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I built a 5.3 from the block up, currently running the stock compression ratio of 9.5:1. I’m about to slap on a set of 243s from a newer 5.3. I am aware that a direct replacement without milling will drop my compression ratio and about .020 mill will put me back at stock but I’m possibly looking to drop compression so I can have a little bit higher boost numbers. Would anyone here know what ratio I would be running with unmilled 243s? Should I keep stock compression or try to bump it down?
and yes it has a genIV timing cover to work with an LS9 cam
Keep the stock heads. 9.5:1 is fine for boost. You won't gain any power with the 243s and boost.
well my old heads are 706s but one of the valve seats is ruined on it. (Old motor blew) and I had the 243s from a good motor we had lyin around and I’m giving them a P&P. But I was wondering what my compression Ratio would be if I didnt have them milled down
Yup, not materially different from the stock 5.3. A bit less but not a whole lot. Not enough to matter in amongst the imprecision and uncertainty of everything else involved. You can prolly buy different head gaskets and mostly hide the difference and maybe even approach the result you would have got if you'd just left it alone and run it. The 243s being inferior in this situation; that is, you can prolly choose head gaskets that will make the 243s almost approach the performance of the 706s, on a 5.3.
You can get reworked ready-to-run 706s, cracks fixed and all, for around $200 apiece. There is no material benefit from buying some other casting. All the "possibly … boost" kinda crap is irrelevant.
Build your motor for what you have NOW TODAY and are doing NOW TODAY. When/if/someday/maybe "boost" comes around, do what ya gotta do AT THAT TIME, to set yourself up for it. Meanwhile, enjoy what you've got, to its fullest, without the penalty of a "boost" motor without boost.
Go drive a turbo car with the turbo deliberately disabled. Any of em, any mfr, any size, any body type. Toyota, GM, Frod, SUV, sedan, compact, truck, w/e. Just plug into the concept of a "boost" designed motor without the "boost". That's EXACTLY what your "boost" idea motor will be like without "boost". It will SUCK.
Yup, not materially different from the stock 5.3. A bit less but not a whole lot. Not enough to matter in amongst the imprecision and uncertainty of everything else involved. You can prolly buy different head gaskets and mostly hide the difference and maybe even approach the result you would have got if you'd just left it alone and run it. The 243s being inferior in this situation; that is, you can prolly choose head gaskets that will make the 243s almost approach the performance of the 706s, on a 5.3.
You can get reworked ready-to-run 706s, cracks fixed and all, for around $200 apiece. There is no material benefit from buying some other casting. All the "possibly … boost" kinda crap is irrelevant.
Build your motor for what you have NOW TODAY and are doing NOW TODAY. When/if/someday/maybe "boost" comes around, do what ya gotta do AT THAT TIME, to set yourself up for it. Meanwhile, enjoy what you've got, to its fullest, without the penalty of a "boost" motor without boost.
Go drive a turbo car with the turbo deliberately disabled. Any of em, any mfr, any size, any body type. Toyota, GM, Frod, SUV, sedan, compact, truck, w/e. Just plug into the concept of a "boost" designed motor without the "boost". That's EXACTLY what your "boost" idea motor will be like without "boost". It will SUCK.
well I’m doing as much of a budget build as I can stick to. We have a head mill and the 243 heads. I have the turbo kit and we are in progress of getting the motor in the truck now. I’m building this based off of what I have available NOW. I would just like to know what compression numbers would work best to get a decent amount of boost
12-31-2018, 09:37 AM
243 heads on Lm7 for boost 
