Compression ratio ??
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From: Louisiana
Compression ratio ?? 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?
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?
Joined: Apr 2006
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From: Grand Rapids, Michigan
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.
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From: Louisiana
Thread Starter
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From: Louisiana
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.
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Combustion chamber volume after port, polish (which does nothing) and milling? No way for us to know...
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From: Louisiana
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.
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Verdad Gallardo TECH Senior Member
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From: Central Cal.
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
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
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Joined: Oct 2022
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From: Louisiana
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
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
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From: Central Cal.
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
With the milled 243 heas you actually ended up with a slightly higher compression tatio than a stock LM7
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From: Central Cal.
Thread Starter
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From: Louisiana
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
Last edited by Tommy42088; Apr 7, 2023 at 09:15 PM.
Thread Starter
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From: Louisiana
Last edited by Tommy42088; Apr 7, 2023 at 09:54 PM.
