Cam Timing vs. Compression Analysis 



“Dynamic Compression Ratio” (DCR) is offered (elsewhere) as a method of approximating how much power will be lost by extended intake closure (i.e., cylinder pressure is reduced). DCR is very useful in predicting what octane is needed at lower speeds to help setting idle (initial) spark vs. advance curve length, stall speed, &c. However, this is only partially accurate due to several errors and misconceptions. 1. The actual power loss is only present below the points where both full capture (where intake reversion ABDC stops) and positive flowthrough at overlap occur (although these may not be close together, depending on engine design). Above that point power is increased (over the milder cam), either by improved volumetric efficiency (and cylinder pressure, since a larger percentage is trapped in the cylinder), higher RPM reached @ the same efficiency level, or both. 2. The power loss is not solely due to reduced cylinder pressure (which is the output of the Kelley, RSR &c. DCR calculators), since the lower pressure is also acting through a shorter effective stroke (measured from intake valve closure ABDC) and therefore suffers from two reductions. My own method factors in the effective cylinder displacement as well, for a closer approximate at how much power is developed at low speed. 3. DCR is widely considered to be an accurate barometer of knock resistance (e.g., “up to 8:1 DCR can be used with XX octane”, &c.). This is not true, and not safe, since (in a high compression motor with its DCR reduced by late intake closure) after the early lowpressure period expires (at the beginning of the torque curve) actual combustion pressure will be at least as high with the bigger cam than it was previously  even though the DCR is lower, and therefore suggests that lower octane is safe. However, in a motor with 14:1 static CR the gas pressure at its torque peak is not at all reduced by a very late intake closing point, although the (lower) DCR may indicate that 92 octane &c. is sufficient. Since the knock will only occur at high speeds it may not be audible, and will reduce power (lower MPH) even if no damage can be detected. In my opinion, the power sometimes gained by retarding spark in high gear is actually an attempt to recover some of this loss  but would be better served by reducing the static ratio slightly. DCR is a curve or slope of cylinder (not combustion) pressure, with Position 0 (the absolute low end) at cranking speed, then a small rise to idle speed, then another rise to the capture point &c. After this point (and especially near the torque peak) the static CR becomes more important, since approximately the full stroke length and nominal swept cylinder volume (or more, depending on VE) is captured and compressed at (below, at, or above) the nominal static ratio, regardless of the intake closing point. Any cam will determine the “slope” (or rate of rise) of the DCR curve. A longduration cam with its attendant late intake closing point will have a high degree of rise, a mild cam less, &c. A longer cam will also extend (stretch) the range of RPM that the slope covers, sometimes over several thousand more RPM. The static ratio determines the height of the cylinder pressure line at Position 0 (cranking speed). With high static ratio the entire curve is higher, with the curve’s upward intensity being governed by the intake closing point. It’s possible to design a DCR that looks promising, but will not provide any more power, by assuming that there is no limit to either static ratio or intake closure  and, of course, neither is true. Some motors cannot turn fast enough (due to stroke length, weak valve gear, high reciprocating weight, &c.) to reach their capture point if the intake closure is too late, and will produce more power with more conservative cam timing. A motor with limited static ratio (flathead) must conserve cylinder pressure by limiting intake closure for the same reasons. Another error in use of DCR calculations for lowspeed power prediction lies in the fact that a smaller volume of mixture being compressed to a higher ratio. Even though the pressure gauge reading taken during cranking or idling is higher, the total of cylinder pressure times the actual mixture volume captured may still be lower (compared to the original milder cam and moderate compression ratio). To sum up: DCR a useful tool, but widely perceived to be of greater worth than can be supported by physics. As a note: the gas present in the combustion chamber @ TDC is presumed to be noncombustible exhaust gas remaining from the previous cycle, and is therefore not included in the mixture volume for our purposes. At high engine speeds (under certain conditions) overlap does cause this remainder to be partially combustible, but this is not true at cranking to moderate engine speed. In short, a cam change can’t be “cured” completely by raising the compression ratio, but it’s still an excellent idea. The “V/P (“Volume/Pressure”) Index” , which is the subject of this Tech Paper, will not predict maximum torque or power, but still provides useful information about conditions present in the motor at cranking speed. These same conditions generally affects lowspeed response & flexibility, knockresistance, &c. for spark advance settings, torque converter stall speed selection, axle ratio choices, &c. Another useful purpose is to anticipate the effect of higher elevation on cylinder pressure. Perform a calculation (as described below) first, then another substituting a lower value than 14.7 psi for “Atmospheric Pressure”. Compare the results to estimate how much adjustment to the nominal compression ratio is needed to (partially) compensate for the lower air density. Click here for a calculation based on differences in atmospheric pressure: V/P Index A reasonable estimate of the relative effects of
compression ratio, rod ratio, and intake valve closing point at cranking speed can be made
by use of a simple formula. Although a “power” (exponent) is used in the
formula, most pocket and online calculators have a function to permit
a fractional exponent to be entered (use Microsoft “Calculator” in W2K, &c. It has this feature  to use it, select “Scientific” view). Try it out  it’s
not that difficult!
 
