If you’re not a follower of high-performance aviation, then Jon Kaase’s cylinder head part numbers may be of only passing interest. But for the warbird faithful, Kaase’s P-38 head for a small-block Ford or his P-51 version for the 385-series big-blocks is his way of acknowledging these earlier piston-driven machines. His latest SR-71 venture is also aimed at the 429-460 family but shoots for even greater heights and certainly appears more than deserving of that high-flying moniker.
This latest cylinder head from Jon Kaase Racing Engines (JKRE) offers the latest in port manipulation aimed at improving power. The Ford faithful are well aware of his abilities, especially after successfully proving out his Boss Nine reanimation of the original Boss 429 head. Moving back into the realm of the wedge style head, Kaase’s (pronounced Kah-zee) latest SR-71 head is a serious upgrade even from his previous P-51 effort.
SR-71 – The Cylinder Head
This head benefits from some of Kaase’s recent experiences in the Engine Masters competition where his air manipulation has proved to be highly successful. The SR-71 merely improves upon the highly touted P-51 head in areas where mere flow numbers don’t begin to tell the story. Kaase says “You can have two heads with the same airflow numbers, yet one has the valves in different places and it makes 50 more horsepower.” This is why Kaase isn’t a big fan of posting airflow numbers. It’s more about the dyno numbers.
We’ll hit the critical features that distinguish this latest version from the also-rans. These figures will back up Kaase’s call, that the SR-71 should out-power any other conventional 429/460 cylinder head. We’ll start with the valve angles. The stock 429/460 Ford head took a cue from its canted-valve Cleveland cousin by tilting the valves inward toward the center of the cylinder. Unfortunately, the intake and exhaust port configurations weren’t all that conducive to high flow. The factory Super Cobra Jet heads were slightly better and the SR-71 valve angles are close but not exactly the same.
The SR-71 heads use an 8.3-degree intake valve angle canted at 4.7 degrees, followed by an exhaust valve angle of 4.7 degrees with a 4-degree tilt. But Kaase then devoted a majority of his time developing a port to improve the high-lift flow. One thing Kaase has learned in his airflow travels is that a generous short-side radius improves the chances that the air will tend to adhere to the floor as it transitions from the port into the bowl area. Raising the floor and the roof of the port in the bowl helps fool the air into thinking it has turned a far less critical corner. This is like road racers negotiating a high-speed corner by making the entry and exit as wide as possible.
Compared to his previous head, the SR-71 has raised the intake port by a generous 0.450-inch – much of which is taken up by the raised port floor in the bowl. As often happens, the raised roof is limited by the other real estate factors, such as the position of the intake valve spring seat. Raising the seat location makes the intake valve longer, so carefully balancing these factors is one key to success. The roof height can be at least partially evaluated by the intake valve length. The SR-71 head demands a 5.700-inch intake valve while the longest stock 429/460 valve is a mere 5.296-inch. Add the roof height change and the numbers come pretty close.
This taller intake bowl area is matched with an even larger intake valve diameter of 2.350-inch, which is a solid 0.125-inch larger than the intake valve in the P-51 head. That size has been carefully balanced with the port’s maximum flow potential. This port will easily feed a 600ci or larger engine with 8,000 rpm potential and valve lift numbers of up to 0.800-inch.
Not all the work was focused on the bowl area, however. Valve angles are all about line of sight flow. This means raising the intake port. But Kaase knows that a custom intake wasn’t part of the plan so he did the next best thing and retained the Super Cobra Jet intake port flange by widening the head at the intake port flange which allowed raising the port opening. This also raises the entire intake (any intake will sit taller than a typical 385 series engine), so Kaase will supply 0.900-inch tall china-wall spacers that will take up the space underneath a typical Super Cobra Jet-style intake.
One way flow port numbers can be cheated is to tweak the area directly underneath the intake valve seat – called the throat. The generally accepted standard for street head designers is the 90-Percent Rule. This establishes the throat diameter under the valve seat at 90-percent of the valve diameter. Increasing this diameter to 92- or 93-percent will increase peak flow numbers and bragging rights. But the cost for this marketing chicanery is a significant loss in mid-lift flow numbers. If you look at the flow numbers from Kaase at 0.400 and 0.500-inch lift on the intake side (see below), you can see that these numbers are seriously improved over previous heads, including the P-51. There’s no trickery here.
Of course, you can’t build an outstanding intake port and then leave the exhaust side untouched. The exhaust valve size is unchanged from Kaase’s previous effort at 1.76-inch, but the top of the port bowl was also treated to a 0.250-inch roof-raising. This was matched with a more efficient exhaust port exit, but Kaase says a Super Cobra Jet set of headers will still bolt directly to the port flange in the stock location.
On the chamber side, Kaase has also made some significant changes. The chamber is actually only 70 cc and if you look closely, you may notice the spark plug has migrated even closer to that triangle area between the intake and exhaust valves. This moves the plug closer to the middle of the chamber and a combination of just these two factors alone could positively affect ignition timing. As the space above the piston becomes more efficient, ignition timing requirements are reduced – meaning the engine needs less lead time to complete the combustion event. Less timing means the engine is doing less negative work.
All this effort has combined to create an outstanding cylinder head for the 385-series Ford engines. With this additional airflow, it would be very easy to create a 521ci stroker with a hydraulic roller valvetrain that could easily make 750 to 800 horsepower with 650 lb-ft of tire-frying torque and still be completely streetable. Push the camshaft a bit and power levels exceeding 800 hp are also achievable.
How much more power? Kaase’s first experiment with these new heads occurred with a 598ci, 15:1 compression race motor that cleared 1,224 naturally aspirated horsepower at 8,000 rpm with over 915 lb-ft of torque on race gasoline. That’s over two horsepower per cubic inch! Remember, these are wedge heads. So this just ratchets up the horsepower war to a crazy new level. There’s a reason Kaase named these heads SR-71 – they are high fliers.
Kaase SR-71 Cylinder Head Specs
Intake port volume: 405cc
Exhaust port volume: 183 cc
Intake valve diameter: 2.375-inch
Exhaust valve diameter: 1.760-inch
Combustion chamber: 70cc
Valve length Intake: 5.700-inch
Valve length exhaust: 5.320-inch
Intake valve angle: 8.3° x 4.7°
Exhaust valve angle: 4.0° x 3.2°
Valve spring diameter: 1.550-inch
Valve guide diameter: 11/32-inch (0.34375-inch)