connecting rod & rod length too stroke info

Discussion in 'Rotating Assemblies' started by grumpyvette, Oct 14, 2008.

  1. Maniacmechanic1

    Maniacmechanic1 solid fixture here in the forum

    Still need some money $$$$.
    10K a good money number for starters.
    Strictly Attitude likes this.
  2. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

    If they are properly designed, and assembled , turbos and centrifugal blowers and nitrous,
    if properly set up, and tuned can add amazing power,
    I've always wanted to install a centrifugal supercharged ,mpfi 572 BBC, in my corvette,
    getting 900 hp/900 ft lbs of torque in a dependable combo that will never hit 6000 rpm is easy
    (no not dirt cheap,
    but with experience and readily available parts,
    easy to design and assemble)

    vaguely similar idea
  3. Strictly Attitude

    Strictly Attitude solid fixture here in the forum

    I know the formula very well $value(X)=HP Value(reliability)
    Indycars likes this.
  4. Maniacmechanic1

    Maniacmechanic1 solid fixture here in the forum

    I wonder what Turbo Phil has been up to Grumpy ?
  5. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

    no, if you have any engine and want too find the true compression you need to deal in verified fact, not assume what the manufacturers suggest is always correct.
    you just cc the heads, combustion chamber volume, place the piston 1" down the bore and seal the rings gap around the pistons,
    above the rings with moly grease or Vaseline and measure that volume and calculate what a bore diameter cylinder 1" tall would
    contain, minus the volume you,ve measured, the piston dome took up.then calculate the true compression.


    read related linked info
    How A Stroker Crankshaft Affects Piston Speed and Inertia.
    [​IMG] Print [​IMG] Email


    An intense look at mean piston speed, inertia, and controlling the massive, destructive forces at work inside your engine.

    Engine builders have long calculated the mean piston speed of their engines to help identify a possible power loss and risky RPM limits. This math exercise has been especially important when increasing total displacement with a stroker crankshaft, because the mean piston speed will increase when compared to the standard stroke running at the same RPM.

    But what if there was another engine dynamic that could give builders a better insight into the durability of the reciprocating assembly?

    “Rather than focus on mean piston speed, look at the effect of inertia force on the piston,” suggests Dave Fussner, head of research and development at Wiseco Pistons.

    Let’s first review the definition of mean piston speed, also called the average piston speed. It’s the effective distance a piston travels in a given unit of time, and it’s usually expressed in feet per minute (fpm) for comparison purposes. The standard mathematical equation is rather basic:

    Mean Piston Speed (fpm)=(Stroke x 2 x RPM)/12

    There’s a simpler formula, but more on the math later. A piston’s velocity constantly changes as it moves from top dead center (TDC) to bottom dead center (BDC) and back to TDC during one revolution of the crankshaft. At TDC and BDC, the speed is 0 fpm, and at some point during both the downstroke and upstroke it will accelerate to a maximum velocity before decelerating and returning to 0 fpm.

    As the piston races from bottom dead center to top dead center, for a brief moment, it comes to a complete stop. This places tremendous stress on the wrist pins. Shown, these Trend pins are offered in various wall thicknesses depending on the application.
    [​IMG]The mean piston speed takes the total distance the piston travels during one complete crankshaft revolution and multiplies that by the engine RPM. Piston speed obviously increases as the RPM increase, and piston speed also increases as the stroke increases. Let’s look at a quick example.

    A big-block Chevy with a 4.000-inch-stroke crankshaft running at 6,500 rpm has mean piston speed of 4,333 fpm. Let’s review the formula again used to calculate this result. Multiply the stroke times 2 and then multiply that figure by the RPM. That will give you the total number inches the piston traveled in one minute. In this case, the formula is 4 (stroke) x 2 x 6,500 (RPM), which equals 52,000 inches. To read this in feet per minute, divide by 12. Here’s the complete formula:

    (4 x 2 x 6,500)/12=4,333 fpm

    You can simplify the formula with a little math trick. Divide the numerator and denominator in this equation by 2, and you’ll get the same answer. In other words, multiply the stroke by the RPM, then divide by 6.

    (4 x 6,500)/6=4,333 fpm

    With this simpler formula, we’ll calculate the mean piston speed with the stroke increased to 4.500 inch.

