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Detonation and Preignition FYI

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  • Detonation and Preignition FYI

    ok, so im wondering which of these 2 explanations is correct in describing what detonation is. is detonation when the air/fuel mixture gets so compressed and hot that it detonates BEFORE the spark plug goes off? or is detonation when a small pocket of air/fuel spontaneously combusts after the spark plug has been set off but has not completed its burn? the reason why im asking is because in Supercharged! by corky bell, he talks about how having an intercooler will cool the compressed air going into the cylendar and make it less likely to detonate. this would lead me to believe that detonation would be the first explanation. in which case if you ran nitrous while supercharging it would lower the cylendar temperature considerably, and thus reduce chance of detonation. i hope this makes sense.

  • #2
    It is my understanding that they are both detonation. You also have pre-ignition. There is a very very fine line between the two. Detonation is described as the fuel combusting from the heat and pressure of the combustion chamber. This could could occur before or after the spark, it doesn't matter. Pre-ignition is describes as an alternate source igniting the fuel, such as a hot spot in the combustion chamber, or a sharp edge on a piston valve relief. Again this could occur before or after ignition. This is my understanding. I may be wrong.

    The bottom line is you have uncontrolled combustion occuring in the combustion chamber. Whether before or after the spark you have flame fronts that will collide or a combustion that get highly compressed because it occurs so early, and the result is extreme stress on the engine components, that will quickly make alloy stew of your motor.
    Bryan Pendleton | LeMons Racing Blog | BPi Flow Stacks | Facebook Anyone?


    • #3
      Yes, both types of problems will occur and are generally referred to as detonation. The two problems are quite different, however, and have different solutions. Corky has a bit of a discussion of both in his other book, IIRC.
      Former PGT-turbo owner... now 2010 VW Golf TDI


      • #4
        Knock and detonation are one in the same. The actual sound of knock is the pressure waves beating the schnizz out of the inside of the cylinder, piston, and valves. Hence, if you hear knock, there are already bad things going on inside the cylinders. I have pictures somewhere of a 4G63 that literally shot 2 rods out of the bottom of the oil pan from it and bent the other two

        Pre-ignition is caused by hotspots (which can also be a plug tip that stays too hot), as was said, and is basically the same as too much timing advance. It's basically still a smooth flame front that may or may not have waves in opposition to the regular spark ignition.

        Detonation is actually ignition without any help from hotspots or spark plugs (auto-ignition..and that's the problem). It's an unsmooth, uncontrolled explosion (hence "detonate")....not a "burn"...and is pretty much guaranteed to have contradicting pressure waves .

        Detonation is only "worse" because of the huge, rapid increase in cylinder temperatures and omni-directional pressure waves. You end up with pressure waves going it all kinds of directions and even against the plug-ignited flame front causing a collision and super-high cylinder pressures/temps.

        As if that weren't bad enough, with both pre/auto ignition, you have combustion out of sync with the crank rotation. Imagine having that super high combustion when the piston is on its upward movement. The combustion will try force the piston down like it normally would on the power stroke, but that's not going to happen. So you end up with a piston that's moving into the pressure wave and neither one wants to budge. With a bunch of pressure and no where to go in any direction, your parts are either strong enough to take the beating or they're not. If you have a weak spot, the pressure is going to find it...that's how you end up with holed pistons, cracked lands, burned valves, blown head gaskets, bent rods, etc.

        As for nitrous, you're thinking is there, but it's not quite the whole picture. You have to remember that it will not only be cold, which will keep cylinder temps down, but you are adding oxygen and a more dense mixture, which needs matching fuel or you'll be super lean and cylinder temps will be through the roof. You will need to add the proper amount of fuel to maintain the ratio for that new air, so at best it will be a wash...and that's exactly how it's supposed to work.

        The only easy ways to control cylinder temperatures is via mixture characteristics or timing. You could pump 10x more air than the stock engine normally uses and if you maintain the proper ratio of fuel for that air like you're supposed to, cylinder temps should not budge, even though the combustion is much larger (more power). If you run too little fuel for the air (leaner), the cylinder temps would increase, if you run more fuel for the same air, you run richer and cooler cylinder temps.

        The other way to play with cylinder temps is to adjust timing. Retarding the timing will ignite it later, give you less of a complete burn and a resultingly weaker combustion (less power) and lower cylinder temps.

        As you can see, it's all a matter of playing with the mixture itself or how completely the mixture is burned. If you get more into it, you can inject things like alcohol, water, or simply go with a higher octane fuel, which again, all change the mixture properties.


