Wallace compression calculator?

[427dart]

Using that calculator to find cranking and dynamic compression ratios give me results that differ from actual readings.
Entering my actual engine build specs and using my Intake Closing Timing at .050 which is 49 degrees ABDC shows I should have 195 PSI on a warm engine which my actual gauge test show. This also claims a 9.1 Dynamic Comp. Ratio using my .050 Intake Close number.

They state to use the Cam Timing events at .006 or advertised duration numbers which my Lunati Cam card does not have.
Called Lunati and tech said the .006 Intake Close for my cam is 72 degrees and when I input that number my cranking compression numbers should be 155 with a 7.4 Dynamic Compress number.

So which is correct since my cranking compression when checked over the last 7 years has been in the 195-200 range?
Seems to run fine on pump 93 Octane with the E-10 around here.

[cgarb]

Is your cam degreed in to those specs on the cam card? If you advance or retard the cam that changes your intake closing point and will reflect that in your cranking numbers. Could be that the cam is advanced some and raising your cranking psi.

[427dart]

Is your cam degreed in to those specs on the cam card? If you advance or retard the cam that changes your intake closing point and will reflect that in your cranking numbers. Could be that the cam is advanced some and raising your cranking psi.

If I recall it's within a degree of recommended install using the multi slot crank gear. I put it together 8 years ago.
Now with a hyd. roller I'm sure some timing and lift might be missing during crank compression test vs. actual running maybe?

[DanE1]

I have been schooled that if you do not have the seat to seat numbers for your intake lobe, use the .050" numbers and add 15 degrees when using the calculator. That will get you close.

[randy331]

Learner's definition of DYNAMIC
1
[more dynamic; most dynamic]
a
: always active or changing
a dynamic city
a dynamic relationship
b
: having or showing a lot of energy
a dynamic [=enthusiastic] speaker
an exciting and dynamic performance
2

technical : of or relating to energy, motion, or physical force
the dynamic theory of heat
dynamic and potential energy


How is knowing the volume above the piston at IVC dynamic ?


Randy

[zums]

Learner's definition of DYNAMIC
1
[more dynamic; most dynamic]
a
: always active or changing
a dynamic city
a dynamic relationship
b
: having or showing a lot of energy
a dynamic [=enthusiastic] speaker
an exciting and dynamic performance
2

technical : of or relating to energy, motion, or physical force
the dynamic theory of heat
dynamic and potential energy


How is knowing the volume above the piston at IVC dynamic ?


Randy

Maybe it relates to what that skinny black kid from good times said "dynomite", lol, dynamic compession+useless data
Tom

[cgarb]

Like others have tried to say...your compression numbers are a real world solid number. How can a computer program simulate that? Wouldn't the speed of the starter, the differences in cylinder head and or intake flow change the compression number? So many variables, I wouldn't get upset if it don't match an internet calculator.

[Stan Weiss]

Using that calculator to find cranking and dynamic compression ratios give me results that differ from actual readings.
Entering my actual engine build specs and using my Intake Closing Timing at .050 which is 49 degrees ABDC shows I should have 195 PSI on a warm engine which my actual gauge test show. This also claims a 9.1 Dynamic Comp. Ratio using my .050 Intake Close number.

They state to use the Cam Timing events at .006 or advertised duration numbers which my Lunati Cam card does not have.
Called Lunati and tech said the .006 Intake Close for my cam is 72 degrees and when I input that number my cranking compression numbers should be 155 with a 7.4 Dynamic Compress number.

So which is correct since my cranking compression when checked over the last 7 years has been in the 195-200 range?
Seems to run fine on pump 93 Octane with the E-10 around here.

What are all of your engine specs?

Stan

[Stan Weiss]

SCR is fixed once you have built the engine DCR is not.Tighten or loosen the valve lash and DCR as well as cranking compression will change.

Stan

[427dart]

Not at all upset about the results but was just curious of what I found plugging in my specs. into the Calculator.
Maybe useful as a guide to get you in the right direction when picking a cam and compression combo.

Couldn't ask for a better running street type performance engine!

[tjs44]

Stan,what do you do with a hyd cam?I had a engine on Big Joes dyno,455,hyd roller cam,9.5 CR iron headed pontiac.Put the cam in on Comps spec.Pumped over 190and from experience with our CA piss 91 gas could be a issue.Had Joe pull the cover and retard the cam 3 degrees,pumped 180 and made a touch more HP.Tom

[David Redszus]

Using that calculator to find cranking and dynamic compression ratios give me results that differ from actual readings.

