Example of Detination

Compression Ratio

The next area to input in the dyno software is compression ratio. What do we want? And how do we get there? The single biggest limiting factor in building a high performance engine is detonation. Detonation is when the air-fuel mixture in a cylinder burns too rapidly or explodes rather than burning as a controlled burn. 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). 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. 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. For those running 87 octane, 8.7:1 is the most that is safe with cast iron heads, 9.7 with aluminum. Race fuel, on the other hand, at 110 to 118 octane will allow compression ratios of 14.5 to 15:1 if tuned properly. 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.

We are frequently asked if running fuel additives would allow running higher compression ratios. 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 talking with one company now, Price Chemical, that sells a nitromenthane additive. We are looking at building another engine that will run live in our booth at the Adirondack Nationals show. We hope to achieve a safe compression ratio of around 13:1 running on either E-85 or 93 octane with an additive. We will keep you posted on our progress.

Compression Ratio Calculator

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 now start inputting compression related information into our dyno software. 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. 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. 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. We will likely be using a dished piston, which will effectively add combustion space to the combustion volume.

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 start out by inputting the chamber size of the head we will be using, in this case 64cc. We now need to input the piston’s dish volume. Let’s start out with a 12cc dish to see where that will put our Compression ratio. We are now asked for the deck clearance. This is the space above the piston to the deck of the block. Generally from the factory, most small block Chevy's deck heights are around .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. We will have a discussion of the benefits of zero decking next week when we show that operation being done. For now let’s just say we are a big proponent of zero decking and try to zero deck any performance engine we build. If we were not zero decking the block, it would be necessary to either do a “mock up” assembly, or measure all the components and calculate the piston height from the crank centerline, to accurately determine what the deck height will be. Finally we are asked to provide the dimensions of the head gasket bore by entering the diameter and thickness of the gasket being used.

Adjust ratio to 17cc

After all this, the software has calculated a compression ratio of 10.2:1. This is over our target of 9.7:1, so we need to change something. Since we want to maintain a zero deck height and the head gasket bore is pretty constant (there are thicker gaskets available but that defeats the purpose of zero decking, more on that next week), we are left with changing either the combustion chamber volume or the piston dish volume. Most aftermarket small block Chevy heads are available in either 64 or 76 cc combustion chambers. With a 76cc chamber we could probably run a flat top piston. Since we have chosen a head that is only available in 64cc chambers, we will have to adjust the dish on the piston. Let’s try 17cc. That puts us at 9.69:1, pretty close to our target of 9.7, but we like to try to hit a little lower to have room to adjust the final number by surfacing the heads until we have the exact cc to hit our target number.

Adjust ratio to 19cc

Numbers published by manufacturers of parts are seldom exactly what they are supposed to be. Mass production doesn’t allow for the accuracy we are looking for to build you an engine we know is safe on 87 octane. We will verify everything ourselves. Also we will be porting and touching up the seats on the heads so there will be a slight change in the chamber volume as well. We are going to look for a piston with a dish in the 18 or 19cc, range. At 19cc you see the compression ratio drops to 9.51:1, which will leave us room for adjustment.