EJ25s: Forged Internals & Low RPM/Load Knock

Andre/Everyone,

I recently upgraded to a built motor in my 2019 STI (an IAG 700 long block to be precise) after the discovery of bearing detonation damage on cylinder #4 (it was technically due to my ignorance for loading it up in 6th at lower RPM on the highway before I knew better, but I won't thank Subaru's factory "emissions is all that matters" calibration for helping). The new engine is obviously forged internals and generates significantly more mechanical noise than the stock engine with the hypereutectic pistons. But more interesting than that is the fact that I have a "concern" that I originally thought was mechanical in nature that is now manifesting with the new engine. Essentially, at low RPM and low load (and for reference this would be from approximately 0.35 g/rev to 0.65 g/rev, and from 1400 RPM to 2800 RPM - we're talking pulling away from the house cruising in first gear at 10 MPH load levels), I can hear somewhat random - though easily reproducible - audible pinging from the engine. Any load/RPM ranges lower or higher than this and the pinging disappears. I have confirmed using a Link KnockBlock that the pinging will reduce and stop with a reduction in ignition timing but this reduction tends to need to be massive. Timing of fewer than 10 degrees is required in some cells to quell this knock which obviously doesn't do drivability any favors and frankly seems excessively low for this level of load.

With that in mind I have a couple of questions I'm hoping you all can help to answer:

1. I'm not particularly new to tuning or performance engines, however, I've only got about a year and a half of EJ experience under my belt. It seems EJs are just noisy beasts to begin with, but forged internals coupled with bigger cams make much more racket than I had anticipated. What are the thoughts/expectations for levels of noise from EJs with forged internals and built valvetrains? Likely normal?

2. What is with the need to pull so much timing to rid these engines of this low load/RPM pinging? Since such a reduction is detrimental to torque in this range, did the Subaru engineers simply figure that the pressures exerted on the internals from detonation at this level of load is not of concern? I understand it's typical to run engines very close to the detonation thresholds at lower loads for power/emissions reasons, but being 20+ degrees over this threshold is not something I'm comprehending well. And why does it only seem to be in this small range of the timing table? If RPM stays relatively the same, load increases, and timing remains relatively unchanged (perhaps dropping by 1 degree due to moving higher into the load axis), it is no longer present. This simply defies the logic of engine behavior in my brain. Is it possible this is something other than knock???

I should reinforce here that this is one hundred percent an issue notice via audible observation and the stock knock system picks up absolutely nothing in this range (since it's below the load threshold for enablement anyway). Also, just trying to be thorough here, I had all of the fuel injectors flow tested to ensure this wasn't something related to fueling. Albeit, the stock engine did this with stock injectors and a stock fuel system as well. The problem occurs on premium gasoline (91 octane in my neck of the woods) and on E85 (which is actually E70 in my area).

Anyway, apologies for the novel. Any thoughts or help would be greatly appreciated!

Thank you all, stay safe and healthy, and have a great day!

Hello interesting issue im sorry i dont really have any advise to help you. its crazy to see its doing it on e85 also and timing removal helps but it takes so mush to do it

Hi Nicholas,

As Ross says that's a very odd one. The only one I can chip on is the noise, I've been around ej's back in the day and forged motors don't sound particularly nice! Customers have come back complaining of the noise and even had the engine rebuild standard and detuned to have a nicer daily driver.

Knock.. I know this is going to make others raise an eyebrow but I don't like any knock listening kits that filter, I make my own sensor with an audio amp and attach that to all motors I tune. Yes this can be noisy but you hear everything and sometimes in my experience can find mechanical faults also. Thursday

I know Subaru do have an odd map from stock, the moment you modify I would map it in its entirety. That should remove all anomalies.

Hope that helps a bit.

Thanks for the feedback guys!

With regard to forged noise, that's what I figured and it isn't an issue for me; just an unexpected observation. Just wanted validation that the EJs are noisy with forged internals and that it's to be expected.

As for the knock suggestion, I'll throw some non-filtered microphonics on there and see if it reveals anything. This is literally one of the most bizarre issues I have ever dealt with; particularly since it's been consistent across two completely different motors that share nothing but the intake manifold, power steering pump, A/C compressor, and various lines/nuts/bolts (everything else is brand new). It crossed my mind at one time that this could be related to AVCS as this does occur in those high intake advance regions on the stock map. But the new build has AVCS at literally zero across the board (not sure if the valves would hit or not on stock values but I wasn't risking it) until it's broken in and I can get it on the dyno to tune.

Regarding low load/speed knock, what is the general consensus on this? Are the pressures involved too small to really be of concern? Or is this definitely something that needs to be addressed and eliminated (pending it is truly occurring of course)?

Rather surprised 'forged' parts are such an issue, I might expect piston slap when cold, but...

This is kinda out in left field, and it would be really odd for two different engines, but come across it on a different vehicle - is it possible there's a heatshield, or other piece of tin or whatever, vibrating at those low engine speeds?

It's indeed piston slap that I'm hearing. Pretty audible at idle and during overrun (even when warmed up). These are 2618 alloy which I've generally understood can swell up quite a bit under high heat... And not surprisingly, when I get a little more than cruising around town heat into them, I've noticed they start to quiet up. Part of it is that I went to much more rigid engine mounts and don't have a comparison with the stock engine using these mounts. It was just more noise than I anticipated is all. Guess it kind of makes sense with a flat engine though since gravity would be putting more lateral force on the piston relative to straight or V setups...?

Anyway, as for my real concern (this pinging sound), this is actually what I had initially thought was going on! But this build is all wrapped and has none of those heat shields or other stock nonsense to rattle around. And also, it definitely goes away when timing is pulled. I've been straining my brain on theories as to what this could be. If it isn't actually knock, then I'm at a loss as it primarily seems to always happen around the 0.40 to 0.50 g/rev area and I've actually observed that the RPM range it occurs in is broader than originally thought. I've heard this from 1200 RPM well up to 3500 RPM at this point (making me less inclined to believe it's mechanical). The only common factor is load and timing. But if I add load and the timing remains relatively the same, it goes away completely. I guess it could be something mechanically wrong with the engine, but to have two completely independent engines - that share virtually no parts - do the EXACT same thing at the EXACT same time (and the new engine literally did this from the very start), it's hard for me to be able to mentally validate that it's mechanical in nature.

I think I need to try a few things... For one, I'm going to richen it up in this area and see what that does. Two, I'm going to temporarily detach the AOS vapor returns and see if maybe this is related to some kind of oil vapor contaminating the intake charge. Three, I need to validate that this isn't temperature related; possibly cool the manifold and see if that changes anything, check for a cylinder running too hot, etc. Four, as Ross mentioned, I'm going to put some none filtered microphonics on the engine and surrounding components to see what I can find. I'm beginning to wonder if it somehow the transmission or something in the accessory system doing this; granted, that makes approximately less than zero sense to me. Grasping at straws here, unfortunately...

Thank you for the feedback! Would love to hear more or from anyone who has thoughts!