How I'm fighting knock at the rev limit
Nov 27, 2013, 03:30 PM
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How I'm fighting knock at the rev limit
Hi all. I'm an amateur / enthusiast. I share these findings because why not? Take it for what it is...
I have a stock turbo Evo X MR lightly modded. Yesterday I went WOT in 2nd gear and bounced off the rev limiter a couple times. Got a big backfire and all kinds of smoke for my fun. I happened to be logging at the time and boo!, saw 13 knock count. Looking at the log and the thing that jumped out is that that fuel cut is not a simple cut (and hold) at off until some reactivation RPM is reached. IDC oscillates on/off. I thought this may have something to do with too high a value for Rev Limiter Low/OFF. So this morning, I reduced the Rev Limiter lower value from 7198 to 6000, crossed my fingers and paid another visit to the fun zone. Maybe RPM bouncing around up there was causing epileptic fuel ON state, and a big reduction should increase the margins, right? No dice, IDC still toggled unabated, just like yesterday. BTW, that run logged 10 knock, fun. Sad log posted - blue line mars entry to rev limit:
Finding: IDC toggling happens meaning there is new fuel being injected while you're riding the rev limiter. You're not going lean for a split second as leftover fuel is scavenged from the manifold. Don't be surprised to see (real?) knock, because there's (silly) combustion as you ride the limiter. So, don't ride the rev limiter.
Now I'll share with you how I'm addressing it. As for now, I haven't found the code (or become comfortable) to suss out how fueling really works at the limit. So I punt and cut boost prior to hitting the rev limiter. Changed boost tables by changing the 8000 RPM row to 7501 RPM and zeroing BWGDC for the entire row. Now, from 7500 onward to 7600 RPM (rev limiter) car is already dumping boost (about 3 psi by the time it hits first cut), and shortly thereafter boosting WG spring pressure. Corresponding reduction in load brings down the knock threshold to where car is happy enough to suffer the fluctuating IDC and operate knock free at the rev limit. Did three pulls is 2nd and 3rd gear back to back to test. Notice WGDC bouncing as you still bounce the rev limiter
Better log here:
I have a stock turbo Evo X MR lightly modded. Yesterday I went WOT in 2nd gear and bounced off the rev limiter a couple times. Got a big backfire and all kinds of smoke for my fun. I happened to be logging at the time and boo!, saw 13 knock count. Looking at the log and the thing that jumped out is that that fuel cut is not a simple cut (and hold) at off until some reactivation RPM is reached. IDC oscillates on/off. I thought this may have something to do with too high a value for Rev Limiter Low/OFF. So this morning, I reduced the Rev Limiter lower value from 7198 to 6000, crossed my fingers and paid another visit to the fun zone. Maybe RPM bouncing around up there was causing epileptic fuel ON state, and a big reduction should increase the margins, right? No dice, IDC still toggled unabated, just like yesterday. BTW, that run logged 10 knock, fun. Sad log posted - blue line mars entry to rev limit:
Finding: IDC toggling happens meaning there is new fuel being injected while you're riding the rev limiter. You're not going lean for a split second as leftover fuel is scavenged from the manifold. Don't be surprised to see (real?) knock, because there's (silly) combustion as you ride the limiter. So, don't ride the rev limiter.
Now I'll share with you how I'm addressing it. As for now, I haven't found the code (or become comfortable) to suss out how fueling really works at the limit. So I punt and cut boost prior to hitting the rev limiter. Changed boost tables by changing the 8000 RPM row to 7501 RPM and zeroing BWGDC for the entire row. Now, from 7500 onward to 7600 RPM (rev limiter) car is already dumping boost (about 3 psi by the time it hits first cut), and shortly thereafter boosting WG spring pressure. Corresponding reduction in load brings down the knock threshold to where car is happy enough to suffer the fluctuating IDC and operate knock free at the rev limit. Did three pulls is 2nd and 3rd gear back to back to test. Notice WGDC bouncing as you still bounce the rev limiter
Better log here:
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