Dynamic compression ratio calculator

[michael hurd]

Anyone used one? From what I have read, ideal max dynamic compression ratio would be 8.0-8.25, 8.5 borderline for a street engine on pump gas. ( no computer controlled ignition, carb )

For the heck of it, I plugged in the numbers for an L30 5.0L

9.4-1 compression,
3.736 bore
3.48 stroke
5.7 rod

Entering the calculated intake closing ABDC point for the '395' or ramjet camshaft ( not sure if there is advance ground in or not )

This is the result I get:

Camshaft, Rod Length, Boost and Altitude Correction to Compression
Static compression ratio of 9.4:1.
Effective stroke is 2.50 inches.
Your dynamic compression ratio is 6.83:1 .
Your dynamic cranking pressure is 129.94 PSI.
Your effective boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 0 PSI is 6.83 :1.
V/P (Volume to Pressure Index) is 85


With the stock cam ( specs from Sealed Power stock replacement listed at Rock Auto )

Camshaft, Rod Length, Boost and Altitude Correction to Compression
Static compression ratio of 9.4:1.
Effective stroke is 3.35 inches.
Your dynamic compression ratio is 8.89:1 .
Your dynamic cranking pressure is 183.23 PSI.
Your effective boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 0 PSI is 8.89 :1.
V/P (Volume to Pressure Index) is 161

 

[Jrgunn5150]

Well that's interesting lol.

 

[michael hurd]

Same L30, Comp Cams XFI 08-465-8 Grind number 260XFI HR13

Camshaft, Rod Length, Boost and Altitude Correction to Compression
Static compression ratio of 9.4:1.
Effective stroke is 2.83 inches.
Your dynamic compression ratio is 7.63:1 .
Your dynamic cranking pressure is 150.31 PSI.
Your effective boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 0 PSI is 7.63 :1.
V/P (Volume to Pressure Index) is 112

Same, Comp Cams Xtreme 4x4 08-411-8 Grind number X4260HR

Camshaft, Rod Length, Boost and Altitude Correction to Compression
Static compression ratio of 9.4:1.
Effective stroke is 2.88 inches.
Your dynamic compression ratio is 7.75:1 .
Your dynamic cranking pressure is 153.41 PSI.
Your effective boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 0 PSI is 7.75 :1.
V/P (Volume to Pressure Index) is 116

 

[michael hurd]

Comp Cams 08-302-8 Grind number 264HR-12

Camshaft, Rod Length, Boost and Altitude Correction to Compression
Static compression ratio of 9.4:1.
Effective stroke is 3.28 inches.
Your dynamic compression ratio is 8.72:1 .
Your dynamic cranking pressure is 178.73 PSI.
Your effective boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 0 PSI is 8.72 :1.
V/P (Volume to Pressure Index) is 154

Comp Cams 08-300-8 Grind number 262HR-12

Camshaft, Rod Length, Boost and Altitude Correction to Compression
Static compression ratio of 9.4:1.
Effective stroke is 3.30 inches.
Your dynamic compression ratio is 8.77:1 .
Your dynamic cranking pressure is 180.05 PSI.
Your effective boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 0 PSI is 8.77 :1.
V/P (Volume to Pressure Index) is 156

 

[michael hurd]

Comp Cams 07-464-8 Grind number 252XFI HR13

Camshaft, Rod Length, Boost and Altitude Correction to Compression
Static compression ratio of 9.4:1.
Effective stroke is 2.92 inches.
Your dynamic compression ratio is 7.85:1 .
Your dynamic cranking pressure is 155.99 PSI.
Your effective boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 0 PSI is 7.85 :1.
V/P (Volume to Pressure Index) is 120

Comp Cams 08-304-8 Grind number 266-HR14

Camshaft, Rod Length, Boost and Altitude Correction to Compression
Static compression ratio of 9.4:1.
Effective stroke is 3.25 inches.
Your dynamic compression ratio is 8.64:1 .
Your dynamic cranking pressure is 176.62 PSI.
Your effective boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 0 PSI is 8.64 :1.
V/P (Volume to Pressure Index) is 151

 

[michael hurd]

OEM cam = 0.2763 intake lift x 1.5 ( optimistic ) rocker ratio = .41445 inch lift.
0.2855 exhaust lift x 1.5 rocker ratio = .42825 inch lift. Lobe separation angle of 111 degrees
Dynamic compression of 8.89


Comp Cams 08-300-8 262-HR12
0.300 intake lift x 1.5 = .450 lift
0.320 exhaust lift x 1.5 = .480 lift Lobe separation angle of 112 degrees
Dynamic compression of 8.77

Obviously, changing to this cam would require a spring change with more seat and open pressure. It would be interesting to compare the ramp profiles and 'area under the curve'.

 

[Jrgunn5150]

I'm interested to know what the direct effect of raising dynamic compression is? I can't seem to find any good info on choosing a cam based on that.

 

[michael hurd]

http://victorylibrary.com/mopar/cam-tech-c.htm
http://victorylibrary.com/mopar/cam-tech-c.htm

 

[michael hurd]

http://www.superchevy.com/how-to/81679-tuning-engine-compression/

 

[Jrgunn5150]

I understand what it is and how it works, what I don't understand is what looking at it as a deciding factor shows us.

An engine with a dynamic compression ratio of 9:1 is better than one with a ration of 7:1 how?