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How do you measure dynamic compression with dual pattern cam
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Author:  Mike_64_Valiant [ Wed Jul 25, 2012 7:59 am ]
Post subject:  How do you measure dynamic compression with dual pattern cam
I have my block ready the crank rods and pistons are together. Last night I was getting ready to install my comp cams 264s cam but for some reason I couldn't find my self able to do it, knowing I have a hotter cam sitting over on the self. The second cam I have is a dual pattern that was ment for my bigger hp motor one day but that day won't be here any time soon and I need a motor together now.

Here is a run down on my specs.
48cc head mild porting 3 angle valve job
Block is std bore 3.400
4.125 stroke
6.700 rod
Pistons .120 in the hole
Felpro head gasket .033 compressed 3.500 bore
From my measurements that should be 9.67-1 static compression ratio.

The cam I have is :
108+2 lobe separation angle
56.0 over lap
Intake opens 31.0 closes 63.0
Exhaust closes 25.0 opens 65.0
Intake lift .510 exhaust lift .487
Duration int. 274 exhaust 270
These are all advertised not at .050"

My question is how do I figure out the dynamic compression ratio to see where the compression will end up. I need this engine to still run pump gas. I originally planed this cam for a bigger bore engine. I will be running dutra dual and holly 390 cfm 4bbl. 2.25 exhaust. Can I run this cam with good effect or should I just use the comp 264 I planned on?
Author:  Joshie225 [ Wed Jul 25, 2012 8:06 am ]
Post subject: 
Dual pattern cam or single, dynamic compression ratio depends on the geometry of the engine and the intake valve closing point.

http://www.kb-silvolite.com/calc.php?action=comp

Also, I think the FelPro head gasket is closer to .039" compressed. What's the lobe separation? I mean what's the +2? Do you mean the cam is ground 2° advanced?
Author:  Mike_64_Valiant [ Wed Jul 25, 2012 9:58 am ]
Post subject: 
Thanks Josh.
One of the articals I was reading said dual patterns where diffrent because of the measurement of timing between the intake and exhaust closing would calculate out diffrent but did not say how to figure it out and confused me that's why I asked.

As far as the felpro gasket number I did a search and had mixed numbers from .033-.040" so I went with the lowest to get the worst case senario for high compression.

With the cam lobe separation angle I asked them to grind it on a 108lsa but they said my exact numbers could not be done and they did 108+2. So yes its 2• advanced.
Author:  Al T [ Wed Jul 25, 2012 10:22 am ]
Post subject:  DCR Calculation
http://www.rbracing-rsr.com/comprAdvHD.htm
Try this calculator. DCR depends on when the intake valve closes (0.050"travel remaining). This is the point at which the compression stroke is essentially sealed and begins compressing.
Image
Author:  Mike_64_Valiant [ Thu Jul 26, 2012 3:35 am ]
Post subject: 
Ok so my SCR is 9.5-1 and the DCR at sea-level should be 7.9 and around 7.3 7.5 at the elevation I will be at most of the time. So I would say I should be safe to run this cam on 91pump gas no problems. My next question Is will this cam be to much for a stock valve head with only minimal porting? Am I going to be hurting my self more than doing good? Would I be better off just using the comp 264 for now?
Author:  DusterIdiot [ Thu Jul 26, 2012 5:39 am ]
Post subject:  You could...
With a DCR in the high 7's, and a SCR in the mid-9's...with judicious tuning and a distributor recurve it is possible to run that on 87 reg.unleaded...I have run engines with a DCR in the low 8's and SCR in the 10's on 87 regular (race it on 92...).

-D.Idiot
Author:  sandy in BC [ Thu Jul 26, 2012 8:18 am ]
Post subject: 
Im with DI on this....run the bigger cam .....prolly on 87......skip to 89 on hot/grade days.


The Comp 264 will likely cause you more grief than the bigger cam........mine did.
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