![]() ![]() ![]() We will have a discussion of the benefits of zero decking next week when we show that operation being done. ![]() 025”, but we are going to enter “0” since we are going to zero deck, Zero decking means the piston at top dead center will be flush with the deck surface. Generally from the factory, most small block Chevy's deck heights are around. This is the space above the piston to the deck of the block. Let’s start out with a 12cc dish to see where that will put our Compression ratio. We now need to input the piston’s dish volume. We start out by inputting the chamber size of the head we will be using, in this case 64cc. We will be using Patriot Performance Freedom Series heads with 64cc combustion chambers and 185cc intake runners (about 190cc after bowl blending) When building a big cubic inch small block, it will be necessary to run a dished piston to keep the compression ratio down to our target of 9.7:1. We will likely be using a dished piston, which will effectively add combustion space to the combustion volume. Since we won’t be using domed pistons, we won’t get into too much discussion about them, only to mention that the dome would take away some of the space in the combustion chamber resulting in higher compression ratios. The combustion volume includes the cylinder head combustion chamber, and the volume of space from the top of the piston to the cylinder head deck, which also includes the volume of the head gasket bore. Compression ratio is the ratio of the volume of the cylinder (called swept volume), plus all the volume that is left once the piston is at top dead center called combustion volume, divided by the combustion volume. We now start inputting compression related information into our dyno software. This 383ci small block Chevy is being built to run on 87 octane, which with aluminum heads will limit our safe compression ratio to 9.7:1. We hope to achieve a safe compression ratio of around 13:1 running on either E-85 or 93 octane with an additive. We are looking at building another engine that will run live in our booth at the Adirondack Nationals show. We are talking with one company now, Price Chemical, that sells a nitromenthane additive. There are several octane boosters on the market that can help in a borderline situation, but we have no firm numbers on how much is needed and what compression can be run with their use. We are frequently asked if running fuel additives would allow running higher compression ratios. Since E-85 has an effective octane of about 110 we are doing research on what we can build using it, keeping in mind some other components will need to be changed to accommodate the use of alcohol. Race fuel, on the other hand, at 110 to 118 octane will allow compression ratios of 14.5 to 15:1 if tuned properly. For those running 87 octane, 8.7:1 is the most that is safe with cast iron heads, 9.7 with aluminum. Because aluminum conducts (dissipates to the cooling system) heat much faster than cast iron does, you can run 10.5:1 with aluminum heads on 93 octane. Since more compression makes more power, how much is safe? We are being told by most piston manufacturers that 9.5:1 is the most you should run in an engine with cast iron heads on 93 octane pump gas. This can be caused by too lean an air-fuel mixture, timing too advanced or compression ratio too high for the fuel being used (or too low an octane fuel for the compression ratio). Detonation is when the air-fuel mixture in a cylinder burns too rapidly or explodes rather than burning as a controlled burn. What do we want? And how do we get there? The single biggest limiting factor in building a high performance engine is detonation. The next area to input in the dyno software is compression ratio. ![]()
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