Product Review: AC Delco 41-950 
Double Platinum Spark Plugs

I put in some new AC-Delco original equipment spark plugs in my 1996 Seville STS this morning.   I used an articulated spark plug wrench, a six inch socket extension, and a three inch socket extension.  I used a spark plug socket with a rubber neck built inside to grasp the plug, which helped a great deal in getting the plugs back out from the deep Northstar plug passages.  I did not have to remove the coil pack, but just worked around it and all was fine.  I did use anti-seize on the plug threads, and di-electric on the plug heads & wires.

Total effort under 1 hour, with a couple of stops for kid related chores.  Much easier than on the 4.9L, as the DOHC engine has the plugs mounted higher toward the centerline.

I measured the plug gap on each plug I removed; .050 inches is the correct gap:

8:  0.067 inches
6:  0.066 inches
4:  0.062 inches
2:  0.067 inches
1:  0.060 inches
3:  0.062 inches
5:  0.050 inches
7:  0.052 inches

So it was well time for these to be replaced, at 85K miles on the car.  All plugs were in fair shape, with some rounding of the center electrode, and some rust on the outside of the plug.  The larger plug gap (caused by wear) may have actually added to the car's performance as long as the coils were able to fire the plugs, which they were.

For the "Before" baseline, the car ran 0-60 mph in 6.75/6.83/6.82 average 6.80 seconds; After the car ran 0-60 mph in 6.75/6.72/6.72 average 6.73 seconds.  Marginal change.  Here are some performance measurements before/after:

The "gear 2" was run 24, with the Automatic kept in gear 2 for the run.  Interestingly, this caused the car to chirp the tires on the 1-2 gear shift, which brought on the traction control.  This surely influenced the run, but overall I can't see an advantage in manually shifting.

Detailed Data -- New AC-Delco Spark Plugs:

Run 22
Weight: 4,095 lbs
Temp: 65 F
Humidity: 97%
Barometer: 29.91s

Start Speed 0.0mph
  mph   s   g    ft  hp 
10.0 0.91 0.51   6  56
20.0 1.84 0.43  27  98
30.0 2.85 0.47  64 160
40.0 3.89 0.40 117 184
50.0 5.21 0.39 205 230
60.0 6.75 0.28 329 206
70.0 8.43 0.25 490 217

Pk Power: 50.5mph 5.26s 208ft 234hp
Peak G: 16.4mph 1.47s 17ft 0.52g

Run 23
Weight: 4,095 lbs
Temp: 65 F
Humidity: 97%
Barometer: 29.91s

Start Speed 0.0mph
  mph  s     g   ft  hp 
10.0 0.91 0.51   6  58
20.0 1.83 0.43  27  98
30.0 2.82 0.46  63 157
40.0 3.85 0.41 116 188
50.0 5.14 0.34 201 202
60.0 6.72 0.28 329 206
70.0 8.38 0.25 487 224

Pk Power: 49.0mph 5.00s 191ft 228hp
Peak G: 15.6mph 1.40s 15ft 0.53g

Run 24
Weight: 4,095 lbs
Temp: 65 F
Humidity: 97%
Barometer: 29.91s
Auto in 2 instead of OD

Start Speed 0.0mph
 mph   s    g   ft hp 
10.0 0.93 0.50   6  57
20.0 1.84 0.46  26 103
30.0 2.83 0.47  63 161
40.0 3.87 0.40 116 186
50.0 5.09 0.30 197 177
60.0 6.72 0.28 329 203
70.0 8.46 0.24 495 212

Pk Power: 48.3mph 4.82s 178ft 264hp
Peak G: 16.8mph 1.53s 18ft 0.52g

AP-22 Performance Meter Review