Thread: Ask a tuner...
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Old 04-01-2010, 08:01 PM   #54
AKSubieDubie
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Join Date: Sep 2008
Chapter/Region: AKIC
Location: Fairbanks Alaska
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2000 RSTi BW8375
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I can see that. I remember reading on a thread about the same theory. I played with it a bunch and didn't really see any difference in learned knock values.

I think I'll be zero-ing them out when I finally build a set of equal length headers, as my current tune seems to be spot on with timing. Been ripping around with the 30r maxxed out for about 20k miles! (motor has 24k on it since rebuild)

Just something to think about.

Thanks!

Quote:
Originally Posted by P3Auto View Post
Ok are you ready for this?

The reason that I at least feel Subaru runs slightly more advanced timing on the back pistons is because of the position of the knock sensor. The engineers wanted slight knock to be heard the best. By advancing the cyls closest to the knock sensor it will hear and detect knock where it might miss slight knock from a front piston.

Ever wonder why the back pistons seem to be the winner for piston damage...If we look at an 05 STI tune we will find the front cyl timing is retarded as much as -3 degrees at certain rpms while the rear cyls are not retarding timing at all.This may not seem like a lot of timing difference but even slight knock can turn severe quickly. In other words 20 degrees may not knock at all, where as 21 degrees may start to knock. Iit might take 3-8 degrees of timing reduction to quiet the knock started by the 1 degree of advance.

This is why many Subarus I have had to tune had "flat spots" around 5000RPM or so. The car was detecting knock but having a knee jerk reaction pulling timing out to stop it. By reducing timing a little in the hot spots the end result is more power and timing since the ECU won't be having to pull gross amounts to stop the knock event.

Sorry got off on a bit of a rant.

To summarzie, I see no reason to mess with the per cyl timing compensation tables. I'm sure there is some optimization that could be done but it would require special equipment to monitor each cyl for temp, pressure, etc.
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