Calculating engine displacement is a simple matter of bore and stroke. You probably recognize the equation from middle school math as finding the volume of a cylindrical shape (in this case, eight of them). A stock engine has a given bore and stroke (left), while a "stroked" engine has a nearly identical bore (cylinder diameter) and a longer stroke (height, or more appropriately, "depth" of cylinder).
LS1 Stroker Basics
If you're looking to up the displacement of an internal combustion engine, there are only two ways to go about it: increase cylinder bore, or increase crankshaft stroke. In the case of "boring," you're increasing the diameter of the piston, and hence, the diameter of the hole you need to fill with fuel and air. In the case of "stroking," the pistons travel further downward into the cylinder, meaning a deeper hole is to be filled up.
Stroking is very popular when it comes to Gen IIIs since the ability to increase cylinder bore is quite limited. A minimum thickness must be kept in the block's stock cast iron cylinder liners, and there isn't a whole lot to work with: maximum overbore is generally accepted to be 10 thousandths of an inch (even less for earlier LS1 blocks). Though "re-sleeving" is possible--installing cylinder liners that can accept larger pistons--this process gets expensive in terms of machining costs, as well as the price of the oversize sleeves themselves.
It should be noted that we are focusing only on the Gen III LS1 and related engines in this article; the Gen IV (LS2 etc.) is a bit different and uses a larger bore from the factory (4.000 inches). While the majority of the information in this article will apply to the similar-architecture LS2, there are too many technical differences to keep track of and still keep this story readable. Hence, our Gen III-only focus.
But "stroking" an engine is not simply a matter of swapping the crankshaft out and calling it a day. Rather, a different crank will require changes to the entire rotating assembly. The piston still must reach the top of the bore during each crankshaft revolution, so the stock LS1 pistons' pin location would dictate an extremely short rod if used with a stroker crank. Besides, the strength of a stock cast aluminum piston is peanuts compared to any aftermarket forged unit and probably wouldn't stand up to the power levels produced by even the most conservative stroker build. And while the stock GM powdered metal steel connecting rods are an appropriate length for most strokers (6.098 inches), a few obstacles stand in the way of reusing them. Aside from the stock rods being press-fit to the piston pins, nearly all aftermarket cranks are filleted around the edges of the journals, whereas the stock nodular iron crank is not. This means the big end of the rod is simply incompatible with most aftermarket cranks.
The Lunati Rotating Assembly
Lunati, a trusted name in aftermarket internal engine components for the last four decades, now offers a line of LS1 Pro Series Stroker kits. These rotating assemblies are designed to fit the needs of any Gen III build, with varying crankshaft strokes and piston designs to help achieve the displacement and compression ratio the customer demands. These assemblies are a matched set and include Lunati's own Pro Series 4340 non-twist forged steel crank, Pro Billet Super Light connecting rods (also aircraft-quality 4340 forged steel), and 4032 forged aluminum pistons. Based on crank stroke and cylinder bore selected, rotating assemblies yielding cubic inches from 347 all the way to 447 can be selected (with a modified block on the high end of course). The kits also include piston rings as well as main and rod bearings, making them 100 percent complete. We'll discuss the high-tech features of each of these components as we proceed with the build.
Since the Lunati rotating assembly is a balanced set, all parts are matched to one another and the assembly is given a unique serial number. Make sure the numbers on all the boxes are the same!
We selected PN EA035-383, which with its 4.000-inch stroke and 3.903-inch bore yields the magic Mopar value of 383 cubic inches (hey, at least it isn't a Ford number). As stated in the main text, the 4.000-inch stroke crank will require minimal clearancing of the engine block to fit, and the 3.903 bore will be achieved with a simple cylinder hone. This rotating assembly is rated at a whopping 1,100 hp, which equates to around 900 at the tires--more than sufficient for any street machine. It carries a suggested retail price of $3,800. Not small change, yes, but the kit is a huge savings over purchasing all of the individual items alone (you'd spend an extra $600). Plus, with a typical engine balancing running at least a couple hundred dollars, this money stays in your pocket as well.
And with the rotating assembly being 100 percent Lunati, you can rest easy knowing your bottom end is made of the best quality stuff on the market, all forged and machined in the U.S. of A.
