Engine Building 101: The Stroker Kit

So you want to build or rebuild an engine. Sounds awesome, and if you do it right, it has the potential to be really awesome. Though if you do it wrong it has equal potential to be really disastrous. That being said, engine building is a process that involves plenty of sweat and blood, but your friends over here at Stage 3 Motorsports are here to help. We're starting out with the basics of a stroker kit and which one out of the very many we list are right for you.

Very basic engine function

We're going to start off with a little remedial learning that most of the people looking at our stroker kits probably already know, but we feel we should go over it anyway. Your standard four-stroke gasoline internal combustion engine works by burning a mixture of fuel and air within an engine cylinder. Inside that cylinder is a piston, attached by a rod to the crankshaft. For a standard four-stroke cycle, the piston has a starting position (Top Dead Center) where it sits at the top of the cylinder. The piston begins its intake stroke downward in the cylinder while air and fuel enters through the intake valve (or valves) that is actuated by a camshaft synched with the rotation of the crankshaft. The piston, driven by the crankshaft, rises from its bottom dead center and compresses the air/fuel mixture on its compression stroke into the combustion chamber where it is then ignited by the spark plug. The small explosion of air and fuel pushes the piston down on its power stroke and the mechanical energy is transferred from the piston to the crankshaft, to the flywheel or flexplate, to your transmission, and eventually to your wheels. After the piston reaches its bottom dead center again, it rises again for its exhaust stroke and the the exhaust valve is actuated by the camshaft and spent exhaust gasses are forced out of the cylinder. The cycle repeats itself until you turn off your engine.

An engine's power is mostly dictated by the amount of air and fuel that can be squeezed into the cylinders, with larger engines with more cylinders being able to produce more power. For instance, a typical 2.0L four-cylinder engine won't be able to produce as much torque and horsepower as a big 5.0L eight-cylinder engine.

Before you can even get into stroker kits themselves, you need to have a block. For our purposes here, we're going to assume you have a Ford Small Block 302 cubic inch V8. There's other engine displacements available for Ford Small Blocks, and we're fairly sure that other car manufacturers made an engine or two somewhere down the line, but Stage 3 is a Ford loyalist, and we don't care much about ''those guys.'' The 302 cubic inches of our particular block refers to the total volume of piston travel from Top Dead Center to Bottom Dead Center that is available across all of the cylinders. Cylinder volume is dictated by the cylinder bore (diameter) of all the cylinders and the length of the stroke (piston travel). A stroker kit actually increases your engine's total displacement by increasing the length of the piston's stroke through the use of a specially-made crankshaft, new rods, pistons and other components. Stroker kits allow your basic 302 to produce more power not only because of the increased displacement, but also because the increased rod length and crankshaft travel has more leverage, and therefore, generates more torque.

The good ol' Ford 302 Small Block can be stroked out to 306 cubic inches, 331 cubic inches, and all the way up to 347 cubic inches. Each kit size has its own benefits and drawbacks and are further broken down into several types based on application.

All of the stroker kits here at Stage 3 are put together by Coast High Performance and are broken up into easy to follow categories based on their horsepower tolerances.

• GT series kits are the baseline stroker kits and are generally built for basic engine rebuilding for stock blocks and light-duty street applications. These kits are only able to handle 450 horsepower, but are the lowest priced out of all our offerings. There's nothing really WRONG with these kits, but engines being built for all-out performance should aim higher.
• Street Fighter series kits have uprated internals and use a cast steel crankshaft that's suitable up to 600 horsepower and are well-balanced between price and performance. The 600 horsepower mark gives you plenty of room to build an engine for autocross, rallycross, and road racing while generally allowing you to throw on middle range forced induction systems with no problems.
• Pro Street kits feature completely forged internals attached to a forged crankshaft that can handle a whopping 800 horsepower. The Pro Street stroker kits are a good $500 more expensive than the Street Fighter series, but can handle just about any aggressive engine build that you can throw at, including powerful superchargers, turbochargers, and nitrous systems with ease. These kits are able to handle almost every application imaginable, except top-tier drag racing.
• Dominator kits are the baddest of the bad and have some of the toughest components out there. These kits can handle over 1000 horsepower and are THE solution for the highest-performing engine builds out there. You can throw just about anything you can at these kits, and they'll keep on going. Just make sure your block is up to par.

