06-09-2011, 12:25 PM
My Jeep: 2009 7.0L SRT8
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Engine class is session....
Andy from PWR posted the following in response to some questions. It was some good info and thought we could benefit here. Grab a PB&J sammich, glass of milk and enjoy:
Working on getting one about heads.
Let me start by saying that there are a couple of ways to build the 440 and we do it differently than everyone else.
Lets look at Crankshaft overlap
With brand X 440, 2.560 main, 1.850 journal, and a 4.25 stroke you get .080" of overlap.
PWR 440, 2.560 main, XXXX journal, and a XXX stroke you get .195" of overlap.
PWR 426, 2.560 main, 2.00 journal, and 4.05" stroke, you get .255" of overlap
PWR 6.1/392, 2.560 main, 2.10 journal, and 3.795" stroke, you get .43" of overlap
PWR 6.1, 2.560 main, 2.125 journal, and a 3.579" stroke, you get .553" of overlap
Here is the equation for determing Crankshaft overlap.
((Main + Journal)/2)-(stroke/2)
Crankshaft overlap is simply how much of a crank's main and rod journal diameters overlap each other. As stroke is increased, moving the rod journals farther away from the main journals reduces overlap and compromises strength and durability. Likewise, smaller rod and main journals reduce bearing speed and friction, but also reduce overlap. The reason why GM increased the size of the mains to 2.65 inches on a 400 SBC compared to 2.45 inches on a 350 was to maintain journal overlap with the longer 3.75-inch stroke.
Another thing to consider is Compression Height.
All figures below are assuming 0 deck height.
Brand X 440: 1.000"
PWR 440: 1.030"
PWR 426: 1.085"
PWR 370: 1.205"
PWR 392: 1.218"
Equation for determining Compression Height.
(((Block Height-(Stroke/2))-Rod Length)-Deck Clearance
When Compression Height gets as small as 1 inch, it often requires narrow piston rings or ring buttons over the piston pin. It may also require the rings to be closer to the top of the piston than you may want for nitrous or endurance use. Short piston height means lighter weight, which is good, but it may also give up reliability you need. Sometimes a compromise for less stroke or shorter rods is prudent.
So looking at all that info, the 6.1 has the most crankshaft overlap which is great for high boost application and it has a great compression height on the piston. This is an awesome motor for high hp applications. Im calling this a 125,000-150,000 mile motor
The 392 also has a great amount of crankshaft overlap and an even better compression height on the piston. This is another great choice for a high hp application. Lets call this a 100,000-135,000 mile motor.
Both of these motors have the wrist pin below the oil ring whereas the 426/440 have the wrist pin in the oil ring and must use an oil rail to hold the oil rings in place. This also creates a rocking motion having the wrist pin this high in the piston which will cause premature wear compared to the 6.1 and 392.
The 426 has good overlap and a good compression height, but its questionable when it comes to long-term durability under a boosted or high nitrous application. Im going to put this in the 75,000-115,000 mile range.
The 440 that we build does have more crankshaft overlap and more compression height than the std 440 that most people have, but I still would not boost it or spray it. Service life of a 440 is not as long as a 426, but it would be very close. Im calling this a 50,000-85,000 mile motor.
The mileage that I have applied to each motor does not mean that the engine will fail at that point. It only means that at that point, the bearings and rings should be replaced. We offer this service for $750 to all our customers.
I hope this helps. Now its lunch time, lol.
Cam Specs and what they mean
Overlap is the amount of time both intake and exhaust valves are open.
LSA is the angle relationship between the intake and exhaust lobes. If you narrow that angle, overlap is increased.
Duration is the amount of time in crankshaft degrees, that the valve is open. More duration with the same LSA will also increase overlap.
The camshaft lobe lift is multiplied by the cars rocker ratio. Most in the industry consider our rockers to have a 1.60 to 1.65 rocker ratio.
Lets say you have the following cam.
220/228 350/340 113+3
220: Intake duration. The amount of time in crankshaft degrees that the intake valve is open.
228: Exhaust duration.
350: Intake lobe lift. (350 x 1.65 = 578 valve lift)
340: Exhaust lobe lift (340 x 1.65 = 561 valve lift)
+3: Amount of advance ground into the cam. In this case, the cam has a 110 ICL.
This particular cam has an advertised duration of 269/278 @ .006 and 220/228 at .050.
Using this cam, you get 47.50 degrees of overlap. (For 47.50 degrees of crankshaft rotation, both intake and exhaust valves are open at same time)
If we leave duration the same and change LSA to 117, we get: 39.50 degrees of overlap.
