Compression question

[dustergal]

I tore my 360 down and I want to figure out the compression. I've used the online compression calculators and I'm questioning the accuracy for static compression. Here's what I got

BORE: 4.030
STROKE: 3.58
PISTONS: speedpro hp116 flat tops w/ 5cc valve reliefs sitting .020 down the hole
HEADS: 915 casting milled. chamber volumes are 68cc 2.02/1.60
GASKETS: MP with a bore of 4.060 and compress to .028.

All the calculator are telling me 10.3:1
Now these pistons are suppose to be between 9.5:1 to 10.0:1 as per the manufacture.

 

[project74]

Even not taking into account the milling job, I'd think the .028 head gaskets would bump you up a bit. Do the manufacturer specs take into consideration milling and non-stock gaskets?




By the way, you didn't actually ask a question hon.

Unless I just missed a ? mark somewhere...[smilie=f:

 

[pigpen]

That was me, wifey was logged in. I'm wondering if the online compression calculators are accurate?

 

[project74]

Oh,I KNEW it was you Piggy :whipnana:

 

[XLR8R]

p74 is hiiiittting on piiiggers! Hon! BWAHAHAHAHAHAHA.

 

[Rob R]

If the head is 68cc's then it is 10.01 to 1...

 

[v8440]

When the piston manufacturers specify a compression ratio they have to make some assumptions. Examples are how far down in the bore the piston is, combustion chamber volume, headgasket thickness, and rod length. Rod length will normally be right-the rest is up in the air, so they have to just try to pick "normal" values for the other stuff. As you see, they aren't always right for a particular engine. And as for compression ratio calculators, yes, they should be accurate as long as your inputs are accurate. Static compression ratio is based on mathematical stuff that can all be nailed down with no leftover variables. It's a different ballgame than with something like hp calculators, where there are umpteen variables that can be out of whack and aren't accounted for.