How much horsepower will a cam swap 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: 5800 rpm.
The best public answer to this question is a Speed Secrets video where one 351C 4V ran two cams on the same SuperFlow dyno with everything else left alone. Factory Boss 351 solid cam: 397 hp and 401 lb-ft. COMP XE284H (240/246 at .050, .584"/.588" lift, 110 LSA), with the springs to run it: 451 hp and 411 lb-ft. That is 54 horsepower for a cam, springs, and an afternoon, on a stock-head 4V with a Performer RPM and a 4-barrel.
Two things make that number honest. The engine was an ordinary .030-over two-bolt with factory 4V heads — nothing exotic was hiding the cam's contribution. And the gain was almost all top end: torque went up 10, horsepower went up 54, because the 284H keeps the valve open long enough to feed the 4V port above 5,000 rpm where the Boss grind (290/290 advertised on a 116 LSA, .477" lift) has already given up.
Richard Holdener ran the same XE284H on a 4V Cleveland at Westech and landed at 450.4 hp — a second data point, different shop, within a horsepower of Speed Secrets. When two dynos agree that closely, the number is real.
Where each one wins
| RPM | stock CJ cam | COMP XE284H | diff |
|---|---|---|---|
| 2000 | 308 | 261 | -46 |
| 2500 | 328 | 274 | -54 |
| 3000 | 351 | 295 | -56 |
| 3500 | 380 | 325 | -55 |
| 4000 | 404 | 361 | -43 |
| 4500 | 404 | 395 | -9 |
| 5000 | 388 | 417 | +29 |
| 5500 | 368 | 417 | +49 |
| 6000 | 274 | 399 | +125 |
Torque, lb-ft, at the flywheel. Everything except the part named is identical between the two runs.
The camshaft is the biggest single lever on this engine and the one people are most afraid of. Going from the factory Cobra Jet hydraulic grind to a COMP XE284H is a genuinely large change: 206/221 at .050 up to 240/246, and .481" of lift up to .584".
You pay for it at the bottom. More duration holds the intake valve open later past bottom dead centre, and at low rpm the piston is moving slowly enough to push part of the charge back out. That is the lope, and it is also the lost torque.
And you must change the springs. The factory springs will not clear .584" of lift, and a cam the valvetrain cannot follow makes less power than the one you took out.
The build
| Block | Factory 351C 2-bolt main (stock) |
|---|---|
| Crank | Stock cast iron — 3.50" stroke (351 ci) |
| 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 | 351 Cobra Jet (Q-code, D1ZZ/D2ZZ-6250) — 270/290 adv · 206/221 @.050 · .481"/.490" |
| Valve springs | Crane single 110 lb |
| 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 |
A cam swap without a dyno tune is a cam swap that gives half its gain back. The Speed Secrets test that anchors this page re-tuned between cams; the calculator assumes you will too, and it is worth 16 hp on this combination. Sources: Speed Secrets 351 Cleveland cam test (SuperFlow, STD); Richard Holdener 351C intake test at Westech (STD).
The other one
| Block | Factory 351C 2-bolt main (stock) |
|---|---|
| Crank | Eagle forged 4340 — 3.50" stroke / 351 ci (~$700) |
| Rods | Aftermarket I-beam forged (Eagle/Scat) — builders' guide ~600 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 XE284H — hyd flat (32-250-4 · 240/246 @.050 · .584"/.588") |
| 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 |
This one makes 456 hp on a block most builders keep under about 450. That figure is shop guidance rather than a measured limit — nobody publishes the horsepower at which a casting lets go, and plenty of these engines run above it. It is where the risk starts climbing, and the block is the first thing to climb. A four-bolt or an aftermarket block buys the margin back if you want it.
Change these numbers yourself