How much horsepower will an RPM Air-Gap 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 | factory 4V iron intake | Performer RPM Air-Gap | diff |
|---|---|---|---|
| 2000 | 250 | 261 | +11 |
| 2500 | 264 | 274 | +10 |
| 3000 | 287 | 295 | +8 |
| 3500 | 320 | 325 | +5 |
| 4000 | 357 | 361 | +4 |
| 4500 | 391 | 395 | +4 |
| 5000 | 410 | 417 | +6 |
| 5500 | 400 | 417 | +17 |
| 6000 | 382 | 399 | +17 |
Torque, lb-ft, at the flywheel. Everything except the part named is identical between the two runs.
This is the intake question everybody asks, and unlike most of them it has a real answer — somebody ran fourteen manifold configurations on one Cleveland with one cam and published every rpm point.
The measured spread from the factory iron 4V manifold to a race single plane was under seven percent at peak. The Air-Gap sits near the top of that, and it makes the most TORQUE of anything tested — more than the single plane that beats it on horsepower.
Which is the useful part. A single plane wins the peak and gives up the middle. On a street car the Air-Gap is usually the faster manifold even though the number on the sheet is smaller.
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 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 | Stock 4V cast iron — factory 4bbl |
| 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 | 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.
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