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Engine Guru's ..... detonation theory/Question?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by twenty_below0, Feb 18, 2003.

  1. twenty_below0

    twenty_below0 1/2 ton status

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    Engine Guru\'s ..... detonation theory/Question?

    Ok all you engine building gurus . Explain what the causes of detonation are and why is it bad ? I know some of the obvious but never really heard the full theory why it's so bad ? /forums/images/graemlins/confused.gif
     
  2. BlaznRebel

    BlaznRebel 1/2 ton status

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    Re: Engine Guru\'s ..... detonation theory/Question?

    Timing to high or to low, low octane. If timing is off can really blow the engine.
     
  3. twenty_below0

    twenty_below0 1/2 ton status

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    Re: Engine Guru\'s ..... detonation theory/Question?

    Thats the obvious I was referring too , But how and why the blown motor , Will a little pinging over a period of time do it or alot at once ?? /forums/images/graemlins/confused.gif
     
  4. BlaznRebel

    BlaznRebel 1/2 ton status

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    Re: Engine Guru\'s ..... detonation theory/Question?

    upon more thought high points and sharp angles in the combustion chamber can cause detonation due to they can get real hot and can sometimes cause the fuel to ignite before the spark from the plug, now for your other ? It is bad for the motor due to the piston still being on the upstroke when the mixture is ignited. With the rest of the motor still forcing the piston up is causes more than normal pressure on the piston rings,piston, rods etc. If your lucky you may only blow some rings but in some cases it's a piston, rod, or crank. Does this help more?
     
  5. twenty_below0

    twenty_below0 1/2 ton status

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    Re: Engine Guru\'s ..... detonation theory/Question?

    Yeah, thats more of the "Meat" I was looking for /forums/images/graemlins/thumb.gif
     
  6. BlaznRebel

    BlaznRebel 1/2 ton status

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    Re: Engine Guru\'s ..... detonation theory/Question?

    Glad to help /forums/images/graemlins/grin.gif
     
  7. heavy4x4

    heavy4x4 1/2 ton status

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    Re: Engine Guru\'s ..... detonation theory/Question?

    Well, BlaznRebel covered most of it, but here's more info to add:
    Detonation is the combustion of the air/gas mixture before the spark is added to it. This has to do with octane rating, for higher compression engines (say 10.5:1 and higher), you need to run a higher octane to prevent the gas/air from combusting only because of pressure. And, like previously stated, when one cylinder fires before it reaches the top of it's stroke and is still somewhat on the way up, well, that explosion wants to send the piston back the wrong way...not good.
     
  8. Thunder

    Thunder 3/4 ton status

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    Re: Engine Guru\'s ..... detonation theory/Question?

    The theroy behind Detonation and pre-detonation.

    During normal operation of the engine, as the piston moves up on the compression stroke and the sparkplug fires the burning of the fuel-air charge produces a steady, smooth push on the pistons of each cylinder. At the instant of ignition by the spark plug, the flame of combustion moves rapidly outward from the plug very much like the waves when a stone is dropped into a pool of water

    With an abnormal condition such as one or more of the following

    Lean fuel mixture
    Fuel octane too low
    Improper ignition timing
    Lugging
    Carbon deposits
    Excessive milling of heads or block, which will increase compression ratio.
    Spark plugs too hot a heat range
    Valves operating at higher than normal temperature because of excessive guide clearance or improper seal with valve seats.
    Overheating
    Ignition crossfiring. Induced voltage in spark plug wires that run parallel to each other for long distances

    Any or all of these problems can cause part of the fuel/air mixture to ignite on one side of the cylinder before the spark plug normally fires.
    When the plug fires and ignites the rest of the fuel. This causes two seperate explosions in the combustion chamber.
    The pressure waves from these seprate explosions collide into each other and revebrate around the combustion chamber They cause very high pressures and tempratures and make the "Knocking" or "Pinging" noise associated with the condition.
    Detonation will cause piston and ring damage, top ring groove wear, scoring, sticking rings, loose head gaskets, burnt valves, loss of power and possible complete engine failure.
     
  9. twenty_below0

    twenty_below0 1/2 ton status

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    Re: Engine Guru\'s ..... detonation theory/Question?

    WOW ......that's what I was looking for !!! Thank You
    Now one more question , My new motor (about 8,000 miles) is at 9.3.1 right now and I'm running premium 90 octane , but at cruise speed 2100 rpm's it will ping sometimes , "quietly-sort of slightly" . Then with a quick throttle , (just tap the pedal for a slight second) it stops and goes for a while without pinging . If I'm on it hard or even medium acceleration it doesn't ping just at a flat cruise . If I Hold the brake and stomp it , it pings for a "second" then stops and I mean just a ping - ing thats
    it. So I checked the distributor and it's timed at 10 btdc and everything is free moving I did notice some side to side play which will be replaced by new distributor when it gets here . But if I drop the timing any more it gets really doggy , I had it advanced more and it screamed but was pinging at cruise speed and since I dont mash it all the time I backed it off . So I'm worried about what Iam going to encounter soon , I have a set of really worked heads and a different cam to put in this week end but would like to know how to deal with this issue I really dont want to do ANY damage to the bottom end since I run it HARD on weekends and ring damage or bearing damage is NOT an option . With the new heads the comp. will be at 10.5.1 and I will step up to 93 octane but the pinging will potentially get worse ......what do I do ? /forums/images/graemlins/1zhelp.gif
     
  10. BlaznRebel

    BlaznRebel 1/2 ton status

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    Re: Engine Guru\'s ..... detonation theory/Question?

    i'm running right now at around 10:1 and as it is my motor still detonates every now and again with 93 Oct. My timing is set at 6 Deg with a vac advance up to around 26 if not mistaken. Wish i could be more help but my motor only has 1500 miles on it. /forums/images/graemlins/confused.gif
     
  11. the professor

    the professor 1/2 ton status

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    Re: Engine Guru\'s ..... detonation theory/Question?

