How much horsepower will headers add to a 351 Cleveland?
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 | stock manifolds | 1-3/4" long tubes | diff |
|---|---|---|---|
| 2000 | 261 | 279 | +18 |
| 2500 | 279 | 298 | +19 |
| 3000 | 300 | 321 | +21 |
| 3500 | 331 | 353 | +22 |
| 4000 | 365 | 388 | +23 |
| 4500 | 392 | 416 | +24 |
| 5000 | 396 | 420 | +24 |
| 5500 | 380 | 405 | +25 |
| 6000 | 359 | 384 | +25 |
Torque, lb-ft, at the flywheel. Everything except the part named is identical between the two runs.
Headers are the most oversold part in the catalogue and still one of the most worthwhile. The honest number on a small block is high single digits to low double digits in percent, not the thirty percent the ad copy implies.
Cleveland exhaust ports are enormous to begin with, which is part of why the factory manifolds are less of a restriction here than on some engines. You still gain, just not as much as the guy at the swap meet says.
Tube size matters less than people think too: on a same-engine test, long tubes from two different manufacturers landed within a horsepower of each other, and adding mufflers cost about one percent.
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 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 | Stock manifolds + 18" pipe — DYNO STANDARD |
| 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 16 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