I can find much on this so I'm wondering what people's thoughts are on the subject.

I have a 2012 Ultra Classic that was a 103 stock.
I've made the following modifications that affect compression.

- bored to 107"
- ported heads that are 84.5cc's in volume
- +3cc domed pistons
- .040" head gasket
- Red Shift 576 cams
- front cylinder deck height is .009" down
- rear cylinder deck height is .011" down

Both cylinders have exactly 3% leak down at 100psi applied.

If I enter my engine specs into the compression calculators that Hammer Performance and Big Boyz have online, I get very close results.

Hammer Performance
Static Compression Ratio = 10.54:1
Corrected Compression Ratio = 9.67:1
Cranking Compression = 209.0psi

Big Boyz
Static Compression Ratio = 10.56:1
Corrected Compression Ratio = 9.68:1
Cranking Compression = 203.3psi

The Red Shift 576 cam has the same timing as a Red Shift 575 cam. It has different ramp profiles for the heavier valves and springs in my heads. According to Zippers, the 575 likes a compression ratio of between 10.2:1 and 10.8:1. So I'm right in the ballpark for this cam as far as compression ratio.

Here's the thing. When I do a compression test with the throttle plate held wide open and turning it over until the needle stops climbing, I get the following results.

Front cylinder 245psi
Rear cylinder 230psi

They are within 10% of each other but well above the calculated cranking compression.

This is the second time I've done changes to this engine, and it was similar the first time also.

Any thoughts on why the measured cranking compression is so much higher than the calculated compression?

 

Simple answer. Calculated vs Actual. The Calculated numbers are just basically close guess work. IF it run like a Bat Outta Hell ON FIRE, don't worry about it. My 07 105" Dyna, (107,000mi) 125tq/100hp.

 

I would get a new gauge..

 

I have a second gauge. Reads close to the same.

 

mr. texas said it.
they are calculations base on standards. i will throw this out just as an example: factor in elevation
the list can go on but you get the picture.
most cam profiles are based on the calculated ratio, not real world. what REALLY matters to a cam is dynamic ratio which will be lower.

 

This is the conclusion I came up with. If I crank the engine until the needle on the gauge stops climbing, it's not an accurate reading of the cranking compression. Its demonstrating how high the cylinder can pump up the gauge and hose, like an air compressor. It also gives an indication as to the health of the rings and how well they are mated to the cylinder walls.

I don't think you can get a super accurate test of the calculated compression with a compression gauge, but you can see if its somewhere in the ballpark. I re-tested it, but this time if I let the starter turn the engine over 5 compression strokes, I ended up with a reading of around 210psi which is close to my calculated compression. I found that 5 compression strokes are where the gauge slowed down movement and it seemed to take more effort to pump up the gauge higher. The service manual suggests 5 or 6 compression strokes for a compression test. It would seem there is a reason behind this instruction.

Anyway, thanks to those who responded, and I'm satisfied with the results of my compression test. Now we need winter to end so we can get back in the saddle.

 

hummmmm
simple, as cyl pressure rises, the less it can take. so 500 cycles vs 5 cycles is moot.
the manual is a forest gump approach.
believe it or not, some will crank the death out of a starter. so if you do not get close to target in five then what you get after is just moot.
even an air compressor with stall. if you have the hp to keep it running it will get to the point of just re-compression, thus providing only heat.
in times past, car engines would hang open a valve to achieve 4/6/8, but not now. the cylinder is just closed and re-compression takes place, seems like a parasitic waste and it is to a degree but less complicated than all the gizmo's and cheaper to produce. they say less than 1% but ?!?
i would place more stock in a leak down than compression.

 

So you used an 0.040 head gasket.

Was the motor hot or cold?

Personally if the motor cranks over OK with the plugs in, I'd run it..

 

I realize I don't have the optimal squish with the .040" head gasket, but I decided to use it to lower the compression. A .030" head gasket gives it a 10.77:1 compression ratio.

 

Then the numbers you gave for cr are not correct..