Detonation problem ace'd!

hytorksc

Registered User
I believe i found another way to eliminate detonation, assuming all other fuel and sensor components are set and working correctly.

Has anyone tried inserting a resistor across the octane plug wires?

i found that there is a wire (red stripe on grey) that connects to the temperature sensor and the octane plug- found it to be the same wire using a electronics tester. in fact this grey and red wire seems to be connected to all of the common sensors that control engine behavior ie; TPS, IAT, CTS... pretty much all of these sensors are based on controlling computer response by resistance varied by heat or mechanical position. the coolant temp sensor controls fuel and timing based on a range of resistance varied by the heated element on the sensor. it also has the biggest influence on engine behavior in terms of fuel and timing curves (aside from using an EEC tuner). resistance goes down as the engine get hotter, thus retarding the timing and changing the fuel curve as appropriate. i measured the resistance between the wires on the octane plug and found it to be 1600 ohms. normally by default the octane plug is closed (connected), and if you pull the plug out it retards the timing by 3 degrees (in effect inserting 1600 ohms of resistance), turning the SC into a performance slug. interaction between the temp sensor and octane plug setting by that common wire indicates that the computer controls the timing under various engine conditions by altering the resistance in ohms. you can control resistance by inserting a certain resistor value across the octane plug thus setting the timing anywhere from default to - 3 degrees. i found you can also alter the timing curve (and richen the fuel curve) slightly by inserting a certain resistor value on the green lead in series coming off the temp sensor.

here are the resistor values i chose that completely eliminated the detonation without perceiving a loss of performance;

inserted (spliced in alligator clips temporarily) a 1300 ohm 1/2 watt resistor across the octane plug leads. this is a very precise adjustment- you can experiment resistor values here and see what works best- higher resistance=less timing, lower=more timing.

inserted with alligator clips a 3300 ohm 1/2 watt resistor in series on the green lead off the temp sensor. you can vary this up to 4500 ohms without running into detonation again. this seems to sharpen throttle response on my SC (similar to when you first start up with a cold engine). I did not notice this resistor to degrade cold start up performance.

if you still get detonation with these values, increase the resistance across the octane plug in increments of 100 ohms until it's gone.

Results??

no detonation whatsoever (could not hear any pinging or clicking sounds) under any hard acceleration loads in any gear or WOT.

At least by my seat of pants feel, the car is much more responsive than ever, and it does not bog down like it used to at certain rpm's/loads. top end power got better also. the car didn't feel as heavy. it's worth it to me-

some of you reading this will probably criticize the hell out of this and think that i'm crazy because i'm not doing it the "right way" with an EEC tuner, but the way i look at it... somebody's got to experiment and try to unlock the secrets of managing control of this car and achieving similar results without too much complication or money spent. I think my way is better because the resistor mods are subtle and it stays closer to stock computer settings assuming you have a mildly modded SC. the computer should not "learn" these changes out because these mods will not set a code or go outside of internal lookup table parameters. Modifying the electronics a little bit does not necessarily make this the "wrong way". You have to understand overall how ford sensors interact and change fuel and timing parameters. I mean if ford did it right the first time we wouldn't be going thru all of this. And if i wanted the ultimate performance machine it would not have bought the SC.
like i said, it works for my SC very well, maybe because the engine is old and carbonized. it's been running it this way for a couple of weeks now with absolutely no drivability or starting problems (cold or hot) other than the leaky injector thing when the engine is hot. i felt i had to share this info. to other SC'ers that are going thru challenging times with their cars.

sorry about the long thread.
 
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Good info there man, when school's out I will start doing some serious crap to my bird and the thing you did might be included in the list, so I might get back with you later and ask some questions.
 
One thing I've never seen discussed here is that the program in the EEC was developed using nominal sensor values. Thermal type sensors are simply not going to be identical from unit to unit or car to car. So trimming the sensors with resistors should allow you to find improvements. The resistance values required will vary from car to car, so what works for you wouldn't necessarily work for anyone else.
 
Explanation

The EEC provides enrichment, like a choke, and timing advance to burn the extra fuel, for "cold" engine coolant temperatures. That's what you do with the resistor in the temp circuit. The octane pin in just a switch. When you put a resistor across that, it's the same as just unplugging it, and you ask for 4 degrees less timing, which just happens to negate the advance for cold engine. Net bottom line is the car is running just a bit richer across the board. Get with Duffy over at TBSCEC if you want the response curves for any of the sensors. Also, be careful with this technique. If you do both the engine temp sensors, your fans will not turn on until it's too late .........
 
RBM91 said:
So trimming the sensors with resistors should allow you to find improvements. The resistance values required will vary from car to car, so what works for you wouldn't necessarily work for anyone else.

I fully agree.

I posted the resistor trim article primarily to try and help anyone here who may be having serious detonation issues with their SC.
Removing detonation will improve performance slightly. The performance improvements are a side effect, like sharpening a tool. I love the consistent, instantaneous response my SC now has. It now seems that after my AOD kicks down WOT at 30mph, the car pins me to the seat and takes 3.something seconds to rocket to 60mph. Now i'm not afraid of making further improvements like a 10% OD pulley for instance because i can always dial out the detonation if any (up to a certain point).
 
I would be concerned that making those changes to reduce detonation simply hide the true cause of the detonation, whatever that might be.

I wouldn't think you need to retard timing to resolve a detonation problem. Sure, it will help, but something else must be going on here.

But if you got it running and are happy, there you go.
 