Dynamic CR (or as I prefer, Trapped CR), is merely a ratio resulting from chamber volume, displacement, and IVC.

In order to obtain compression pressure, and more importantly compression temperature, we need to know the temperature of the inlet air and pressure at IVC, and the polytropic air constant. Now an accurate prediction can be made.

However, when compared to actual pressure measurement, we have not considered ring and valve leakage and carbon deposits. In a running engine we also must consider resonant inlet pressures. If we are measuring dry air without fuel, our results will be different than if fuel is included due to a change in the polytropic air constant and inlet charge cooling.

Either we measure our contribution variables accurately or we accept measured approximations.

[Stan Weiss]

Stan,what do you do with a hyd cam?I had a engine on Big Joes dyno,455,hyd roller cam,9.5 CR iron headed pontiac.Put the cam in on Comps spec.Pumped over 190and from experience with our CA piss 91 gas could be a issue.Had Joe pull the cover and retard the cam 3 degrees,pumped 180 and made a touch more HP.Tom

Tom,
While tighten the valve lash and retarding the cam will both move the IVC point and reduce cranking compression that does not mean that the results are the same. Retarding the cam moves all 4 valve events but leave the overlap the same. Tightening the valve lash also moves IVO and increases overlap and also will increase by a small amount the area under the intake lift curve.

Stan

[RamblerRebel6]

<r><QUOTE author="427dart" post_id="746185" time="1529343943" user_id="25831"><s>

</s>
Using that calculator to find cranking and dynamic compression ratios give me results that differ from actual readings.<br/>
Entering my actual engine build specs and using my Intake Closing Timing at .050 which is 49 degrees ABDC shows I should have 195 PSI on a warm engine which my actual gauge test show. This also claims a 9.1 Dynamic Comp. Ratio using my .050 Intake Close number.<br/>
<br/>
They state to use the Cam Timing events at .006 or advertised duration numbers which my Lunati Cam card does not have.<br/>
Called Lunati and tech said the .006 Intake Close for my cam is 72 degrees and when I input that number my cranking compression numbers should be 155 with a 7.4 Dynamic Compress number.<br/>
<br/>
So which is correct since my cranking compression when checked over the last 7 years has been in the 195-200 range?<br/>
Seems to run fine on pump 93 Octane with the E-10 around here.
<e>

</e></QUOTE>

Unfortunately, when it comes to the number of methods which try to get an accurate value for DCR based only on a handful of data, the immortal words of Thomas Edison ring true: "I have not failed. I've just found 10,000 ways that don't work."

[David Redszus]

<r><QUOTE author="427dart" post_id="746185" time="1529343943" user_id="25831"><s>

</s>
Using that calculator to find cranking and dynamic compression ratios give me results that differ from actual readings.<br/>
Entering my actual engine build specs and using my Intake Closing Timing at .050 which is 49 degrees ABDC shows I should have 195 PSI on a warm engine which my actual gauge test show. This also claims a 9.1 Dynamic Comp. Ratio using my .050 Intake Close number.<br/>
<br/>
They state to use the Cam Timing events at .006 or advertised duration numbers which my Lunati Cam card does not have.<br/>
Called Lunati and tech said the .006 Intake Close for my cam is 72 degrees and when I input that number my cranking compression numbers should be 155 with a 7.4 Dynamic Compress number.<br/>
<br/>
So which is correct since my cranking compression when checked over the last 7 years has been in the 195-200 range?<br/>
Seems to run fine on pump 93 Octane with the E-10 around here.
<e>

</e></QUOTE>

Unfortunately, when it comes to the number of methods which try to get an accurate value for DCR based only on a handful of data, the immortal words of Thomas Edison ring true: "I have not failed. I've just found 10,000 ways that don't work."

Don't be discouraged. We have never lost, we've just run out of time.
If you have the following data, DCR can be determined reasonably accurately.
Bore
Stroke
ConRod
Chamber volume including piston and crevices
IVC using seat to seat numbers.
The above are all that is necessary to calculate DCR.

To obtain compression pressure and temperature we also need:
inlet temp at IVC
inlet pressure at IVC
polytropic constant (dry air or fuel vapors)
now we can plot compression temp and pressure at every crank angle.