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Nitrous and Blower Motor Tech Tips

Introduction:
Two aspects of engine building that relate to nitrous and supercharged use will be discussed. Those that allow the motor to survive under nitrous or supercharger boost and those that enhance performance. General principles are discussed as definitive answers to such FAQ's as "will my motor survive with an xxx shot of nitrous" or with "xxx pounds of boost" are surprisingly hard to answer. There are just too many variables involved to provide general answers to such questions.

Engine Block:
Factory blocks vary all over the map in terms of their basic strength. Different materials are used and designs vary from wimpy to beefy. And for certain motors, the aftermarket provides numerous alternative designs with the same basic architecture. Examples include the GM "Bowtie" blocks and the Ford "R" blocks from the OEM's and the Dart and World Products aftermarket pieces. The main areas to concentrate on are the strength of the cylinder walls and the design and materials used in the main bearing support areas; the caps and webs.

To illustrate these points, let's look a little bit more closely at the venerable small block Chevy, small block Ford, big block Chevy, and the new Gen III small block as well as some other commonly encountered engines.

The original SBC is not a strong design. Priorities of the design were light weight and low cost. The SBC was not designed as a race or even a high performance motor. Designed over 50 years ago the designers lacked the advantages of modern CAD systems. And in addition, much has been learned since then about metallurgy and production techniques. Fortunately, by staying in production for more than 50 years and becoming the darling of the performance community, GM made many changes and the aftermarket became heavily involved. In fact, the performance aftermarket was in a sense created to address the deficiencies of the small block Chevy design!

The two areas of most concern, the cylinder walls and the main bearing support, are two that are compromised by the low weight/low cost design priorities for the SBC. Cylinder walls are thin to save weight (and to promote heat transfer). Nitrous and SC use drastically increases peak cylinder pressures. This can lead to bore distortion with blow-by and loss of power. If the cylinder walls are even thinner due to boring and production tolerances and if a large nitrous shot or high boost is used, cylinder walls may split, which for practical purposes destroys the block. When building a nitrous or blower SBC, this leads to a rule that really applies in principle to all nitrous and blower motors: overbore as little as possible to preserve cylinder wall thickness.

The newer castings (since ~1987) have even thinner cylinders than older motors. And core shift with lack of inspection causes a lot of variation in production block cylinder thickness. If building up newer block, do not go beyond an 0.030-0.040"" overbore and it is preferred to stick with the smallest overbore possible. The huge variety of SBC parts makes it practical to obtain pistons and rings in overbores as small as 0.005", though 0.010" is the most common small overbore size. Older blocks may be ok with larger overbores of up to 0.060". When large overbores are being considered, having the block sonic checked may be well worth your time. Blower and nitrous motors should have AT LEAST 0.200" minimum cylinder wall thickness, and more is better.  

Cylinder block fill should be considered if a large overbore must be used or very heavy nitrous use is contemplated. A complete fill makes the block unsuited to anything other than drag strip use. But a 1/2 fill or less provides some added support for the cylinders and can be used on the street. A partially filled block will run hot though. Overheating may occur and an upgraded cooling system using parts such as an electric water pump, high capacity radiator, and a better fan may be needed. Agents that promote heat transfer such as "Water Wetter" are also helpful.

Before starting any expensive block modification, the block should be checked by a competent machine shop for cracks and casting defects such as core shift. Ultimately, if extra displacement is wanted, it should be attained by installing a stroker crank and not by excessive use of the boring bar.

Besides keeping the cylinders as thick as possible, the main cap area also needs attention in the building of a nitrous/SC engine. Stock SBC with 2-bolt caps are adequate for basically stock motors with small amounts of nitrous or boost (in the 100-150hp range). As noted above, block filler will help here also, keeping in mind the potential for overheating that relates to using fill. Block filler will stabilize the crankcase webs and help prevent main cap "movement" under load. The term "movement" refers to a consequence of block flexing and if excessive this will quickly destroy the main bearings and then the crank. Highly modified small blocks will need 4-bolt main bearing caps. The stock iron 4-bolt main caps are a significant upgrade compared to stock 2-bolt caps. They are all that will be needed for typical street or street/strip use. Very highly stressed engines (>250+hp nitrous/>20psi of boost) will benefit from 4-bolt steel caps. Caps with splayed outer bolts offer only a small advantage on stock blocks. Aftermarket blocks, with reinforced webs, will benefit most from the 4-bolt splayed outer bolt steel caps.

An analogous situation exists with respect to the SB Ford. This is an excellent motor in many respects, but the stock Ford small block has significant weaknesses, even more so than the SB Chevy. Most hot rodders consider these blocks as good to only ~500-575hp range depending on the exact combo before the strength of the basic block becomes problematic. Aftermarket main caps, main stud girdles, and block fill are all options for strengthening these motors. 

The deficiencies of the stock small blocks have been addressed by both GM, Ford and the aftermarket. For both Fords and Chevys, the factory and the aftermarket produce a variety of heavy duty "race" blocks. These typically are made of stronger alloys, offer thicker cylinder walls, strengthened crankcase webs, upgraded main caps, and thicker deck surfaces. All are quite advantageous when a max-effort motor is being built. In addition to accommodating larger bores, these blocks are often designed to also allow use of longer stroke cranks than their OEM counterparts. The primary downside is cost. Typically, from $1,500 and up depending upon the model chosen. However, these blocks are stronger than even a maximally prepped OEM case. And compared to the cost of fully prepping a production block, the total cost may actually be less if a factory machined race block is selected. Having a stock block fully "blueprinted" is not cheap by any means and no amount of machine work can compensate for the basic deficiencies of the OEM design.

The "new generation" of small block motors are the future of the domestic performance market. Both the Gen III Chevy and the Ford "modular" motor offer significant advances over their traditional predecessors. While most of the advancement has been in the cylinder head area, the "bottom end" and cylinder blocks of these motors also have been improved. These stronger blocks have increased potential for handling high power without resorting to extensive modifications compared to the earlier models. Cylinder wall thickness is still an issue though, so overbore should be kept to a minimum if high boost is contemplated.

Besides the ubiquitous SBC and SBF, there are a huge number of other engines that may be modified by the enthusiast. Each has it's own strengths and weaknesses. Especially in the import market, many seemingly similar car models will have different cylinder blocks, depending on the exact model and year of production. The number of possibilities is so large that it is not possible to generalize in an article of this length. Small domestics also cover a wide range. We suggest you consult us further if you need information about a specific model. The same applies to "exotic" motors such as the Viper and "Triton" V-10's and European models.

Before leaving the subject of cylinder blocks, domestic big blocks deserve a few words. These motors are not currently used as original equipment in cars though they may still be found in light trucks. The big block Chevy in particular is a much stronger piece than its' small block counterpart. The stock BBC can withstand nitrous boost in the 300+hp range with no difficulties due to thick cylinder walls, larger main bearings with thick crankcase webs and strong 4-bolt main caps. As with the small block, both GM and the aftermarket offer "race" versions that are immensely strong though also expensive. It would be very unusual for any street or street/strip big block setup to require a replacement block. But if you want to experiment with huge amounts of nitrous or boost (500hp range) they will be necessary. If very large displacement is needed, an aftermarket block will also be needed.

Next Installment: The Crankshaft 

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