 |
| 4000 | 373 | 388 | +15 |
| 4500 | 400 | 416 | +16 |
| 5000 | 404 | 420 | +16 |
| 5500 | 390 | 405 | +16 |
| 6000 | 369 | 384 | +15 |
Torque, lb-ft, at the flywheel. Everything except the part named is identical between the two runs.
The ports are the same. The chambers are not — 75.6cc open against 61cc closed — and on the same pistons that is worth better than a point and a half of compression.
So most of what people call the closed-chamber advantage is compression, plus quench that lets you run the timing to get it. Swap the chambers in the tool and watch the compression readout move before the horsepower does.
Which matters because you can buy compression other ways. If closed-chamber castings are three times the money, a piston change on open chambers gets you to the same place.
The build
| Block | Factory 351C 2-bolt main (stock) |
|---|---|
| Crank | Eagle forged 4340 — 3.50" stroke / 351 ci (~$700) |
| Rods | Stock I-beam (factory) — builders' guide ~450 HP |
| Pistons | Generic flat top (0cc) |
| Head gasket | OEM compressed (.040") — neutral |
| Cylinder heads | 4V open chamber (75.6cc) — 70s CJ/Q-code |
| Porting | As-cast / stock |
| Camshaft | COMP XE274H — hyd flat (32-246-4 · 230/236 @.050 · .562"/.565") — Holdener +44 HP vs Boss cam |
| Valve springs | COMP 916-16 dual |
| Rocker arms | Stock stamped 1.73 rockers |
| Intake manifold | Edelbrock RPM Air-Gap |
| Carburetor | Holley 750 cfm — street/strip sweet spot |
| Exhaust | Long tube 1-3/4" — hot street |
| Ignition | MSD Pro Billet distributor + 6AL box — ~8,000 rpm |
| Fuel | Sunoco 260 GT 100 — unleaded, street legal, cat/O₂ safe |
| Air cleaner | Stock air cleaner, LID FLIPPED — DYNO STANDARD |
These numbers assume a proper dyno tune — timing and jetting set on a dyno, not guessed at in a driveway. It is worth 17 hp on this combination, and it is the one line on this page you cannot buy: we build the parts list, you pick the tuner. Leave it untuned and the engine will run, it just will not read what it says up top.
The other one
| Block | Factory 351C 2-bolt main (stock) |
|---|---|
| Crank | Eagle forged 4340 — 3.50" stroke / 351 ci (~$700) |
| Rods | Stock I-beam (factory) — builders' guide ~450 HP |
| Pistons | Generic flat top (0cc) |
| Head gasket | OEM compressed (.040") — neutral |
| Cylinder heads | 4V closed chamber (61cc) — the coveted quench head |
| Porting | As-cast / stock |
| Camshaft | COMP XE274H — hyd flat (32-246-4 · 230/236 @.050 · .562"/.565") — Holdener +44 HP vs Boss cam |
| Valve springs | COMP 916-16 dual |
| Rocker arms | Stock stamped 1.73 rockers |
| Intake manifold | Edelbrock RPM Air-Gap |
| Carburetor | Holley 750 cfm — street/strip sweet spot |
| Exhaust | Long tube 1-3/4" — hot street |
| Ignition | MSD Pro Billet distributor + 6AL box — ~8,000 rpm |
| Fuel | Sunoco 260 GT 100 — unleaded, street legal, cat/O₂ safe |
| Air cleaner | Stock air cleaner, LID FLIPPED — DYNO STANDARD |
These numbers assume a proper dyno tune — timing and jetting set on a dyno, not guessed at in a driveway. It is worth 18 hp on this combination, and it is the one line on this page you cannot buy: we build the parts list, you pick the tuner. Leave it untuned and the engine will run, it just will not read what it says up top.
Change these numbers yourself