Higher or lower compression? Have two piston choices...
Thread Starter
Joined: Apr 2006
Posts: 3,891
Likes: 1,137
From: Grand Rapids, Michigan
Higher or lower compression? Have two piston choices... So, I've decided to go ahead and purchase a Summit Pro LS piston and rod kit since my LH6 needs the bores opened up and I don't want to have any issues down the road if I really lean on this thing (.030 over 5.3 with a 78/75 turbo ). The thing is, they list two different piston styles in the kits, a 2CC dome and a 7CC dish, which, with my 243 heads, comes out to either 10.4:1 or 9.4:1 compression ratio. I was hoping to be dead on 10:1, which is basically what this thing was stock. I do plan on running a flex fuel sensor and E85, but probably not all the time, I'd like to run 93 octane premium as well. Thing is, there's nothing that says what head gaskets or deck heights would be used to obtain those compression ratios, so they're probably a rough calculation. I used a compression ratio calculator, and it seems I'd need the pistons .008 out of the hole in order to hit the numbers Summit has listed. Not sure where they'd end up, but I know most factory LS pistons are out of the hole. I'm assuming Summit's pistons would be less due to thermal expansion of forged versus cast. I'm leaning towards the 2CC dome because I think I'd be dealing with some tolerance stacking in any case, and my actual compression ratio would be closer to 10:1 than 10.4:1. I can even drop about .1 by installing BTR LS9 gaskets with a 4.100 bore VS. the BTR small bore LS9 models with a 3.940 bore. This doesn't even take into account the lower DCR I'll have by installing a cam. So, am I on the right track here? Or will I hate life with 10:1+ and boost?
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Joined: Jun 2002
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So, I've decided to go ahead and purchase a Summit Pro LS piston and rod kit since my LH6 needs the bores opened up and I don't want to have any issues down the road if I really lean on this thing (.030 over 5.3 with a 78/75 turbo ). The thing is, they list two different piston styles in the kits, a 2CC dome and a 7CC dish, which, with my 243 heads, comes out to either 10.4:1 or 9.4:1 compression ratio. I was hoping to be dead on 10:1, which is basically what this thing was stock. I do plan on running a flex fuel sensor and E85, but probably not all the time, I'd like to run 93 octane premium as well. Thing is, there's nothing that says what head gaskets or deck heights would be used to obtain those compression ratios, so they're probably a rough calculation. I used a compression ratio calculator, and it seems I'd need the pistons .008 out of the hole in order to hit the numbers Summit has listed. Not sure where they'd end up, but I know most factory LS pistons are out of the hole. I'm assuming Summit's pistons would be less due to thermal expansion of forged versus cast. I'm leaning towards the 2CC dome because I think I'd be dealing with some tolerance stacking in any case, and my actual compression ratio would be closer to 10:1 than 10.4:1. I can even drop about .1 by installing BTR LS9 gaskets with a 4.100 bore VS. the BTR small bore LS9 models with a 3.940 bore. This doesn't even take into account the lower DCR I'll have by installing a cam. So, am I on the right track here? Or will I hate life with 10:1+ and boost?
Of the two choices I'd go 10.5:1 if you intend on running E85.
You can still run on pump easily at that compression ratio and E will be the fuel where you make serious power anyways.
You can still run on pump easily at that compression ratio and E will be the fuel where you make serious power anyways.
Thread Starter
Joined: Apr 2006
Posts: 3,891
Likes: 1,137
From: Grand Rapids, Michigan
I thought the same, so I guess my choice is made. Gotta place my order soon so I can get the block to the machine shop. I'd like to have the shortblock buttoned up by the end of February.
It won’t make much difference at all. I’ve had both and even run as low as 8.6:1 on a 5.3. If you are new and want a cushy tuning window, go with lower compression. MUCH less likely to hurt something at 9:1.
You are talking about losing maybe 4% power form 9:1 to 10:1. On a 350hp 5.3 that’s what, like 14hp? 1lb of boost more than makes up for that. Way too many people jump n the big compression band wagon. Its not a max effort class racing car with a single 78/75… Personally I’d run what’s safest. Diff between 9: and 10:1 is nothing. FWIW my 8.6:1 5.3 was going 8’s at 3100lbs.Was a gen3 "weak" rod and piston combo as well.
You are talking about losing maybe 4% power form 9:1 to 10:1. On a 350hp 5.3 that’s what, like 14hp? 1lb of boost more than makes up for that. Way too many people jump n the big compression band wagon. Its not a max effort class racing car with a single 78/75… Personally I’d run what’s safest. Diff between 9: and 10:1 is nothing. FWIW my 8.6:1 5.3 was going 8’s at 3100lbs.Was a gen3 "weak" rod and piston combo as well.
