I dyno'd two 347's on Friday. I got a lot of good data to pass along...
Engine 1 was a D9 302 block, Scat cast crank, Scat I-beam rods, Mahle pistons, 10:1 compression, 1.5/1.5/3mm ring pack. RHS heads, out of the box with a good valve job, with careful setup and assembly. Used a Weiand intake, pretty short dual plane, with a Quick Fuel Slayer 650. Cam was a custom Comp hydraulic roller, 230/236 @.050", 110 LSA, 104 ICL, .608" lift.
It made 425 hp @ 5750 with 445 lb-ft at 4500. Not too shabby for a mild engine. Going to a customer on the island of Malta...
Engine 2 was an E7 5.0 block, Scat cast crank, Scat I-beam rods, Diamond pistons, 9.5:1, 1/16, 1/16, 3/16 ring pack. CHI Boss 185cc heads, hand-ported, along with a hand-ported CHI Boss intake manifold. Basically a Boss 347. Used a Quick Fuel Slayer 650 again. Cam was a Comp hydraulic roller, lobes hand picked, 223/227 @ .050", 110 LSA, 104 ICL, .635" lift.
It made 517 hp @ 6200 with 463 lb-ft at 5200.
Now, I know it's not a "control" experiment with just a head swap, but I'll tell ya, the cylinder heads make the engine. Not only did we make basically 100 hp more, but the peak hp rpm was 600 rpm higher with a shorter duration cam.
The funny thing is that I built Engine 2 for a car builder in Mississippi. He originally wanted to put it in a '69 Boss 302, so I had Joe port the heads and intake and I wanted to put a real nice cam in it to match the setup. He got rid of the Mustang before I got the engine build started, and bought a '52 Ford F1 pickup to sell and decided to put this engine in it. His very words were, "Make this a very mild engine..." I was upset at first because we spent the time and money to have the heads and intake ported, and I basically thought we were wasting a good engine.
With 9.5:1 and a short cam, I really thought this was going to be turd-puller, but it goes to show you that sometimes you get what you didn't expect.
The lower compression showed its head though, as this combo took about 4-6 degrees more timing than the usual CHI headed builds do. It actually picked up 25 hp going from 30 degrees total to 34 degrees total.
Was also very happy with the cam. I tried some new lobes that I hadn't tried before and they were very quiet, but was not getting any evidence of valve float, even at 6500.
This was the first 347 that made 425hp....
And the Boss 347....
