Horse Power Potential of Cylinder Heads

When I was researching cylinder head intake flow's impact on power I ran across this set of thumb rules for relating HP to CFM:

Horsepower per cylinder =
0.43 x airflow @ 10" of water,
0.275 x airflow @ 25" of water, or
0.26 x airflow @ 28" of water (This last is 2.08Hp / CFM in an 8 cylinder engine).

You will note that one has to know the test conditions to derive the potential of a given head (i.e., how hard was the flow bench sucking on the head :D ). One also has to take into account other aspects of the test set-up like how often the flow bench is calibrated, whether putty was used around the entry of the port (or extensions on exhaust ports), and the size of the bore the head is being sucked over (large intake valves can be partially shrouded by the cylinder wall. Another point is that this does not take into account exhaust flow; you can have 320 CFM @ 0.600" lift on the intake port, but if the exhaust ports top out at 150CFM your never going to make the predicted numbers even if the rest of the package is optimized. Finally different shops (both being completely honest) can disagree on flow by 10%-15% on the same head due to all the variables involved in the testing procedure.

So yeah 320 CFM @ 28" of water can support about 665HP +- 30 or so, but the rest of the package has to match up to it and that also will likely require a race gas tune to get there(all of this leaves out the variables involved in actually measuring HP too :banghead: ).

All that said here's a real world example, the 318 combo that's in my '66 Dart:

'69 318 0.050" over bock, K1 rods, Scat 4340 crank, custom forged pistons (forget by whom right now), Hughes Engines main stud girdle.

EdleBrock LD4B Intake with a 0.5" semi open spacer and QFT road race 750

The heads are ported 1985 and up 318 ...302 castings with 1.94" intake and 1.6" exhaust valves by Ryan Johnson at Shady Dell speed shop in central
PA; they flow:

LIFT IN / EX
0.100" 66.1 / NA
0.200" 132.8 / 90.4
0.300" 189.9 / 126.2
0.400" 218.4 / 148.8
0.450" 227.9 / 152.9
0.500" 232.8 / 159.8
0.550" 238.0 / 163.9
0.600" 239.1 / 169.8

at 28 Inches of water (these will support no more than 497 HP). This motor previously made 330 HP at the wheels with the same cam & heads but less compression (was 10.1:1 now 10.25:1).

Comp Mech Roller Cam:
Lift-0.630"/0.630"; [email protected]"-254*/260*; and LSA-107*.

Harland Sharp Rockers. Magnum rocker adjusters, push rods, and lifters (basically pressure oiling the push rod/rocker pivot point from the lifters and through the push rods like a Ford or Chevy small block).

Headers used on the engine dyno were 1"&5/8" to 3" collectors, in the car I have the TTI early A-body step headers (1.625", 1.75", 1.875" to 3" collectors), 3" to 2.5" X-Pipe and a pair small Oval canister Borla mufflers dumping in front of the rear wheels.

Milodon road race pan, Brad Penn 20w50 Oil, Oberg canister filter, Oil cooler, and 3 quart Accusump.

On 93 octane pump gas at the crank:

RPM HP FtLb
==== === ====
3100 216 365
3200 221 364
3300 229 364
3400 237 366
3500 246 370
3600 255 373
3700 265 377
3800 275 381
3900 285 383
4000 293 385
4100 302 387
4200 309 387
4300 316 387
4400 324 387
4500 332 387
4600 340 388
4700 348 390
4800 357 391
4900 364 391
5000 371 390
5100 378 389
5200 384 388
5300 391 387
5400 396 385
5500 401 383
5600 406 380
5700 411 379
5800 416 376
5900 419 373
6000 421 368
6100 422 364
6200 424 359
6300 425 354
6400 423 347
6500 421 340
6600 419 333
6700 416 326
6800 415 320
6900 414 315
7000 413 310

Dave

And at the rear wheels: