Discussion in 'Performance Mods' started by jaguar, Jan 15, 2014.

  1. jaguar

    jaguar Well-Known Member

    I finally got my first case of detonation in my Suzuki 100cc. Yeah I know it isnt a motorized bike but it is a small 2 stroke engine and will serve as a good example in this case. Detonation is similar to pre-ignition and these are the main factors contributing to it:
    1) excess cylinder pressure
    2) overly lean fuel mixture
    3) excessively advanced ignition timing
    4) high air temperature
    5) fuel being lower in octane than is required for existing engine compression

    Reasons for my case of detonation: I had increased the cranking pressure from 120 psi to 150, there was an air leak at the cylinder base gasket causing a lean mixture (evidence for which was blackened oil deposits on the underside of the piston crown), I was running WOT in the middle of a hot summer day, and the octane ratings here in South America are always questionable.

    Signs there was detonation: erosion of the piston in the center, tiny globs of aluminum on the spark plug. (see photo)

    The reason for this post is that many people are buying those high compression cylinder heads while keeping the stock CDI which has abnormally advanced ignition (which is marginally OK before you modify with higher compression and better carburetor). And some of those heads give ridiculous pressures, often around 185psi. Hey, motocross bikes only have 150 psi. If you already have one and dont want to trash it just buy a dremel rotary tool (or Black&Decker in WalMart) and use a fiberglass disc to grind away at the center dome to make it wider and allow more compressed area (which lowers the pressure). I highly recommend buying a cylinder pressure tester. Adding another head gasket is undesireable because that defeats having a small squish area. Also the stock carb is impossible to jet correctly for all three RPM ranges and gets air leaks very easily where it mounts to the intake manifold.

    2 stroke engines are very finicky and can go bad easily once you start to modify them for increased power. I dont want people to get scared and leave them pathetically stock but rather to be informed and maintain balance between the different engine systems when modifying. After modifying you should ride it hard for a short time and then dissassemble the engine and look under the piston to make sure it isnt black. A lean fuel mixture or an air leak causing a lean condition is the most pernicious cause of problems in a two stroke engine. Also oil on the top cylinder air fins indicates an air leak where the head and cylinder mate. Use sandpaper and glass to plane both surfaces and then use a fiber gasket (home-made). Also checking the crank seals every few months is a good idea. And check the conrod bearings and crank bearings for any up/down play.


    A quote from that site was new info for me: "Engines are much more prone to detonation while "lugging" when at high loads and low RPMs. This is mostly due to the extended burn times at low rpms". That situation exists for me when I give my room-mate a ride in town.

    In the pícture you can see the detonation crater in the center of the piston and the speckles of dirty aluminum on the spark plug:
    Last edited by a moderator: Dec 18, 2015

  2. HeadSmess

    HeadSmess Well-Known Member

    its under the spark plug, its not detonation.

    look at the oil! the carbon deposits! youre trying to say this was a "lean-induced" detonation?

    maybe people that dont know better will fall for it :)

    "An engine can live with detonation occurring for considerable periods of time, relatively speaking. There are no engines that will live for any period of time when pre-ignition occurs. When people see broken ring lands they mistakenly blame it on pre-ignition and overlook the hammering from detonation that caused the problem. A hole in the middle of the piston, particularly a melted hole in the middle of a piston, is due to the extreme heat and pressure of pre-ignition."

    of course, just one article isnt the be-all and end-all... but why repeat the same information ?

    all diesels "knock" or detonate to some degree, and honda worked out that bit about raising the speed to avoid detonation.... running 70 octane with silly CR's at 18000+...

    is this just another advertising ploy?

    feed em halftruths and tell em youve got the wondrous cure-all...

    "The typical pre-ignition indicator, of course, would be the hole in the piston. This occurs because in trying to compress the already burned mixture the parts soak up a tremendous amount of heat very quickly. The only ones that survive are the ones that have a high thermal inertia, like the cylinder head or cylinder wall. The piston, being aluminum, has a low thermal inertia (aluminum soaks up the heat very rapidly). The crown of the piston is relatively thin, it gets very hot, it can't reject the heat, it has tremendous pressure loads against it and the result is a hole in the middle of the piston where it is weakest."

    sounds pretty logical to me...
  3. jaguar

    jaguar Well-Known Member

    from the article you referenced:

    "Detonation causes three types of failure:
    1. Mechanical damage (broken ring lands)
    2. Abrasion (pitting of the piston crown)
    3. Overheating (scuffed piston skirts due to excess heat input or high coolant

    Mine had number 2 and 3.

    "That's where you will see the effects of the detonation; you might see it at the hottest part of the chamber in some engines, possibly by the exhaust valves."

    In a 2 stroke the hottest place is the center of the combustion chamber. My piston eroded there, just a bit off-center. (When I was testing engine heat effects of different spark plugs a while back I noticed that the temperature reading at the spark plug base was always higher than under the cylinder stud nuts.)

