well, that would be long and short stroke.  

yeah very funny! i meant long stroke/ short stroke  

chevy Small Block Height 9.00
355  3.48  5.700  1.64:1  1.550
355  3.48  6.000  1.72:1  1.250
383  3.75  5.565  1.48:1  1.561
383  3.75  5.700  1.52:1  1.433
383  3.75  6.000  1.60:1  1.125
395  3.87  5.700  1.47:1  1.363
395  3.87  5.850  1.50:1  1.213
395  3.87  6.000  1.55:1  1.063
408  4.00  5.700  1.42:1  1.300
408  4.00  5.850  1.46:1  1.150
408  4.00  6.000  1.50:1  1.000

400 GM Block 4-1/8 Bore
352  3.25  6.000  1.85:1  1.375
377  3.48  6.000  1.72:1  1.250
406  3.75  5.700  1.52:1  1.433
406  3.75  6.000  1.60:1  1.125
420  3.87  5.700  1.47:1  1.363
420  3.87  5.850  1.50:1  1.213
420  3.87  6.000  1.55:1  1.063
434  4.00  5.700  1.42:1  1.300
434  4.00  5.850  1.46:1  1.150
434  4.00  6.000  1.50:1  1.000


maximum piston speed of 4500 feet per minute for a well built (internally balanced, 4340NT crank, 4340 rods with cap bolts, steel pins, forged pistons, and a suitable valvetrain) street/strip engine is around the limit for reliability. Production engine's are generally good up to around 3800-4000 feet per minute. Racing engines such Nextel Cup, F1, Indy, and even sportbikes have piston speeds exceeding 4800 feet per minute and maybe exceeding 5000.

Piston Speed in Feet per Minute = (stroke x 2 x rpm)/12

Figure 1
Calculating Maximum Safe RPM

Max. Safe RPM = Mean Piston Speed (ft/min) x 6
Divided by Stroke in Inches

Example for a budget aftermarket forged crank in a 4-inch stroke small-block Chevy:
4,800 x 6 = 7,200 rpm
4

Maximum Mean Piston Speeds for Above Formula:
Factory cast-iron cranks 3,750 ft/min
Aftermarket cast-steel cranks 4,500 ft/min
Factory forged cranks 4,600 ft/min
Budget aftermarket forged cranks 4,800 ft/min
Typical race aftermarket cranks 5,500 ft/min
High-dollar custom endurance race cranks 6,000 ft/min
ProStock/Mountain Motors 7,500 ft/min
Formula One 7,500+ ft/min
 

Effects of Long Rods


Pro:
»

Provides longer piston dwell time at & near TDC, which maintains a longer state of compression by keeping the chamber volume small. This has obvious benefits: better combustion, higher cylinder pressure after the first few degrees of rotation past TDC, and higher temperatures within the combustion chamber. This type of rod will produce very good mid to upper RPM torque.
»

The longer rod will reduce friction within the engine, due to the reduced angle which will place less stress at the thrust surface of the piston during combustion. These rods work well with numerically high gear ratios and lighter vehicles.
»

For the same total deck height, a longer rod will use a shorter (and therefore lighter) piston, and generally have a safer maximum RPM.





Con:
»

They do not promote good cylinder filling (volumetric efficiency) at low to moderate engine speeds due to reduced air flow velocity. After the first few degrees beyond TDC piston speed will increase in proportion to crank rotation, but will be biased by the connecting rod length. The piston will descend at a reduced rate and gain its maximum speed at a later point in the crankshaft’s rotation.
»

Longer rods have greater interference with the cylinder bottom & water jacket area, pan rails, pan, and camshaft - some combinations of stroke length & rod choice are not practical.
   To take advantage of the energy that occurs within the movement of a column of air, it is important to select manifold and port dimensions that will promote high velocity within both the intake and exhaust passages. Long runners and reduced inside diameter air passages work well with long rods.
   Camshaft selection must be carefully considered. Long duration cams will reduce the cylinder pressure dramatically during the closing period of the intake cycle.  

Effects of Short Rods


Pro:
»

Provides very good intake and exhaust velocities at low to moderate engine speeds causing the engine to produce good low end torque, mostly due to the higher vacuum at the beginning of the intake cycle. The faster piston movement away from TDC of the intake stroke provides more displacement under the valve at every point of crank rotation, increasing vacuum. High intake velocities also create a more homogenous (uniform) air/fuel mixture within the combustion chamber. This will produce greater power output due to this effect.
»

The increase in piston speed away from TDC on the power stroke causes the chamber volume to increase more rapidly than in a long-rod motor - this delays the point of maximum cylinder pressure for best effect with supercharger or turbo boost and/or nitrous oxide.
»

Cam timing (especially intake valve closing) can be more radical than in a long-rod motor.





Con:
»

Causes an increase in piston speed away from TDC which, at very high RPM, will out-run the flame front, causing a decrease in total cylinder pressure (Brake Mean Effective Pressure) at the end of the combustion cycle.
»

Due to the reduced dwell time of the piston at TDC the piston will descend at a faster rate with a reduction in cylinder pressure and temperature as compared to a long-rod motor. This will reduce total combustion.  

