View Full Version : Compression Ratio Help

06-14-2010, 08:08 PM
I punched in my cam specs in the calculator above at all three locations

Here is the ratios I got calculated for me:
Static Compression Ratio 10.37:1
Dynamic Compresion Ratio 7.91:1
Rod to Stroke Ratio 1.68:1
Bore to Stroke Ratio 1.08:1

Can someone explain to me meaning of those ratios, the good and the bad? Also how can I make the better?

The Car is a stock LS1 LTs ORX, Fast 78mm, Stock TB, 853 heads, El torro cam.


06-25-2010, 03:34 AM
I'm not sure how much detail you need, but here's a start:

The static compression is a fixed mechanical ratio between the
volume of the cylinder when the piston is at Bottom dead vs. Top dead

This value alone means very little until you compare it with dynamic

Dynamic compression indicates how much of the static ratio is actually
used when the intake valve closes on the compression stroke. This is a
function of the camshaft design and phasing with the crank.

By looking at the static and dynamic numbers together you can see
how aggressive your valve timing can be and estimate power curves.
Based on the numbers you show, it's telling me your intake duration is
fairly high (285 ish) and your torque will peak late in the band (let's
say 3800 RPM).

Sub 8's for dynamic is safe for gasoline; some will say 8.5 or thereabouts
is pushing the envelope for pump grade fuels. You're not too far off
the mark for a street / strip engine.

In most cases, if the cam duration is high the static compression will
increase to get more squeeze to produce enough heat, to help
light off the mixture and create decent force during the initial degrees
of the power stroke.

Another sign to look for is the spread between the static and dynamic
numbers. If the static is higher while the dynamic stays fixed, you can
estimate that the engine will be efficient in higher RPM ranges.

IE: 8.0:1 DCR with 10.0:1 SCR will tend to operate in a lower band than
8.0:1 with 12.0:1 SCR

Bore and Rod stroke is another novel, and a gray area for most (me!). I've read
different opinions on the use of these ratios.

More bore is a plus for unshrouding valves and getting more air into the cylinder.

More stroke allows for higher piston speeds at lower RPM (lower torque peak
all else being equal)

A combo of the two creates your desired displacement. Comparing similar
cube engines:

An engine with a longer stroke, smaller bore tends to peak sooner because
the piston speeds are faster allowing more air to move at lower RPM.
Downside is stress on the reciprocating parts (g's).

An engine with short stroke, big bore will peak later because the same
piston speed requires more RPM. RPM is rolling the dice with engine
longevity...but the trade-off is chamber design (bigger valves, air entry, etc.)

Then of course, most of the above is all dependent on the cam, heads, intake,
exhaust setup so don't bet the house in all scenarios.

06-25-2010, 12:54 PM
Cool man, that was just what I was looking for.

I really wasn't sure what those things meant until now.