Building A Big Block for Nitrous Oxide - Building For A Big Hit

Over the years, we've printed quite a few stories in ST covering the addition of nitrous oxide to sport trucks. In nearly every case, the engine was stock, and the hit of N2O was a small, safe one. That's cool, but what happens when you want to throw a larger shot of laughing gas at your engine? Is it safe? Can you repeatedly feed your engine 200, 300, or more extra horsepower without melting pistons and blowing the air cleaner through the hood of your truck? The answer is yes! There is a catch though: If you want it to live longer than a few seconds after you hit the fun button, then you're gonna have to build your engine a bit differently than you would otherwise. You're also going to be spot-on when tuning your engine.

Injecting more than a 100 horsepower shot of nitrous into an engine requires the engine to be built to handle the extra cylinder pressure and heat that will be produced. So where do you start? The build tips presented here apply to any forced-induction application, including supercharged or turbocharged engines, as well as a nitrous engine. Follow along as we turn the Big Block From Hell into a guinea pig for large-dose N2O testing. Next month, we'll address the various ways to tune your engine for safe and reliable nitrous operation.

Part One: The Bottom End

1. Put simply, a cast rotating...        read full caption 1. Put simply, a cast rotating assembly will not stand up to the force exerted upon it by a high horsepower shot of nitrous oxide. To that end, we outfitted our BBC with quality forged components from Lunati and CP. After a quick polish at Castillo's Crankshaft service, our 4.375-inch Lunati Pro Billet stroker crankshaft was ready for service once again.	2. The pistons used in the...        read full caption 2. The pistons used in the old version of this engine were never meant for a forced-induction application. Although they were machined from a quality forging, there are areas in the piston design that need changing in order to survive a big shot of gas. On the left is one of the old pistons. Notice the skirt depth is much greater than the new piston on the right. CP shortened the piston skirt to keep as much of it in the cylinder at bottom dead center of the stroke as possible. This keeps the piston from rocking in the bore too much and side-loading the skirt. The barrel shape of the nitrous piston is also different. It's not round, its an ovoid shape that's meant to expand under heat without sticking in the bore.	3. The extreme heat generated...        read full caption 3. The extreme heat generated in the combustion chamber from the additional fuel and nitrous oxide will cause the aluminum piston to expand at a greater rate than a piston in a naturally aspirated engine. To keep the piston from sticking in the bore during a nitrous run, the diameter of top of the piston is reduced, which is visible in the taper shown in the piston on the right of this photo.
4. This photo shows a couple...        read full caption 4. This photo shows a couple of important design changes: First off, the area below the wristpin bore has been beefed up for extra support that will help keep the wrist pin from flexing, galling the rod bushing, and sticking in the pin bore. Second, you can see that in the new piston the ring locations have been moved downward from the top of the piston and the ring lands are much thicker. This helps support the rings and move them slightly away from the violent combustion event occurring in the chamber when the nitrous is introduced.	5. The new piston also features...        read full caption 5. The new piston also features CP's "slipper" design below the wristpin bore. This flat design eliminates sharp edges that can turn into stress points under load. Sharp spots are the first place a crack will develop.	6. You'll notice that the...        read full caption 6. You'll notice that the piston on the right has a smaller diameter dish than our old piston. This is because we'll be running this engine on race fuel now. The higher octane fuel is necessary when using a large hit of nitrous, and we figured we might as well gain some compression and horsepower by making the dish smaller since we were going to have to buy expensive race fuel anyway. The smaller dish will up the compression ratio from 12.29:1 to 13.7:1.
7. This photo also reveals...        read full caption 7. This photo also reveals that our intake valves were contacting the pocket of the old piston. We showed the contact area to CP and the company used this info to determine how much larger and deeper the pocket in the new piston should be. You'll also notice that the top of the pocket is not perfectly flat. The material at the outer edge of the piston is thin here and under the extreme heat of a nitrous hit, this material could become a hot spot and melt away, causing engine damage. CP puts a radius cut on top of the pocket, eliminating the thinnest portion of it to avoid this potential damage.	8. The wristpin on the right...        read full caption 8. The wristpin on the right side of the photo is from the old motor. It's a lightweight design that tapers from end to end to reduce mass. This is a trick way to make the rotating assembly lighter and easier to accelerate, which translates into more power. But, that wristpin will flex enough to wear out quickly in a nitrous application so we ordered straight wall wristpins that are better suited to our application.	9. Piston rings are another...        read full caption 9. Piston rings are another part of the build that require a different approach to sealing the cylinders up. Again, the heat and pressure built up in the combustion chamber plays a role in not only what material the rings should be made from, but also how large the end gaps should be and how much ring tension is needed to maintain a good seal. We replaced our old low-tension rings with standard tension rings from Akerly and Childs.
10. We bought the rings through...        read full caption 10. We bought the rings through CP and at its suggestion, went with a Hellfire top ring with a .030-inch gap and a ductile iron second ring with a .035-inch gap. We file-fit our rings to each individual cylinder bore using this Summit Racing manual ring filer.	11. When it comes to ring...        read full caption 11. When it comes to ring sealing, the finish of the cylinder walls is just as important as the end gaps. We took our cylinder block to Paul Pfaff Racing in Huntington Beach, California, because the shop has state-of-the-art equipment to precisely bore and hone our Brodix aluminum engine block. The diamond stone tools used to hone the cylinders left a cross-hatch finish matched to our ring package.	12. We went with Clevite main...        read full caption 12. We went with Clevite main and rod bearings sourced through Calico Coatings. Calico stocks bearings in many sizes and the company's CT-1 Dry Film Lubricating not only reduces friction in the rotating assembly to free up horsepower, but it also protects the bearing material.
13. We used Proform Parts...        read full caption 13. We used Proform Parts billet aluminum rod vice and a dial bore gauge to check the oil clearance between the crankshaft and connecting rod bearings, setting the clearance at .0025 inch. We also double checked the main bearing clearances, setting the clearance at .0030 inch. With the clearances where we wanted them, we put the bottom end together and moved onto the valvetrain updates.