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Compression Ratio - Ask Anything

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Effective Compression Ratios

Derrick Morris; Pearland, TX: OK, guys, I need to know this information, as I can't find the answer anywhere. Basically, I have a 12:1-compression, 414ci LSX motor using a 6.0L iron-block with a 4.060 bore and 4.00-inch stroke with L92 heads. While the motor was originally intended for carb/nitrous, I want to put boost to it, but I don't want to change pistons. I know my cam is not ideal for a turbo, but it's new, so it's getting put to use. The cam is a Comp hydraulic roller 260/268 at 0.050 0.646/0.646 on a 112 LSA. I've got good parts, including Manley Inconel valves, T&D shaft rockers, and ARP head and main studs. I've also converted to fuel injection with a Victor Jr. intake.

I just came across Ask Anything in my Mar. '12 issue and read the letter titled "Effective Compression." Using the formula: [(boost psi/14.7) + 1] x static compression = ECR, I come up with the following for my engine: 15 psi/14.7 = 1.02 + 1 = 2.02 x 12(compression) = 24.2:1 ECR, which is obviously high. So what ECR is too high? I did some digging and came up with a stout motor from Pro Line in the Lynch/Petty Mustang from June '08 issue of Hot Rod (3,500 hp on 10.5W). They are running 34 psi on roughly 9:1 and using above formula, which comes out to an ECR of 29.8:1.

Obviously, we are dealing with cylinder pressure and staying away from detonation, so my short version of the question is whether there is such a thing as too much ECR? I know why guys are running lower static compression when running boost, as the higher the compression, the lower the boost, and you can make more power with lower compression and higher boost. Obviously, you want as much compression as you can get, and I realize there is a point at which you can only run so much. But if motors are staying together with ECR of 29.8:1, and since they say in that article that they were going to step it up to 40 psi, and that there are higher ECRs out there, it all comes down to my 12:1 motor. Can I shove 15 psi into it or not? What about 20 psi that would generate a 28.3:1 ECR? Thanks!

Jeff Smith: It appears your question has really more to do with octane rating, avoiding detonation, and high-quality race fuels than about effective compression. It would probably not be a good idea to place too much faith in that formula. I think it's more useful as a comparator than anything else. But having said that, I recently applied that formula [(boost in psi /14.7) + 1] x static compression ratio to a 1,200hp, 4.8L LS engine in our sister book, Hot Rod. That engine had a static compression ratio of 9:1, used a whopping 25 psi of boost, and came up with an ECR of 24.3:1. That engine ran on 118-octane race gas and had no detonation issues, at least on the dyno, where there is good control over most variables. On the dragstrip or the street, many different variables come into play. For example, on the dyno, load is constant as is the acceleration rate of the engine, while both are completely different in the car.

Let's start with boost. In this equation, it's used as an indicator for cylinder pressure, and there are several problems with that. In this case, boost--or pressure in pounds per square inch (psi)--is merely an indication of a restriction to flow. Part of the ECR equation computes something engineers call pressure ratio. This is merely the calculation of psi divided by 14.7. That means 15 psi of boost represents a pressure ratio of 1.02--exactly what you calculated in the first part of the ECR equation.

The problem is that pressure ratio is merely an efficient way to express pressure that is above atmospheric levels. What it does not tell us is how hot the air becomes on its way to creating that ratio. There are ideal gas laws (called Boyle's Law) that relate pressure to volume. In an ideal situation, raising the pressure of air will result in a rise in temperature (this is referred to as 100 percent adiabatic efficiency), which means this is the minimum temperature rise (without aftercooling). Turbochargers are generally about 75 percent efficient when it comes to creating pressure. That means that by creating pressure (boost), our turbocharger also inevitably inputs heat into that pressurized air. When air is heated, it loses density because the molecules are more active and space themselves farther apart. So a cubic foot of air at 200 degrees F is less dense than a cubic foot of air at 100 degrees F at the same pressure. In other words, cooler air is always better. When it comes to horsepower, the real reason we increase boost (pressure ratio) is to shove more oxygen into the combustion chamber (that's how nitrous works--by adding oxygen). The equation uses pressure ratio to compare with static compression but doesn't take into account the discharge temperature, so we have a huge variable here. And this is just one variable; it would take the rest of the year for this column just to touch on the hundreds of variables that also have an effect on combustion efficiency.

So really, we're talking about the ability of the fuel to resist detonation (among other things) that will allow us to build an engine that makes more cylinder pressure and not detonate. In the Hot Rod story, that 1,200hp engine ran on 118-octane race gasoline. This is where chemistry begins to cloud the issue of combustion. So perhaps a better question should be, what kind of fuel would help me achieve my goals? The two major rating forms for octane are Research (RON) and Motor (MON). Of the two, Motor is far more important to a race engine, as that rating takes into account how the fuel deals with heat and temperature in a real engine. The anti-knock index (AKI) is rated by adding the Motor and Research Octane numbers together and dividing by two: (R + M) / 2. One critical factor we uncovered researching this answer was that the closer the two octane numbers are, the more stable the fuel. So for a stable, 100-octane fuel, we might see the Motor number at 96 and the Research number at 104. A less stable fuel could exhibit numbers such as 90 and 110. As an example, VP's C-16 leaded race gasoline has a MON of 117 and a RON of 117 with a R+M/2 AKI (referenced sometimes as pump octane number or PON) of 117. Clearly, this is a highly stable fuel with excellent anti-detonation characteristics. In your search for a high-quality fuel, choose the one with the higher MON number.

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Another SUPERB Car Craft article that successfully makes me understand a subject by teaching DIFFERENT aspects of an issue from a DIFFERENT approach. I've read hundreds of articles and textbooks on compression ratio. Yours is better. Thank you.

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