The document analyzes the stress on piston and connecting rod components from increasing the compression ratio on a small block V8 engine from 9:1 to 11:1. Finite element analysis showed that at a 9:1 ratio the maximum stress was 45,810 psi, while an 11:1 ratio would cause maximum stress of 105,400 psi, below the alloy steel safety factor of 90,000 psi. The conclusion is that increasing the ratio to 11:1 would likely cause cracking and failure of the connecting rod.

1. Piston Compression Ratio Stress
Analysis
08/14/17 rev B
Don Blanchet
3B Associates
dwb3298@verizon.net
3.4 inch dia

2. Study
 A local hot rod friend wants to know if
he can safely plane the heads on hi
small block V8 engine.
 He wants to raise the compression
ratio from 9/1 to 11/1.
 Will the piston and connecting rod
withstand the increased preesure?

3. Solidworks FEA Model
Piston top pressure
750 psi at
Crank shaft position
Top Dead Center
For the baseline 9/1
compression ratio
Connecting rod
Fixed at journal
bearing
900,000
Degrees
Of
freedom

4. Results:
9/1
compression
ratio
max stress
45,810 psi
in the
connecting rod
Max stress
In
Connecting
rod

5. Results:
11/1
compression
ratio
max stress
105,400 psi
in connecting rod

6. Conclusion
 Raising the compression ratio from 9/1
to 11/1 on this design will eventually
cause cracking and fatigue failure at
the base of the connecting rod.
 Alloy steel factor of safety
90,000/105,400 = 0.86
 This modification is high risk.