Choosing the Right Starter for High-compression Engines
Engine builders often don’t think about the starter during the planning process or while completing a build. In fact, the starter may not be given much thought until it’s time to install the engine. For some, a simple trip to the local parts store will do the trick. Most OEM starters may be suited to handle the job when turning over just a mildly modified short-block. However high-compression engines, those with tight clearances and racing engines need a more tailored approach than factory offerings.
Three of the leading suppliers high-performance starter are Powermaster, MSD, and Proform, and their experts can provide plenty of information to engine builders, racers and enthusiasts when selecting a starter for high-compression, high-performance engines.
Many OE style starters boast a great deal of torque, but do so at very low timing on fuel injected engines, so they are generally not stout enough to handle big-cube motors without the possibility of kickback.
— Brady Basner, Powermaster
Torque is the name of the game when it comes to starting power. Just as it takes torque to spin the wheels and get a car down the track, it also takes torque to crank over an engine. Every component inside the engine that generates friction is working against the starter. Compression also makes it more difficult for the starter.
Although OEM starters may carry a high torque rating, the higher compression ratio of many racing and high-performance engines can cause kickback and actually break a poorly made or OEM starter.
“Many OE style starters boast a great deal of torque, but do so at very low timing on fuel-injected engines. So, they are generally not stout enough to handle big-cube motors without the possibility of kickback,” warns Brady Basner of Powermaster.
“After a few starts on a high-compression engine, one of those starters is probably going to fail,” says Basner.
“Higher compression necessitates more foot-pounds of cranking torque,” concurs MSD’s Joe Pando.
This is why Powermaster, MSD, and Proform all offer starters with extremely high torque ratings, ranging anywhere from 160 ft-lbs for an engine with around 10:1 compression ratio, and up to 250 ft-lbs for engines with over 18:1 compression ratio.
Supporting electrical demands
Just as a fuel system should be tailored to the engine’s output, the electrical system must be up to the task of supplying power to the starter. Upgrading to a low resistance, high output battery is always a good idea. Also the starting switch should be able to handle any current or voltage running through it. The same holds true for the battery cable and starter cable. All of these components should be properly suited for the job. Don’t however make the mistake of thinking higher cranking amps will equal more starting power, no matter the output to the system the torque will always remain the same.
Gear reduction is another factor that contributes to a starter’s torque output as well as how fast it can crank the engine. Think of gearing in a rear differential. The higher the number numerically, the greater the torque multiplication from the starter’s electric motor to the gear that engages the flywheel or flexplate ring gear. Gear reduction ratios are used to better match starter peak torque to starter peak electrical output. Typical gear reduction ratios are 4.4:1 and 3.73:1 for many high-torque and high-speed starters, respectively. The numerically lower ratios will typically generate a higher cranking rpm, while the higher numbers will generate more cranking torque.
Gear reduction is accomplished with a set of planetary gears inside the starter, much like in an automatic transmission. The planetary gears rotate around a center sun gear and inside an external ring gear. The gear reduction is the difference between the ring and sun gears. For example, if the ring gear has 88 teeth and the sun gear has 20 teeth, the gear reduction is found by dividing the ring by the sun gear. Doing the math in this example gives us an answer of 4.4. The electric motor’s torque is multiplied 4.4 times inside the starter. This torque is then multiplied again by factor computed from the number of teeth on the flywheel ring gear divided by the starter pinion gear.
A starter’s peak torque also needs to be optimized to its electrical power peak. By doing this, heat is drastically reduced. When the cranking demand exceeds the starter’s available torque, the additional energy that is supplied is wasted as internal heat. This heat build up is what eventually causes failures in many starters.
Fitting a Starter
Whether it’s clearance for oil pan kick outs, large tube headers, a turbocharger or other plumbing, getting a starter to fit on a high-performance engine can often be a tricky affair.
Modern gear-reduction starters in most cases use fairly compact electric motors and gear-reduction systems. As a result the starter bodies themselves are typically very small. They are also lightweight, with many coming in between eight and eleven pounds.
Something that we’ve seen become a more popular option lately on many starters has been the option to clock the starter. Using precision machined mounting blocks, many starters can be rotated 360 degrees around the mounting block. This allows for additional clearance.
Higher compression necessitates more foot pounds of cranking torque.
— Joe Pando, MSD
This feature can also allow the starter to be rotated away from high-heat sources in some cases, which can help extend the life of the starter. Powermaster, MSD and Proform have starter offerings that feature clockable billet mounting blocks.
“Adjustable starters are also very beneficial since no two engine packages are the same,” says Basner.
Manufacturing and Quality
The increased potential for kickback, coupled with the high stress of cranking a high-compression engine, indicates that a stater must be made with high-quality components.
According to Basner and Pando, high-quality bearings and steel components should be used in the manufacturing of a high-performance starter. Also, the mounting blocks require precise machining, especially those that allow for clocking the starter.
If a starter is properly matched to the engine, and the electrical system is up to the task, Pando says, “MSD starters will easily last 10 or more years.” That sentiment was also echoed by Powermaster and Proform.
“We build a large number of the OEM high torque starters for General Motors, and Chevrolet Performance” says Rick Hobbs of Proform, a testament to the company’s quality standards.
Matching Compression to the Starter
Just as an engine can’t have too much horsepower, the same holds true for starters. According to Basner, a 200 ft-lb starter will work as well on a street car with a near stock engine as it will on a 12:1 compression race engine. For budget considerations though, the one size fits all approach is not always the best one to take.
The primary factors to consider are the engine’s compression ratio and cubic inches. Cubic inches affects starting in that the greater the rotating mass, the more torque it may take to get it spinning, while compression increases resistance to the effort of the starter. Below are the general recommendations made by Powermaster on the torque needed for various compression ratio engines:
- Up to 10:1 160 ft-lbs
- Up to 12:1 180 ft-lbs
- Up to 18:1 200 ft-lbs
- Over 18:1 250 ft-lbs
Packaging and weight are the next areas of concern. Many high output and racing engines will utilize components that cause starter interference. The starter must be compact enough to fit in these tight spaces while generating enough torque and speed to start the engine. This is where clocking the starter comes into play. Everyone in racing is concerned about weight, and as such manufacturers are constantly looking ways to make their starters lighter while retaining strength. The use of lightweight materials such as billet aluminum, and advancements in electric motor technology over the paste several years, has helped to reduce the weight of some starters.
By selecting a starter that matches the engine properly, engine builders, racers, and enthusiasts can ensure that their engine starts reliably. Given proper care, and a solid electrical system a high torque starter may likely outlast the life of an engine, depending on its intended usage as well.