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Atmospheric pressure = 14.7 psi (pounds per square inch, or 29.92 inches of mercury). This is a constant for our purposes here at sea level, but should be adjusted downward for extreme elevations.
 
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Intake valve closing point (ABDC). At cranking speed, intake flow reversal still takes place until the valve is actually closed, so the “paper duration” (nominal) closing point is used for calculations. For intake closing points of common MP factory camshafts, click here: . By the way, at operating speeds the general consensus is that the intake valve is not effectively open until valve lift reaches .050”.  
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Theoretical compression ratio. I have found that (in general) manufacturer’s nominal compression ratios are a bit optimistic, and that for purposes of calculation about 5 to 10% should be added to the chamber volume as a correction for loose factory tolerances. For example: a bigblock 906 head is nominally 78.5cc. In practice, the volume is generally larger, up to 85cc. The calculations made below assume accurate chamber volume  if you're not certain of your actual chamber volume and use your original (nominal) compression ratio when calculating, your actual cranking pressure will be a bit lower than the figures resulting from the formula. Click here to review the factors, as described, supra: .  
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Stroke remaining at intake valve closing point, which is determined by the engine’s stroke, and rod to stroke ratio (“n”). This is calculated by dividing the rod length by the stroke length. To find the rod ratio of common Mopar engines, click here: .  
Results:  
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Corrected (true) compression ratio.  
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Cranking pressure, absolute (in psi, zero elevation std.).  
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Gauge pressure (in psi, zero elevation std.).  
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Corrected cylinder volume (cylinder displacement trapped when the intake valve closes).  
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V/P Index  
The V/P Index is a mathematicallyderived figure, the product of the corrected volume and cranking pressure. It’s a useful barometer of the motor’s lowspeed power, and may be solved with new variables for a “pre & post” analysis when modifications are contemplated. Anyone planning a cam or compression ratio change would definitely want to know how much the lowspeed flexibility would be affected. Some useful insight may also be derived as to the motor’s tolerance of pumpquality gas (ping & knock resistance). The results of these comparisons are generally quite surprising!To calculate the V/P Index for a specific motor, you’ll need the exact piston position at all relevant crank positions from BDC to about 90° ABDC, the compression ratio, and the intake valve closing point.  
If you'd like to calculate piston positions for your own motor, the following formula has been contributed by a reader:  
SE = (S ÷ 2) + R + ((S ÷ 2) × cosA)  SQRT ((R^{2})  ((S ÷ 2) × sinA)^{2})  
where “SE” is the effective stroke, “S” is the nominal (full) stroke, “R” is the rod length, “A” is the crankshaft angle in degrees ABDC from 0 to 90°, and “SQRT” (or “SQR”) is the square root function.
 
To make a V/P Index calculation, find your intake closing point from the cam manufacturer’s data. For common MP stock & performance camshafts, click here: . Locate this figure as the crank position, and take the stroke from that position to calculate the “effective”
cylinder volume (“VE”), which is amount trapped
by the closed intake valve; which is always less than the nominal volume (“VN”).
Using this, calculate the effective compression ratio (“CRE”);
also lower  the combustion chamber volume is unchanged, but the cylinder
volume is less. At cranking speed, the absolute cranking pressure (“CP”)
is a function of the 1.2 power of the effective compression ratio (i.e.,
for 8:1 compression ratio, use 8^1.2)
times atmospheric pressure (14.7 psi @ sea level, &c.). This adjustment (1.2 power) is a polytropic value used in preference to the traditional adiabatic value (1.4) for the ratio of variable heats for air and similar gases at the temperatures present. This compensates
for the temperature rise caused by compression, as well as heat lost to the cylinder. 1.2 is not accurate in all cases, since the amount of heat lost will vary among engines based on design, size and materials used, but provides useful results for purposes of comparison.