    "What typically happens is that when detonation occurs the piston expands excessively, scurfs in the bore along those four spots and wipes material into the ring grooves. The rings seize so that they can't conform to the cylinder walls. Engine compression is lost and the engine either stops running, or you start getting blow-by past the rings. That torches out an area. Then the engine quits. In the shop someone looks at the melted result and says, "pre-ignition damage". No, it's detonation damage."

    Mine was on its way to having aluminum crowd and seize the rings which would cause a complete piston seizure. Mine had the cylinder scuff marks at the four corners. When it sounded like it was seizing I turned the engine off and coasted to a stop. Kickstarting it revealed almost no engine compression.

    "The definition of pre-ignition is the ignition of the fuel/air charge prior to the spark plug firing. Pre-ignition caused by some other ignition source such as an overheated spark plug tip, carbon deposits in the combustion chamber and, rarely, a burned exhaust valve; all act as a glow plug to ignite the charge."

    I had a cold number 8 NGK in it, fairly new and recently cleaned, so I doubt it was over-heating and pre-igniting the fuel mixture. The piston top did not have a lot of carbon on it.

    The article goes on to say that a hole in the middle of the piston and a melted spark plug are signs of pre-ignition. Mine had the first sign but not the second. But they also said that detonation causes the most damage where the heat is greatest, which can also be in the center.

    "As for detonation, it can get hammered on for seconds, minutes, or hours depending on the output of the engine and the load, before any damage occurs. Pre-ignition damage is almost instantaneous. When the piston crown temperature rises rapidly it never has time to get to the skirt and expand and cause it to scuff. It just melts the center right out of the piston. That's the biggest difference between detonation and pre-ignition when looking at piston failures. "

    So I think the piston scuff marks on my Suzuki cylinder indicate detonation and not pre-ignition.

    The article goes on to say that a loose spark plug can get abnormally hot. Mine wasnt loose.

    Great article! We can both learn from it because now, after I read the following paragraph, I think we might of both been technically wrong. I think this might be describing what happened to mine:

    "There is a situation called detonation induced pre-ignition. Imagine an engine under heavy load starting to detonate. Detonation continues for a long period of time. The plug heats up because the pressure spikes break down the protective boundary layer of gas surrounding the electrodes. The plug temperature suddenly starts to elevate unnaturally, to the point when it becomes a glow plug and induces pre-ignition. When the engine fails, I categorize that result as "detonation induced pre-ignition." There would not have been any danger of pre-ignition if the detonation had not occurred. Damage attributed to both detonation and pre-ignition would be evident."

    Mine had much detonation-induced aluminum mini-globs on it which could of acted as glow points to cause pre-ignition.
  4. HeadSmess

    HeadSmess Well-Known Member

    im personally inclined to agree with the last section, but rather than detonation induced, im saying carbon deposit induced preignition, caused by the lean mixture from the leaking base gasket. happens all the time when someone "tunes" an engine with some major carbon buildup... always remove the head and clean the insides out when its got some hours on it!!

    all that carbon also tends to break off, get into the edges of the ports and makes a right mess of things. graphite might be pure carbon...but so is diamond!

    its rare to find a dirty encrusted piston with (severe) detonation...the hammering effect tends to strip everything off the surface!

    if thats out of the A100, i suuspect theres a nasty oil leak down around the disc valve, cus that thing is SOAKED in oil... cant remember if they run an injector or not, and most people ignore that and just run mixed anyway... if it does, and you dont...needs adjusting. another reason most people never use them! but, two cables is nothing compared to the five on the suzuki rotary!
  5. jaguar

    jaguar Well-Known Member

    the AX100 uses oil injection. I just tried to lower the amount of oil it uses but it comes stock set on "minimum".
    Keep in mind too that I do a fair amount of putting around at minimum throttle in town which induces a rich condition in 2 strokes. It was worse before I increased the slide cutaway.
    HS, have you ever heard of the do it yourself system that corrects for that? Connecting a tube from the intake between carb and reed valve, and running it to the float? Put a aquarium valve adjuster on the breather tube of the float. Put the same type adjuster on the first tube? I did it as a gas saving device on my Honda car and it worked really good. But in this case it is to eliminate the rich low speed cruising mixture that causes that oh-so-irritating rat-a-tat-tat sound of eratic combustion.
    I am going to put it on my to-do list.
  6. Fabian

    Fabian Well-Known Member

    Jaguar, can you please show a photo of the aquarium valve adjuster setup.
  7. jaguar

    jaguar Well-Known Member

    No, it is only in my head.
    i know how to draw it though, but actually getting around to doing that is another matter.
    here is a picture of an air valve for small aquarium air tubing:
  8. jaguar

    jaguar Well-Known Member

    A penpal sent me this photo of his detonating piston which was caused by a base gasket air leak and slightly overly advanced ignition:
  9. Fabian

    Fabian Well-Known Member

    That looks much more like bearing failure, with needle bearing rollers being forced through the transfer ports and then rattling around inside the cylinder; becoming trapped in the squish area between cylinder head and piston.

