Geometry Vs Engine Breathing P M V Subbarao Professor Mechanical Engineering Department Its not engine Volume, but Mass of air Decides the Power Output ….

Improper Breathing in an Engine Creates A Pumping Cycle

Anatomy of Intake System

The Intake System and Instantaneous Mass of air in Engine Cylinder

Piston Speed The speed of the piston Piston Displacement :

Better than using rotational speed (RPM) Gas flow velocities in the intake manifold and cylinder all scale with mean piston speed. Should be between 8 and 16 m/sec. Lower end is typical of large industrial engines High end is typical of automotive engines Mean Piston Speed

Cylinder Geometry Vs Breathing Issues The smaller bore reduces the area available for valves in the cylinder head, requiring them to be smaller or fewer in number. These factors favor lower engine speeds, under-square engines are most often tuned to develop peak torque at relatively low speeds. An under-square engine will typically be more compact in the directions perpendicular to piston travel but larger in the direction parallel to piston travel. An over-square engine allows for more and larger valves in the head of the cylinder.

Effect of Rod Ratio on Piston Speed

Rod Ratio vs. Intake Efficiency A “R” 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. The “R” value can be used as a correction factor to better “match” the intake to the manifold. Low “R” numbers ( ) are produced by short rods in relation to the stroke. High “R” numbers ( ) are produced by long rods in relation to the stroke.

SHORT ROD : Intake Stroke Intake Stroke -- Short rod spends less time near TDC and will suck harder on the cylinder head from 10 o ATDC to 90 o ATDC the early part of the stroke. Will not suck as hard from 90 o ATDC to BDC as a long rod. Will require a better cylinder head than long rod to produce same peak HP. Will require stronger wrist pins, piston pin bosses, and connecting rods than a long rod. Short rod spends more time at the bottom which may reduce intake charge being pumped back out intake tract as valve closes. May permit longer intake lobe and/or later intake closing than a long rod.

LONG ROD Intake Stroke -- will draw harder on cylinder head from 90 o ATDC to BDC. Compression Stroke -- Piston travels from BDC to 90 o BTDC faster than short rod. Goes slower from 90 o BTDC to TDC--may change ignition timing requirement versus short rod as piston spends more time at top.

Rod Angle “R” Ratio ExamplesComments 13½° High speed motor with small ports. Best breathing with small ports 14° ½° Long rods for good breathing with small ports 15° Long rods to help breathing with small ports. Responds well to stroke increases (“n” value too large for intake port size) 15½° Responds well to stroke increases (“n” value too large for intake port size) 16° Mopar 383/400Approximate “ideal” compromise between stress & breathing (1.81-1) 16½° Chevy 327Good choice for motors with good breathing

17° Mopar 360 Ford 302, 351W, 460 ”Safe” limit for thrust angle. Approaching practical limit for street motors 17½° Approaching practical limit for street motors 18° Chevy BB 396/427 Approaching practical limit for street motors. Good power due to large intake port 18½° Limited street use 19° Chevy BB 454Good power due to large intake port 19½° Not practical for street use due to short pistons 20° Chevy SB 400Poor peak power. Longer rods are used in any serious application

The Valve and its Inlet Conditions