How much horsepower does a 1971 351 Cobra Jet have?
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: 5200 rpm.
The Q-code Cobra Jet is the 4V with closed-chamber heads and the bigger 4300-D carburetor. The cam is D1ZZ-6250-A — hydraulic flat tappet, advertised 270/290, .481"/.490" lift. Ford rated it 280 gross.
The closed chamber is the part worth having. Quench, better compression for the same piston, and a chamber shape that tolerates timing. It is the head everybody hunts for and the reason a CJ core costs more than an M-code core.
Note this is a hydraulic cam. Only the Boss and the 1972 HO got solid lifters from the factory — everything else Cleveland is hydraulic.
The build
| Block | Factory 351C 2-bolt main (stock) |
|---|---|
| Crank | Stock cast iron — 3.50" stroke (351 ci) |
| Rods | Stock I-beam (factory) — to ~450 HP |
| Pistons | Generic flat top (0cc) |
| Head gasket | Fel-Pro .040" MLS — 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 | Stock 4V cast iron — factory 4bbl |
| Carburetor | Autolite 4300-D 4bbl — Boss 351 / Cobra Jet (600 cfm) — built 1971-74 only, reman/NOS |
| Exhaust | Stock manifolds + 18" pipe — DYNO STANDARD |
| Ignition | Stock breaker points — floats ~5,200 rpm |
| Fuel | Sunoco 260 GT 100 — unleaded, street legal, cat/O₂ safe |
| Air cleaner | Stock air cleaner, LID FLIPPED — DYNO STANDARD |