The short answer is a stock GenII can safely handle 20 to 40 shots for several seasons
IF the motor is well tuned, on good gas, and the motor is not on the spray for more than 8 seconds. Others have gone bigger with success, but your playing with fire.
The slightly longer answer...
In the 20 to 40 HP range, engine failures still do occur. A backfire in one cylinder can pressurize the airbox momentarily, and lift an intake valve on an adjacent cylinder. A piston comes up and meets the floating valve. The valve spits a keeper. If your lucky only one keeper falls out, and the shim gets spit out with no bent valve. Motor has a big ticking noise, and you find that you need to reinstall the keeper and shim. If you are unlucky, both keepers fall out, the valve drops into the cylinder, frequently resulting in bent valves and guides, holed pistons, damaged cylinders, and even bent cams and cracked heads. Backfires can be caused by bad tuning, bad fuel, or extended time on spray creating hot spots on pistons. Even dyno tuning may not be good enough if you plan to spray on a regular basis. The tune you get at 0 mph on a dyno in march might be totaly different from what you need in August at 160 mph of ram air. Dataloggers are a good idea. 98 octane minimum is a good idea. Most of the dry spray threads you see are about how to get the N2O in with spraybars and such. That the easy part. Its getting the fuel right that is the difficult and important part.
In the 50 to 80 HP range, failure modes are different. The genII has crazy light springs and Ti valves to reduce parasitic horsepower loss. Valve float (due to mini-backfires) is common, so bumping spring pressure to 50 or 60 (or even 70! lbs) is recomended. I have heard that the Ti valves can take this spring pressure, but I have no first hand knowledge. At that point it makes sense switching over to SS valves. Remember, you have tricked the intake valve into accepting 25% more air and fuel than the engineers ever designed it for, but the exhaust valve gets HAMMERED. Generally it is a good idea to increase the exhaust valve size by minimum 1mm. Its also a good idea to advance the exhaust cam by several degrees, and add duration if possible. By rolling the lobe centers forward from 105 to about 107, you give the exhaust more time to get rid of all that extra exhaust that the engineers never accounted for. You will need adjustable cam sprockets. Head work on the exhaust side is also a good idea. Finally, you will need to retard ignition some amount depending on the fuel that you plan to use, or you will ping the motor for sure and blow it up. Remember that the GenII is a pain to advance ignition. Need to purchase an ignition module for your power commander.
In the 70 to 100 HP range, failure modes are yet again different. The wrist pin on the GenII is smaller than the prior design. Rods, Pistons, wrist pins and ring lands are likely to fail very quickly. I suspect that the skirt on the cylinder of the GenII is weaker due to all of the changes to improve pumping loss. Good aftermarket rods and pistons are a requiement to stand up to the occasional detonation. You will need a piston with a thicker ring land, and lower compression. You will need to take 1 to 2 points of comression out of the motor either with low compression piston, or an .040 spacer plate. Ceramic coated pistons and coated skirts are a good idea to stand up to the extra heat. I also increase the valve seat land to help get the heat out of the exhaust valve. Most of the heat transfer out of the valve is through the seat, and a traditional valve job is not appropriate.
125 to 150 HP range.
Perhaps you buy 4 motors and line them up for quick changes? I don't know but there are people that do.
Remember, failure builds integrity.
