How do I determine my compression ratio
gen 3 5.3 iron block
Gen 4 cross top piston
243 heads Cnc port, polished and milled .030
head gasket .051
what other info is needed?

 

https://www.summitracing.com/newsand...ion-calculator

 

You'll also need a burette of at least 75CC to properly check the volume of your combustion chamber, or you can fudge it and google what the CCs are for a 243 milled .030", but that's sloppy. Need to know your piston deck height, which will take a bridge and dial indicator, or you can also fudge it if it's an SBE and say .006 out of the hole, which is about average for an LS. You can do it precisely, or you can get a rough estimate. Up to you and how important it is for you to accurately determine your compression ratio.

 

I tried using that and for some reason it wasn't working, thats what made me ask

 

I'm just looking for a rough estimate, im thinking my cc should be around 58 with the milling work. My truck is a nitrous truck and I know nitrous likes compression so I'm just hoping to see a difference in the changes I've made

 

What is a "cross top piston"?? Either flat top, dished, or valve eyebrowed... never heard of cross top...
Combustion chamber volume after port, polish (which does nothing) and milling? No way for us to know...

 

cross top is what people call them but they are dished but it's a style they made that has a cross on top of them, and thats why i asked in the original post what else specs need to listed. If you read the post I said I wasn't sure how to figure my compression.

 

This is an example of the style i have

 

Do you know where those pistons came out of?
All Gen IV 5.3 engines came with either plain flat tops (no dish) or flat top with valve reliefs (3cc volume/piston)
Your pistons are obviously dished with the cross on top. NOT any OEM Gen IV 5.3 piston.
You sure your pistons ARE Gen IV?
Most Gen III 5.3 came with 8cc dished pistons, which yours appear close to being.
Assuming these are actually Gen III pistons, you have a stock spec Gen III 5.3 lower end, stock spec .051 gasket, and 243 heads milled .030.
That should give you 60cc chambers.
Your compression is 9.71:1, guessing .005 out of the hole

 

Agree, around 9.7ish if info provided is correct.
only way to know exact is accurately measure.

 

oh ok, it's cause I bought this setup from someone else and that's what I was originally told. The more I've gotten into this motor the more lies I've found. I guess they are gen 3 pistons because thats definitely how they look. Ive already put the motor back together and have the truck at the tune shop so i think its a little late for measuring. That's why I ask. Now I'm not even sure which iron block I have cause I could of been lied to on that. It was supposed to be a gen 3 block with gen 4 pistons and rods

 

You likely have an LM7 or similar short block, which is a very common iron block 5.3 with dished pistons.
With the milled 243 heas you actually ended up with a slightly higher compression tatio than a stock LM7

 

That's an LQ4 piston. Are you sure that is what you have?

 

OOPS! Looks like he has a 6.0, not a 5.3.
Described in Summit specs as "dished, 6.7cc" No mention of the cross...
This also ups the compression to 10.53:1

 

I am 100 percent that my pistons are like that. I just put the heads back on maybe less then 2 weeks ago and i seen that my pistons looked like this and i did some research on them trying to figure out what pistons they were. Something told me to take a picture of them but they definitely do look like that. What made me notice is because they were supposed to be flat top and when I removed the heads and changed the lifters I noticed the cross

 

I say cross because I seen a few people when I searched them calling them cross top. I wasn't sure the proper name, how could I tell which motor I have without pulling it back apart?

 

Gm 12576612 is the part number they are 5.3 pistons

 

Look for a block casting #....

 

The front of the block is going to say 6.0 or 4.8/5.3. That's how you know.

 

where is it located on the block