Wallace compression calculator?

[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."

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.

Thanks for the encouragement. I think the 10001st might be the charm.
I've used Pat Kelly's http://members.uia.net/pkelley2/DynamicCR.html] calculations for DCR in a BASIC program I've written. (That's right, BASIC. I know, I'm a relic! )
When it comes to cranking compression, I just can't seem to get a solid value for gamma (the ratio of specific heats) without fudging.
The formula I have is:

Cranking compression = 14.73 (or local air pressure in lb per sq.in.) x DCR ^ gamma (dynamic CR to the gamma-th power)

I've seen (or derived) the following values for gamma: 1.20 / 1.27 / 1.30 / 1.33. Not a big spread numerically, but it causes a pretty significant margin of error when you take the lowest gamma value and the highest. For instance, if I take a DCR of 8:1, using 8 ^ 1.33 x 14.73 I get approx. 233.6 lb/in^2. With a gamma value of 1.2, I get about 178.2. So about a 24% difference between upper and lower values, which I don't think is acceptable. And therein lies my problem.

[David Redszus]

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

Thanks for the encouragement. I think the 10001st might be the charm.
I've used Pat Kelly's http://members.uia.net/pkelley2/DynamicCR.html] calculations for DCR in a BASIC program I've written. (That's right, BASIC. I know, I'm a relic! )
When it comes to cranking compression, I just can't seem to get a solid value for gamma (the ratio of specific heats) without fudging.
The formula I have is:

Cranking compression = 14.73 (or local air pressure in lb per sq.in.) x DCR ^ gamma (dynamic CR to the gamma-th power)

I've seen (or derived) the following values for gamma: 1.20 / 1.27 / 1.30 / 1.33. Not a big spread numerically, but it causes a pretty significant margin of error when you take the lowest gamma value and the highest. For instance, if I take a DCR of 8:1, using 8 ^ 1.33 x 14.73 I get approx. 233.6 lb/in^2. With a gamma value of 1.2, I get about 178.2. So about a 24% difference between upper and lower values, which I don't think is acceptable. And therein lies my problem.

Not a problem.
The formula is correct, the inputs need some changes.
The ratio of specific heat, or gamma (k), varies depending on the composition of the gases.
For air, use a value of 1.4. If fuel vapor is included, use a value of 1.3-1.33.

Local air pressure is rarely 14.73, especially at the inlet valve. It will change with altitude and barometric pressure.

If you shut off any fuel into the system, use the correct gamma value and correct air pressure, your readings should be very close to correct. Remember the calcs are for pressure in units of psia (absolute) not psig (gauge).
If fuel is included, you must change gamma and account for evaporative cooling effects.

Errors often result from mere leakages. If in doubt, perform a pressure leakdown test.

[Turbo231]

The DCR calculation, in my opinion, is mainly just used to get your camshaft intake duration and centerline in the ballpark based on your static compression ratio. Or it can work vice versa. And it is not as important if you are using race fuel or alcohol. It is more important if you are running pump gas. If you are close on recommended DCR, you can fine tune it by moving the cam centerline. Retarding the cam will reduce your DCR. If you are not adverse to moving your cam later, I recommend you install it retarded a few degrees and test on dyno or track and see if you are getting any knock. Like any new engine running on pump gas, you want to be as conservative as possible at startup whether it's boost, SA, or A/F.