The performance cars like Preludes say on the dash board to use only Premium fuel

Discussion in 'Prelude' started by LovingPerson, Feb 9, 2004.

  1. no offense Paul, but you are mis-informed (guessed)...the ideal time
    for the plug to fire depends on many things(compression ratio, rpm,
    octane rating, etc)...the timing *advances* as engine rpm
    increases...the mixture is fired BEFORE TDC which will compensate for
    the *time* it takes for the "explosion" to take takes time
    from when the plug fires to when the explosive forces are exerted on
    the piston....and in that time the engine continues to rotate....the
    ideal time for the explosion to begin forcing the piston back down is
    indeed at TDC(technically at the point *halfway* between the rod's
    "dwell" at tdc) pre-ignition, whether caused by the timing being too
    far *advanced*(fires earlier) or too low of an octane fuel for the
    timing setting can be very hard on the moving parts of the
    engine(crank, rods, piston wrist pins, and pistons) and in a sever
    case can cause catastrophic engine failure....when a modern vehicle's
    knock sensor senses "spark-knock" it does indeed *retard* the timing
    to avoid the forces of the "explosion" being exerted on the upward
    moving piston....
    Just a simple example of timing, my old aircooled vw engine is
    timed at 7.5 degrees BTDC(before top dead center) at idle and tops out
    mechanically at around 32-34 degrees BTDC over 3000 rpms...add to that
    the vaccum advance that advances the timing more at low load and it
    can register as much as 40-42 degrees BTDC total advance....
    Joey Tribiani, Feb 9, 2004
  2. LovingPerson

    Paul Bielec Guest

    Ok, so I was wrong about the advancing the timing. It just made sense to me
    that you'd like to fire it up before it auto ignites. My mistake.

    But you're full of shit when you say that it cannot auto ignite.
    If it couldn't, all engines would be using the same octane rate. But they
    My CR-V has 9.6:1 compression ration and uses regular fuel. The same engine
    is used in the Acura TSX, but it has 10.5:1 compression ratio.
    So you have to use premium fuel because it would knock because it would self
    Paul Bielec, Feb 9, 2004

  3. LOL..okay Paul disreguard *most* of the post i just posted a minute is clear that you *do* understand how the combustion process
    works, but you are confused on "advance" and "retard"....advancing the
    timeing causes the ignition to fire *earlier* while retarding
    obviously means it will fire later...knock sensors do indeed *retard*
    Joey Tribiani, Feb 9, 2004
  4. LovingPerson

    Paul Bielec Guest

    I was going with what I know and some common sense.
    I was right about guessing that the base timing would be set to little after
    Now, what I'm not getting is, why retarding the ignition prevents the
    Initially, I thought that retarding the ignition would result in more
    chances of auto ignition...
    Paul Bielec, Feb 9, 2004
  5. LovingPerson

    John Ings Guest

    Which it does. But our 'engineer' claimed this would reduce
    performance. See the first line above.

    What really happens is that the plug fires as the piston is still
    coming up, but full pressure resulting from combustion doesn't develop
    until the piston has come to TDC and is starting down. At high RPM if
    you waited until after TDC to fire the plug there would still be
    uncombusted fuel in the cylinder when the exhaust valves opened.

    Our would-be guru is wrong about three things--
    First, if you've got detonation, you want to retard the ignition,
    fire the plugs later.
    Second, when the plugs fire governs only how long there will be
    combustion pressure on the piston, it's not timed so as to fire when
    compression is highest.
    Third, pressure in a cylinder is not directional, not "from the top of
    the cylinder pushing down on the piston." It's equal in all
    John Ings, Feb 9, 2004
  6. LovingPerson

    Paul Bielec Guest

    Didn't say that right neither, I mean that we want the explosion to push on
    the piston just after the TDC.
    A guess again, this would probably require to fire up the fuel mixture just
    a litle before the TDC as it probably takes some time before it starts
    pushing downwards.
    Paul Bielec, Feb 9, 2004
  7. LovingPerson

    Paul Bielec Guest

    no offense Paul, but you are mis-informed (guessed)...the ideal time
    I understand that the thing we want to avoid is to have a detonation that
    would push on the piston while it is still moving up.
    Now, what I don't get is how retarding the ignition prevents knocking caused
    by the too low octane fuel?
    I mean, if the fuel ignites because it cannot sustain the compression ratio,
    how does retarding the ignition help the situation?
    Is it because, by sparking later, the encounter of the two flame waves
    happens later i.e. after the piston reached TDC?
    Paul Bielec, Feb 9, 2004
  8. LovingPerson

    John Ings Guest

    I don't need to read it to know that ADVANCING the ignition while the
    piston is on its way up the bore is hardly the way to cure knock, or
    that it will not force "the mixture explosion to happen correctly i.e.
    from the top of the cylinder pushing down on the piston."

    Combustion chamber forces are not directional, they don't push from
    the top down.
    Well let's see now, in my mispent youth I owned in sucession
    and did all my own maintenance on...
    1953 MG TD
    1953 Jaguar XK-120
    1953 Jaguar Mk 7
    1953 Jaguar XK-120M
    1963 Jaguar XK-E
    Plus assorted Morrises and Austins, all with vacuum/centrifugal
    advance distributors.
    "Don't try to teach Grandma to suck eggs dearie"
    John Ings, Feb 9, 2004
  9. it will only prevent knocking caused by too early firing of the
    sparkplugs....that is what it make the most power most
    efficiently the ecu fires the fuel injectors and plugs at the optimum
    time(and with the injectors for the optimum duration). Pre-ignition
    based on over advanced timing causes the computer to delay firing the
    plugs(retard the timing..) spark knock caused by carbon build up, lean
    condition, or overheating can only be compensated for by the computer
    to a certain today's autos if you have carbon build up,
    enough overheating in the combustion chamber to caues pre-ignition, or
    spark knock due to a lean condition you have been ignoring the "check
    engine" light for some time...after all if the sensors the computer
    depend on are not working properly then the computer can't do its job
    properly...most computers will automatically go into a "safe mode"
    that is basically a one size fits all situation concerning the fuel
    and spark systems....the performance suffers badly, but engine damage
    is not as likely...
    Joey Tribiani, Feb 9, 2004
  10. LovingPerson

    John Ings Guest

    We probably do. What are you driving?
    John Ings, Feb 9, 2004
  11. Paul, im no expert in the matter, but ihave built my fair share of
    "performance" engines....if you have pre-ignition due to compression
    ratio, then you have either gotten bad gas or you are driving a
    diesel...<G>....lower octane gas has a more "explosive" temperament.
    Octane is basically a gasoline's *resistance* to ignition....therefore
    cars tuned to run with a higher octane fuel usually have a more
    advanced timing(to overcome the resistance to ignition) and higher
    compression ratio....if the compression is so high that the heat
    produced when the air is compressed causes pre-ignition, whether your
    computer retards or advances the timing doesn't matter much because
    you are probably going to experience the joy of looking under the car
    to see why all the oil is running out the side of the block, by that
    mangled up piece of metal sticking out....<G> in a sense you
    can't stop that type of pre-ignition by retarding the timing, but if
    you advanced the timing at sparkknock detection as you initially
    stated you guaratee sparknock and hammering of the rods and crank....
    Joey Tribiani, Feb 9, 2004
  12. JOHN...exactly..detonation and auto-ignition...the words i needed to
    use to make my explanation more clear....funny how you can draw a big
    blank when trying to type out something that is almost second nature
    in your head...LOL....
    .....and yes 19:1 or *more*....most diesels use a minimum of 17-20 to
    Joey Tribiani, Feb 9, 2004
  13. LovingPerson

    John Ings Guest

    OK, I'll get off your case before too much hostility develops.
    You're confusing detonation (combusting gas suddenly exploding) with
    auto-ignition (gas/air mixture combusting due to extremely high
    compression ratios.)
    No, because it would detonate. Different thing.

