turbos and fuel octane.

[Dan]

This is for all those of you out there that are constantly asking if you
really need premium (92+ octane) in you turbo wrx, baja, forester, or
STI. Share this with your friends and disregard at your peril.

Ok, let me explain about forced induction (turbo or supercharging) and
how it relates to octane ratings.
First of all the octane rating of gas is it's resistance to what's
called detonation. Higher rating=more resistance=better burn
When you're running any forced induction application this becomes
critical in that you start running a lot more air into the engine.
Remember, all that a turbo does is force more air into your engine so
that there is more air to burn. Now if you run any gas with too low of
an octane rating you are basically now running too lean on the engine.
It's just as if the engine wasn't getting enough fuel. Hence detonation.
Very bad juju. Too much air+not enough/wrong fuel=BOOM! Forget
knocking, we're talking about destroying your pistons, and blowing the
head on the motor. Trust me, this information came very painfully to me.

 

[Guest]

Ok, let me explain about forced induction (turbo or supercharging) and
how it relates to octane ratings.

I suggest you restrict you explanations to subjects you have some knowledge
of.

 

[Ross]

This is for all those of you out there that are constantly asking if you

really need premium (92+ octane) in you turbo wrx, baja, forester, or
STI. Share this with your friends and disregard at your peril.

Ok, let me explain about forced induction (turbo or supercharging) and
how it relates to octane ratings.
First of all the octane rating of gas is it's resistance to what's
called detonation. Higher rating=more resistance=better burn
When you're running any forced induction application this becomes
critical in that you start running a lot more air into the engine.
Remember, all that a turbo does is force more air into your engine so
that there is more air to burn. Now if you run any gas with too low of
an octane rating you are basically now running too lean on the engine.
It's just as if the engine wasn't getting enough fuel. Hence detonation.
Very bad juju. Too much air+not enough/wrong fuel=BOOM! Forget
knocking, we're talking about destroying your pistons, and blowing the
head on the motor. Trust me, this information came very painfully to me.

Its got nothing to do with how rich or lean the engine is running the ECU
takes care of this by monitoring the exhaust gasses, simply forcing more air
into the engine will not generate more power, you need to get more fuel in
there too. So, the more air that is forced in by the turbo in the more fuel
is pumped in too the more fuel and air pumped in mean higer compression in
the cylinder this means more heat buildup in the compression stroke.

Ther higher the octaine rating of the fuel the more the fuel/air mixture can
be compressed before it ignites due to its better resistance of heat build
up. If you use low octaine fuel the heat build up from compression will
ignite the fuel before the spark (Ever used a fire pump?), a tiny fraction
of a second later the spark will ignite another area of the fuel/air mixture
and you get Kock/detonation, two or more flame fronts in the combustion
chamber colliding and causing undesired shockwaves to travel through the
engine.

When the engine detects this it whacks back the timing so as the air fuel
mixture is ignited prior to preignition by heat buildup, this allows a
controled burn with one flame front. When this happens you lose power and
efficiency as the timing is so far back that the fuel and air is ignited
before maximim compression and is burning and trying to expand when it is
still being compressed (IE piston moving up) the engine needs to fight this
and compress the burning fuel. the optimum point for ignition i think is
just before compresion is complete this gives the best burn and efficiency.

Turbos are high compression, variable compression engines. They they need
higher octane fuel not because they are turbos but because they can run at
very high compression and generate a lot of heat from the large mass of air
getting forced and compressed into a relatively small combustion chamber.

Ross

 

[FNO]

me.

Its got nothing to do with how rich or lean the engine is running the ECU
takes care of this by monitoring the exhaust gasses, simply forcing more air
into the engine will not generate more power, you need to get more fuel in
there too. So, the more air that is forced in by the turbo in the more fuel
is pumped in too the more fuel and air pumped in mean higer compression in
the cylinder this means more heat buildup in the compression stroke.

Ther higher the octaine rating of the fuel the more the fuel/air mixture can
be compressed before it ignites due to its better resistance of heat build
up. If you use low octaine fuel the heat build up from compression will
ignite the fuel before the spark (Ever used a fire pump?), a tiny fraction
of a second later the spark will ignite another area of the fuel/air mixture
and you get Kock/detonation, two or more flame fronts in the combustion
chamber colliding and causing undesired shockwaves to travel through the
engine.

When the engine detects this it whacks back the timing so as the air fuel
mixture is ignited prior to preignition by heat buildup, this allows a
controled burn with one flame front. When this happens you lose power and
efficiency as the timing is so far back that the fuel and air is ignited
before maximim compression and is burning and trying to expand when it is
still being compressed (IE piston moving up) the engine needs to fight this
and compress the burning fuel. the optimum point for ignition i think is
just before compresion is complete this gives the best burn and efficiency.

Turbos are high compression, variable compression engines. They they need
higher octane fuel not because they are turbos but because they can run at
very high compression and generate a lot of heat from the large mass of air
getting forced and compressed into a relatively small combustion chamber.

