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Compression Ratio - Ask Anything

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So if you wanted to build an engine that would have 15 psi of boost and a 12:1 compression ratio (ECR = 20:1), you would want it to run on gas that has a strong octane rating but also one with Motor and Research numbers as close together as possible. My recommendation would be to sneak up on this combination. Start with relatively conservative boost numbers with a high-quality fuel, and see how that works. Then gradually increase the boost and use careful evaluation of the spark plugs to get an idea of what is happening in the combustion chamber. Unfortunately, if something goes wrong, the first indication will probably be a piston that begins to disintegrate. The hint earlier in this discussion was all about temperature. The lower your inlet air temperature, the more power you will make, and the engine will be much happier. Alcohol is a good fuel for this type of application, as its cooling effect is dramatic. E85 is a bit less aggressive with 85 percent ethanol and 15 percent gasoline, and it enjoys an increasingly strong following. The general consensus is that pump-gas E85 hovers around 105-octane. E98 (or 98 percent ethanol) is around 115-octane. One disadvantage to pump E85 is that you don’t really know the actual percentage ratio of ethanol to gasoline or the quality of gasoline that is added to the ethanol. According to Tim Wusz at Rockett Brand Racing Fuel, because ethanol has such a great octane rating, pump gas tends to be mixed with low-grade gasoline, which would push the RON and MON numbers farther apart. On the other side of the technology coin is E85’s excellent cooling effect. This is called latent heat of vaporization. An example of this process is when you spread a small amount of isopropyl alcohol on your skin and notice that as the alcohol evaporates, it pulls heat away from your skin, creating a cooling sensation. When used in an engine, alcohol boils (evaporates), and its physical state changes from a liquid to a gas, which removes heat and cools the inlet charge. We see this when using E85 in draw-through supercharged engines in which the discharge temperature of the pressurized inlet is radically lower than that of a similar engine running on gasoline. The point of this discussion is that as the inlet air temperature becomes lower, the engine becomes less sensitive to detonation. A 10-degree reduction in inlet air temperature is a big move. If you’ve ever driven an engine that is overheating, it will detonate like crazy because the entire engine is hot and this heats the inlet air temperature and contributes to the engine rattling.

This has been another long walk off a short pier, but as you can see, there are plenty of variables that contribute to engine detonation—and we’ve only just begun to dig into this topic. Combustion-chamber shape, quench-area effectiveness, piston-to-head clearance, mixture distribution in the combustion space, piston-ring sealing, oil contamination, and a few dozen other variables all contribute to the witches’ brew that constitutes the combustion process. As you have added injectors to a Victor Jr. intake, are you aware that injector placement within the induction system appears to be very critical? I’m not privy to any in-depth research, but it’s clear that the farther upstream you can place the injectors, the more time the fuel will have to cool the incoming air.

This answer has probably created more questions than answers, but I would caution you to move slowly and carefully and encourage you to call the tech people at VP Racing fuel and Rockett Brand Racing Fuel for more specific application questions.

More Info

Rockett Brand Racing Fuel; 714/694-1286;

VP Racing Fuels; 512/621-2244;


Anthony Guy; via I’ve been reading Car Craft since I first started driving in the mid ’70s. It was an article written during that time that showed the basics of airbrushing in the height of the kustom van craze that got me into airbrushing. I currently have a two-wheel-drive ’94 S-10 pickup that had some serious body lean in the corners. I installed a rear antisway bar that flattened out the roll problem, but now it has some very noticeable oversteer. I don’t have a garage, so any projects I take on have to be completed during daylight hours over a weekend. Eventually, I’d like to lower the body a couple of inches, so I haven’t tried replacing the shocks for fear of having to replace them twice. Is there an easy solution to this oversteer problem, or will I just have to deal with it? Thanks.

Jeff Smith: You should never just “deal with it,” Anthony. Improving the handling not only makes your truck more fun to drive but also makes it immensely safer. To simplify this somewhat, when you added a rear sway bar, you increased roll resistance in the rear of the vehicle. So in the middle of a corner when you try to accelerate, the stiffest end of the car (in your case, the rear) will break free, causing the oversteer condition you described. Let’s take a basic look at both the front and rear suspensions and then we can figure out a way to reduce the body lean, improve your handling, and make your little S-10 pickup a hoot to drive.

Springs are designed to support the weight of the vehicle. Entering a corner places greater load on a given side of the suspension. In a left-hand turn, weight transfers from the lefthand side of the vehicle to the right with more weight moving to the front as the vehicle enters the corner. Stock suspensions always involve a compromise between ride quality and handling. Generally, production spring rates are decidedly on the soft side to improve ride quality. Given a softer spring rate, the spring will compress more under load when you negotiate a corner. The spring compresses until the force stabilizes, creating excessive body roll, which dramatically affects the front suspension geometry (and to a lesser extent, the rear). Pickup trucks add another variable into this equation, as they are designed to carry a given payload in the bed. This means the rear springs will be a significantly higher rate compared with the spring rate of a similar-sized passenger car. For example, a ’94 S-10 has a load-carrying capacity of 1,500 pounds. Even with a spring rate of 300 pounds per inch (300 lb-in), the rear suspension will drop 5 inches with that amount of load in the bed. Even if the leaf-spring rate is closer to 250 lb-in, that’s still a very stiff rear spring rate for the vehicle when there is no load in the bed. Adding a rear sway bar increased the effective spring rate (by adding resistance to roll) when entering or exiting a corner. So it’s no wonder that your S-10 wants to oversteer. If you want to reduce body roll by retaining a sway bar, there are a couple of options. First, reduce the diameter of the rear sway bar. Next, reduce the rear spring rate by removing one or possibly two leaves from the springs. This will soften the spring rate and reduce the tendency to oversteer. Of course, this will reduce the load capacity of the bed as well. It’s all a compromise.

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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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