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Old 01-14-2010, 08:07 PM
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Dynamic Compression Calculation

I am trying to determine dynamic compression ratio. The Kb Silvolite calculator asks for intake closing point at .050 lift plus 15 degrees.
The Comp Cams website lists it intake closing point at .006 lift, Lunati doen't tell you what their number is.
So here's my figures so far:
65 cc Head volume
Pistons have 5cc vavle reliefs
Head gasket is .015
Pistons are .025 in the hole
Gasket bore is 4.100
Cylinder bore 4.030
Stroke is 3.48
Rods are 5.7 inches

Intake closing point in the Comp Cam is 67 ABDC @.006 lift, duration is 230 and it a Comps XR282HR
On the Lunati intake closes at 41.5 but it doesn't give the lift but duration on this cam is 231 @ .050. Its a Lunati Voodoo 60112

Any help would be appreciated.
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Old 01-14-2010, 08:18 PM
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Static compression is 10.2 to 1 Dynamic is 8 to 1 with the comp cam(70 ABDC). The calculator says you need 8.5 to 1 or less dynamic compression for pump gas.

(edited)

Last edited by stovepipe699; 01-14-2010 at 09:11 PM.
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Old 01-14-2010, 08:21 PM
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The Lunati spec card says timing figures obtained @ 0.050 lift.

Last edited by stovepipe699; 01-14-2010 at 09:12 PM.
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Old 01-14-2010, 08:31 PM
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The united/Keith Black calculator comes out much diferent? 10.15 static?
exact combo I am building however 67 cc heads for 9.9.. I figured about .030 down the hole with my vernir caliper.. Eyes getting too old for that! I picked the Summit 1103 cam for 8.785 dynamic.. is this wrong?
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Old 01-14-2010, 08:32 PM
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The intake valve closing point ABDC is calculated in the following manner. Divide the intake duration by 2, add that to the lobe separation and subtract out any ground-in advance. Lastly, subtract 180 degrees.

The XR282HR has 282 degrees of "advertised" duration (0.006" lift at the cam lobe, or 0.009" at the valve with 1.5:1 rockers) on the intake side, 230 degrees of duration at 0.050" lift (at the lobe), 113 degrees of lobe separation, and 4 degrees of "ground-in" advance.

For XR282HR you get an intake valve closing point of 70 degrees
ABDC: 113 + (282 /2) - 4 - 180. Using advertised duration, since air in the cylinder is still being bled-off with a valve opening of only 0.009".

The calculator asks for the intake closing point at 0.050", plus an additional 15 degrees. For not knowing the exact valve closing point its an ok substitute (the exact closing point is just a guess in the first place as how would you know if intake valve is leaking cylinder pressure at 0.009 of lift but not at 0.008?). The 15 degrees the calculator tells you to add is small for is too small for a steep roller cam (adding 25 degrees would be good) (the difference in advertised and 0.050" duration being 52 degrees). For a flat tappet cam around 30 degrees added would be good.
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Old 01-14-2010, 08:44 PM
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Thanks for the formula Jaguar, that puts the Lunati Voodoo 282 at 67 ABDC. (282/2) + 110 - 4 - 180 = 67 That makes a lot more sense.

Last edited by stovepipe699; 01-14-2010 at 09:13 PM.
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Old 01-14-2010, 08:47 PM
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Thanks.. That's something that ticked me off too..

If the KB calculator is correct I would think the tighter quench would be a bigger benifit than going with a composite head gasket and losing the tight quench?
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Old 01-14-2010, 08:51 PM
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I get 10.27 and 8.06 for regal beagle's setup?
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Old 01-14-2010, 09:03 PM
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Yes you got it right Airboat,
I entered a 5cc dome instead of a dish on the program(forgot the -). I'll go back and edit my replies to the correct info.

Last edited by stovepipe699; 01-14-2010 at 09:16 PM.
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Old 01-14-2010, 09:17 PM
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Quote:
Originally Posted by stovepipe699
I don't know abou this software, the cam calculator may not be calculating compression properly... sorry if I gave incorrect compression. My dyno program comes up with 10.3 to 1. IT's not the software, I entered a 5cc dome instead of a dish on the program. I'll go back and edit the info to stop the misinformation...
When built my static cr was calculated at 10.3:1 so your calculation there is dead on. I currently have a Comp XR270HR, advertised duration 270, LSA 110, duration at .050 is 218. Based on this my current dynamic cr would be 7.440. That's using 61 ABDC plus 15 degrees for a total of 76.
Have I got this right?
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Old 01-14-2010, 09:34 PM
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110 LSA + (270/2) -4 -180 equals 61 + 15 equals 76, Had to do it myself just to learn.. Yup.. 7.440 that's what I get too! That's cool!!

Mine is a little closer to the edge but at 2200 aSL should be OK!
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Old 01-14-2010, 09:41 PM
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airboat, I don't know how much ground in advance the summit 1103 has, but if it had none (272/2) +112 - 180= 68 ABDC static comp of 9.9 and dynamic comp of 7.7

if it has 4' advance ground into it, then ABDC is 64, and dynamic comp is 7.93

Hope this is of help

Last edited by stovepipe699; 01-14-2010 at 09:47 PM.
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Old 01-14-2010, 09:43 PM
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Figuring putting it in straight up regardless..
Hwy 2 would be a good one to check top speed of this Porsche!

Last edited by airboat; 01-14-2010 at 09:59 PM.
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Old 01-14-2010, 10:07 PM
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Regal, using the formula from above the 270HR would have 61 ABDC, and that's what on the comp spec card, listed with valve timing @ 0.006 lift(not @ 0.050) so in this case adding 15 isn't correct. Your dynamic compression should be 8.4 to 1
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Old 01-14-2010, 10:17 PM
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Quote:
Originally Posted by stovepipe699
Regal, using the formula from above the 270HR would have 61 ABDC, and that's what on the comp spec card, listed with valve timing @ 0.006 lift(not @ 0.050) so in this case adding 15 isn't correct. Your dynamic compression should be 8.4 to 1
I am confused.....the Kb Silvolite calculator says to use the "intake closing point (degrees) at .050 lift plus 15 degrees.

So that would put my current cam (Comps XR270HR) at???

And the Lunati 282 dynamic cr at ????
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