Stage 3 currently carries 302 / 306 rebuild kits, 331 stroker kits, and 347 stroker kits for the Ford 302 Small Block V8s. The very down and dirty and simple breakdown is as follows:

• The 302 / 306 stroker kits only up your displacement to 306 cubic inches and are good for engine rebuilding and general use on street cars and the like. Engines with a smaller strokes are also more efficient at high rpms, and uprated Street Fighter and Pro Street 306 stroker kits are excellent choices for autocross, rallycross and road racing builds.
• 331 stroker kits are a good choice for giving your 302 Small Block an excellent power output bump without pushing the tolerances of your block the way larger stroker kits can when not installed or maintained properly. 331 kits also have a slightly more efficient rod ratio, though in high-power engine builds that becomes more of an afterthought. 331 kits make excellent power and have goo performance across a huge range of applications.
• 347 stroker kits are the big boys of the Small Block 302 and make massive horsepower when properly set up and excel at producing tons of low-end torque. However, the large amount of piston travel generally makes them less efficient at high rpm and the long length that the piston has to travel within the cylinder causes more wear. Still, if you need tons of horsepower and torque in a jiffy, then a 347 cubic inch stroker kit is for you.

When 347 stroker kits first hit the market there were pretty serious reliability and performance issues associated with them. The first 347 stroker kits burned a ton of oil that led to many engine failures because the wrist pins of the pistons came too close to the piston rings and caused oil burn-off and premature piston failure. These early issues gave the 347 stroker kits a bad reputation that extends to this very day.

Luckily, all of our 347 stroker kits use piston designs and kit configurations that make oil loss and piston failure a thing of the past. The 347 kits are now just as reliable as any other stroker kit, assuming your block is capable of handling their horsepower.

Your stroker kit requires balancing. Period. One way or another, you're going to need balancing to keep your engine from shaking itself apart. ''Balancing'' a crankshaft is a process where an engine builder ensures that the crankshaft's counter weights have exactly the same weight as the pistons and rods to create a perfectly smooth rotation inside your engine's crankcase. All of our stroker kits can come balanced out of the box or not balanced at all if you or your engine builder are intent on doing the job.

The cast steel crankshafts of the GT and Street Fighter can come with an external 28oz balance that requires a 28oz balancer to be mounted on the end of the crankshaft. These balancers are either SFI-certified or non-certified. The certified balancers have higher strength thresholds and are often a requirement for most racing leagues, but are a bit more expensive than non-certified parts.

Forged steel crankshafts found on the Pro Street and Dominator stroker kits can come with a 28oz external balance, like the lower-tiered stroker kits, or come internally balanced where material is added or removed from the crankshaft's counterweights to offset the weight of the pistons and rods. Internally balanced engines offer a slight performance gain, due to the fact that they tend to rev up slightly faster than their externally balanced counterparts.

All of our stroker kits are broken up by the type of pistons that come with it. Needless to say, there's a huge variety of piston types with dozens of different top configurations that yield different results in different situations. As if this wasn't confusing enough. For all intents and purposes, there's only a few things you need to know. Certain piston tops will generate different compression ratios depending on their displacement.

The compression ratio is the ratio between the size of the combustion chamber when the piston is at Top Dead Center versus Bottom Dead Center. For instance, a piston in a cylinder with 1000cc of total volume that leaves 100cc of volume in its combustion chamber at the top of its stroke will have a 1000:100 compression ratio that can be reduced to 10:1.

Your pistons come into play by helping dictate the size of the combustion chamber (along with your heads). For example, take two of our Street Fighter 331 stroker kits. Our first kit comes with -4.0cc flat top pistons, while the second kit comes with -14.2cc reverse dome pistons. The first kit has a standard compression ratio of 10.25:1 when using 58.0cc heads. The second kit only has a 9.21:1 compression ratio with the same sized head. The secret is in the piston design. A -4.0cc piston top leaves less room in the combustion chamber than the -14.2cc piston tops of the second kit. In general, a higher compression ratio will generate more power, but high power levels at high compression ratios cause excess pressure on the piston and rod that can make them fail, especially with forced induction systems like supercharging, turbocharging, and nitrous. Lower compression ratios are perfect for forced induction systems, and can more adequately handle high boost pressures.

Each of Stage 3's stroker kits comes with an option for bore size. As we mentioned earlier, the bore is the overall diameter of the cylinder. Pistons can match the ''standard'' piston bore or can have an ''over bore'' of +.030'' or +.040''. The cylinders of your block leave a slight amount of room between the piston and cylinder wall when using pistons with a standard bore. Overbored pistons close that gap to generate higher compression ratios. However, large overbore can cause cooling issues, because there's less air available for heat transfer out of the cylinder and into the block's water jacket (or coolant jacket). That being said, a +.030'' to +.040'' overbore should not cause serious heat problems.

Well, that's a ton of information, but building an engine that won't blow up requires a certain amount of information. That amount being ''a hell of a lot.'' This is only the tip of the iceberg, given we haven't even begun on blocks, electrical, cooling, and all the other fun stuff that'll make your engine build run on its own. That stuff can wait till ''Engine Building 102.'' In the mean time, if you have questions, comments, concerns, corrections, or if you need any advice, don't hesitate to contact Stage 3 directly. Most of our sales staff has experience in engine builds and components and would be happy to get you started on your own tire-shredding kit.