Symbol 
Meaning 
Definition or Calculation 
B 
Bore 
Piston diameter, in inches 
S 
Stroke 
Full (nominal) crankshaft stroke; TDC to BDC measurement (180° rotation), in inches 
SE 
Stroke, Effective 
Stroke measured from intake valve closing point (less than 180°) to TDC [always less than “S”, supra] 
VN 
Cylinder Volume, Nominal 
B^{2} × S × .7854 (1 cylinder) 
VE 
Cylinder Volume, Effective 
B^{2} × SE × .7854 (1 cylinder) [always less than “VN”, supra] 
VC 
Chamber Volume 
VC = VN ÷ (CRN  1). Total volume (1 chamber) above piston @ TDC, in inches. 
N 
Number of cylinders 
(4, 6, 8, 10, &c.) 
AP 
Atmospheric Pressure 
14.7 psi @ sea level (zero elevation); use the correct lower figure for higher elevations 
CRN 
Compression Ratio, Nominal 
CRN = (VN + VC) ÷ VC 
CRE 
Compression Ratio, Effective 
CRE = (VE + VC) ÷ VC [always less than “CRN”, supra] 
CP 
Cranking Pressure (absolute) 
CP = (CRE^{1.2} × AP) 
GP 
Gauge Pressure 
GP = (CRE^{1.2} × AP)  AP. To predict gauge pressure, subtract atmospheric pressure (14.7 psi @ sea level, &c.) from absolute pressure. See supra: . 
V/P 
Volume/Pressure Index 
V/P = CP × VE × N × .3% (.3% or .003 is a correction factor to return a useful 2 digit number roughly proportionate to torque) 
Note 1: Mark Juric, a reader & patron has generously supplied a Microsoft Office 2000 Excel spreadsheet for easily solving these equations. To download a copy, click here:  
Note 2: RB Racing has a similar calculator (and many others) online, click here for a look: 
For a baseline on our test 340 motor, let’s use 10:1 compression (4.72” or 77.4cc chamber volume), and locate the intake
closing point of the original manual transmission cam (P4452782; 268°
intake) at 66° ABDC. The effective stroke at this point is 2.52”, for an effective volume of 258.4” (for all 8 cylinders). 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.

We have a baseline on our test 340 motor in sea level atmospheric pressure of 14.7 psi (supra). The usual reduction for lower air density is approximately .49 lbs/sq.” (1 In. hg) per 1000 foot elevation above seal level (consult your local weather station or barometer). Let’s do a calculation with elevation at 5,000 feet: atmospheric pressure is reduced to 12.24 psi The same effective compression ratio as supra (7.84:1) now gives a reduced effective pressure of 144
psi absolute (132 psi gauge pressure). The V/P Index is 224, a 16% drop.
This explains the extreme loss in power caused by the “thin” air at high elevations.

Let’s try a really big LA motor: 4.100” bore & 4.00” stroke = 422”. For maximum power, let’s set the compression ratio at 12:1 (4.8” or 78.7cc chamber). Let’s use the MP 312° mechanical cam (P4120657), with an intake closing point of 80° ABDC giving an effective stroke of 2.673”, and an effective volume of 282.3”. The effective compression ratio is 8.35:1. The cranking pressure is 187 psi absolute (172 psi gauge pressure). The V/P Index is 317. As you see, where the cam is this big, even high static compression ratio and a large displacement don’t produce results at cranking speed. 
How about a “Mega” RB motor: 4.50” bore & 4.50” stroke = 573”. For pump gas, let’s set the compression ratio at 10:1 (7.95” or 130.3cc chamber). Let’s use the MP 296° mechanical cam (P4120661), with an intake closing point of 76° ABDC giving an effective stroke of 3.156”, and an effective volume of 401.6”. The effective compression ratio is 7.31:1. The cranking pressure is 159 psi absolute (145 psi gauge pressure). The V/P Index is 385. Again, the late intake timing reduces the effective pressure quite a bit, but the large size makes up for it. Looks like fun! 
How about a comparison of different compression ratios in the 440 motor, using the 292° .509” hydraulic cam (P4120237). Bore & stroke are 4.32” by 3.75”. The 509 cam closes the intake valve @ 74° ABDC, giving an effective stroke of 2.636” and an effective volume of only 309.1”.

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