        • #5
          Found some of the pics of the detonation results. If you have weak stomach, don't look. Looking at these pics is like getting kicked in the nellies :o

          This is one of the rods that came through the pan. As you can see, it shot out sideways no less. The wrist pin literally blew a hole in the pan and the small end of the rod tore a huge gash in it.

          That's the gash from the rod. Through it, you can see big end of the rod is still attached to the crank. The shiny mess in the middle is where the rod snapped off.

          This is the rod as pulled from the pan (first pic). The piston was truly "free floating" around the crankcase somewhere


          • #6
            I see where I was misreading you. I was thinking of it in terms of heat from combustion and not just overall temps.

            Adding nitrous generally isn't thought of as a way to cool cylinder temps because it's not a full-time solution, but yes, given the temperature of nitrous, with the proper fuel, it will cool the intake air temps and that would have an impact on overall cylinder temps. That benefit would be multiplied if you were boosted w/o an intercooler. So, I'd have to say yes, with the proper fuel to cover the extra air in the cylinders, nitrous would have some cooling effect on cylinder temps (I have no idea how much), but as I said it's not really a feasible full-time solution.

            Given the cost of refills, the duration a bottle will last and the extra fueling requirements, it's definitely not the way to keep cylinder temps cool(er) on a street car. On a track-only car, it's another story entirely. Some of the fastest cars run huge boost, non-IC'd with alcohol and nitrous for cooling (Abel Ibarra comes to mind) or intercooled w/ nitrous on gasoline.

            For the street, get a FMIC to get compressed air temps as close to ambient as possible and then use fuel/timing tuning to keep cylinder temps in check....the tried and true, 24/7 "right" way to do it....and it's a hell of a lot cheaper that daily bottle refills in the long run


            • #7
              well if i ever did add nitrous onto my non-existant supercharger i would only be using it to add a little extra hp when at the track or in a really good race. i just wanted to make sure that it would be safe to do so. but now that i think about it, it could kinda work as a detonation deterrent all the time couldnt it? i mean when youre just driving around between 1500-3500 rpm its not like youre getting that much boost anyways. so you could set the boost so that when you get to like 5000 rpm its running a little higher boost than the point you get detonation at, and just always have your nitrous switch in the "on" position. so you should be getting detonation, but the nitrous would cool down the charge. i doubt that made much sense, but in my head it would work... not saying i would do it, like my engine a little too much.


              • #8
                Well, there are limits and that won't only be left up to tuning, but fuel characteristics as well (octane). You'll need to add fuel for boost whether you have nitrous or not. If you add nitrous, you'll need to add fuel for that on top of the fuel for boost. That's a LOT of gas. You may not think you'll be on it much now, but when my PGT was boosted, the pedal was on the mat....often

                I definitely wouldn't call nitrous a "detonation deterent", far from it. I know a lot more people who have screwed their engines on nitrous than with boost. Combining the two will make tuning, and the ability to supply enough fuel, that much more crucial.

                Retarding the timing, richening the mixture, running lower boost, increasing the octane, cooling the intake charge or any combo thereof are detonation deterents. They all lower cylinder temps and decrease the chance of knock. You've taken ONE aspect of those (lower IATs) and are looking for the most costly and difficult way to achieve it

                If done properly, what you're talking about IS feasible and will cool temps to a point (still seriously doubt the benefit out-weighs the cost), but that's definitely not the most practical way to achieve the goal. For the price of a nitrous kit, you could have a FMIC, have ambient intake temps and not have to pay for refills. That, and a good A/F mixture under full boost and you're cylinder temps should be in real good shape without adding to the fuel demand.

                Running two power adders is not for the meek. If you do look at those track cars that run boost/nitrous, you'll see all of them run huge injectors, sometimes 2/cylinder, have fuel pumps that will empty a swimming pool in 15 min., and don't care about MPG or need to pass emission tests.

                Intercooling, proper fuel supply and management is all it takes. You're making it much more difficult and expensive than it needs to be. You would be much better served with a water injection kit if anything. It will drop temps, water is the ultimate anti-knock solution, and it won't increase the fuel demands any more than the little extra needed for the denser intake charge. It's freely refillable and most kits activate immediately on positive manifold pressure or at a set boost level.

                Running nitrous with boost on an engine that will only hold 7-12psi seems ludicrous to me, especially for the still apparent lack of fuel supply parts and tuning ability. I think you'll have enough trouble getting and tuning good fuel for either one separately, let alone both together.