Bearing Clearances, and How to Measure Them at Home
When we say bearing clearance, we're talking about the amount of space between the fast-spinning crankshaft journal and the bearing (whether they be rod or main bearings). This space must be occupied by a certain amount of oil to ensure proper lubrication. Too much main bearing clearance and you lose oil pressure; too little and oil can't properly flow into and out of the space.While it's possible to assess this by carefully measuring and comparing the bearing bore diameter to the bearing thickness and crank journal diameter, the problem is that all but the most well-equipped machine shop lacks the kind of super-expensive, precise measuring equipment needed to do this. All of the tools in our possession are only accurate to about half of a thousandth of an inch; for each item you're trying to measure and compare, the error only compounds. Therefore, we're going to go with the tried-and-true method of the home hot rodder and professional engine builder alike: Plastigage.
The Plastigage we chose was manufactured by Sealed Power. We needed the type that measures clearances in the range of 1 to 3 thousandths: PN SPG-1. Plastigage is available by mail order or at any machine shop. It's cheap, too; heck, our machine shop gave us our piece for free.
Basically, the Plastigauge gets squeezed between the bearing and the crankshaft journal when the main or rod cap is tightened. By measuring the width of the smooshed piece of Plastigauge, we know the bearing clearance. More smoosh = less clearance for oil to flow through. It's simple, but this stuff is quite accurate and has been used in engine assembly for decades. We've detailed the procedure for checking main bearing clearances in the photo captions; the process of checking rod bearing clearances is virtually identical.
The Lunati Friction Package
We asked Mark Chacon, Lunati's East Coast Regional Rep., about the reasoning behind Lunati's choice to include bearings in its kit that provide somewhat greater clearances than those used by GM.
"As a general rule, high performance or race engines may need different clearances than a factory engine. Such engines will generate more bearing heat than a stock engine, and the engine builder may decide to establish bearing clearance to whatever he is comfortable with. Everyone has their own take here and you will get a different opinion depending on who you talk to. If a given bearing clearance works well for a given environment that an engine is being used in, and no excessive bearing wear has been seen once the engine is torn down, it's been my experience that the engine builder will continue to do what he has had success with.
"That said, my opinion on bearing clearance for a high performance street or race engine would be 2.25 to 2.50 thousandths on the rod bearings and 2.50 to 2.75 thousandths for the main bearings. The engine may need to run a slightly heavier weight oil to keep the necessary oil pressure (10 psi per 1,000 rpm), but this is a good area for compromise: there's simply more protection inherent in a slightly heavier weight motor oil. Sure, it might cost a small amount of horsepower to drive the oil pump, but in my view it's worth it. After all, engine building is nothing more than a series of well-thought-out, strategic compromises.
"Oil volume and pressure are the lifeblood of your engine, and the type of oil used can affect this to some degree. For example, synthetics may cause you to lose some oil pressure because they are slicker and a bit more effective at lubrication. I may be inclined to be a bit tighter with my tolerances if using synthetic oil; but we're not talking big amounts here. Similarly, I might be inclined to go a little wider on clearances in an endurance application; for example, a circle track or marine application where the engine will be at mid to high rpms most of the time. All of these decisions should be based on application as well as the oil type used."
"In my view there are some dangers if you get 'cute' with main and rod bearing running clearances in a street engine. On a short-life, all-out race engine where you're looking to extract every last hp (and with regards to engineering a friction package specific to a total race application), I might want to run some super-light, zero-weight motor oil, and might look at adjusting the bearing clearances to accommodate this. But on the street, who knows when you are going to get stuck in gridlock traffic on a 100-degree day--I like the idea of 20W-50 in my street engine for the extra protection this type of oil will provide. Dry sump engines require altogether different considerations as well. Again, it's all about the environment the engine will be used in.
"On a final note, I feel that I need to give credit where credit is due, for I am but the student: the real expert here is master engine builder and thinker Bob Mendenhall, who took the time 30 years ago to teach a 15-year-old kid the finer points of engine assembly and advanced engine theory. Thanks again Bob!"We should note that we'll be installing a high-volume SLP oil pump in the next segment of this story, and that we'll be adjusting it to give increased oil pressure. So stay tuned for more on this topic!