    (4.5 x 6,500)/6=4,875 fpm

    As you can see, the mean piston speed increased nearly 13 percent even though the RPM didn’t change.

    Reducing piston weight plays a huge role in creating a rotating assembly that can sustain high rpm. The seemingly insignificant gram weight of a piston is magnified exponentially with rpm.
    Again, this is the average speed of the piston over the entire stroke. To calculate the maximum speed a piston reaches during the stroke requires a bit more calculus as well as the connecting rod length and the rod angularity respective to crankshaft position. There are online calculators that will compute the exact piston speed at any given crankshaft rotation, but here’s a basic formula that engine builders have often used that doesn’t require rod length:

    Maximum Piston Speed (fpm)=((Stroke x ?)/12)x RPM

    Let’s calculate the maximum piston speed for our stroker BBC:

    ((4.5 x 3.1416)/12)x 6,500=7,658 fpm

    By converting feet per minute to miles per hour (1 fpm = 0.011364 mph), this piston goes from 0 to 87 mph in about two inches, then and back to zero within the remaining space of a 4.5-inch deep cylinder. Now consider that a BBC piston weighs about 1.3 pounds, and you can get an idea of the tremendous forces placed on the crankshaft, connecting rod and wrist pin—which is why Fussner suggests looking at the inertia force.

    “Inertia is the property of matter that causes it to resist any change in its motion,” explains Fussner. “This principle of physics is especially important in the design of pistons for high-performance applications.”

    When the connecting rod is lengthened, it provides a softer transition as the piston changes direction. The longer connecting rod also reduces the compression height of the piston and can help pull weight out of the rotating assembly.
    The force of inertia is a function of mass times acceleration, and the magnitude of these forces increases as the square of the engine speed. In other words, if you double the engine speed from 3,000 to 6,000 rpm, the forces acting on the piston don’t double—they quadruple.

    “Once started on its way up the cylinder, the piston with its related components attempt to keep going,” reminds Fussner. “Its motion is arrested and immediately reversed only by the action of the connecting rod and the momentum of the crankshaft.”

    Due to rod angularity—which is affected by connecting rod length and engine stroke—the piston doesn’t reach its maximum upward or downward velocity until
    about 76 degrees before and after TDC with the exact positions depending on the rod-length-to-stroke ratio,” says Fussner.

    Stroker cranks such as this forged LS7 piece from K1 Technologies, are a great way to add displacement. However, when the stroke is lengthened the piston must accelerate faster each revolution to cover the larger swept area of the cylinder wall.
    “This means the piston has about 152 degrees of crank rotation to get from maximum speed down to zero and back to maximum speed during the upper half of the stroke. And then about 208 degrees to go through the same sequence during the lower half of the stroke. The upward inertia force is therefore greater than the downward inertia force.”

    If you don’t consider the connecting rod, there’s a formula for calculating the primary inertia force:

    0.0000142 x Piston Weight (lb) x RPM2 x Stroke (in) = Inertia Force

    The piston weight includes the rings, pin and retainers. Let’s look at a simple example of a single-cylinder engine with a 3.000-inch stroke (same as a 283ci and 302ci Chevy small-block) and a 1.000-pound (453.5 grams) piston assembly running at 6,000 rpm:

    0.0000142 x 1 x 6,000 x 6,000 x 3 = 1,534 lbs

    With some additional math using the rod length and stroke, a correction factor can be obtained to improve the accuracy of the inertia force results.

    Crank Radius÷Rod Lenth

    “Because of the effect of the connecting rod, the force required to stop and restart the piston is at maximum at TDC,” says Fussner. “The effect of the connecting rod is to increase the primary force at TDC and decrease the primary force at BDC by this R/L factor.”

    For this example, the radius is half the crankshaft stroke (1.5 inch) divided by a rod length of 6.000 inches for a factor of .25 or 383 pounds (1,534 x 0.25 = 383). This factor is added to the original inertia force for the upward stroke and subtracted on the downward movement.

    Both the crank on the left and right are at the same point in their respective rotations. However, the piston on the left will have to travel much faster to reach top dead center at the same time as the piston on the right.
    “So, the actual upward force at TDC becomes 1,917 pounds and the actual downward force at BDC becomes 1,151 pounds,” says Fussner. “These forces vary in direct proportion to the weight of the piston assembly and the stroke to rod length and they also vary in proportion to the square of the engine speed. Therefore, these figures can be taken as basic ones for easily estimating the forces generated in any other size engine.”

