Nitrous Rats
Jay Jones, currently in Iraq: I own a '76 Nova. It has been gutted and treated to a set of CalTrac bars and adjustable shocks in the rear. My Nova is powered by a 468ci big-block Chevy that I have been more than happy with. I have two problems, though. About six months ago I put a Nitrous Express Hitman kit on the big-block and set it up to deliver a 150hp shot. I want to put more nitrous to the engine, but I am told that there are two things that will hold me back from doing that. First, I did not put a forged crank in the car. Although the Scat crank is stronger than production, I'm not sure if it will hold up to much more gas. The second thing is I am told that the ring seats on my pistons are too high (I don't know what that means) and that the pistons, even though they are a good forged set, cannot or should not have more than 175 hp of nitrous applied to them. Do you think that the bottom end will handle maybe 200 hp of nitrous? Also, I am running the Edelbrock Performer 454-0 heads on the engine along with a Team-G intake and a Race Demon carb. If you don't think the bottom end will hold up, what rotating assembly would you advise me to get? I do not want to get into another overly expensive build, but I know that you must pay to play. If I go with a new assembly I would like it to be a 496.
Measuring top ring endgap is important when building any engine, even if the rings are already clearanced for the bore diameter. Make sure to square the ring in the bore to ensure that measurement is accurate.
Jeff Smith: Great question, Jay. As with some information you get from friends and Internet experts, there are a few grains of truth amid the chaff. Let's look at the crankshaft question first. Our experience with Scat cranks, even the cast units, has been universally good. The downside to a cast crankshaft is that castings tend to crack when hit with a bending moment, while forgings, generally speaking, are more flexible and resilient. While everyone thinks about horsepower with nitrous systems, it's actually torque that has a big impact on the crankshaft. Consider that a 200hp shot delivered to the engine at 3,500 rpm is the equivalent of an increase of 300 lb-ft of torque. That's roughly a 75 percent increase in torque at that rpm. That's why the nitrous companies tell you not to engage nitrous at engine speeds lower than 3,500 to 4,000 rpm. In your case, I think that your crankshaft is OK as long as you don't rev the engine much past 6,000 rpm. With cast cranks, engine speeds above 6,500 rpm are a common cause of crankshaft failures, especially when combined with heavy pistons. The g-force loads of heavy rods and pistons changing direction at higher engine speeds are what may eventually fail a crank, not necessarily the addition of another 200 hp.
As for the pistons, top ring position is very important to increasing piston and ring life when using nitrous. The closer the ring is to the top of the piston, the higher the temperature from the heat of combustion that transfers to the top ring. Nitrous increases cylinder temperature, which puts more heat into the top ring, causing it to expand. If the ring expands enough to push the ends of the ring together, the ring seizes in the groove and causes all kinds of ugliness to cylinder walls and pistons. In the worst cases, it will break the piston. We've seen a hypereutectic piston with the entire top half broken off from just above the wristpin as the result of a butted top ring. All good engine builders will automatically increase the top ring gap to accommodate mild nitrous use. One way to reduce the heat that hits the top ring is to move away from the piston deck surface, away from the heat. A less expensive step would be to increase the top and second ring endgaps to accommodate the additional heat. Since you've already successfully run your engine with a 150hp shot, the additional 50 hp may not do any damage, but short of tearing down your engine there's no way to be sure. Unfortunately, you did not include enough information to make an educated guess.
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Here are some areas that need to be addressed: If you are running forged pistons, I would call the piston manufacturer and get its recommendation for nitrous use. If the pistons are hypereutectic, I would not recommend running any more than your current 150hp shot. While hypereutectic pistons are stronger than cast, they are more brittle and tend to break rather than bend when subjected to detonation. If you know what the top piston ring gaps were set at when the engine was built, this will also help determine your course of action. For example, if your ring endgaps were set tight, then that additional nitrous is asking for trouble. But if the ring endgaps were set loose, then the additional nitrous may be OK. If you don't know your ring endgaps, you'll be taking a certain amount of risk at levels around 200 hp. I've included a piston ring endgap chart courtesy of JE Pistons that makes recommendations for ring endgap based on normally aspirated and nitrous use. Note that the chart uses bore size times a given clearance that increases as horsepower and cylinder temperatures increase.
Other recommendations for more nitrous would include going one step colder on your spark plug heat range and changing (if you have not already done so) to nonprojected-nose spark plugs. Projected-nose spark plugs use a longer ground strap, which can turn into a glow plug that can preignite the incoming mixture and cause all sorts of engine damage. I'd also recommend using a copper-core-style spark plug since the center electrode does not tend to glow when subjected to higher cylinder temperatures. It's also advisable to reduce your total ignition timing by at least another 2 to 3 degrees to prevent detonation. Of course we probably don't need to tell you that you will need higher-octane fuel and sufficient fuel flow and fuel pressure to feed that additional 50 hp.
| RING ENDGAPS |
| DESCRIPTION | TOP RING |
| Normally aspirated street engine | Bore x 0.0045 |
| Moderate street turbo/nitrous use | Bore x 0.0050 |
| Nitrous race only | Bore x 0.0070 |
Example: Moderate street nitrous use for a top ring in a 4.250-inch bore is 4.25 x 0.0050 = 0.0212-inch top ring endgap.
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