The Gen III Hemi engines in production today have evolved from a long line of performance/race Hemi engines that also did double duty as production engines. These new Hemi engines also share many features with the Chrysler/Mopar small-block engines.
Since the engine has been out in the public for almost 20 years, they are easier to acquire, easier to rebuild, easier to afford, and it’s easier to build horsepower than it has been in the past.
In this article, we’ll cover ways you can build your new Hemi into a Big Cubic Inch monster.
A crankshaft with a stroke that is longer than stock is generally considered a stroker. As long as the original crank is repairable then it can be used in the reassembly, but this is a good opportunity to increase displacement. With the engine pulled and torn down, most of the work has already been done. Now there is just the expense of a new crank.
This Tech Tip is From the Full Book, NEW HEMI ENGINES 2003-PRESENT: HOW TO REBUILD. Here are some additional New Hemi books that may interest you as well!
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The term stroker is somewhat of a hot rod term. In a technical sense, the engine’s stroke is the total axial movement of a piston in its cylinder bore, or how much the piston moves up and down the bore (the distance from one extreme of travel to the other). This movement is directly related to the amount of offset in the crankpin. Most production engines have a commonly used stroke length. For example, the 5.7L and 6.1L Gen III Hemi engines share a 3.58-inch stroke, while the 6.4L 392 Hemi and the 6.2L Hellcat share a 3.72-inch stroke.
If you use a crank that has a longer stroke than the original version, you are installing a stroker or increased-stroke crank. This technique is not limited to hot rodders and racers; production engineers also do it. For example, in 2011 Chrysler engineers made the 6.4L 392 by increasing the Gen III Hemi’s stroke from 3.58 inches to 3.72 inches. This crank from Molnar Technologies has a 4.08-inch stroke.
Once you have a specific block, it is difficult to change the engine’s displacement by overboring because the blocks tend to be limited by how much the bore walls will allow (about 0.020 inch in the Gen III Hemi production blocks). Therefore, increasing the stroke is an easy way to gain displacement, which is an easy performance gain.
Why do performance builders want larger engines? Generally a larger engine will make more power and more torque. For example, the early 5.7L engine at 345 ci makes 345 hp, or 1 horsepower per cubic inch. So if you added 15 ci using the same hardware, you would assume that you would gain 15 hp.
The Displacement Equation
The engine’s displacement is the sum of each cylinder’s displaced volume. In other words, this volume is the area of one piston multiplied by the stroke length multiplied by the number of cylinders (eight for our Gen III Hemi engines). With the term volume, you might think of a large box or a cube, but the 5.7L displacement is actually the sum of eight smaller volumes. The small volume is the amount displaced by one piston going through one revolution. Therefore, the engine’s displacement is the sum of the eight small cylinder displacements.
Since displacement is a volume, it is generally discussed in the units of cubic inches (imperial) or liters (metric). The early 5.7L Gen III Hemi is referred to as 5.7 liters or 345 ci (calculated but not used), while the 6.4L is referred to by both the liter version and the 392 ci description (the 392 version seems to be more popular today).
To actually calculate the engine’s displacement, you need an equation that relates the engine bore and stroke. The equation is as follows:
D = 0.7854 × B × B × S × N
D = Displacement in cubic inches
B = Cylinder bore in inches
S = Stroke of the crank in inches
N = Number of cylinders
For example, the Hellcat engine (called a 6.2L) has a bore of 4.09 and uses a 3.58-inch stroke. This makes the actual displacement: D = 0.7854 × 4.09 × 4.09 × 3.58 × 8 = 376 cubic inches.
The engine’s block height tends to be one of the limiting factors in determining how much stroke can be built into a given engine. The Gen III Hemi engines have piston heights of around 1.18 to 1.21 inches. This height is pretty close and it is not easy to build pistons with less since the piston’s deck height is already just a few thousandths of an inch. So within a given block, the stroke increases tend to come with rod length changes (shorter) to keep the balance or the piston would come out the top of the block.
BH = 1/2 stroke + rod length + piston height + deck height
An engine’s block height (BH) is at 9.25 inches and doesn’t change. The piston height is around 1.20 inches and can’t vary much. The deck height is almost zero. This means the crank and its stroke become a team with the length of the rod and indirectly the piston. If the crank’s stroke goes up then the rod length should get shorter. If you swap the 3.58-inch stroke crank for the 3.72-inch stroke crank, then the rod should be shorter. The stroke difference is 0.140 inch, which means the 1/2 stroke number is 0.070 inch. The difference in rods from the 5.7L and the 3.58-inch crank (6.24-inch rod) to the 6.4L and the 3.72-inch crank (6.20-inch rod) is 0.040 inch. There is an additional 0.030 inch in piston height difference (1.21 inch versus 1.18 inch). The 0.040- and 0.030-inch heights make up the whole 0.070-inch height required to keep the top of the piston at the same location.
The 3.72-inch stroke crank installed in the basic 5.7L block makes a 361-ci engine. This makes the engine 16 ci larger. This means that the engine change will make about 16 to 19 hp more than it did originally.
