Compression Ratio Calculation Help - LQ4 with 243 Heads

[evan]

This is a stock LQ4 6.0L short block with 243 heads, and stock GM head gaskets. It says 9.98 : 1.

Did I screw it up somewhere? I was assuming the dished pistons with the 65 cc cylinder head volume would make for lower compression so was thinking about taking some thickness off the cylinder heads when they get surfaced. If it's accurate I'll just have them take off as much as necessary to level them out.

[scottofksu]

Are you familiar with the concept of dynamic compression?

I run a 383 with 10.5:1 STATIC compression (i.e., the metric you shared) on pump gas. I did this intentionally as the cam I run bleeds off significant amounts of this pressure where it is dangerous to the motor (i.e., at lower rpms) while retaining it at higher rpms for the production of power.

[evan]

Originally Posted by scottofksu

Are you familiar with the concept of dynamic compression?

I run a 383 with 10.5:1 STATIC compression (i.e., the metric you shared) on pump gas. I did this intentionally as the cam I run bleeds off significant amounts of this pressure where it is dangerous to the motor (i.e., at lower rpms) while retaining it at higher rpms for the production of power.

I was not aware of dynamic compression. Is it overlap that bleeds the compression at lower RPM?

[scottofksu]

Correct! And THAT is the number that determines many aspects including drive ability (e.g., there so much overlap that there is no power or throttle response until 3-4,000 RPM) and octane requirements in part with timing and other component (e.g., fuel air mixture) playing a role. Obviously, where you are starting with static compression plays a big role in how much "work" you need to do to keep the engine alive on the street. The LQ9 - basically an LQ4 with different pistons to increase compression and deliver about 10 more HP and TQ - has a static compression ratio very similar to yours; so you should be fine.

[biketopia]

Are you shooting for a very low CR for a turbo build? The LS will take more static and dynamic CR than a typical SBC will and run fine on pump gas. What are you trying to accomplish with your build?

[evan]

Originally Posted by biketopia

Are you shooting for a very low CR for a turbo build? The LS will take more static and dynamic CR than a typical SBC will and run fine on pump gas. What are you trying to accomplish with your build?

It's a motor for my '79 C10 (approx 3,900 lbs) with 4L80E and 3.73 gears. It's a weekend cruiser with a street legal drag night now and then. Going to be running on 91 because there's no 93 or e85 near me. Naturally aspirated, stock LQ4 short block, 243 heads to be ported by the same machine shop that will be surfacing them. I'm still deciding on the cam but I've been looking at 220-230's intake and 550-620 lift. Possibly the TSP Madd Maxx if I can find any more info on it or possibly the BTR Red Hot. When I called BTR about the Hot Rod they asked what motor it was for and when I said LQ4 they suggested bumping up to the Red Hot. It's wilder than I was originally considering but I'll be getting a new torque converter anyway.

[biketopia]

Ok cool, first the BTR cam recommended is designed around an LS3 top end with square port heads, I'd tend to steer away from that with your 243's and intended use. The Madd Maxx could be a good choice but might be a little roudy for a cruiser. If I were you, I would call summit and talk with them about their Pro LS cams. They get a lot of love on the net and do really well in the real world. Since a lot of the cams, especially truck cams are based around a 5.3 the added cubes of a 6.0 help move the power band where you'll want it when driving around. I'd look at their stage 3 truck cam, 222/232 112 at .550 lift, still uses LS6 springs, will have nice power, a noticeable idle but good street manners.

Edit, I meant to link this threat from LS1 tech, there's some good back and forth with Summit on recommendations and combos. https://ls1tech.com/forums/generatio...urbo-cams.html

[evan]

Originally Posted by biketopia

Ok cool, first the BTR cam recommended is designed around an LS3 top end with square port heads, I'd tend to steer away from that with your 243's and intended use. The Madd Maxx could be a good choice but might be a little roudy for a cruiser. If I were you, I would call summit and talk with them about their Pro LS cams. They get a lot of love on the net and do really well in the real world. Since a lot of the cams, especially truck cams are based around a 5.3 the added cubes of a 6.0 help move the power band where you'll want it when driving around. I'd look at their stage 3 truck cam, 222/232 112 at .550 lift, still uses LS6 springs, will have nice power, a noticeable idle but good street manners.

Edit, I meant to link this threat from LS1 tech, there's some good back and forth with Summit on recommendations and combos. https://ls1tech.com/forums/generatio...urbo-cams.html

Thanks. I'll check that out.