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Old 02-19-2006, 11:29 PM   #1
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Stock LT1 DCR? Intake closing point?

Dynamic compression ratio on a stock LT1? Just curious.
Tried a search, but it isn`t working. Anybody do a Dynamic compression ratio calculation? Or does anyone have the stock LT1 cam intake closing point?
Thanks
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Old 02-20-2006, 08:41 AM   #2
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Re: Stock LT1 DCR? Intake closing point?

.............Anyone have the valve timing specs?? anybody?
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Old 02-20-2006, 09:26 AM   #3
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Re: Stock LT1 DCR? Intake closing point?

I have also asked for this before. Part number on the nose of the my 95 cam is GMPT 242 BC, and I should have degree'd it in when I re built the motor.

Please...someone doing a motor right now degree the cam in and get the valve timing data....
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Old 02-20-2006, 09:53 AM   #4
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Re: Stock LT1 DCR? Intake closing point?

From my math, the intake valve closing point is between 60 - 64* ABDC. This puts the DCR between 8.3 - 8.6. Hopefully someone else chimes in with a concrete number.
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Last edited by 97bowtie; 02-20-2006 at 10:01 AM.
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Old 02-20-2006, 10:41 AM   #5
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Re: Stock LT1 DCR? Intake closing point?

97bowtie, what specs did you use? here`s what I used, and the results are similar to yours:
54cc chambers
8cc piston head volume (did a search on this)
gasket .049
gasket bore 4.125 (just a guess here)
bore 4.00
deck .015 (guess also)
stroke 3.48
rod 5.7

With the above data the CR comes out to 10.455. I sorta reverse engineered the deck, and gasket bore to get the known CR for an LT1 (10.4/10.5).
Then with the 60-64* ABDC I got: 8.396 - 8.643.
How did you get the 60-64*?
ADV DUR minus .050" DUR divide by 2 and add that to the INT CLOSING @ .050"?
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Last edited by cdb95z28; 02-20-2006 at 10:44 AM.
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Old 02-20-2006, 10:52 AM   #6
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Re: Stock LT1 DCR? Intake closing point?

I used:

54cc combustion chamber
8cc piston (valve reliefs)
.046 gasket thickness
4.1 in. gasket bore
.02 deck clearance (piston .02 in the hole)

This gives a static compression of 10.344 and the DCR values I posted earlier.
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Old 02-20-2006, 11:09 AM   #7
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Re: Stock LT1 DCR? Intake closing point?

Quote:
Originally Posted by cdb95z28
How did you get the 60-64*?
ADV DUR minus .050" DUR divide by 2 and add that to the INT CLOSING @ .050"?
I couldn't find any date on the advertised duration of a stock LT1 cam, so I used the specs @ .050" from a '97 LT1. The cam has 201* of duration @ .050". I looked at the "typical" difference (percentage) between advertised duration and duration @ .050" of the Comp magnum lobes (figured the stock cam would have a lobe profile similar to the magnum lobes - but I could be wrong). The percent seems to be ~ 79%. Using this, I figured the advertised duration of the LT1 cam to be ~ 254*. I then took this number and the 117 LSA of the cam and figured the DCR off of a ~ 10.35 SCR:

(((254/2)+117)-4)-180 = 60*

Some other sites I found said to figure the intake closing point using the duration @ .050", then add 25-30* to that to figure the actual intake closing point. That's where I came up with the 60-64* range.

It would be nice if someone knew the advertised duration of a stock LT1 cam from any year.
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Old 02-20-2006, 11:34 AM   #8
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Re: Stock LT1 DCR? Intake closing point?

Thanks 97bowtie,
I use SStrokerace`s easy method for the intake closing point. Maybe some of the cam gurus here might chime in!
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Old 02-20-2006, 11:39 AM   #9
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Re: Stock LT1 DCR? Intake closing point?

Quote:
Originally Posted by cdb95z28
Thanks 97bowtie,
I use SStrokerace`s easy method for the intake closing point. Maybe some of the cam gurus here might chime in!
What is his 'easy method'?
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Old 02-20-2006, 11:50 AM   #10
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Re: Stock LT1 DCR? Intake closing point?

Advertised Duration minus the .050" duration, divide by 2 and add that to the Intake closing @ .050'
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Old 02-20-2006, 12:14 PM   #11
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Re: Stock LT1 DCR? Intake closing point?

Quote:
Originally Posted by cdb95z28
Advertised Duration minus the .050" duration, divide by 2 and add that to the Intake closing @ .050'
This works out very close to the other formula. However, we still don't know the advertised duration.
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Old 02-20-2006, 12:34 PM   #12
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Re: Stock LT1 DCR? Intake closing point?

Cool.


-We may never find out the actual specs
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Old 02-20-2006, 12:34 PM
 
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