CPDave said

Thought I'd add a little data to this discussion. First we all know that bigger, better flowing heads make more power, but there is a lot of other stuff that has to go into the combination to exploit heads that flow 270-320 CFM @ 0.600" & 28" of water.

So, say your running a dual plain intake, and 8.8-10.5:1 compression, want to run pump gas, have a nice wide torque curve, don't want to spin the motor past 7000, and aren't running a stroker... Now a smaller head may begin to make more sense. So how much power can you get out of a ...302 casting? That depends on the flow available at the intake valve, so here are some numbers (all @ 28" of water):

STOCK 302 1.78"/1.5" (Shady Dell)
0.100" 62.8 NA
0.200" 125.6 70.9
0.300" 158.4 92.8
0.400" 169.3 99.6
0.450" 170.6 101.0
0.500" 170.6 102.4
0.550" 170.6 102.4
0.600" 170.6 102.4

As Cast "302" 1.78"/1.50" (MD Performance Center)
0.100” 51.8 36.5
0.200” 92.5 70.7
0.300” 130.1 104.3
0.400” 163.9 113.2
0.450” 169.1 115.2
0.500” 170.4 117.3
0.550” 172.2 119.0
0.600” 172.5 120.4

Ported
"302" 2.02"/1.60" (Shady Dell)
0.100” 71.8 42.5
0.200” 148.4 86.3
0.300” 212.2 113.9
0.400” 241.5 131.1
0.450” 244.6 137.7
0.500” 246.7 139.7
0.550” 242.6 144.9
0.600” 240.8 144.9

Ported "...302" Heads 1.94"/1.60" (Shady Dell)
on a 4.040" Bore (heads currently on my Dart's
0.050 over 318 (326Cid):
LIFT IN / Out
0.100" 66.1 / 48.4
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

Same 1.94/1.6 302 head on MPC's bench on a 4.030" Bore
LIFT IN / EX
0.100" 65.7 / 49.6
0.200" 128.6 / 94.5
0.300" 174.7 / 130.6
0.400" 208.6 / 157.5
0.450" 216.6 / 166.0
0.500" 205.7 / 171.1
0.550" 210.0 / 173.3
0.600" 213.1 / 173.2

Some other Small chamber heads:

273 Cleanup Port/Valve Job 1.78"/1.50"
(pretty much a "stock" class vavle job)
0.100" 49 43
0.200" 98 85
0.250" 124 100
0.300" 145 112
0.350" 162 120
0.400" 179 125
0.450" 188 129
0.500" 190 132
0.550" 185 134
0.600" 177 135

273 Ported 1.88"/1.60" (note the biggest intake
gains are at lower lift, suspect the intake valve get's
shrouded against the side of the combustion
chamber at higher lifts):
0.100" 60.93 48.78
0.200" 122.75 99.78
0.300" 179.68 137.16
0.400" 197.02 157.11
0.500" 185.91 168.20
0.600" 187.24 179.29

So how much power can a stock 302 head support? Well they flow roughly 170 CFM @ 0.500 & 28" of water, the thumb rule for this is:

Horsepower per cylinder = .43 x airflow @ 10" of water,
.275 x airflow @ 25" of water, or
.26 x airflow @ 28" of water.

So 0.26 x 170 x 8 = 353 HP at the crank (if everything is optimized you
might be able to 10% better, say 385 HP).

What is the impact of porting on this? Taking the average of MPC and Shady Dell's numbers we get 226 CFM @ 0.600" and 28" of water:

0.26 x 226 x 8 = 470 HP at the crank (again, if everything is optimized you
might be able to do 10% better, say 523 HP).

So that's theory, how'd it really work out? Below are a series of chassis dyno plots (all for the same short block and intake manifold (LD4B)). The first set of three shows a baseline (Green) with 9.1:1 compression, unported 302 heads, 0.420" lift mech cam, and stock early A-body exhaust manifolds dumping into a single 3" exhaust, 750 Holley. The Red trace shows the impact of TTI headers, 3"-in-2.5"-out X-pipe, glass packs dumping in front of the rear wheels, and Hughes roller rockers. Next (Blue) is the swap to the ported 302 castings, with a 0.005" clean up cut these cc'd slightly smaller than the unmolested 302s and bumped the compression to 9.25:1. A tidy gain of 54 Hp and 46 FtLb without so much as pulling off the water pump. So Time for a Cam...

The Existing cam was a flat tappet Mechanical:
Intake Exhaust
Valve Lift 0.420" 0.420"
Advertised Dur: 262° 262°
Dur @ 0.05" Lift: 219° 219°
Lobe Sep: 110°
Intake Center Line: 106°

The new cam is a Comp Cams Mech Roller:
Lift: 0.630" 0.630"
Dur@0.050": 254° 260°
AdvDur(@0.006"): 284° 292°
LSA: 107°
Int Center Line: 102°
Lash: 0.022"-0.024".

Additionally the heads were milled 0.030, which netted roughly a 10.1:1 CR, and we replaced the vacuum secondary 750 Holley with a QFT 750 Double pumper... The second image below (black & white plot) compares the blue plot from the previous graph to the results of the cam, compression, and carb change; net 85 HP & 15 FtLb :D

Sooo, then I hurt the motor and we rebuilt it with lighter forged pistons (previously was 0.045"over KBs about 0.020" down the bore), a 0.050" over bore (only needed a 0.005" hone to clean up), a compression ratio of 10.25:1, and Harland Sharp rockers replacing the Hughes rockers which were getting tired too quickly in this application (road race/street). I added a 0.5 partially open carb spacer and we also replaced glass packs with some 16" long oval Borla mufflers (quieter!). This netted another 33 Hp and 47 FtLb (the blue traces on the final graph). Still digests pump 93 gas.

Is there more there with better heads and intake, you bet, but this is plenty to pull to 150MPH on the front straight at Summit Point. My rule book is a little restrictive, I have to use a 273/318/destroked 340 (less than 5000cc), Iron heads, and one of the following manifolds: LD4B, LD340, Performer, or Performer RPM.

So the final question might be, what would the stock 302 heads be like on a 360? Another Thumb rule says:

RPM at peak horsepower = (N / one cylinder displacement (ci)) x CFM

N = 2,000 @ 10" of water
N = 1,267 @ 25" of water
N = 1,196 @ 28" of water

So a 0.030" over 360 cylinder = 45.66 ci

(1,196 / 45.66) 170 = 4452 RPM

and Ported:

(1,196 / 45.66) 226 = 5919

and a .045 over 318 = 40.66 ci

(1,196 / 40.66) 170 = 5000 RPM (in the real world with headers it peaked about 5300)

and Ported:

(1,196 / 40.66) 226 = 6647 (ITRW It peaked about 6300)

So yes, you could make more power with better heads, but looking at the torque curves and considering the throttle response that a high flowing but smaller port can provide, the high turbulence/quench combustion chambers also provide greater efficiency and tolerance for lower octane fuel, so there is a place for the 302 head in a street performance or moderate competition build.

Dave

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