Compression Ratio Change 12.5:1 to 13.5:1
 

I did a stock rebuild, only changing connecting rods and pistons. No machining or tuning modifications. The goal was to just get a little more power and efficiency out of E85 on an NA motor for track use.

Differences:

• E85 cold starts are just like on 91AKI now (first starter button press).
• About 2 MPG better on the freeway.
• Lost 8 HP. :thumbdown:

Conclusions:
Don’t increase your compression ratio hoping for power. I kinda wish I did 14:1 or 15:1 CR with how minimal the differences are. Also I found out the car can still run on 100 octane unleaded with the stock tune and pass California smog, even if not technically legal. The engine is wonderfully smooth with an incredibly flat torque curve, but at $2900 combined for pistons and connecting rods, it’s not a great value.

Setup:

• 2017 FA20 manual with 43,000 miles and 63 track days
• Custom 13.5:1 CP Carrillo pistons, optimized for lightweight NA track use
• CP Carrillo SCR7548-4 connecting rods
• JDL equal length exhaust manifold
• JDL over pipe
• JDL dual resonated front pipe
• OpenFlash Tablet E85 tune
• TRD lightweight flywheel
• Fluidampr harmonic damper
• King race main and rod bearings

The high compression ratio these engines have is for improved efficiency at cruising speeds with small throttle opening. It's limiting the maximum power on a stock engine.
12.5:1 is already extremely high and only possible because of the direct injection and clever electronics. Lowering to 11.5:1 or 12:1 and then retuning would likely give more top end power at the expense of cruise efficiency.

I wonder if that's from pumping loss or less squish band?

I’m assuming a few factors. Curious what the timing looks like on the upper end.

Peak ignition advance looks to be 30 degrees.

A few data logs here: https://datazap.me/u/mkodama/log-174...4?log=0&data=1

I think mine is like 32-34. I’m on mobile so I can’t zoom in on it really well.