For example, let’s assume that you want to use one of the readily available 4.00-inch stroke cranks in your engine. The 1/2 stroke number is 2.00 for the 4-inch crank, which is 0.140-inch longer than the 3.72 crank discussed above. So let’s pick a 6.00-inch rod, which is 0.200-inch shorter than the 3.72-inch rod (6.200) but still readily available. You could select a piston with a height of 1.24-inch (higher than the 1.18 stock), which would keep the piston at the same deck height. The problem is there is too much compression ratio, so you might pick 1.21 inches for the street version, which drops the ratio back around the stock 10.5:1.
Crank, Rod, and Pistons
The production engines define a specific package of hardware for the block and displacement. The bore can only be increased slightly, maybe 0.020 inch, and that will usually allow the block to be reused in your rebuild. The 0.020-inch overbore amounts to about 4 ci, which is only going to change the performance characteristics of the engine maybe 1 percent. As mentioned previously, if the stroke is changed, then you may have to change the rod and perhaps even the piston to get the engine to work together as a team again.
The aftermarket cranks use full-radius journals (rods and mains), and the production designers seem to have solved this potential problem by making the production bearings slightly narrow. Always check to make sure the crank has the necessary clearance to turn after the bearing has been installed and torqued.
For a rebuild project, you must consider the time and the costs involved. Since the engine is already apart, the time to take it apart is not an issue. Time to get the parts could be an important aspect, but if you plan ahead it should not be of any concern. The real deciding point is cost. Let’s focus on the three major parts: crank, rods, and pistons. At a rebuild, new pistons will be required, so the costs would be similar regardless. You were going to use the stock rods with new bolts compared to new rods. Finally, you were going to use the stock crank reground 0.020/0.020 inch undersize compared to a new crank.
Ordinarily, you would have the costs of pistons balanced against the costs of all three (crank, rods, and pistons). However, there is an exception to this analysis. If you found a used 392 crank that could be repaired at 0.020/0.020 inch for a reasonable fee, then you might be able to obtain the 392 rods at the same source for a similar fee. Now you have two of the three pieces for a reasonable cost. Now you just need to have the piston’s height to be made at 1.18 inches in addition to the 3.93-inch bore size.
With any rebuild project, the bores will have to be bored oversized to freshen the block, and this requires new pistons. To keep project costs down, try to limit the new parts to the pistons. However, there are three other aspects of the pistons that you have to consider: compression ratio, compression height, and ring package.
Let’s look at the stroker example detailed previously. Keeping the top of the piston the same (dome, notches, etc.), keeping the piston’s deck height the same, you can calculate that the stock 5.7L’s combustion chamber volume or VTDC is 76.56 cc. If you keep everything the same, this volume should not change. However, the longer stroke of the 392 crank makes the displacement larger or 739.46 cc. Therefore, the new CR is 10.65 or it is almost half a point higher. Try to drop the piston about 0.015 to 0.020 inch to get the ratio back to 10.2:1.
The second feature of the piston after bore size is its compression height (CH). The piston height (PH) or compression height is the distance from the center of the pin to the top of the piston. Typically, this number is given to you by the piston manufacturer. You can measure it directly by using the piston and pin together.
PH = CH = HT + PD/2
PH = Piston height or compression height (CH)
HT = Piston head thickness measured from the top of the pin bore to the top of the piston
PD = Pin diameter
These measurements can be taken with a dial vernier caliper.
The piston height is typically used to balance the rod length to make things fit. If the rod is 0.040 inch shorter, then you can make the piston 0s.040 inch taller to keep the top of the piston in the same place. This keeps the compression ratio, valve clearances, and piston-to-head clearances the same. The compression height of the 5.7L is 1.24 inches with its 3.58-inch stroke, while the 6.4L uses a 1.20-inch compression height with a 3.72-inch stroke.
A popular package today is a 4.00-inch crank (around 4.030 to 4.050 inches specifically) for the 5.7L engine, which makes 390 to 400 ci. This kit is offered by Manley. (Photo by Manley)
The final feature of the piston is the ring package. The newer engines use a 1.2-mm ring package, while the older ones use a 1.5-mm ring package. As long as rings are available for the piston bore size that you have selected, matching the rings to the piston is the only concern. Racers like to use rings that are considered low-tension and file-fit. For a rebuild you do not want either of these features.
To make stroker cranks more cost-competitive, some of the crank manufacturers offer what is called a stroker kit. These kits feature everything you need to make a conversion in a given engine. Currently, they are available from Eagle, Modern Muscle, and Manley, among others. The crank, rods, and pistons make up a kit, but you always want to check to see what is included and what other options you may have (gaskets, bearings, rings, etc.). For example, Eagle offers a kit for the 5.7L engine based on a 4.03-inch stroke that yields around 390 ci. The 4-inch cranks fit into the Gen III Hemi crankcase reasonably easily. However, the longer 4.125- and 4.250-inch cranks are more of a challenge. These bigger packages are just being developed on the racing side but new hardware is coming constantly.
Written by Larry Shepard and republished with permission of CarTech Inc
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