To Dr. Fred- need clarification

So what you're saying is the measured resistance across the octane plug does not mean anything? I used several resistor values ranging from 1500 ohms to 1200 ohms when i tested this and the detonation progressively dissappeared as the values got higher. On some resistors the detonation was still there but to a lesser degree. I also noticed that the car got a little less responsive as the resistance got much bigger.

Are you saying that anything less than the 1600 ohms measured across the octane plug leads just switches timing to -4 degrees like digital zeroes and ones? If it's just a digital contact switch relying on whether the circuit is opened or closed, why would there be a measured difference of 1600 ohms between the leads and why is it connected in the same path electronically as the temp sensor? Doesn't the temp sensor vary resistance thru a certain range to incrementally adjust timing and fuel instead of just switching?

I agree with you on the cooling fan situation, but my fan is hard wired so i won't ever have that issue. If anyone else does this they may want to consider how it will effect fan operation before they try it.
 
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The octane adjust plug is simply a shorting plug on circuit 359, which is one of several signal return circuits (a "ground" reference for the EEC, although it is not the same as battery ground). Circuit 359 includes the ECT, ACT, TP, HEGO, KS, and BP sensors. When the octane plug is installed, circuit 359 is tied to pins 46 and 29 of the EEC, and with the plug removed, pin 29 is floating, and 359 is only on pin 46. The 1600 ohm reading you got across the pins is meaningless, it's just the resistance of whatever circuits are between pins 46 and 29 in the EEC.

The octane plug does not simply add resistance to the circuit, based on the wiring diagram I would say the EEC tests pin 29 to determine whether the plug is in or not.

I'd guess that inserting a resistance into the circuit affects the values measured by the EEC for all those sensors on circuit 359. Definitely not the right way to tune or fix anything. Messing with the HEGO signal is especially a bad idea, as this is a very sensitive and important signal. Although it sounds like you are probably running richer, not leaner, it's still not a safe plan.
 
thanks Rob

Well, i guess that idea got shot down royally. i'm just trying to fix my detonation issue, and the funny thing about this it seems to change the behavior of my SC engine to actually work for me. But i don't want to screw up anything in that circuit either.

i don't know what else to do- i don't want to leave the octane plug out- the car is just too slow like that. with it in, it pings like the devil. all sensors are new except for the intake air temp sensor and the knock sensor. i can't afford an EEC tuner right now.

My apologies to everyone for waisting your time on this post.
 
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Thermal sensors function by varying resistance with temperature. No two sensors are completely identical. I know nothing of the octane plug but the thermal sensors can be trimmed by increasing/decreasing resistance. This is the reason that, for optimum performance, the vehicles program needs to programmed uniquely for that vehicle.
 
RBM91

You are correct in that no two sensors are identical. However...... .looking at the FORD response curves for those resistive type sensors the values given are stated to vary by up to +/- 15% so the EEC programing already takes that possible variance into account in the calibration programing. Apdative Strategy will cancel out variances over time as well as the sensor ages.
 
i just drove my SC to the post office to send off my taxes (yeah, i'm a lazy one) and gave some thought to the postings above warning me about modifying the overall circuit resistance of the sensors. at first after reading these posts i thought i imagined the performance improvement, but it's actually for real. judging by the car's behavior it seems to really like what i did. if all i did was richen the fuel as indicated in the above posts, i don't understand why it acts like it has more timing WITHOUT DETONATION. why does my car run so much better if this is not the right thing to do? i'm not throwing any codes, as far as i know all of my sensors are working (there were no codes before this setup), the engine does not bog or hesitate, nothing seems to be out of adjustment, i even noticed my gas mileage went up to about 320 miles per tank mostly highway driving. i drive my SC about 450 miles per week and i have been running it like this for 2 weeks now without incident. i even went to the beach last weekend with the setup. i just don't get it. if it is working for me why can't i keep my settings? if the HEGO part of the circuit is adversely affected, does it ruin my o2 sensors or something? Or will it prevent me from passing emissions? if this was the case shouldn't it throw codes at me? what gives??
 
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I'm never for changing sensor resistance, or doing things to fool the EEC. If your car has detonation problems, it's best to get it to the dyno to test her out. Over the weekend I saw two lightly modified SC's pull an a/f ratio above or at 14:1 at WOT. One had a bad fuel pump, the other just had C&L that just leaned it up too much. In both of those scenarios, the cars will drive better, have more power, and last longer when the real issue is fixed.

A SC on the stock computer shouldn't ping. The stock timing is not that agressive. If it is pinging, look at getting an I/C fan to help cool down the air charge. Sometimes the stock and spearco intercoolers get heat soaked, and cause the intake temps to rise.

Beyond that, don't drive the supercharger too fast. If you don't have a MPII or ESM supercharger, 10% is more than enough, even 5% is enough is some cases.

Those are my recommendations for somthing like this.

Brian
 
Maybe you are lean to begin with and by richening that curve, you balance out the equation. If your 02 sensor is new I would guess you have a vacuum leak, small enough not to notice but enough to cause small detonation because of excess air entering the engine. I would install an A/F gauge so that you know for sure.
 
my o2 sensors are new. maybe i have a pin hole leak in my headgasket on one of the cylinders causing the pinging. it pings only when the octane plug is connected with no sensor mods (stock setup). i spent a very long time (almost the whole time i owned the SC) trying to figure out why this thing pings.

i guess when i get some money i will have the engine tested for compression leaks and de-carbonization, then get an EEC tuner and dial it out- assuming nothing else is wrong.

thanks for everyones input.
 
I must be confused about how the EEC works. From what I understand the computer runs closed loop (feedback from oxygen sensors etc) on part throttle. But at WOT it reverts to a map that is only modified by the various thermal sensors and MAF with no mixture feedback. Is this incorrect?
 
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