Combustion chamber size vs compression ratio
10-06-2018, 11:33 AM
#1
Combustion chamber size vs compression ratio I have an LQ4, I'm ditching the 317 heads for a set of ported LS3 heads (couldn't pass up the deal). My question is, how much compression will I lose switching heads. The factory 317 heads are a 71cc combustion chamber, the new LS3 heads are 77cc. Being an LQ4, the compression is already on the low side. I want to run a fairly aggressive cam (Looking at a Cam motion Junkyard Dog Rectangle Port V1 LS Camshaft (229/245-112+4) for single plane or a custom grind from them and PTV will be a concern (I will check PTV once installed). I was thinking about having the heads milled to shrink the combustion chamber if only a little. What are your thoughts/opinions? Thanks in advance.
10-06-2018, 04:17 PM
#2
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The LS3 heads like compression. Is 93 octane available in your area? If so, I’d shoot for 10.5-11.0:1. It will pick up power across the board. You’ll need to check ptv. I’m building my second LQ4 this year with LS3 heads, this go around using the GMPP ported castings. Compression is your friend here.
10-06-2018, 09:18 PM
#3
The LS3 heads like compression. Is 93 octane available in your area? If so, I’d shoot for 10.5-11.0:1. It will pick up power across the board. You’ll need to check ptv. I’m building my second LQ4 this year with LS3 heads, this go around using the GMPP ported castings. Compression is your friend here.
10-06-2018, 10:39 PM
#4
I have an LQ4, I'm ditching the 317 heads for a set of ported LS3 heads (couldn't pass up the deal). My question is, how much compression will I lose switching heads. The factory 317 heads are a 71cc combustion chamber, the new LS3 heads are 77cc. Being an LQ4, the compression is already on the low side. I want to run a fairly aggressive cam (Looking at a Cam motion Junkyard Dog Rectangle Port V1 LS Camshaft (229/245-112+4) for single plane or a custom grind from them and PTV will be a concern (I will check PTV once installed). I was thinking about having the heads milled to shrink the combustion chamber if only a little. What are your thoughts/opinions? Thanks in advance.
10-06-2018, 11:22 PM
#6
I did buy them used. So I’m not certain that they haven’t been milled already. I won’t lie. I haven’t looked to see how to check the cc’s on the chamber.
10-06-2018, 11:29 PM
#7
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10-07-2018, 12:20 AM
#9
Aggressive cam + milling the heads and you still may be limited on compression by PTV clearance. First step would be to install the cam and check PTV with the LS3 heads. You will then need to calculate how much you can take off the heads, then calc what CC that would yield on the chambers. You may find you don’t have enough room to reach your target compression ratio. You could fiddle with different thickness gaskets and even consider reworking the bowls to tighten the combustion chambers but at the end of the day, it might be easier to just start with a set of ported heads with smaller combustion chambers.
10-07-2018, 09:49 AM
#10
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Alternately you could swap pistons to domed and relieved to gain compression and then lightly mill to achieve compression.