Rod Ratio vs. Intake Efficiency
    An “n” value of 1.75 is considered “ideal” by some respected engine builders, if the breathing is optimized for the design. Except for purpose-built racing engines, most other projects are compromises where 1.75 may not produce the best results. There will be instances where the choice of stroke or rod has not been made, but the intake pieces (carburetor, manifold, and head) have been selected. Some discretion exists here for making the rod and/or stroke choice compatible with the existing intake. The “n” value can be used to compensate for less-than-perfect match of intake parts to motor size & speed. The reverse is also possible: the lower end is done, but there are still choices for the top end. Again, the “n” value can be used as a correction factor to better “match” the intake to the lower end.
   The comments in the following table are not fixed rules, but general tendencies, and may be helpful in limiting the range of choices to those more likely to produce acceptable results. Rather than specify which variable will be changed in the lower end, “n” values will be used. Low “n” numbers (1.45 - 1.75) are produced by short rods in relation to the stroke. High “n” numbers (1.75 - 2.1) are produced by long rods in relation to the stroke.



Planning a 383 Motor
   This engine is generally overlooked in selecting a high-performance project. The motor has an excellent bore to stroke ratio: 1.26-1 (similar to 327” SBC, better than 340). The short stroke allows high RPM without destructive piston speed (7100 RPM = 4000 ft./min., the accepted “safe” limit for piston stress). The large bore permits big valves (2.14” intake, 1.81” exhaust).
   A potential method of increasing peak power is to substitute the longer 440 6.768” (LY) rods for the original “B” 6.358” rods on the original crank. This has the following effects:
»
Increases the rod ratio (“n”) from 1.884-1 to 2.005-1
»

Reduces the piston compression distance to about 1.525” for a useful weight savings
»

Slightly reduces piston acceleration
   This should allow an advantage in peak power. For a start in piston selection, take a look at the KB224 for BBC: flat top, CD = 1.52” (just below zero deck), and .990” pin for more weight savings and moderate cost. There may also be “possibles” for the 400 (4.34” bore), but not discovered yet. Ideas?  

Angle Limitation
   The angle of the rod at 90° ATDC is a good indication of how much stress the piston and cylinder wall will be subjected to with a specific rod/stroke selection (this is not the angle of maximum thrust, which occurs when the rod’ beam axis is at 90° to the crank throw or journal, typically between 70-76° ATDC; however, the math is easy to do). Angles beyond 17° (where the rod axis is 90° to the crank throw at 73° ATDC) promote excessive wear at the piston major thrust surface, and piston breakage could be the result. Before you purchase connecting rods that are shorter than previous or increase the stroke of the crank, calculate the new rod angle. High rod angles will require quality rods that have been checked for cracks and have quality (ARP, etc.) fasteners. Piston selection will be critical for the life expectation of the engine; maximum skirt length below the pin is desired.
Sine of Rod Angle = Stroke ÷ (Rod Length * 2)

(or)

Sine of Rod Angle = .5 ÷ R/S
#     To make your own calculations using the Microsoft Calculator (every Win95/98/00/ME has it): Double-click the “Calculator” icon to open it
# Click “View”, then “Scientific”
# Input the result from the formula above
# In the left margin of Calculator, look for the check-box that says “Inv” - check it
# Make sure the box marked “Degrees” (not Radians) is checked
# Click on “sin”
# The rod angle in degrees will show in the window



Rod Angle

“n” Ratio

Examples

Comments
13½°

2.142-1



High speed motor with small ports. Best breathing with small ports
14°  

2.067-1




14½°

1.997-1



Long rods for good breathing with small ports
15°  

1.932-1



Long rods to help breathing with small ports. Responds well to stroke increases (“n” value too large for intake port size)
15½°

1.871-1



Responds well to stroke increases (“n” value too large for intake port size)
16°  

1.814-1

Mopar 383/400

Approximate “ideal” compromise between stress & breathing (1.81-1)
16½°

1.760-1

Chevy 327

Good choice for motors with good breathing
17°  

1.710-1

Mopar 360
Ford 302, 351W, 460

”Safe” limit for thrust angle. Approaching practical limit for street motors
17½°

1.663-1



Approaching practical limit for street motors
18°  

1.618-1

Chevy BB 396/427

Approaching practical limit for street motors. Good power due to large intake port
18½°

1.576-1



Limited street use
19°  

1.536-1

Chevy BB 454

Good power due to large intake port
19½°

1.498-1



Not practical for street use due to short pistons
20°  

1.462-1

Chevy SB 400

Poor peak power. Longer rods are used in any serious application  

Cubic inches make torque.So why not put a 400 crank in a +.030 400 block and make a 406.The reasons I used 5.7(350)rods A. More plentiful than 400 rods B. Rod ratio causes excess loading on thrust side of piston-cylinder wall due to mechanical disadvantage. The lower the rod ratio the more torque wasted internally turning the engine. C. The most important reason for me, pistons for the 5.7 rod were 3 or 4 dollars cheaper apiece compared to the short rod units. Yea I had to clearance them to miss the cam,no big deal.I used a preformer intake and a CC 268 cam,couldn't tell much difference in this and the LS6 454 I put it in place of.