    To auto-ignite you need a compression ratio like that of a diesel.
    For instance the valves on a Volks diesel are flush with the surface
    of the head. There is no combustion chamber volume at all. The piston
    tops are flat and come level with the top of the block at TDC. To
    adjust the compression ratio you use different thickness head gaskets.
    I dunno what the compression ratio is, but it's way the hell and gone
    up there! 19 to 1 or so? Something like that.
    John Ings, Feb 9, 2004
  14. LovingPerson

    223rem Guest

    Not to nitpick, but the Prelude is hardly a performance car...
    223rem, Feb 9, 2004
  15. LovingPerson

    MaxAluminum Guest

    If you were to use 87 in an attempt to save money, would the reduced
    mileage offset a 10ยข per gallon price difference over 89?
    MaxAluminum, Feb 9, 2004
  16. You're describing pre-ignition; gasoline octane affects spark knock or
    detonation which is not the same thing and happens AFTER the mixture has
    been ignited by the spark plug. As the flame front travels across the
    cylinder, the heating of the unburned mixture can cause it to start burning
    spontaneously before the flame front reaches it, and the colliding flame
    fronts produce the noise and potential damage.
    Robert Hancock, Feb 10, 2004
  17. Knocking and pre-ignition aren't the same thing - pre-ignition happens when
    the mixture is ignited before the spark plug fires (by hot carbon deposits,
    for example), while knocking (a.k.a. detonation or spark knock) occurs after
    the mixture is ignited and the unburned part basically detonates instead of
    burning smoothly across the cylinder.

    As for whether you can use regular, most fuel-injected cars that call for
    premium will retard the spark timing to prevent knock when running on
    regular gas; however this will reduce engine performance.
    Robert Hancock, Feb 10, 2004
  18. LovingPerson

    dold Guest

    Neither is my Honda Civic Hybrid. Or maybe it is, depending on what
    performance you're after...

    But, I have a question.
    In reading these posts, and looking at the web pages, there are comments
    about what octane rating you might predict is needed for a given
    compression ratio. I was surprised that my Civic has a 10.8:1 compression
    ration, and yet it calls for regular gas. It does have a knock sensor. Is
    it possible that premium fuel would result in more than the screaming 85
    horses that I already have? Maybe I'll try some. If I get more power, and
    that increase my overall mileage up from 46mpg, it would be worth the extra
    money that premium fuel costs.
    dold, Feb 10, 2004
  19. Clarence, it is not easy to explain alot of stuff in writing....there
    are combustion chamber designs and matching pistons that are designed
    to enhance the "squish" of the combustion charge...these designs are
    supposedly more efficient and can handle more compression and more
    timing advance(to squeeze the most power out of a small engine)
    without detonation....don't know the inside or even outside your
    engine so i can't really say....but i can say that *most* cars
    designed to use 87 octane fuel really do not benefit from higher
    octane fuel...
    Joey Tribiani, Feb 10, 2004
  20. If I could add a little light...

    An explosion (aka detonation) is what you don't want ever. Under
    optimum circumstances, the compressed fuel/air is ignited by the spark
    plug and burns with a (relatively) slow, steady flame front through
    the volume of the cylinder. Because it is slow, you have to start it
    early (BTDC) so that TDC coincides with combustion that is well
    underway. Because the piston is moving faster at higher rpm, you have
    to start the ignition much earlier than at idle. If ignition is too
    late, you lose efficiency as the piston is already decompressing when
    the flame front is just getting going.

    OTOH, if the ignition is too early, the rapidly expanding flame front
    combines with the compressing piston to develop excessive pressure in
    the cylinder. At a critical pressure, the speed of the flame front
    will exceed the speed of sound and that is pretty much the definition
    of detonation. The ability to detonate is the pretty much the
    definition of a high explosive. You can see why you don't want this
    in your engine.

    So that critical pressure, the one where the charge detonates, that is
    what is measured by octane. If you use high octane, you can ignite
    earlier for increased efficiency without fear of detonation. If you
    use lower octane, the computer will detect the detonation and retard
    the timing. Efficiency (power and fuel economy) are reduced but the
    engine is spared. Maybe.

    IIRC, my GS-R manual says that 87 octane is acceptable if higher is
    not available. I get the impression that a steady diet might not be
    good (could cause increased engine deposits, emissions, damage to
    catalyst.) Besides the extra twenty cents buys 25 miles of grinning.
    Gordon McGrew, Feb 10, 2004
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