Compression Ratio Change 12.5:1 to 13.5:1 - Toyota GR86, 86, FR-S and Subaru BRZ Forum & Owners Community

mkodama · 07-04-2025, 11:39 PM

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.

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

Eaton PSI · 07-05-2025, 07:04 AM

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.

Lantanafrs2 · 07-05-2025, 01:01 PM

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

NoHaveMSG · 07-06-2025, 01:46 PM

Quote:

Originally Posted by Lantanafrs2

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.

mkodama · 07-06-2025, 03:18 PM

Quote:

Originally Posted by NoHaveMSG

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

NoHaveMSG · 07-09-2025, 01:37 AM

Quote:

Originally Posted by mkodama

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.

07-04-2025, 11:39 PM	#1
mkodama 86 Challenge Join Date: Dec 2016 Drives: 2017 BRZ Location: SF Bay Area, CA Posts: 133 Thanks: 38 Thanked 78 Times in 49 Posts Mentioned: 0 Post(s) Tagged: 0 Thread(s)	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. 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 Attached Images

07-05-2025, 07:04 AM	#2
Eaton PSI Member Join Date: Mar 2023 Drives: 2013 86 Location: melbourne VIC Posts: 80 Thanks: 3 Thanked 46 Times in 26 Posts Mentioned: 0 Post(s) Tagged: 0 Thread(s)	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. __________________ 2014 GT86, 2.2L big bore, harrop supercharged

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07-05-2025, 01:01 PM	#3
Lantanafrs2 Senior Member Join Date: Jan 2018 Drives: 2013 frs red Location: South Florida Posts: 3,575 Thanks: 2,584 Thanked 3,138 Times in 1,688 Posts Mentioned: 6 Post(s) Tagged: 1 Thread(s)	I wonder if that's from pumping loss or less squish band?

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