How will compression affect the tune

[Cat in Hat]

Hi All
I have been running with the same engine setup and tune for years, and it has been working well.
The engine has now been rebuilt with some new internals including JE 9.5:1 compression pistons. The intention is to up the boost a bit as well (was at 1.4 bar wanting to go 1.6) My question is how will the increased compression work with the old tune?
Should I expect to add fuel? pull-back on timing? Both?

 

[Alin]

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[HolyCrapItsFast]

To simplify the explanation... assuming the same fuel and head gasket thickness, by increasing compression, you reduce the tolerance for knock. So you will have to reduce either timing or increase fuel or reduce your boost or any combination of the three.

However this is not absolute because you need to consider cams and other components that effect your dynamic compression. If you have stock cams then you have both increased static compression along with dynamic compression and should probably reduce timing or boost. But if you have larger duration higher lift cam, it is conceivable that your dynamic compression actually decreased and you might even be able to increase timing and or boost. It really depends on allot of factors.

You really need to let your tuner determine what is safe to increase or decrease IMO.

 

[WRB_STi]

^ +1

There are many factors in deciding on what to do. Your old tune was based off of the old motor. I would recommend a full retune for the modifications. With the change in compression and different pistons, you are changing the combustion chamber. As previously stated, this changes the risk of knock. Also, changing the compression will change your AFRs in any driving.

I dont have a ton of experience on these engines and tuning but an EVO I did last year had an issue of knock after a rebuild. We bumped the compression .25 and it had knock at a higher level of boost. We pulled timing and the knock went away. It also made another 20whp. For what thats worth it could be anything that caused the increase in hp. But the higher compression and pulling the timing back caused increased results. Mind you that nothing else has changed. Same turbo, cams, exhaust, intake, etc...

The point of all of this is that the fueling and timing may need to be adjusted to keep that motor together for the long run.

 

[Cat in Hat]

Thanks for the replies guys
My engine has been run-in now (I am not a big believer in a long run-in period) and this is what I have noticed. For the record I use an Autronic ECU with live tuning and logging facilities. I have an AFR gauge wired into the Autronic which is logged and I also make use of a Phormula Knock Sensor. Fuel = E50

For the most part fuelling has not had to change. There are a few cells where I have had to add a bit of fuel, these are around the spool-up zone before peak torque. Looking at the values that needed changing, I suspect that my tuner previously didn't pay much attention to these cells because they fell out of the load/rpm range that my engine was accessing. Now I am getting to higher load cells at lower rpm and found that this is where the engine was running lean.

I have not yet made any changes to the ignition timing but will start out by pulling it back a few degrees at higher RPMs and slowly advancing it to find the sweet spot. My target AFRs are 11.5 at peak torque and high RPM.