dibbons
Well-Known Member
This is kinda technical and I am a novice, so keep that in mind as you read my comments. Our Mopar brother AJ/FormS mentioned a website in another thread that explains how to calculate the volume/pressure index which can be used to compare different engine combinations by changing camshafts and/or compression ratios:
Cam Timing vs. Compression Analysis
I ran some figures and came up with some interesting results (assuming my math was correct).
According to the article, a factory stock 340 running 10:1 compression:
"The effective compression ratio is 7.84:1. The effective pressure is 173 psi absolute (159 psi gauge pressure). The V/P Index is 269. This explains the good torque on these motors, and why they require premium gas."
Then I calculated a de-stroked 340 (305 cubic inches) using a 340 profile camshaft with 10:1 stock pistons and 12:1 domed pistons:
stock pistons: effective compression 7.054:1, absolute cranking pressure 153 psi (140 psi on gauge). The V/P index is 104. (Note: Static compression in the 305 is only 9:1 with the factory 10:1 pistons because de-stroking caused a one point loss in compression)
domed pistons (+0.30): effective compression 8.46:1, absolute cranking pressure 190 psi (175 psi on gauge). The V/P index is 130. (Note: Static compression in the 305 is only 11:1 with the 12:1 domed pistons because de-stroking caused a one point loss in compression)
Then I calculated a 318 running 8.6:1 compression/stock camshaft and a 318 running 8.6:1 compression/340 camshaft:
stock camshaft: effective compression 7.54:1, absolute cranking pressure 166 psi (150 psi on gauge). The V/P index is 136.
340 profile camshaft: effective compression 6.78:1, absolute cranking pressure 146 psi (130 psi on gauge). The V/P index is 120.
What does all this mean? It was a fun exercise. Higher compression increases low end torque and hotter cams and de-stroking diminish low end torque. But we already knew that.
The 305 will probably need the domed pistons to match the low end torque of the stock 318. The stock 340 is king.
Cam Timing vs. Compression Analysis
I ran some figures and came up with some interesting results (assuming my math was correct).
According to the article, a factory stock 340 running 10:1 compression:
"The effective compression ratio is 7.84:1. The effective pressure is 173 psi absolute (159 psi gauge pressure). The V/P Index is 269. This explains the good torque on these motors, and why they require premium gas."
Then I calculated a de-stroked 340 (305 cubic inches) using a 340 profile camshaft with 10:1 stock pistons and 12:1 domed pistons:
stock pistons: effective compression 7.054:1, absolute cranking pressure 153 psi (140 psi on gauge). The V/P index is 104. (Note: Static compression in the 305 is only 9:1 with the factory 10:1 pistons because de-stroking caused a one point loss in compression)
domed pistons (+0.30): effective compression 8.46:1, absolute cranking pressure 190 psi (175 psi on gauge). The V/P index is 130. (Note: Static compression in the 305 is only 11:1 with the 12:1 domed pistons because de-stroking caused a one point loss in compression)
Then I calculated a 318 running 8.6:1 compression/stock camshaft and a 318 running 8.6:1 compression/340 camshaft:
stock camshaft: effective compression 7.54:1, absolute cranking pressure 166 psi (150 psi on gauge). The V/P index is 136.
340 profile camshaft: effective compression 6.78:1, absolute cranking pressure 146 psi (130 psi on gauge). The V/P index is 120.
What does all this mean? It was a fun exercise. Higher compression increases low end torque and hotter cams and de-stroking diminish low end torque. But we already knew that.
The 305 will probably need the domed pistons to match the low end torque of the stock 318. The stock 340 is king.
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