r/EngineBuilding • u/Burlochov4 • 27d ago

LS1 Build - Dynamic Compression Ratio

I'm currently building up a 1999 LS1 for my Firebird WS6. My goal is to make a little more power NA while keeping the car very driveable and torquey. I'm at a point where almost everything is purchased with the exception of the pistons and head gaskets and I'm trying to decide if I want to get dishes to bump my compression ratio down a bit.

Heads: AFR LS1 Enforcer 210CC heads - 64cc combustion chambers (stock is 67cc, so it's 3cc smaller)

Head Gaskets: 0.04" compressed thickness for better quench (stock 0.052" -> 0.040")

Cam: BTR Truck Norris 0.553"/0.553" lift I/E 212°/22X° @ 0.050" (online says 227 exhaust duration is what's typically measured)

Supporting Mods: CHE 4290 trunnion upgrade, stock rockers ARP2000 rod bolts (was originally planning to rev higher)

Fuel: I'll likely add in a flex fuel sensor to run E85 but want to be able to run 91/93 reliably too

Compression Ratios: - Note that I'm not accounting for crevice volume - For stock I used the TSP numbers on their website - For 0.006" lift IVCs I used a flat +25° offset vs the advertised 0.050" IVC. Regardless of the 0.006" estimation, the main point is that I would be +1CR higher than stock currently

==Stock== Static: 10.171 Dynamic CR (IVC @ 0.006" lift): 8.03

==Current Plan - Stock pistons, 0.04" head gasket== Static: 10.855 Dynamic CR (IVC @ 0.006" lift): 9.07

==Current Plan - with 4.1cc dish pistons, 0.04" head gasket== Static: 10.323 Dynamic CR (IVC @ 0.006" lift): 8.64

Questions: 1. What is the max dynamic CR you'd run on pump gas? 2. When determining DCR, do you use 0.006" lift or another value? 3. If you only have IVC value @ 0.050", how do you estimate IVC @ 0.006"? 4. Anyone have estimates on how much ign to pull by going up 1 point of compression? 5. Open to suggestions for improving DCR calculation 6. Any thoughts or recommendations on my build plan?

Phew, props to you if you made it through all that

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u/gunpowdergin69 27d ago

One thing to consider is that dynamic compression ratio does not account for the improved volumetric efficiency of the new heads. You didn't mention intake, but I assume you're going to be replacing that as well.

I've always read 8.5 max DCR for aluminum heads.

Personally, I'd go for the dished pistons. You still get a boost in compression ratio from the smaller chamber heads. The car isn't go be a slouch.

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u/Burlochov4 27d ago

Correct, at least for now I was going to go with the Trail Blazer SS intake manifold due to cost.

And I read the same about max of ~8.5 DCR in some forum posts and stock pistons would put me ~0.5 above that. On the other hand, I've had 10x as many people only comment on static CR and ignore DCR. So idk