                K.I.S.S. is by far the best philosphy...


                • #9
                  Corky's book gives a lot of good info on calculating the heat created by a supercharger and the heat from compressing the air. There is and equation to get that plus the normal heat from the compression ratio and you get your final cylinder temp. I figured out what mine is (can't remember off the top of my head), but what is safe and what is not? On 93 octane and factory timming, what temps are ok if you have a good A/F ratio (about 13.5:1)? Thats what I want to know.

                  Supercharged Probe GT


                  • #10
                    First of all, a good A/F ratio is 12.5:1 ... 13.5:1 is actually fairly lean, at least for running under boost. Also, all the graphs I've seen for power vs A/F ratio put peak power at 12.5:1.

                    Second, the detonation temperature of 92-94 octane gas is approximately 500-860oF. Yes, this is a very wide range -- that is because it depends on a lot of factors including quality of gas, how well it is injected, A/F mixture, etc. I got the figure from Howard Chu's turbocalc spreadsheet, and I think his source was gas company publications or something like that. Your mileage may vary (so to speak).
                    Former PGT-turbo owner... now 2010 VW Golf TDI


                    • #11

                      Yeah, those calculations are available everywhere. They're probably pretty close in most respects, but bench racing is no sustitute for real world feedback (dyno/logging/gauges).

                      Most people don't measure cylinder temps per se, exhaust gas temp (EGT) is probably one of the most commonly used methods (which measures "cylinder temps" outside the cylinder . Mount the probe within a few inches of the leanest cylinder's exhaust port and go from there. "Safe" for me will be about 1400-1500°F on pump gas. On race gas, I'd let it peak about 1550-1600F right before the shift.

                      13.5:1 isn't what I would call a good A/F. It takes fuel to make power. If you're are running stock timing and it's not being pulled from the knock sensor with that A/F, I would be VERY surprised. A "lean" part of the curve on my car would be 12.0:1 and "rich" would be less than 11.5:1. If you're looking at it from an A/F gauge point of view (which isn't really the best feedback for tuning), that's going to be pretty much pegged the hell out.

                      Like I was saying in another thread, if you tune fuel for good timing, EGTs will almost always have to fall in line. If the EGTs get too high, it's most likely going to knock first, which you'll see on a datalogger. Richening up the mixture will combat knock and drop EGTs. Tuning fuel just rich enough to hold the maximum timing the ECU will give (or as close to that as you can) without knock is what you want to shoot for. If you get there and EGTs aren't insane (shouldn't be at all), that's the best you can hope for without additional timing control.


                      • #12
                        Yeah, I ment 12:0:1 - 12.5:1. Thats what Corky's book says. It was late when I typed that. So safe temps on 93 octane with a 12.5:1 A/F ratio would be anything less than 850 deg F and EGT of anything less than 1500 deg F. That comes out to 1350 deg R absolute cylinder temp, which is about what Corky said is the safe limit, but I would have to look.

                        Supercharged Probe GT


                        • #13
                          i had also heard that playing with the individual cams timing on ur twin cam changes temps alot aswell - espically on the exhaust cam on boosted engines.....which in turn also effects EGT's...i had also heard that an EGT of around 800 degrees is ideal for most turbo's...??

                          this sound right 2 others out there?
                          WAS: 1989 MX6 2.2 Turbo 4ws

                          NOW: FE3 MX6 - 2 Cams is better then 1.....much better!


                          • #14
                            For the Probe....

                            To combat detonation.....

                            I know:

                            We can increase FP to almost injector static limit.
                            We can run cooler plugs.
                            We can buy bigger injectors.
                            We can decrease boost.
                            We can intercool.

                            Does octane booster really work or only true race gas?
                            Is there a way to retard timing on the stock ECU without MSD?

                            It sucks that we can be rich on our AF meter but still get detonation do to hotspots.

                            I AM A BOOT CAMP NON-BOOSTER ....but willing to learn!


                            • #15
                              racing gas will fry your o2 sensors, and make your engine run rich.........which initally sounds like a good thing, more fuel is better for boost. I just dont like the idea of having my ECU not knowing o2 levels in a boosted engine, would just make me nervous. I dont think octane booster would be very reliable either. Since you just add a can into your gas tank, theres no way to assure that it will mix well, leaving you with a range of octanes. Which again, on a boosted engine, i wouldnt want.