After a thorough review of all the dyno pulls we made that day, it looks like the engine probably started detonating on the first 150-hp pull we made (three consecutive pulls at each power level were made for all tests). Average horsepower dropped from 581 to 572 to 567 on each 150hp pull, and we think the initial damage set in then. When we later tore down the motor, we found two things that went wrong, both a result of the continued hammering from detonation.
The No. 7 cylinder bore the brunt of the load, and its moly-coated ductile-iron top ring and forged piston suffered fatally. Also, every connecting rod bearing and most of the main bearings were about to give out, showing more copper than any street bearing should. This further proves the nitrous experts correct in that 150 additional nitrous horsepower is the max safe point for even a stout motor when running on pump gas. Any more nitrous should at least be combined with a mix of race gas to get your octane well over the 100 mark. And you should always pull out at least 4 degrees of advance for every 50 hp added, that means at least 12 degrees should come out with a 150hp setting and 16 degrees should be removed for 200 hp and so on.
Keep in mind that for this test only, we had the benefit of not worrying about hurting the motor (even our meager budget can allow us a few mistakes from time to time). You'll probably never have that benefit, and we hope you learn from our prodigious misuse of nitrous oxide. We sacrificed this engine for your benefit, so we don't want any letters from you crying how you blew your motor up making the same mistakes we did.
We first ran the motor sans nitrous to dial in the carb and get some baseline power figures. These are the engine specs used in all tests.
Short block: 391cid, 3.75-stroke4340 forged C.A.T. crank, 4.030-bore size, 5.7-inch H-beam 4340 forged C.A.T. rods,10.25:1 forged JE pistons and moly rings, Clevite H-series bearingsHeads: Holley Street Avenger, ported by Valley Head Service, 2.05/1.60 valves.1.6:1 Lunati roller rockersIntake Manifold: Holley/Keith Dorton-series Strip Dominator PN 300-110Cam: Lunati solid flat-tappet 254/268 duration @ .050, .570/.612 lift, 110 lobe separation, installed straight upCarb: Holley HP 830cfm double pumper PN 0-80785Ignition: Holley Annihilator, 36 advanceFuel: 92-octane, 76-pump gas
Peak HP 494 @ 5,700Peak TQ 482 @ 5,100
Peak Power Levels With Nos KitsTop Shot100HP settingFuel PSI 5.25Timing 34HP 583 @ 5,500TQ 590 @ 4,600