    Damage, huuummmm:

    Well that depends on how "harsh" the detonation is...

    An at what rpm

    It is actualy much worse at lo-rpm, because the sudden shock to the piston, can break the oil film on the rod and even main bearings...and the BMEP (explained below) is usualy high

    It may improve with engine break-in..oil tends to increase detonation, and blowby on new rings makes the pistons run hotter..also increasing the chance of detonation

    Retard you timing 3deg at a time, don't go past 0deg

    Installing a "hotter cam" or hi-ratio rockers will improve things, because:

    It will lower the BMEP...Brake Mean Effective Pressure...

    This is the rpm when the cylinder fills the best, and you get the highest combustion pressure..AND usualy the highest torque...(twisting power)

    As simple as I can put it for now, I am tired

    Mild cam= lo-rpm...hi- BMEP..less overlap, less duration that the valves are open..More torque..Less horspower..(there are fewer of the hi-BMEP hits)

    Hotter cam= higher-rpm..LOWER BMEP...more overlap, more duration...the BMEP is lower WITH THE SAME COMPRESSION RATION, but since there are more hits because the engine is turning faster, you get More horse-power (the ability to do work over time) but Less torque (twisting power, as in torque wrench)

    This is why engines that run up in the 7000rpm bracket have such high compression..to keep the BMEP high, with the long duration, high lift, high overlap cams..

    Bottom line..10:1 is a very high compression with a "mild" or OEM and PARTICULARLY with an Emissions cam, and you will probably get detonation..

    When the spark plug fires, at what normaly would be the correct time, the pressure spike, with the hi-BMEP causes the mixture to explode instead of burning..

    SB Chevy engines usualy run a total mechanical advance of about 48-52deg..not counting vacc..the total should come in about 2500rpm for the street

    Does the engine "ping/knock" with the vacc adv disconnected..??

    Try this..dissconnect the VA...drive the truck...retard adjust the timing until the ping/knock stops....by ear, a very small amount at a time...

    Check where the timing is with a timing light...if its near 0 and runs OK 1/2 throttle or more..leave it...

    If its way back before 0, try pulling the cap and rotor, and pulling one of the advance springs off..put it back together and see if it pings...

    If it is worse, you need to try stiffer springs..

    Get an Edelbroc, Crane or somebodys kit with an adjustable advance unit too

    If you fixed it...check you timing for reference and use that in the future

    Now, hook you vacc advance to manifold vacc..

    It is best to have an adjustable vacc advance..

    If you do, you can adjust the unit in, until it releases the advance at a higher and higher vacc.(less and less throttle)..until it stops pinging...
    --
    This is a pain without a distributor machine, or at least a degree tape on the damper..
    --
    But what you are trying to do, is get the advance to come in at a different point...

    Because a lean mixture, or a lo-BMEP (combustion pressure), burns SLOWER that a max power mixture, or a hi-BMEP

    Ignition timing is LESS at lo-rpm and MORE at hi-rpm...following the BMEP curve...as it drops off at hi-rpm you need more timing, because the mixture burns slower...the piston is also traveling faster...

    You need MUCH more timing at PART THROTTLE, because the BMEP is lo, due to lo air flow...and usualy lean part throttle mixture. and so the mixture burns slowly....that is what the vacc advance does...pushes the timing way up..

    I can't go on, I am too tired, i hope this helps..
     
  12. twenty_below0

    twenty_below0 1/2 ton status

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    Re: Engine Guru\'s ..... detonation theory/Question?

    I'm LIKING THIS...... Thank You for the info I am running a mild cam with specs as follows 216-228 @ .050 I dont remember lift right now I'm tired too , but very thankful for the schooling and interested in more , one thing I did do that I feel a little uneasy about is set the cam at 4 degrees advanced on the shaft , I should degree it I know but dont have the kit and was taught the old school way was to do this...not knowing any better I have always done it this way ? So If more info is needed let me know . OH I am running 1.6 crower rollers with ToTaL Seal rings so both of those suggestions are done . I wouldn't think the rings are creating that much blow by ....well there at about 4% with a leak down test which is a little bit more than the 1% they say but I wanted it a little loose to deal with the 6,800 rpm's it see's on a weekend basis in the mud and sand . It also has 6" rods will this afect the bmep , I know it makes for a longer dwell? (what is BMEP) As you can see I thought I was fairly smart and have built several engines but I was not near as smart as I thought ....LOL Kind of like theres always someone bigger and meaner /forums/images/graemlins/eek.gif But for real , Thank you for the time your taking to explain this , very cool and I'm sure others are learning with me , maybe there should be a write up on the topic... "hint", Steve or Grim /forums/images/graemlins/grin.gif !!
     
  13. Goober

    Goober 1/2 ton status

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    Re: Engine Guru\'s ..... detonation theory/Question?

    Hey professor,

    Dude, that was beautiful! You give a really simple explanation of what is happening and a simple way to adjust everything to run well.


    If you add a few pictures it would make a great tech article!
     
  14. Thunder

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    Re: Engine Guru\'s ..... detonation theory/Question?

    I cant add much to what the professor said. But when you put on those heads and up the compression ratio. Be sure that the cam you have will work.
    Make sure that cam is ground for the 10.5 to 1 comp ratio.
    If it is a cam profile for a lower compression ratio it will "Ping". Cams without enough duration at high comp ratios usually cause unwanted detonation. Not enough duration causes too high of pressure in the combustion chamber.
     

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