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Small Block Chevy Dynoed Combos
(Combos 77-84)
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Combo 77.
Combo 77 comes from pages 64-67 of the April 1991 issue of Chevy High Performance. Combo 77 uses a 350 cu. in. engine with reworked GM #186 heads and a Comp Cams 268° camshaft. The source states that the camshaft has 0.474 in. of lift—however, the Comp Cams High Energy 268° cam only has 0.454 in. of lift, and the Comp Cams Xtreme Energy 268° cam has 0.477 in. of lift. Perhaps Comp Cams used to offer this version back in '91. Anyhow, this combination makes 383 HP and 404 ft-lbs of torque. |
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Combo 78.
Combo 78 comes from pages 65-68 of the April 2001 issue of Chevy High Performance. GM Vortec cylinder heads are used with a Comp Cams Xtreme Energy 262° camshaft to produce 390 HP and 426 ft-lbs of torque. These numbers are made with 87 octane, while most of the combos on my site used 92 or 93 octane gasoline. Again, I am not sure how much of a power difference the lower octane makes.
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Combo 79.
Combos 79 and 80 also come from the April 2001 issue of Chevy High Performance. These two combos are on pages 72-74 of the magazine. Both of the combos use the same old-style small block crate motor from GM, the ZZ4 350 cu. in. The only difference between the two combos is the valvesprings. Combo 79 uses stock-type springs offered by Comp Cams, while Combo 80 uses springs that are larger and more appropriate for the moderately aggressive camshaft. Combo 79 uses the popular GM ZZ4 crate motor with Air Flow Research 190 cylinder heads and a Comp Cams Xtreme Energy 274° cam. Stock-type Comp Cams 980 valvesprings are used on the heads—these have a seat load of 91 pounds and 229 pounds of load at 0.450 in. of valve lift. This engine produces 388 HP at 5500 rpm and 380 ft-lbs of torque at 5000 rpm. The torque peak is at 91% of the HP rpm peak—the stock springs have encountered valve float and dropped the HP peak from an expected speed of 6,000 to 6,500 rpm down to a lower 5,500 rpm. Past the peaks, at 6,300 rpm, the torque has dropped to 221 ft-lbs and the power has dropped to 265 HP. We can definitely see that the valves are not closing quickly enough at high rpm's with stock springs.
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Combo 80.
Combo 80 uses the same engine as Combo 79, but better valvesprings are used. Instead of the stock Comp Cams 980 valvesprings, with a seat load of 91 pounds and 229 pounds of load at 0.450 in. of valve lift, Comp Cams 981 springs are used. These springs have 105 pounds of seat load and 295 pounds at 0.450 in. of valve lift. This extra amount of increased force is plenty to keep the lifters on the cam. This engine produces 408 HP at 6300 rpm and 388 ft-lbs of torque at 5000 rpm. Since only the springs are changed, the early torque and HP curve is very similar to the previous combo. After 5,000 rpm, the stronger springs have zero valve float, and the curves are more normal. The torque peak is down to a more expected 79% of the HP rpm peak. At 6,300 rpm, the torque is dropping at a more steady rate; it is only down to 340 ft-lbs. This is 119 more ft-lbs than the combo with weaker springs at the same rpm level. The power peaks at 6,300 rpm at 408 HP, compared to the valve-floating 265 HP of the previous engine. This is 143 extra horses right when you are about to shift—with Combo 79, you would have to shift in the early 5000's before the engine began to fall on its face. Therefore, it is very important to follow the manufacturer's spring recommendations when purchasing a camshaft.
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Combo 81.
Combos 81-84 come from pages 40-46 of the July 1997 issue of Hot Rod Magazine. These combos compare rebuilt GM #882 heads and L31 Vortec heads. The baseline engine makes 275 HP and the best Vortec engine makes 375 HP. Combo 81 is the baseline engine for the test. The #882 heads have 2.02/1.60 in. valves and 1.5:1 rockers. A low-performance, stock camshaft is used with only 0.390/0.410 in. of lift at the valves.
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Combo 82.
Combo 82 changes the stock cam from Combo 81 to a Federal Mogul grind, with 0.472/0.496 in. of lift at the valves. The cam swap results in 21 extra HP.
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Combo 83.
Combo 83 is much different than the previous two combinations. The heads are swapped for GM L31 Vortecs, and 1.6:1 roller rockers are used. A Holley 750 cfm carburetor is used, and the power jumps to 375 HP with 413 ft-lbs of torque. Note that this combo is very similar to another combo from a separate test, Combo 78. Combo 78 has 2 less compression points and a slightly less aggressive cam, but it still makes 15 more HP and 13 more ft-lbs of torque. This is interesting, and it shows the variations between engines and dynos.
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Combo 84.
Combo 84 takes the previous combo and swaps the cam to one with more intake duration. The result is a loss of 11 HP and 29 ft-lbs of torque. This loss is very large for only a 10° change in the intake duration at 0.050 in. of lift. The small increase in valve overlap usually does cause the cylinder pressure, and thus the peak torque, to drop. However, because of easier breathing, the peak power usually increases with such a change. The power increase did not occur with this combination though.
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