Rods

[Gary Derian]

    With no cylinder pressure, the greatest rod/piston stress is at TCD.
High rpm, long stroke and short rods make this worse.  On the power stroke,
gas pressure pushes the piston down and relieves the stress.  Inertia loads
increase with the square of rpm.  Gas loads are proportional to bmep.  What
Corky meant is a good turbo engine at mid rpm (3,000 to 4,000) has less
rod/piston stress than trying to get the same power by turning an atmo
engine at 6,000+.  My comment had to do with high boost at low rpm (1,500)
in which case the gas loads are much higher than the inertia load and cause
a high compressive load on the rods.  This will increase fatigue, squeeze
oil out of bearings and can cause the rods to buckle.

Gary Derian <gderian at cybergate.net>


I wrote:
>>    Don't forget that too much boost at low RPM will cause great stress on
>>the connecting rods and crankshaft.  Boosted motors need the inertia of
the
>>pistons to balance out some of the gas loads.  My question is,  why do you
>>want to have all of this complication?


Fred wrote:
>Unless I misunderstood something, I was under the impression from Corky
>Bell's Turbo book, that higher boosts at lower RPMs caused less stress than
>less boost at higher RPMs, because most of the rod stress was from the
>piston being jerked down immediately after the exhaust cycle.