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Small Block Chevy Engine LS Conversion - Coil Conversion Therapy

DIS for SBCS

Have you ever wondered why GM, Ford, and Chrysler converted from distributors to distributorless ignition systems (DIS)? Sure, they eliminated the whirling mechanical device that spins a rotor that aims high voltage to the plugs. But then the OEs had to purchase eight expensive ignition coils—one for each cylinder. When buying millions of these units every year, the piece price is ridiculously low, but it’s still an investment. Let’s take a quick look at why they went this route. Distributor technology goes back to a man named Charles Kettering not long after the first internal combustion engine made noise. Today, with a simple crank and cam sensor, the computer knows exactly where the engine is in the firing order and can very accurately trigger when the spark should occur. With this increased accuracy, the next biggest limitation was coil energy at high engine speeds. At 6,000 rpm, the engine is firing all eight cylinders 3,000 times per minute or 50 times per second. This leaves 0.02 second to charge the coil and fire that spark energy through the secondary side of the ignition system. As a result, spark energy is drastically reduced at higher engine speeds. Of course, this was acknowledged back in the ’70s when capacitive discharge ignition systems first became popular with drag racers. See our sidebar on inductive versus CD ignitions and the relative advantages of each.

Small Block Chevy Engine So what does this have to do with building a stronger street engine? Imagine if you could convert an original small block Chevy engine over to DIS. What advantages would there be for the average enthusiast? Sure, you’d have a much more accurate spark that’s not affected by backlash in the cam drive system and all kinds of issues with distributors. You’d also have a much hotter spark at higher engine speeds because now you have a single coil for each cylinder, so it has plenty of time to recharge and fire a much stronger spark to the cylinder. But you might think, My engine doesn’t misfire now with my HEI, so I don’t need a stronger ignition. In actuality, your engine misfires hundreds of times in just one pass down the dragstrip and thousands of times just driving down to the auto parts store for an oil filter. You may not feel it as a dropped cylinder, because a misfire is actually defined as incomplete combustion, meaning a spark initiates combustion, but the cylinder never achieves complete combustion. The result is reduced power from the combustion event. But what if you could produce a stronger spark delivered at precisely the right moment throughout the engine’s entire operation range? Would it be possible to make more power?

DIS ignition is partially responsible for the LS, new Chrysler Hemi, and Ford mod motors making more power per cubic inch and being more fuel efficient than their 50-year-old cousins. So why not use LS engine technology to give a leg up to older engines to make more power? That’s exactly the question that led Mike Noonan to form EFI Connection. His company makes a conversion system that uses either the 24x or later 58x crankshaft trigger wheels to indicate crankshaft position within the engine. In either case, a spinning shutter wheel on the crankshaft is read by the crankshaft position sensor to indicate engine rotating position.

Because each piston arrives at TDC twice within the four-stroke cycle, the computer needs to know which of these two events represents the power stroke. That’s the job of the camshaft sensor. Noonan and his team developed a shutter wheel that adapts to a small block Chevy engine (including LT1/LT4) using a GM Vortec engine front timing cover, a crank sensor, and a sealed distributor that is only used to indicate cam position for firing on the No. 1 cylinder. A late-model GM LS computer is used to manage all these components using an EFI Connection wiring harness.

Our friends at Tuned Port Induction Specialties (TPIS) built a 383ci LT1 engine using some excellent parts that promised to make in excess of 500 hp. Given the engine’s potential, TPIS wondered if there might be a performance advantage in converting to the GM DIS system and LS powertrain control module (PCM—aka computer). Previous dyno tests had shown gains of 15 to 20 lb-ft of torque in the mid to upper rpm ranges, so TPIS decided to try the EFI Connection system on its latest engine. TPIS baselined the small block Chevy engine in its LT1 configuration: a GM factory TPI computer, a new MSD Opti-Spark distributor, and a 6AL box to light the fire. The rest of the engine was built using 11.2:1 compression, a TPIS solid roller cam (242/242 duration at 0.050 with 0.600-inch lift and a 112-degree lobe-separation angle). The company also bolted on a set of 210cc AFR aluminum cylinder heads and topped the engine with a TPIS Mini-Ram LT intake with a Mono Blade throttle-body. As you can see from the dyno test results, this version made 550 hp at 6,500 rpm and 484 lb-ft of torque at 5,300.

Next, TPIS installed the LS coils and PCM. After firing the new combination and making a few minor air/fuel adjustments, the company leaned on this engine again and were rewarded with solid improvements in torque curve across the entire test range, from 2,500 to 6,800 rpm. Amazingly, TPIS saw gains of more than 30 lb-ft of torque at various points, and an average torque improvement of a stunning 18.7 lb-ft. An ignition change that’s worth an average of nearly 19 lb-ft of torque is an impressive achievement.

So what really happened here? The TPIS guys and our own research point to several possible explanations. First and most important, we think the power gains are the result of greater spark energy delivered to the engine over a longer period of time, enhancing the combustion process. In addition, a portion of the gains can likely be attributed to improved spark-timing accuracy using LS engine technology on this LT1-style engine. This requires a deeper appreciation of the combustion process.

Car Craft family member Louie Hammel pointed out what we think is the best explanation. According to Hammel, there have been hundreds of tests performed on internal combustion engines that have measured actual, in-cylinder pressure for each individual combustion event in a given engine, and the results indicate that the combustion process produces varying levels of cylinder pressure. Sometimes, the cylinder completely misfires, producing no appreciable cylinder pressure, while at other times this same cylinder produces high cylinder pressures. Over a very short period of time, this cylinder produces widely differing pressures. In a V8, this cylinder and its seven sisters combine to produce a given power number at a given rpm. As you can see, this single-power number (at a given rpm) is an average of multiple cylinder firings within an extremely short time frame on the dyno.

To make this easier to understand, let’s equate horsepower to your grade-point average in school. If you earn a B grade (3.0) in one class, a D grade (1.0) in another, followed by two grade Cs (2.0), your overall mean grade average is a C (2.0). If you work smarter in the next semester and improve that D and one of those Cs up to a B, your average grade point improves to a 3.0 or a B average, and you begin dating the prom queen because she thinks you’re not only smart but you have a hot car! If we apply that concept to our engine, Hammel’s theory is that low cylinder pressures can be partially attributed to poor spark performance that produces a mediocre combustion process. By not only improving the accuracy of the spark timing but also increasing the spark energy over a longer period of time (inductive versus CD ignition), the average power output increases because we have more spark energy to help complete the combustion process. The greater spark energy from an individual coil for each cylinder doesn’t raise the cylinder pressure higher than what the engine is capable of producing under ideal conditions. Instead, the greater spark energy raises the engine’s grade-point (horsepower) average by reducing the number of partial misfires, thereby raising the engine’s overall average torque, which is seen as greater power across the entire rpm band.

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