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Old 06-19-2013, 01:25 PM
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a tad of dcr help

Is the DCR really 8.937?
The scr looks right, at 9.5

COMPRESSION RATIO CALCULATOR
Static and Dynamic Compression Ratio
(Considers Cam Timing and Rod Ratio)


ENTER YOUR DATA CALCULATED DATA
Cylinder Head Volume (cc) - 59cc

Piston Head Volume (cc) - +26cc

Gasket Thickness (in.) - .028

Gasket Bore (in.) - 4.000

Cylinder Bore Diameter (in.) - 4.030

Deck Clearance (in.) - .007

Stroke (in.) - 3.75

STATIC COMPRESSION RATIO

9.502



OPTIONAL DATA
Rod Length (in.) - 6.0"

Intake Closing Point (degrees)ABDC @ 0.050 lift plus 15 degrees - 35.5????

DYNAMIC EFFECTIVE COMPRESSION RATIO

8.937



Cam Spec Card :: Lunati Power

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Old 06-19-2013, 01:51 PM
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Or am I just forgetting to add the 15 degrees to intake closing point.

So it would be

Intake closing point 50.5
DCR- 8.357



?
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Old 06-19-2013, 01:55 PM
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Looks like im just a dingle bat and ive answered my own question. 9.5scr/8.3dcr .
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Old 06-19-2013, 03:15 PM
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Quote:
Originally Posted by 0trbo4myCHEVUICK View Post
Looks like im just a dingle bat and ive answered my own question. 9.5scr/8.3dcr .
This is something of a rampy cam so KB's add 15 degrees is probably a bit optimistic as a closing point. The valve actually doesn't seat for another 25.5 degrees beyond -35.5 which makes the actual close occur at -61 which is 10 degrees more than the KB calculator uses. So the extreme DCR at valve on seat is more like 7.8. One needs to keep in mind that the piston is reverse pumping the mixture with a lot more force than you would interpret from flow bench data of .050 lift flows through the intake valve so the flow between still .050 open and really closed can be significant.

I rather feel the 15 degree thing of the KB calculator is more representative of a street roller cam especially an OEM cam or a fast cam like the Comp XE flat tappet series than it is of more typical hot rod or even speedway cams that tend have quite long ramps. An easy way to get to the ramp closure point it the timing card shows the valve on seat or the .006 of seat timing as well as the .050 timing is to subtract the .050 from .006 or closed point then divide by 2. This added to the .050 closing point will give you the rest of the ramp.

I would suppose the reasonable DCR answer is some point between the .050 close and the truly closed point but after doing this for half a century I still don't know exactly where that is because it varies with rocker ratio as well as port and valve size, and altitude/moisture/temperature density of the air drawn into the engine. Somewhere somebody's probably computer modeled this in a computation flow dynamics class.

Bogie
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Old 06-19-2013, 03:41 PM
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I was thinking this was considered a street roller cam? Your saying to add 25 degrees instead of 15? I will pull out the cam box and look at the card for a
.006 close point.. And thank you very much for the informative post.
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Old 06-20-2013, 11:02 AM
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Called Lunati and the tech told me to use 61* as my intake valve closing point.

That puts me at 7.6 dc / 9.5 sc

Is 7.6 too low for street performance?
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Old 06-20-2013, 11:19 AM
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Quote:
Originally Posted by 0trbo4myCHEVUICK View Post
Called Lunati and the tech told me to use 61* as my intake valve closing point.

That puts me at 7.6 dc / 9.5 sc

Is 7.6 too low for street performance?
General consensus says that 8 to 1 for the street with street automobile type gearing and weight (somewhere in the range 3.0 gearing and 2-1/2 tons of vehicle wt. with an engine using modern cast iron heads similar to the L31 Vortec.

A data point where the DCR is too far off for the cam can be found in the light to heavy duty TBI Swirl Port engines where except for compression and main cap bolt 2 vs 4 the engines are identical. The light duty engine uses a 64 cc chamber the heavy duty a 76cc chamber. The light duty is rated at 210 hp the heavy duty at 190. That's a 10.5% improvement for about a ratio between these engines, neither pushes the DCR vary hard, but the HD version is quite low about 6.6 where the light duty engine is about 7.6. So this shows that power is availble when the compression ratio is sub optimized for the cam.

Bogie
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Old 06-20-2013, 12:18 PM
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7.6dc is discouraging.
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Old 06-22-2013, 08:46 PM
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a tad of dcr help

Quote:
Originally Posted by 0trbo4myCHEVUICK View Post
7.6dc is discouraging.
The DCR range can be from 7.5-8.5. I have read that most try for a 8.2 DCR. Remember this is a guide to an engine that will run on 91 octane fuel. So if you have a 7.6 DCR you might get away with using 87 or 89 octane gas.
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Old 06-22-2013, 09:14 PM
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SCR is 9.52:1
DCR with 50.5 IC is 8.37:1
DCR with 60 IC is 7.84:1
Run it. Down the road, when the rings are worn in, do a compression test.
A good max target for pump gas is 160.
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Old 06-22-2013, 09:18 PM
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Quote:
Originally Posted by 0trbo4myCHEVUICK View Post
7.6dc is discouraging.
Is the quench distance optimized?
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