Thread: Internet CR calculators question

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How do you determine what # to use as the INTAKE CLOSING POINT with these calculators to determine the DYNAMIC CR? The one I've been using says to use the intake closing point @ .050" and add 15 degrees.
Some say that is not correct for determining dynamic cr. Do you use the 'advertised' figure, .050 figure, or what?

The figure on the cam card is the figure you use,its when the valve is closed.It would not be called the intake valve closing point if it was open any at all? Gatta give ya a hard time atleast ....after all the pay on this job sux!!!!
If the web site or cam card has a spec for valve events,they should list intake opening degrees ,intake closeing degrees/exaust closeing degrees,exaust opening degrees....this is exactly the degree of the cam rotation that those events occure.The .050 and all other specs are not closing or opening points ,they are for duration and lift caculations not a specific point in rotation.
Hope this helps!!

On the KB calculator, it asks for intake closing point at 0.050" tappet lift plus 15 degrees as you stated. What they're suggesting is that there will be about 15 degrees of crankshaft rotation on the closing ramp from 0.050" tappet lift to the advertised duration closing point which is calculated at 0.006" tappet lift.

In other words, if you had a cam with 220 degrees of duration at 0.050" and 250 degrees advertised duration at 0.006", the difference would be 30 degrees, 15 degrees on the opening ramp and 15 degrees on the closing ramp. So to get to the point where compression can actually start (intake valve on the seat), they use an arbitrary figure of 15 degrees to be added to the 0.050" figure.

Personally, I thing the KB calculator is conservative, so I still use it as they suggest, but instead of looking for a DCR of 8.00 to 8.25, I have adjusted my personal choice to between 8.25 and 8.50. That makes up for the conservancy in my opinion.

Shawnlee recently built his 305 to a DCR of 8.40, so I think he's thinking along the same lines as I am.

Spec sheet lists the 'advertised' or SAE intake closing point at 86*, so is this the figure to use with the calculator?

That should be the number...but it seems awful big...It may help if you do what Denny said and post what cam this is.....and some other specs about the motor.This is the time to get it all figured of with no doubts!!!!
Theres alot of people on here with calculators and converters to help and they may not be familiar with that specific calculator/converter.

The cam I used had a number of 57 degrees and called for a static of 9.5 to 1 to 10.5 to 1 compression.I also used rhodes lifters which lowers this number further under 3000 rpms.

Cam is modern replica of the L79 hydralic,447/447 lift, duration @ .050 222/222 with 290/290 adv.duration (some list at 306 instead of 290???);valve timing @ .050 intake 1*/41* (open/close); exhaust 49*/7* (open/close); valve timing advertised, intake 40*/86* (open/close);exhaust 88*/38* (open/close).
Motor is 4.00" bore/ 3.00 stroke; pistons are original static 11:1 when used with 64cc heads but I "might" try to use a set of 58cc heads "if possible",if not then 64cc but I am using 58cc for the 'cyl.head vol.' figure in the calculator. Gasket thickness is .016"; gasket bore 4.166"; deck clearance is a 'measured' .025"; and 5.7" rod length. This will be a weekend cruiser on race gas,cam 2 or so,,,
So Shawnlee 28 if you or anyone else would be so kind as to compute and advise it would be much appreciated.And notice my italics,,,,I know a degree wheel and precise building measurements will be required.Thanks,Jerry.

Here is one that I used ,it calls for the .050 numbers that you already have!!!
http://www.wallaceracing.com/dynamic-cr.php

So which number,does anyone know? .050 +15 (41+15),or the advertised ICP 86*,or the advertised duration number 290?

On the link shawnlee gave, use just the 0.050" figure. Do not add anything to it. On the KB calculator that I have been using, add 15 to the 0.050" figure.

On that other cam that you say has a closing point at 86 degrees ABDC, that would be one hellacious camshaft, somewhere on the order of 290 degrees duration @0.050" tappet lift (about 330 degrees advertised). A cam like that would only be used in a 12.5 to 14.0 static compression ratio motor and would make power from about 5,500 to 9,000 rpm's.

Thanks techinspector1.
The 86* was advertised,not @.050"
And the 290* duration was also advertised,not @.050".