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

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 place...it 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....
JT

 

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
aren't.
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
ignite.

 

LOL..okay Paul disreguard *most* of the post i just posted a minute
ago...it 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*
timing....
JT

 

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
TDC.
Now, what I'm not getting is, why retarding the ignition prevents the
knocking?
Initially, I thought that retarding the ignition would result in more
chances of auto ignition...

 

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
directions.

 

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.

 

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?

 

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"

 

it will only prevent knocking caused by too early firing of the
sparkplugs....that is what it does...to 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 extent...in 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...
JT

 

We probably do. What are you driving?

 

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>....so 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....
JT

 

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
1...
JT

 

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.

 

Not to nitpick, but the Prelude is hardly a performance car...

 

If you were to use 87 in an attempt to save money, would the reduced
mileage offset a 10¢ per gallon price difference over 89?

 

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.

 

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.

 

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.

 

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...
JT

 

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.