OEM ECU w/ Compression Bump
 

Hoping to get some input on my setup, I have an F20 with aftermarket pistons, compression about 12.3:1 but I can't afford an EMS and tuning right now. My question is will the OEM ECU work ok with the compression bump or will it cause problems? All else is stock except for super tech springs and Ti retainers. Interested in hearing any input.

You'll be running lean as a mother f@#ker and have det everywhere. EMS is needed to get that AFR back down to safe target zones. The stock ECU is coded with the expectation of certain compression. I'm not sure it's capable of pulling timing enough to offset the knock that is going to be created.

I don't believe that it will adjust your AFR, however due to the extra heat generated by the extra compression knocking and detonation may happen. I believe that higher octane fuel can help. I.e. we require 91, so run 94 if available.

It is interesting to note that the JDM S2000 runs more compression but they have better fuel in Japan, so they have 250 hp at the crank factory.

Running lean was my concern as well. But (and correct me if I'm wrong), it's my understanding that during WOT the system is no longer Closed Loop and can add some fuel to an extent if necessary, and in Closed Loop mode during part throttle and low rpm its a little rich anyways.

My other thought was to temporarily add a V-AFC to add fuel, and hook up a wide band to monitor AFR in case it gets nasty.

Let me know what you think!

You aren't adding any air, a compression bump does not really alter the intake or exhaust. You are just squishing the air/fuel mix in the combustion chamber for a more powerful bang. This creates more heat though. When you squish a gas it warms up, when it expands it cools down. The greater heat can cause the fuel mixture to light off prematurely in the combustion chamber causing detonation. A higher octane fuel can provide more protection against this. Another few ways to prevent this is to richen up the fuel mixture or run cooler heat range plugs. These things basically take heat out of the combustion chamber.

A VAFC cannot add fuel reliably unfortunately. I think a few people have added a percent or two, but it is not always the case. I would always recommend running a wideband when making changes to a motor.

Personally, I would do more research on raising compression in the F20C. I do not know all the repercussions from doing this.

Correct, the vafc wont be an option for adding fuel if you need it, a working inexpensive alternative to that would be replacing your fpr with a aftermarket adjustable one. That aside, detonation would be my biggest concern, so higher OCT fuel as well as one step colder plugs would make me feel better since you have no way to retard timing.

Closed loop will self balance your afr for the added compression if need be, as well as the added fuel pressure of the adjustable fpr should you go that rout, and you can run the vafc to then trim the fuel across the rpm band for fine tuning in open loop/WOT. But honestly unless you have a higher OCT fuel option, I wouldn't do any of this.

Thanks for the input! I've had mild succes with a V-AFC in the past on this engine when it was all stock and I was able to add some fuel, but indeed it wasn't much because the ECU wouldn't allow it past a certain point. I'm in Seattle where premium is 92 octane and I've heard of some places where you can get 94 but I'm not entirely positive where. Colder plugs I can do, thanks for the tip on that. I do also have an adjustable FPR to go on it as well. Since this is a temporary solution, I was thinking of maybe running a thicker head gasket for awhile until I can get the EMS. Thoughts?

Thicker head gasket will lower compression.

I do not know what extra thickness will lower back to stockish compression.

You could also get the crowns machined back to standard JDM compression ( 11.5) and run on the stock ECU.

I have an aftermarket adjustment FPR for sale let me know if you need it. It's probably your best bet for safety before you get tuned