Volume II, Issue 2, Page 5


2/15/2007

How Cool Is Big Power?

In my last column I talked about how force is applied to a piston and rod assembly, and why a power adder engine can take so much force if it is applied at the right place and time. Just as important as mechanical force, is the thermal stress an engine takes. An engine can take quite a lot of thermal load if it’s applied evenly and you avoid “hot spots.”

A couple of things to consider: how long is the engine actually going to be making big power, and what type of cooling system is used? In a marine application the engine is at wide open throttle, sometimes for hours, but it has a massive cooling system (the water it’s driving over) and the engine's cooling system has been substantially modified when compared to a car. Also, marine engines are designed to operate at considerably less RPM because of the incredible friction generated by high-speed operation. The friction of the rings, bearings, valve guides, and other components not only causes substantial localized heat, it obviously wears out the parts much faster. Minimizing engine speed substantially lowers the heat of these rubbing and turning items.

Most of you have street engines or drag strip engines. At the track you’re only on it for a matter of seconds and then it cools down for sometimes an hour or more.  On the street an engine is fully warmed up and driven for extended periods, but typically only under massive acceleration for short periods. There are a lot of excellent aftermarket radiators, fans and cooling system component upgrades you can purchase, in fact they’re a lot better these days than just 15 or 20 years ago. Even so, what allows an engine to run hotter, typically has very little to do with a short burst of supercharged power. That’s right, a blown 1200-horsepower street engine, which is only leaned on for short periods, will have a negligible effect on a decent cooling system. This may sound counterintuitive, but I’ll explain.

In order to understand how the cooling and heat transfer work in a big power blown street engine I first must refresh your memories about the basic air/fuel mixture. Many today are familiar with air/fuel ratios and oxygen sensors. But most people don’t really fully understand why a particular air/fuel ratio is required, or sometimes aren’t interested. In simple terms the air/fuel ratio is just the ratio of fuel to air in weight or pounds. The perfect or ideal ratio is 14.7:1. At this ratio, ideally the fuel has the best chance to completely burn. But there’s also an excellent chance you can burn up the engine, whether or not it’s blown. This is considered a very lean ratio (not enough fuel), so almost all gasoline engines operate at a few points richer, in the twelve’s. The extra fuel, which most certainly does not burn is colder and called “quench” fuel. This quench fuel, although a very small extra percentage, keeps the temperatures of the combustion chamber in check.  It also helps the engine from burning off the tip of the spark plug and other parts, and helps avoid detonation.

As I said, a big power blown engine with a decent cooling system doesn’t have to run hot at all. In fact in some cases pony for pony, an 800 or 900-horse small-block may operate substantially cooler on the street compared to a highly built big-block that makes several hundred horsepower less. The big-inch high-compression motor is continually generating substantial heat because of the significant friction generated by the large rotating parts. Additionally, if the engine is substantially overbored, there isn’t much block left to allow good uniform cooling. The small engine under cruise conditions acts like it is normally aspirated. With today’s advanced newer blower technology (with substantially less heat produced, especially intercooled), the engine can be forced to easily flow three times as much air at a reasonable RPM with low compression.

The real bottom line here is how much extra air and quench fuel can be added, while still making big power. Interestingly enough, with a modern supercharger, the engine can actually be “detuned” and still flat smoke the tires on command. There is so much air (which is free) available with a modern blower, that by adding a few extra pennies worth of fuel, for the few seconds of all-out acceleration, does it really matter if you’ve detuned it by 60 or 70-horsepower when you’re still making more than a thousand, on pump gas? The additional fuel and air as a mixture, as with the engine, have to be heated with combustion. This takes time, and drag racing doesn’t last a long time (especially with this type of power). It takes so much energy to heat up this additional quench fuel, that in a modern blown application, with only 15 pounds of boost, a carbureted Chevy engine can easily produce twice the power as normally aspirated, while in many cases operating with 100° less exhaust gas temperature. So although for a very short period there may be a significant thermal stress on the ENTIRE engine, at the localized combustion chamber if tuned properly, most of the parts exposed to the combustion gases may remain cooler than before the engine was blown, and there just isn’t enough time to heat up the entire assembly. Obviously, these big power engines would overheat with automotive cooling systems if run all out for many miles. But with greater than a thousand horsepower, it doesn’t take much time to run out of road! 

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