With the 117LSA and less overlap, you can expect:
a. Wider powerband, more peak power, smoother idle
b. Improved low-rpm responsiveness, better fuel efficiency, engine may run hotter
With the 113LSA and more overlap, you can expect:
a. Increased mid-range torque, faster acceleration, narrower powerband
b. lower fuel efficiency, potential for reversion. Low vacuum.
More Lobe Separation: Wider powerband, smoother idle
Less Lobe Separation: Increased mid-range torque, faster acceleration, narrower powerband
More Duration: Powerband moved higher in rpm range
Less Duration: More low-end torque
More Overlap: lower fuel efficiency, potential for reversion. Low vacuum.
Less Overlap: Improved low-rpm responsiveness, better fuel efficiency, engine may run hotter
What camshaft overlap is optimal for you?
10-30: Street towing
20-40: regular street
35-55: street performance
95-115: Pro race
The cam in my Charger has 65 degrees of overlap.
Most of my NA 426 cams have around 55-60 degrees.
Most of my NA 6.1 cams have around 48-52 degrees
More displacement and greater leverage means more torque. This concept is obvious when you compare the torque ratings between factory small block motors and factory big block motors. However, nowadays it is not necessary to suffer the time and switching costs of leaping to a larger block if you are only after more displacement. Displacement is just a factor of bore and stroke, by increasing the stroke of your current motor you can enjoy the satisfaction of more torque disguised in the same package.
Formula A. Displacement. Simply a factor of bore and stroke. Increase the stroke of your current motor and reap the benefits of more torque.
Widespread awareness of the facts above and an abundance of aftermarket stroker kits have made the stroker option extremely popular. If you are out for performance, a stroker is a wise alternative to building a motor that only meets the factory displacement. Whether you have already built a stroker motor or are simply researching them, take a little time to learn the basics and understand the benefits and possible compromises of the now popular engine building practice.
Stroker Motor (def.)
A motor that has greater than stock displacement due to an increase in the factory crank throw. An increase in crank throw increases stroke (the difference between the piston's top dead center and bottom dead center position).
The illustrations below show the difference between a stock and a stroked rotating assembly. Study the differences and you can see what makes up a typical stroker motor. Though a bit exaggerated for effect, the stroked cross section in Figure 2 incorporates:
Increased Crank Throw (distance between C and D)
Increased Rod Length (distance between B and C)
Decreased Piston Compression Height (distance between A and B)
Keep in mind that rod length does not affect the displacement of the engine, it is common to have a stroker motor that uses an increased crank throw, decreased piston compression height, and stock rod length to achieve additional stroke. We'll discuss why longer rods are often used in stroker motors later in the article.
Figure 1. Stock CylinderFigure 2. Stroked Cylinder
The animation below helps visualize the effect of increased stroke and rod length on piston travel and speed.
Figure 1. Stock Cylinder
Figure 2. Stroked Cylinder
Stroker engines are nothing new, and in fact they are not even an aftermarket invention. If you look closely at factory engine offerings, you'll see that changes in displacement are often nothing more than a change in stroke. This was a cost effective way for the factory to increase power for larger vehicles, or future models, while reusing the same block and accessory components.
Figure 3. Offset Grinding. The rod journal is offset ground to move the centerline of the rod journal further from the centerline of the main journal. Result is increased stroke.
Performance enthusiasts then caught on and they found that creative machining and parts matching could yield more cubes while hidden in the stock block to fool fellow racers.
One of the methods used to increase stroke with a stock crank, is called offset grinding. By offset grinding the rod journal you move the centerline of the rod journal away from or toward the centerline of the main journal. This will result in increased or decreased stroke. Figure 3 above illustrates the case we are interested in, the rod journal is ground in a manner to increase stroke. Keep in mind that when the rod journal is offset ground it now has a smaller diameter. The motor will require special connecting rods with correctly sized bearing bores. Additionally, if the rod journal is ground too much it becomes weak. Unless you add material and regrind, you can only stroke a motor so far with a stock crank.
Due to a demand for more stroke than offest grinding a stock crank could achieve, many aftermarket companies developed specialized cast and forged cranks with relocated rod journals. The specialized stroker crank has dramatically increased the amount of stroke you can add to your stock bottom end. Stroker cranks require a shorter piston to keep the factory sized piston from extending beyond the deck surface, it is also shortened to accommodate a longer rod. In the past the only way to complete a stroker motor was to find the right combination of rod lengths and piston heights. This often meant researching other factory motors for the right dimensions. It was not uncommon to have a Small Block Ford stroker motor consisting of Pinto rods and Chevy pistons.
