GDI VS EFI Knock Thresholds

I was tuning a car today that was converted from Direct Injection to Port injection and ran into something I have not seen before: No matter how much timing was pulled out of it, it would constantly read knock under load/boost. I cannot hear any knocking, and it is registering a knock event every 100-250 rpm.

The stock ECU is monitoring knock and I was going to use that as reference whilst tuning. I had the car making a very healthy amount of power at 14* btdc and 22psi, but after seeing the knock values, i started pulling timing out, and even down at 10* it was reading the same amount of knock, and power was down 13%, so it clearly liked the timing. This is on gasoline by the way. I regularly tune these specific cars with DI on the same turbo and fuel system to the power the car is currently making at the same boost levels, and timing levels.

Before i continue i just need to see if someone can verify a theory: I am thinking that the combustion frequencies for DI is very different from PI, and because the oem ECU is designed for a DI system, the sound of the new injection system is just making the ECU think it is knocking. Which if that's the case, i will need to get a knock listening device to listen for knock while I tune, or I need to adjust the knock sensitivity threshold in the oem ecu accordingly.

I've never had standard port injectors cause false knock, so I'd say it's more likely that without the benefits of DI you're getting to the knock threshold far sooner than you're used to, which is to be expected.

I would drop boost from 22 to perhaps 5-10 psi, along with very conservative timing, and see if the knock goes away.

I ended up finding the culprit. I hooked up the haltech knock listening device and figured it out.

The fuel, though clear and seeming okay, was either low octane or was bad. I swapped in some fresh 93 and the knock is gone.

Great!