I've read on another forum (SRT4 i think), that running too rich and with too little timing can cause what they called "rich misfires" that register as knock. Could this phenomenon actually occur? Does it occur with our platform? Could running too ruch with too little timing cause REAL knock to occur? I understand that generally running rich and with lower than optimal timing can lead to excessively high EGTs, and generally torture the exhaust valves, but can this condition lead to a type of real knock?

In either case, when tuning a forced induction set up, if you start out with a rich tune and pull some timing until the fuel is dialed in (this is what i tend to do), you may start off seeing an assload of knock retard, and pull more timing to combat it, only to make it worse right? This is kinda a new concept to me, but i'd love to hear from those with more experience on logging KR on a rich tune with less than optimal timing. I wish i had a specific example to shaer (i may have to do some searching), but if you are running AFR's in the 10s or 11s and pulled ,say, 3 degrees in the highest load cell and still see KR what would be your first course of action?

Sorry for speaking in such generality, it's just a few threads i've read on other forums that seemed to contradict the usual

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10s or 11s and pulled ,say, 3 degrees in the highest load cell and still see KR what would be your first course of action?

Well depends. If I have already pulled a good amount of timing out (to keep it safe), then I will work on the fuel more and get it where I want it. Usually 11:7-12:0 range, then work on the timing.



FU Tuning

Wouldn't that technically be preignition because of the temp being too high???

Its called fuel knock, and I know a few dsm friends that are scared it'll eventually happen to them and they wont be able to get rid of it. I honestly never plan on running a boosted car that rich, might be because I come from the mindset of assuming you'll make more power with an ok amount of timing and a leaner fuel mixture than a couple more degrees of timing and a rich fuel mixture (which can only be proven as to whats better on the dyno).


1994 Saturn SL2 Home Coming Edition: backup car
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leafy, do you have any specific references? I googled the term fuel knock and came up with mostly diesel pheonomena. Do you have any links to cases in gas engines? I want to read and learn.

Ive saw this "rich knock" occur on Phil's car when were having VE scaling issues way back a few years ago, typically it wouldnt happen unless it was 10.8AFR or lower, when the car was leaned out into mid 11s the knock went away. However if the AFR went above 11.9 then knock would return.



1989 Turbo Trans Am #82, 2007 Cobalt SS G85

Rule of thumb ive always been told is get your AFR right before even touching timing



1989 Turbo Trans Am #82, 2007 Cobalt SS G85

Rodimus Prime wrote:Rule of thumb ive always been told is get your AFR right before even touching timing

You are correct.... However, timing adjustments can change your AFR.....





P&P Tuning
420.5whp / 359.8wtq

So to revisit this, I did some pulls last weekend in my M62 L61. Very basic build, stock pulley. Water/meth off, AFRs are about 11.5-11.8:1, max timing at WOT is 16*. It was nice and cool out, maybe 55-60*F. NO heat exchanger, and after repeated beatings with the water/meth off, max IATs were about 130* and there was ZERO KR. Of course the IAT quickly cooled off throttle.

Now, i turn around to come home, engage the water meth (~150cc/min 80/20 water/meth), and the AFRs drop a little - maybe 11.0-11.2:1, IATs barely exceed 100*F. But there are specs of KR 1-2* here and there. There was no reflash in between the runs - same tune. So even though on the surface the conditions were less favorable for knock (in my opinion) on the return trip with the water/meth on, there is something going on there. I suspect if i ether add some more timing and/or lean it out a hair, the water/meth on will produce clean pulls as well. Is this approaching the phenomenon discussed above? Is the water just slowing the burn too much, causing minor/unnoticeable misfires?

Now, water on or off, the car pulled great with the cold ambient temps, and there was really no perceivable difference in performance either way. And it's not that 1-2* blips are overly worrysome, it's just totally counter-intuitive that they would be observed with the water/meth on, cooler IATs, and slightly richer mixture, while the other condition was totally clean.

I don't know anyone that runs 80/20, 60/40 is the most water I have seen run. I would maybe give it some more Meth and see how she responds. I could definitely see too much water causing a misfire.

I'm increasing the methanol concentration in the mix now for two reasons. The first you mentioned, the second is although water pulls more heat upon evaporation per molecule than methanol, methanol evaporates much faster and does so more in the intake rather than the combustion chamber like water. Honestly i'm probably going to end up reciprocating that ratio - 80/20 meth/water.