Longer rods are often required to increase leverage and minimize the high degree of rod angularity created by the increase in stroke. The longer rod also prevents the piston from being pulled out the bottom of the cylinder bore. Rod Ratio and rod angularity are especially important issues to consider before simply choosing the stroker kit that yields the largest displacement for your application. We will discuss these topics in the following section.
Rod Ratio (Rod to Stroke Ratio)
Rod Ratio or Rod to Stroke Ratio is the figure achieved when dividing a motor's rod length by its' stroke. This is an important calculation to understand since it informs us about a motor's rod angularity. A low Rod Ratio yields a high rod angle. For example, a motor with a 5.400" rod length and a 3.000" stroke yields a rod ratio of 1.8:1. If we maintain the same stroke and shorten the rod length to 5.000" we get a 1.7:1 rod ratio. The rod angle has increased.
A high rod angle or low Rod Ratio creates a greater potential for accelerated wear to cylinder walls, pistons, and piston rings. The illustrations below show why this is so. Figure 5 is exaggerated for effect but clearly shows how an extremely low Rod Ratio can drive the piston into the side of the cylinder wall.
Figure 4. Low Rod Angle. (High Rod Ratio)
Figure 5. High Rod Angle. (Low Rod Ratio)
By lengthening the rod, as stroke is increased, we can offset the increased rod angle. However, this requires further shortening of the piston. The further the piston is shortened the more likely the piston pin will intersect the oil ring groove, creating a potential for increased oil consumption. See Figure 6 below. Many piston companies however have engineered pistons to avoid this problem with tighter ring packs and Figure 6. Shortened Piston. The further the piston is shortened the more likely the piston pin will intersect the oil ring groove. bridge rings.
Either way, there comes a point when you cannot shorten the piston any further before dependabilty is compromised. As in the discussion about offset grinding, we have reached a limit to how far you can stroke a motor before some component or function is sacrificed.
The consensus amongst engine manufacturers is that a ratio of 1.50" is the lowest acceptable rod ratio for a street motor. Realistically, rod ratios between 1.65" - 1.80" are ideal. See the tables in the following section about stroker kits. Notice how the Rod Ratio decreases as stroker displacement increases.
Piston Dwell Time and Piston Speed
An often overlooked factor that contributes to the advantage of a stroker motor has to do with piston dwell time, the amount of time the piston remains at the top and bottom of the stroke. The increased stroke and rod length of a stroker motor yields a longer piston dwell time. Longer dwell time allows for better flow of combustion and exhaust gases since the piston accelerates slower in the transition between "up" and "down" strokes. Intake gases have a longer time to enter the cylinder while exhaust gases are given more time to escape. This translates into more natural torque over a longer range of rpm. Power and torque can also be enhanced with valve event timing and cam profile.
Even though the piston accelerates slower in transition, the piston ultimately reaches higher speeds to cover the additional stroke. This increase in piston speed means greater component strain. Another factor to consider before simply going with the kit or components that give you largest stroke increase.
Stroker Building Considerations
As you may have guessed, there are certain issues which must be addressed when actually assemblying any stroker engine. First and foremost is the issue of clearances. Due to the increased stroke and rod length changes, it is common for the rod and crank to interfere with cylinder bore end, pan rails, piston skirts, windage trays and other areas inside the block. Therefore it is mandatory that you preassemble the engine components, mark the areas needing grinding for clearance, dissasemble and make the neccesary clearances, and then reassemble and check again. As a rule of thumb you should have at least 0.030" clearance between any interfering points. Another set of considerations unique to stroker engines is rotating assembly balancing. Whether the stroker kit is custom made, or off-the-shelf, the use of new or offset ground cranks, longer rods, and stroker specific pistons ensures that the assembly is not going to spin evenly. Any stroker kit, even off-the-shelf ones, must be balanced by a competent machine shop. Not doing so is a recipe for failure. Always perform the balancing with the harmonic balancer and flywheel you intend to use.
Many of the issues that arise when planning a stroker motor are solved by using a kit that provides a crank, connecting rods, and pistons. Rather than purchasing the components separately, you can purchase predetermined safe combinations for your block. You will get a thousand differing opinions regarding the best stroker for your application. We urge you to gather opinions from fellow enthusiasts and engine builders. Also use the information about rod angularity in this article to make your decision. Stroker displacements remain fairly consistent from kit provider to kit provider. We have highlighted the most popular stroker displacements for Ford blocks in the tables below.
2012 300 SRT
2011 GC Overland
2009 Modern Blue Pearl (for sale)
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