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Old 03-10-2008, 01:00 PM
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Cam Duration vs CR

Hey guys,
I have heard conflicting information on CR calc's.

Some say use the cam spec duration at .050" and
some say use the advertised duration which is generally
more. Since compression doesn't start till valve is seated
using .050" should NOT be used. This number makes the
dynamic CR vary a lot. For a good street car you want you
dynamic CR to be between 7.50 and 8.50

Can anyone clear this up for me please????

thanks

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Old 03-10-2008, 01:19 PM
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Looky here
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Old 03-10-2008, 01:37 PM
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Dynamic CR and Cam

Hey RippinRon thanks

So it was suggest to me from a reptiable speed
shop in the area that I would be happy with a
280H Compcam for my 4 bolt, 454 .030" with closed
chamber heads 101cc chambers milled to 97cc chambers
with flat top pistons. The dur at .050" is 224, the dur advertised
is 280. The 2 different numbers DO CHANGE the dynamic CR.
http://www.projectpontiac.com/ppsite...nt/view/16/30/

On the site above is the Pontiac CR calc table. If a guy plugs is
the chev 454 specs and uses 224 duration hit calculate at the bottom
watch the dynamic CR, then type in 280 duration and go to bottom again
hit calc and watch the dynamic change.
The result is 224 make it higher than 280. So which should a guy go for??
Thank you
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Old 03-10-2008, 02:15 PM
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Quote:
Originally Posted by Fastest65
Hey RippinRon thanks

So it was suggest to me from a reptiable speed
shop in the area that I would be happy with a
280H Compcam for my 4 bolt, 454 .030" with closed
chamber heads 101cc chambers milled to 97cc chambers
with flat top pistons. The dur at .050" is 224, the dur advertised
is 280. The 2 different numbers DO CHANGE the dynamic CR.
http://www.projectpontiac.com/ppsite...nt/view/16/30/

On the site above is the Pontiac CR calc table. If a guy plugs is
the chev 454 specs and uses 224 duration hit calculate at the bottom
watch the dynamic CR, then type in 280 duration and go to bottom again
hit calc and watch the dynamic change.
The result is 224 make it higher than 280. So which should a guy go for??
Thank you
On the link I sent use there is a calculator you can download, use that one it uses the .006 specs. With it I get 7.68 dcr and 9.64 scr using 4.380"x.041" head gaskets, zero deck clearance with 2cc valve reliefs. It should be fine with cheap pump gas.
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Last edited by RippinRon; 03-10-2008 at 02:21 PM.
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Old 03-10-2008, 02:26 PM
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dynamic CR vs Static CR

Hey RippinRon..thanks for the reply and no-how

My engine specs are:
4.28" bore (which is .030" I believe)
4.0" stroke
6.135" rod length
.022" steel shim gasket
.040" deck height
97cc Combustion chambers
4.9cc valve reliefs in piston (2 of them)

I am not getting the CR you said. I am down around 9.09-9.14 CR
????
http://www.rbracing-rsr.com/compstaticcalc.html
that is if you use the site above, if you use the Pontiac site the
duration is the only other variable.

The responce you sent did not have a link to any site.

Thanks
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Old 03-10-2008, 02:33 PM
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I was guessing on some things. Look at the bottom of this page for the download http://www.empirenet.com/pkelley2/DynamicCR.html. You definitely want to keep you quench or squish as its sometimes called (deck clearance + head gasket thickness) between .035" - .044". This improves fuel/air mixing via turbulence and improves detonation resistance.
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Old 03-10-2008, 02:41 PM
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CR/DCR vs Duration

Hey Ron,
I cannot download the link...I work for a place that you need
IT clearance.
Anyway, my deck clearance is .040" already and with a .040"
gasket your looking at .080" and with a .022" gasket your looking
at .062" which with even 97cc heads (which is good for high CR)
creates a not too positive CR for a good engine. That 280H Comp
is going to make this thing fall right on it's face I think.
Unless I am way off on CR calc. You have the numbers that I have.
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Old 03-10-2008, 02:51 PM
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Having a quench of .062" with say 9:1 will make it worse in the detonation department than having a .040" quench and 9.6:1. I'm saying you should have the block decked to maintain proper quench. What pistons did you use? I'm guessing jobber type with .020 reduced compression height which isn't too good.
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Old 03-10-2008, 02:58 PM
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deck clearance..quench??

yeah is doesn't sound good from what your saying.
This is a REMANF straight from AC DELCO and the clearance
from wrist pin to dome is as you say jobber??
I didn't do the engine I picked it up pretty cheap with only 40K
on her and wanted to just beef it up some. She is already sitting
in my 65 Malibu waiting for my valve job/milling to get done. Darn-it
I was thinking through my calcs with .040" deck and .022" gasket with
97cc heads, 4.9 reliefs that I would be in the low 9.0's on CR.
Did you look at the CR calc though? it says roughly 9.14 or so.
Now I am back pedaling if what you say is true. It contradicts what
others have said...slightly
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Old 03-10-2008, 03:03 PM
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Quote:
Originally Posted by Fastest65
Hey guys,
I have heard conflicting information on CR calc's.

Some say use the cam spec duration at .050" and
some say use the advertised duration which is generally
more. Since compression doesn't start till valve is seated
using .050" should NOT be used. This number makes the
dynamic CR vary a lot. For a good street car you want you
dynamic CR to be between 7.50 and 8.50

Can anyone clear this up for me please????

thanks
Very complex subject, dynamic compression ratio is really compression pressure. Assuming a fixed air pressure, temp and humidity; compression pressure is variable with throttle opening, cam timing mostly intake closing point and LSA, port size, carb or throttle body size, RPM, rod length, static compression ratio. And the list goes on and on.

A quick and dirty free calculator is here http://www.kb-silvolite.com/calc.php?action=comp.

Bogie
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Old 03-10-2008, 03:13 PM
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Okay deck clearance is from the flat part on top of the piston to the deck of the block at top dead center. Compression height is from the center of the wrist pin to the flat part on top of the piston. Here are some good calculators http://kb-silvolite.com/calc.php. But I don't use their dynamic compression calculator. The block should have been decked some to square it up if not i'd start to wonder. Regardless you need to maintain proper quench. Be it replacing the pistons with ones with a higher compression height or decking the block .02" to go with .022" head gaskets. Read up on quench on google or http://kb-silvolite.com/article.php?action=read&A_id=35.
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