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12) IVC/BP (In Hg) - Graph change in Intake Valve Closing for change in Barometric
Pressure (In Hg) with a fixed Cranking Compression Pressure. Using Bore, Rod Length,
Stroke, Compression Ratio.
13) IVC/CR CGP - Graph Intake Valve Closing against Compression Ratio with a fixed
Cranking Compression Pressure. Using Bore, Rod Length, Stroke, Barometric Pressure
(In Hg), Wrist Pin Offset and Cranking Compression Pressure.
14) CGP - Graph change in Cranking Compression Pressure against change Intake Valve
Closing against Compression Ratio with a fixed Compression Ratio. Using Bore, Rod
Length, Stroke, Barometric Pressure (In Hg), and Wrist Pin Offset.
15) Cylinder Pressure - Graph change in Cylinder Pressure (Decay - without anymore burn
or any heat loss and no EVO) against Crank Rotational Angle Using Bore, Rod Length,
Stroke, Compression Ratio, Crank Angle (Degrees ATDC of Max Cylinder Pressure),
Compression Gauge (Max Cylinder Pressure) and Wrist Pin Offset.
16) Graph First will set up the X-Axis and Y-Axis ranges and Produce a graph based on
the selected option.
17) Graph +1 will add another Graph line to the present Graph; this will produce good
results if the same option is selected.
18) Calculate Intake Valve Close ABDC using Bore, Stroke, Rod Length, Compression
Ratio, Dynamic CR and Wrist Pin Offset.
19) Calculate Intake Valve Close ABDC using Bore, Stroke, Rod Length, Compression
Ratio, Barometric Pressure, Temperature, Humidity and Compression Gauge Reading.
Note: Intake Valve Closing is when the valve actually closes. Compression of the air/fuel mixture cannot start until the intake
valve is closed Lets take a (ex 1) SBC using 1.5:1 ratio rocker arms. That has a valve lash of .030" for the intake (solid lifters).
That means at .020” of cam lift the valve closes. Lets take a (ex 2) BBC using 1.7:1 ratio rocker arms. That has a valve lash of
.017" for the intake (solid lifters). That means at .010” of cam lift the valve closes. Add an extra .004 for valve train flex. Using
a degree wheel you need to find where the intake lifter measures .014” lifter on the closing ramp. Hydraulic lifters are another
deal and preload, spring seat pressure and oil pressure all can come into play, for general use .004 to .006 cam lift is a good
starting point. If you change the Intake centerline or valve lash you have to recalculate your dynamic compression ratio. Your
Dynamic Compression Ratio (DCR) can never be higher than your Static Compression Ratio (SRC). But in a racing engine your
DCR is generally much lower than SCR. Like the SCR, the DCR, is fixed when the engine is built. But unlike the SCR the DCR
can change during the operation of the engine. Thing like pushrod flex, and timing belt stretch can alter the cam timing events
and that will change your DRC. For street and street/strip motors a DCR in the range of 8-8.5:1 is normal. This should work
well with pump gas and yet not have any detonation problems.
I use dynamic compression ratio to calculate cranking pressure. If I have an engine at sea level and than take it up to Denver
the DCR does not change. Let's say I have an engine with 7.6:1 DCR and at 70 degrees no humidity and Barometric Pressure
of 29.92 it cranks 186.4 psi. Then I change locations and have 25.95 Barometric Pressure it will only crank 161.5 psi.
Dynamic compression is the actual physical compression that takes place after the intake valve closes and this generates the
cranking pressure. NOTE - this is all happening at starter motor RPM's
Roughly this is how I do it.
1) Calculate displacement of one cylinder
2) From CR and displacement of one cylinder #1 above I calculate volume above piston at TDC
3) From IVC I calculate piston position and then dynamic stroke
4) From dynamic stroke #3, bore and volume above piston ATDC calculated in #2 I calculate Dynamic compression ratio
5) From Dynamic compression ratio #4 and atmospheric variables I calculate cranking pressure.
52 - CARFOR Performance Software by Stan Weiss / World Wide Enterprises