I've been reading and learning about DCR and have run into some things that are very frustrating.

(1) Some Cam Manufacturers list their Cam Specs at .006" lift, while others specify .050" lift.

(2) Some online DCR Calculator programs say to take the .050" figures and ad 15 degrees to them and plug them in, while others say this is inaccurate because the Ramp Rates aren't known.

(3) MANY of the online DCR Calculator programs give DIFFERENT answers from each other

These are the ones I've been playing around with, in no particular order . . .

http://www.uempistons.com/calc.php?action=comp2

http://www.uempistons.com/calc.php?action=comp

http://www.projectpontiac.com/ppsite...tio-calculator

http://www.empirenet.com/pkelley2/DynamicCR.html

http://www.tbucketeers.com/threads/d...culator.12683/

https://docs.google.com/file/d/0B7Vt...Ums/edit?pli=1

This is very frustrating . . . Is there an ACCURATE Dynamic Compression Ratio Calculator online ?

Thanks, in advance !

 

The Kelley one is the only one I have found accurate.

 

They're all a bit of guesswork, in my opinion. In theory, dynamic compression is the compression ratio as measured from the moment the intake valve stops having any meaningful amount of flow past it. Now, if you know where that happens and at what point on the cam that exact lift is achieved, you're in high cotton. But unless you are an engineer with very specific knowlege of the cam and cylinder heads, you don't know what you need to make a truly accurate measurement. So you run some on line calculators and hope for the best.

And for the record, different cams can be rated at different advertised duration lift event points. .006 .004 .001 and, believe it or not, .000 (some GM cams)!! It's all over the map.

 

That's really difficult I think to get completely accurate considering we can have two identical cams if you eyeball the specs, but they're ground completely different... which would in turn affect your compression. You see a lot of cams rated the "same" at .050 figures, but that doesn't tell you what happens before/after those numbers of measurement... which obviously has a lot to do with it. I suppose it's potentially possible to get all those measurements and then make a prediction accordingly, but you'de most likely end up making an average in some way or form would be my guess.

I'd honestly like to know more about this also. This would probably be a question for YellowBullet, heh.

 

Do you think they tend to err on the "high side" or the "low side" ? Or, are they "all over the place" ?

I'm not comfortable using "guesswork" . . .

 

http://www.crankshaftcoalition.com/w...pression_ratio

Read the part about the specs not being well known and how to figure it out.Btw one author of this is Techinspector 1 who is one of forerunner in math calculations for high performance.

 

How were you able to verify your DCR, after the motor was assembled, and compare it to the Kelley predictions ?

 

PM sent.

 

Use the kelley one
Do not use .050 plus 15

Use seat values whether they are at .004 or .006. That is accurate enough.
If you dont know these ask the cam manufacturer.

Or measure it with a degree wheel and dial indicator

Verify by doing cranking compression check. Generally under 210 psi is what i have seen as a upper limit for pump gas but it could vary

 

http://www.proracingsim.com/purchasejac2.htm


Desktop Dyno 5 has all the math you want / need.
hours of engine building and dyno fun for 50 $


then you can test them in the 1/4 or 1/8 mile
using Desktop Drag 5 for another $50 bucks.

I did this years ago, scary how accurate it can be..


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LOL

 

Hmm . . . So, I take it that program is a "good thing" ? LOL !