    As the piston reaches top dead center on the exhaust stroke, there is no cushion of compression to help slow it down. Instead, the connecting rod takes the full brunt of the force which pulls on its beam and tries to separate its cap. Quality connecting rods are paramount to a high-horsepower, high-rpm engine.
    “We know a common measure used for many years to suggest the structural integrity danger zone of a piston in a running engine is mean piston speed,” sums up Fussner. “As the skydive instructor told his student, it’s not the speed of the fall that hurts, it’s the sudden stop. And so it is with pistons. So rather than focus only on the mean piston speed, let’s decide to also consider the effect of inertia force on the piston, and what we can do to reduce that force. And if that is not possible, make sure the components are strong enough to endure the task we have set forth.”

    This article was sponsored by Wiseco. For more information, please visit our website at

    Last edited: Jun 18, 2019
  6. Maniacmechanic1

    Maniacmechanic1 solid fixture here in the forum

    Longer Rods affect the engine output slight, different camshaft timing profiles net better or worse results.
    Seen that on my own using The Original HO Racing Website Calculators Grumpy.
    Different that what we use here.
    Plugged Chevy Engines in & Works.
    Though Kern Osterich wrote the programs Just for the Pontiac V8.
    He also listed RPM Limits of Production & Factory Racing parts in His Books.
    No on dares Posting all from his books online......Big Sin.

    6.625" - 6-5/8" Pontiac Rods do very well.
    Not such a big fan of using Chevy BB 6.700 & 6.800 In Pontiacs.

    Wrist Pins are another Topic.
    With High Boost levels They need to be super strong.
    44 psi + being done in Chicago.
    At Least the Race shop I worked at.
    Super High Output.
    Super expensive too.
    Proud to say Engines I built from a bare block stayed together.

    7K has to be done by me.
    I did it for 10
  7. Maniacmechanic1

    Maniacmechanic1 solid fixture here in the forum

    Street engines should stay at 6000-6500.

    Drag Race 7K Ok....Its to Win.
    Shorter life is expected.
  8. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

    heres a bit of info, in a couple video that might be useful.
    on building a reasonably priced BBC combo using a 454 block and forged steel 396 crank, ID strongly suggest you use aftermarket 6.385" rods with 7/16" ARP bolts and pistons matching the applications requirements,
    6.385" longer aftermarket rods and pistons, and related info on a similar big block build

    be aware there are counterfeit ARP bolts for sale
    related info
    Last edited: Apr 1, 2019
  9. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

  10. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

    Last edited: May 26, 2019
  11. johnnmo

    johnnmo Active Member

    Just for discussion, there is a relationship to valve and port size that works with high and low rod ratio's.
    Take a big rectangular port big block chevy, with a long rod and the port is lazy.
    Take a big block mopar with stock 906 ports and a low rod ratio and it cannot breath at RPM.
    The Mopar relies more on velocity in the port than the chevy will.
    This all effects power/torque + hp peaks.
  12. Maniacmechanic1

    Maniacmechanic1 solid fixture here in the forum

    I have read many articles here in Grumps and elsewhere on connecting rod lengths that affect R/S.
    The dwell time at TDC and BDC.
    People focusing TDC.

    BDC affects alot.
    Some simulation programs I have used show 40 Hp more.
    Others notta.
    I think longer rods help.
    Build it and test.
  13. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

    one advantage of individual stack fuel injection is easy changes in rpm/ram induction tuning to compensate
  14. johnnmo

    johnnmo Active Member

    Longer rods will reduce friction.
  15. Maniacmechanic1

    Maniacmechanic1 solid fixture here in the forum

    Yes that is True.
  16. Maniacmechanic1

    Maniacmechanic1 solid fixture here in the forum

    Need the pistons made for 4.21" -4.25" - 4.500" - 4.75 " stroke crank engines with high crank rpms.
    Going to be in the danger zone.
    Have to hit drag race.
    2618 premium forged and Full Anodized.
    All the bells and whistles you can afford.
    $1k - 2k for a set of 10. Always buy 2 extras for race pits emergency repairs.

    Street can do fine with 4032 forged.
    Even Keith Black Hypers or Mahle Hypers.
    Stick to Grumpy safe Zone rpm and it will last.

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