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Ignition timing on supercharged SOHC EJ25 with centrifugal supercharger and cams

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First time poster so I apologize if this is in the wrong spot? I am working on building up a Subaru SOHC EJ25 with a Vortech V3 Si. The bottom end is currently stock (10:1) motor and I run 50/50 water methanol injection on USA 93 octane E10 fuel. I currently run a 2.4" pulley and 5.22" pulley which puts me around 14psi at 6200rpm, with the impeller going up to 48546 rpm (well below redline). I am currently tuning it on the street but will eventually build the motor and dyno tune it. (Eventually I'd like to drop the static CR a bit, do a forged EJ257 bottom end and Supertech valvetrain and rev it out to 7000+ and I think it will really respond well to the cenfrifugal SC.)

I recently added Delta 1500 cams which are a regrind of the stock cams that add about 10(intake)-15(exhaust)% more lift in the same duration and same (none or very little) overlap as stock cam. My problem is that even after adding fuel back, I ended up having to pull what seems like an amazing 9 degrees of ignition timing at redline in order to get it to stop knocking (and we're still right on the edge of knocking). My wideband reads in the mid to high 10s (this is the "gasoline" AFR including both gasoline and water-meth mixture) with a Snow #5 nozzle (375 ml/min) with the controller set to max spray at 16psi and start at 4psi.

Is this expected behavior? I understand that MBT will be lower with the more efficient cams, but the amount of ignition timing retard to avoid knock in my case seems drastic. Is a typical centrifugal SC ignition timing chart flat, negative or positive as RPM and boost increases? Would I be better off swapping to a larger pulley for less boost and running a more "normal" amount of ignition timing rising (or flat) with RPM instead of dropping? Attached are pre and post camshaft install timing charts.

It feels a bit like this is false knock and I am going in circles...but I am not sure how to validate it is not real knock.

Attached Files

If you're improving the engine's VE, which should be the case with a cam upgrade if you've made the right choice, you'll have a larger combustion charge and in this case naturally we'd expect the timing to need to be a little lower to achieve MBT. What i think you've probably stumbled across however is the point where your high static CR and low quality base fuel is probably becoming your limiting factor and this is why the engine has become suddenly more knock sensitive. You may find that by lowering the boost you can make the same power as you previously had (hardly the aim really), however the better option would be a superior fuel which has a higher octane rating or a lower compression ratio - 10:1 is very high for a boosted application on gasoline based fuel.

Thanks - Just for clarification, it is 93 AKI base gasoline which would be like 98 RON.

What I am struggling with is confirming it is real knock. The reduction in timing above 5200rpm just seems very drastic, compared to the rest of the RPM range, which only needed a minor reduction in timing, if any.

On the same day(same weather conditions), I took a pull with a little knock and then on the next pull, I purposely turned UP the ignition timing btdc to get it to induce a little more knock to confirm that it is real knock. On the next pull it did not knock at all, and made more "load" (aka more MAF flow at the same rpm which I presume to be more power).

Is it possible to knock (or for the ECU to register knock) from too little ignition timing?

You may well be right. I'd strongly recommend considering some audio knock detection equipment so you can validate the knock count. I always do this before I trust the results from electronic means in a data log.

Also you can correlate your knock data by looking at your spark plugs, is not son difficult , some reading and practice requiered but it helps.

There are a lot of info and pictures on the web to look for .


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