Kaase P-51d Cylinder Heads

Jon Kaase discusses the technical features of the company’s new P-51d cylinder heads.  He also draws comparisons between it and the SR-71 as well as the original P-51, a derivative of his Super Cobra Jet contribution.

Kaase Racing Engines’ original P-51 cylinder head remains a fine product; in fact, an impressive technical achievement, it produced more power in its classification than did its rivals.  Though ideal for big-block Ford enthusiasts who harbor desires of affordable power increases, those free of crank and rod changes—stroking ambitions—its production was beset by persistent foundry delays.

Late deliveries of castings had led to non-deliveries that endured for months.  To address the constraints, the company began developing an improved P-51 with hallmarks that included larger internal ports and other concepts that necessitated revised foundry patterns.  Now after three years of revisions and testing, a competent finished product has emerged.  They say that the fourth revision is usually the best, hence its classification P-51d.

Kaase P-51d flow chart (at 28 inches of water) reveals exceptional mid-range flow

Valve Lift (inches) Intake (cfm) Exhaust (cfm)
0.200 165 113
0.300 265 157
0.400 341 196
0.500 381 224
0.600 406 243
0.700 424 254
0.750 428 257
0.800 433 260


Says Jon Kaase:

“In many areas, the new head is the same as the SR-71. The biggest change is reflected in its lower intake port entry and also its port length, which is the same as the P-51 and all stock intake ports.  Unlike the SR-71, the casting does not require a China Wall spacer thus the intake manifold sits in the stock position; consequently the carburetor pad affords a one-inch advantage of additional hood clearance.

After extensive dyno testing, we adopted the 2.300 inch intake valve for the P-51d. The 2.375 inch SR-71 valve made the same power on our 521ci test engine. Usually, if an increase in valve size does not increase power output, the engine will accelerate slower because the air speed is slower. The P-51d valve guides are in the same locations as the SR-71; therefore if the heads are fitted to a bigger engine or a more aggressive build we can easily adopt the 2.375 inch valve.

In engines with bore sizes under 4.500 inches, we use a smaller exhaust valve: 1.700 inches. This valve features a 60-degree seat angle and performs well with the premium seat material and same seat bore ID as the 1.76 inch valve.  Larger bore engines use the 1.76 inch valve with a 45-degree seat. Extensive dyno testing on an 1100hp-plus engine comparing the two exhaust valve sizes produced exactly the same power!

The intake and the exhaust port openings and locations are the same as the stock Ford cast iron 429 Super Cobra Jet, the aluminum SCJ, and the P-51. The exhaust port is .250 inch taller at the short-turn radius. The intake port is also .250 inch taller at the short turn as well as being more squared-up and wider at that point.  Also, the new P-51d heads use the same length valves, pushrod guide-plates and stud girdles as the SR-71.

To conduct the P-51d dyno tests, we used our 521ci stock block engine operating with 11.5:1 compression ratio and camshaft specs of 273-280.  Older P-51’s recorded 880hp compared to the SR-71’s at 940hp with the P-51d’s close at 925hp.

By the way, the original P-51 heads aren’t dead, just resting. To revive them the tooling needs some revisions and then we’ll get a few sample castings poured and machined.

It’s important to mention that these new P-51d heads are cast at one of the best and most modern aluminum foundries in the US; to their credit, it’s not an establishment that casts patio furniture followed by a run of cylinder heads! Casting premium cylinder heads is a craft—some inspiration but mostly know-how and hard work.

Compared to the original P-51 head, a question will likely arise regarding the cross sectional area of the P-51d’s larger intake port and if its street- and low-end performance will be impaired? The answer to this is that I don’t think so.  In the 19 years of competing in the Engine Masters Challenge, it was the camshaft that made the greatest difference in performance between 2500 to 4500rpm as well as the headers and intake manifold lengths. The port size made almost no difference at lower engine speeds.  Often I’ve made intake ports smaller but found no advantage.

Finally, to accommodate all the machining and finishing work in-house, we’ve been purchasing machinery and expanding our factory space, and (though we have learned to remain hushed about new cylinder head designs until product is ready to ship) P-51d finished parts are now available with many more raw castings in stock.”

For pricing click here.