    I know this look quite well, because i used to own a Yamaha RZ 350, and i specialised in blowing big end connecting rod bearings; more than once.
  10. FurryOnTheInside

    FurryOnTheInside Well-Known Member

    I'm not understanding the link between a base gasket leak and detonation. :(
    Though I am "getting" (rightly or wrongly) that detonation occurs around the area close to the outside edges piston crown, and pre-ignition occurs where a very hot particle gets stuck, i.e. at the fouled plug.
    I'm still learning.
    I would (rightly or wrongly, this is just AFAIK so far) clean everything really well, and reduce the piston-head clearance to 0.035" (for an uncoated piston) while maintianing the CR by dremmelling out the dome.. to improve the end gases cooling in the squish area.
    I wouldn't know how to tell if the ignition was slightly too advanced, so would just check that I'm operating within recommended settings.

    EDIT: moment of :dunce: -ness there. The base gasket leak causes lean condition. However I have to agree with headsmess, it doesn't look like there's been a lean condition.. at least to my untrained eye.
    *goes away to study more*
    Last edited: Mar 18, 2014
  11. jaguar

    jaguar Well-Known Member

    Fury, base gasket leaks allow a lean mixture which contributes to detonation.

    Fabian, a picture sometimes is not a thousand words but trust me, this was detonation. He said the edges of the piston were eroded away although we see that eroded aluminum resting on top of the piston which means it can be mistaken for something else such as chewed up bearings. But look closely at the picture to see that the residue is much finer than chewed up bearings.
  12. FurryOnTheInside

    FurryOnTheInside Well-Known Member

    I got that, thanks Jaguar. But I was thrown at first by the oily look and the rest of the thread was a bit much to take in all at once, for me.
    Was the lean condition confirmed? Was detonation confirmed? Was (another means of) pre-ignition disproved?
    I'm only asking because I'm keen to learn and I have no idea how I would tell the difference if I opened and engine to inspect and was confronted by this. At the moment I would just have to guess.
  13. keatonx

    keatonx Member

    Wait, to the float or the float bowl?

    wouldn't that cause it to just suck gas from the float bowl making it even richer?
  14. HeadSmess

    HeadSmess Well-Known Member

    correct me if im wrong, but i believe the idea is full throttle, jet correctly, use the main. at idle/very low speed...rather than use the emulsion tube and jet, allow some air to bypass through the bowl... it picks up vapor, not liquid that has to then be vaporised.

    bit of tweaking between the idle speed and the bypassed air to get it in, leave the idle circuit on the lean side. iunno. ive heard of bubbling air through somehow, i forget the details...

    but that is a perfect picture of detonation. a perfect ring of peppering in the squish, while conrod bits would show most damage near the ports, and by the time enough material got up there to do such damage, the engine would have locked solid. also note theres no skirt damage, and the very small "pores" over the entire piston. near the thumb, the crown has eroded away enough to start exposing the ring.

    makes me recall that once upon a time i did this to a nitro RC car...shame i threw that engine out now :(
  15. keatonx

    keatonx Member

    I read that's the elusive "100mpg carburetor"; just bubbling the air through the gas. I guess they change the mixture (richen it) by putting resistance before the "carb" so that there's more vacuum and the gas evaporates faster. Seen lots of vids of mowers running like this
  16. jaguar

    jaguar Well-Known Member

    Fury, if your piston ever looks like this then look for an air leak, retard the ignition a bit (if your CDI is adjustable), make sure the jetting isn't too lean (richen the mixture and see how it runs), and check that your cranking pressure isn't more than 140psi.
    FurryOnTheInside likes this.
  17. FurryOnTheInside

    FurryOnTheInside Well-Known Member

    Thanks jaguar, will do. Oh it will be adjustable and properly retarded, not going to build without one of your CDIs. :)
    You've given me another question to ask now.. Can the cranking pressure go up? I'll build with it set to a reasonable pressure but I wasn't aware anything could then raise it except perhaps for loss of volume through carbon build up.
  18. jaguar

    jaguar Well-Known Member

    raising pressure:
    1) lathe off metal from the head mating surface
    2) planing cylinder and head mating surfaces and then using copper coat instead of a gasket
    3) use the Honda piston I recommend on my site which seals better due to the ring ends being closer

    lowering pressure:
    1) using your rotary tool to raise the exhaust port (for higher peak RPM)
  19. FurryOnTheInside

    FurryOnTheInside Well-Known Member

    I haven't been able to find out from others the CR of the engines sold here, but I can see the logic in your approach to altering pressure. When I actually get my hands on the engine I'll check compression and think about which route to take.
    I think when I get around to playing with replacement pistons (WELL into the future) I'd be going with the Hoca Minarelli and all the expensive non-DIY mods that go with that.

    Actually my question was about UNintentional increases to the pressure... i.e. during use. I was just unsure how to read what you said about checking the pressure isn't over 140psi.. but I guess from your response you were just referring to during the build.
    I know I will need to check the pressure during the build, and then later at times if I think there may be a head gasket leak (if it isn't obvious from oil stains on the cylinder/head).
  20. Fabian

    Fabian Well-Known Member

    It appears that i was wrong in my assessment of debris from connecting rod bearing failure getting trapped in the squish band area. Consensus says that it is detonation related damage.
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