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Old 10-06-2008, 10:00 PM
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383 compression ratio?

I am building a 383 with afr heads and a comp xr270hr cam. My question is what can I run for a compression ratio with this cam on 91 octane? And will rod length effect my camshaft for clearance with scat wave locks. I would like to build it with .040 quench but that leaves me with 7cc piston at 10.4 or 18cc at 9.2. Any help is appreciated

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Old 10-07-2008, 04:23 AM
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Your cam timing at 0.006" is 29-61 / 72-24. At 0.050", it's 3-35 / 46-(-2).

I've been using the KB DCR calculator for a couple of years and have found that a DCR of around 8.0 to 8.4 has shown to work well on pump gas according to the feedback from those who have built such motors. It's not definitive, but until someone else comes up with a better way, it's what I've used. The instructions on the calculator say to add 15 degrees to the 0.050" lift intake closing point. That would make it 50 degrees after bottom dead center on your cam. Using that data, at 10.4 SCR, the DCR calculates to 9.182....probably too high for pump gas. At 9.2 SCR, the DCR calculates to 8.138....leaving some on the table, but most assuredly operable on pump gas. Using the constant 10.4 SCR, you'd have to delay the intake closing for an additional 15 degrees (increase intake duration by 30 degrees) over the cam you are planning to use in order to get the DCR down to 8.336....or....you could choose a cam that closes the intake valve sooner by 5 degrees (decrease intake duration by 10 degrees) to arrive at a DCR of 8.341 with your 9.2 SCR.

I know there must be some valid calculators somewhere on the web that will convert data into cranking cylinder pressure, but I haven't found such an animal yet. Crane Cams recommends a CCP limit of around 165 psi for operation on pump gas. Here's their blurb on it.....
"In very basic terms, the more cylinder pressure we make the more power the engine will produce. But look out for the fuel! Today's pump gas is too volatile and cannot tolerate high compression ratio (above 10.5:1) and high cylinder pressure (above approximately 165 PSI) without risking detonation. Fuel octane boosters or expensive racing gasoline will be necessary if too much cylinder pressure is generated."
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Old 10-07-2008, 06:47 AM
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A lot of that also depends on your head selection. the chamber shape, layout, and surface have a LOT to do with detonation tolerance. If you were running old iron "camel humps" then I wouldn't go over 10:1 on pump gas, but with some new AFR 195's you may be able to push it all the way to 11:1. That point of grey area depends on what your comfortable with. Also, steep gearing or lighter viehicle weight, and a looser stall all contribute to allowing more compression.
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Old 10-07-2008, 08:03 AM
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With that cam, heads, and octane I would try to run 10.0:1 cr.

And I would use a D-shaped dished to maintain a quench area.

(785 + 64 + 8.5 + 3.5 + X) / (64 + 8.5 + 3.5 + X) = 10

X= piston cc
785= cylinder cc
64= head cc
8.5 = gasket cc (0.040")
3.5 = piston below deck cc ( assuming your engine has a - deck height)

solve for X
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Old 10-07-2008, 10:48 AM
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Afr 0916 heads are 68cc and the two combinations of pistons available to me are 7cc and 18cc. I havent decked the block yet so theres a option for deck hieght.
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Old 10-07-2008, 11:42 AM
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deck the block to maintain .040 quench. You're better off running higher quench with higher compression than lowering both.
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Old 10-07-2008, 12:05 PM
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Using KB197 12cc pistons with 5.700" rods will yield 9.87:1 SCR and 8.72 DCR with 68cc heads and the 270 cam.

You didn't say what rod length is, but if you're limited to piston choice by 6.000" rods, maybe you need to read this by Iskenderian. Tech Tip- 2005.
http://www.iskycams.com/techtips.php
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Old 10-07-2008, 12:08 PM
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Quote:
Originally Posted by 454C10
With that cam, heads, and octane I would try to run 10.0:1 cr.

And I would use a D-shaped dished to maintain a quench area.

(785 + 64 + 8.5 + 3.5 + X) / (64 + 8.5 + 3.5 + X) = 10

X= piston cc
785= cylinder cc
64= head cc
8.5 = gasket cc (0.040")
3.5 = piston below deck cc ( assuming your engine has a - deck height)

solve for X
HuH? LOL I"m lost! Give me fractions any day!!
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Old 10-07-2008, 01:55 PM
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ok, here is another way to find piston volume needed to get 10:1 cr

785cc= 383 cylinder volume

(785cc + x)/x=10
x = 87.2

87.2 is the total volume in the chamber

so 87.2= 68cc + 8.5cc + 3.5cc + piston cc

68cc is the head chamber, 8.5 is the head gasket, 3.5 is the deck height

therefore piston cc should be 7.2cc to get 10:1 cr

87.2 - 68 - 8.5 -3.5 = 7.2
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Old 10-08-2008, 11:05 PM
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The heads are aluminum but what is the limit with pump gas. AFR does make a very effecient head but that and 91 octane will only go so far. Is anyone running a similar combo and with what compression ratio? With this roller cam and heads on a 350 Iam running 9.8:1 with no problems even on 87 octane.
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Old 10-09-2008, 06:14 AM
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If you're putting it in a truck (assuming from your name) then 10.4:1 is too high even fro AFR's. about 9.8: looks about right, mostly due to the extra load your truck will put on thte engine.
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