cdb95z28

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

cdb95z28

.............Anyone have the valve timing specs?? anybody?

VintageMuscle

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....

97bowtie

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.

cdb95z28

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"?

97bowtie

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.

97bowtie

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.

cdb95z28

Thanks 97bowtie,
I use SStrokerace`s easy method for the intake closing point. Maybe some of the cam gurus here might chime in!

97bowtie

What is his 'easy method'?

cdb95z28

Advertised Duration minus the .050" duration, divide by 2 and add that to the Intake closing @ .050'

97bowtie

This works out very close to the other formula. However, we still don't know the advertised duration.

cdb95z28

Cool.


-We may never find out the actual specs