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More machine than man
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Discussion Starter · #1 ·
I know the rule of thumb is to aim for a DCR of 8.2 - 8.4:1...on an older engine. I am curious how these numbers hold up to modern engines that are all aluminum, modern combustion chambers, fuel injected, and computer controlled?

For instance, I am rebuilding my LS3 and am sticking in a bigger cam. I calculated the stock OEM DCR and found it to be 9.13:1 (and that is after I poured the stock cast chambers and found them to be 75cc and not the published 70cc figure). The numbers might be a little off, but it is the best I could do to find the published OEM cam specs.

After surfacing the heads and with the bigger cam, I am looking at a proposed DCR of 9.38:1...not a major change over stock.

What do you think?
 

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The LS with aluminum heads is pretty tolerant of octane .
Newer engines have variable valve timing which changes intake opening/ closing events as well as exhaust valve timing.
Notice the newer engines dont have an EGR valve? That is becaus ethe exhaust cam/valve timing can be changed to help dilute incoming air/fuel charge with exhaust gasses to emulate EGR operation.
 

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More machine than man
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Discussion Starter · #3 ·
The LS with aluminum heads is pretty tolerant of octane .
Newer engines have variable valve timing which changes intake opening/ closing events as well as exhaust valve timing.
Notice the newer engines dont have an EGR valve? That is becaus ethe exhaust cam/valve timing can be changed to help dilute incoming air/fuel charge with exhaust gasses to emulate EGR operation.
I'm not sure I follow what you are saying about the EGR...I thought they were used for emissions not detonation control...and now fuel injection, sensors, computers, and catalytic converters made the EGR extinct.

However, more to my point, the factory LS3 has neither VVT or an EGR.
 

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I generalized on the statement in your first Paragraph ie: Modern engines. Not on the example in your second paragraph.

9.1 ish DCR sounds high for a modern pump gas motor, but the aluminum heads can keep detonation away better than cast Iron. Kinda doubt 9.1 is correct. What is the intake valve closing event @ ABDC. ?
 

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More machine than man
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Discussion Starter · #6 ·
What is the intake valve closing event @ ABDC. ?
As best I can tell from internet sources, the OEM LS3 cam card shows:
  • IVC is 43º ABDC (@ .050")...in the screen shots below you can the DCR calc determined this value to be 44.73º
  • IVC is 80º ABDC (@ .004")

Here are screen shots from the DCR calc I used.
Setup screen #1
Setup screen #2
SCR Result
DCR Result

Here are the full LS3 cam card specs I found...again, I cannot speak to the accuracy here...
 
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