Ross

You're very close to being correct. Most forced-induction engines have a
lower compression ratio than a normally aspirated engine. In a WRX, it is
8:1, and in an STi, it is 8.2:1. However, because the air at the intake is
compressed before going into the cylinder (14.2 PSI or 1 atmosphere in an
STi), the effective compression is higher (the number of PSI during the
maximum compression during the compression stroke of the engine); in an STi,
nearly double that of a normally aspirated engine of the same compression
ratio. The ratio is still the same. The compression ratio is simply the
ratio of the volume of the cylinder with the piston at the bottom, with
respect to the volume of the cylinder with the piston at top dead centre.
It is fixed in all engines no matter how they breath.

The forced-induction cars (turbocharged, supercharged, and ram-air) differ
in the method of how they compress the air at the intake, and how much
pressure they can generate at the intake (boost pressure), and all of them
run the risk of detonation with insufficient octane. The octane rating is
its resistance to detonation under compression, and the speed at which it
burns. Low octane fuels are explosive, while higher octane fuels burn more
slowly as well as resist detonation under compression.

 

[Ross]

You're very close to being correct. Most forced-induction engines have a

lower compression ratio than a normally aspirated engine. In a WRX, it is
8:1, and in an STi, it is 8.2:1. However, because the air at the intake is
compressed before going into the cylinder (14.2 PSI or 1 atmosphere in an
STi), the effective compression is higher (the number of PSI during the
maximum compression during the compression stroke of the engine); in an STi,
nearly double that of a normally aspirated engine of the same compression
ratio. The ratio is still the same. The compression ratio is simply the
ratio of the volume of the cylinder with the piston at the bottom, with
respect to the volume of the cylinder with the piston at top dead centre.
It is fixed in all engines no matter how they breath.

The forced-induction cars (turbocharged, supercharged, and ram-air) differ
in the method of how they compress the air at the intake, and how much
pressure they can generate at the intake (boost pressure), and all of them
run the risk of detonation with insufficient octane. The octane rating is
its resistance to detonation under compression, and the speed at which it
burns. Low octane fuels are explosive, while higher octane fuels burn more
slowly as well as resist detonation under compression.

Stop being picky, I was at work and im not a mechanic ;o)

Bottom line is the pressure at TDC potentially is higher aand the charge is
hotter as the air has been squashed through a (not to mention very hot)
turbo first then compressed further by the piston. Higher octaine fuel
allows you to get more air/fuel into the chamber without preingnition and
spark timing retardation!! More air in the chamber means higher pressure
which means hotter charge. but the benefits are bigger bang and more power!!
There are many factors that justify a turbo requiring higeher octane fuel.

I think my WRX just 'goes' better on it!!! Its the AWD in the wet thats
really puts the smile on my face and a puzzled look on anyones face who is
(trying) to follow!!!

Ross

 

[Ragnar]

I suggest you restrict you explanations to subjects you have some knowledge
of.

Then tell us what he was wrong about.

 

[Nige]

This is for all those of you out there that are constantly asking if you
really need premium (92+ octane) in you turbo wrx, baja, forester, or
STI. Share this with your friends and disregard at your peril.

Ok, let me explain about forced induction (turbo or supercharging) and
how it relates to octane ratings.
First of all the octane rating of gas is it's resistance to what's
called detonation. Higher rating=more resistance=better burn
When you're running any forced induction application this becomes
critical in that you start running a lot more air into the engine.
Remember, all that a turbo does is force more air into your engine so
that there is more air to burn. Now if you run any gas with too low of
an octane rating you are basically now running too lean on the engine.
It's just as if the engine wasn't getting enough fuel. Hence detonation.
Very bad juju. Too much air+not enough/wrong fuel=BOOM! Forget
knocking, we're talking about destroying your pistons, and blowing the
head on the motor. Trust me, this information came very painfully to me.

I have a turbo Diesel - how does this effect me?

Nige

 

[Henry Paul]

Good stuff. I love to read information like this.

 

[FNO]

I have a turbo Diesel - how does this effect me?

Nige

Sort of. Diesel already has a high octane rating and the compression is
what ignites the fuel. Diesels have very high compression ratios. Where it
would affect you is if you tried to burn anything other than diesel in it;
which has been tried

 

[NitroTrike]

I have a turbo Diesel - how does this effect me?

Actually, diesel has a "cetane" rating, whatever that means. I have a 94 dodge
pickup with a cummins diesel (mechanical pump) and it will run on anythhing i
put in the tank (kerosene, heating oil, corn oil, cooking oil, mazola etc). Ive
run it on all of them for shits and giggles. Smells like a deep fried zuchini
out of the tailpipe, but it will get me home. I usually "recycle" my waste oil
from my wrx, s10 blazer and jeep cherokee, and dump it into my fuel tank
(strained of course). No problems, almost 300k. Prob not real good for the fuel
filter though

 

[Ignignokt]

Actually, diesel has a "cetane" rating, whatever that means. I have a
94 dodge pickup with a cummins diesel (mechanical pump) and it will
run on anythhing i put in the tank (kerosene, heating oil, corn oil,
cooking oil, mazola etc). Ive run it on all of them for shits and
giggles. Smells like a deep fried zuchini out of the tailpipe, but it
will get me home. I usually "recycle" my waste oil from my wrx, s10
blazer and jeep cherokee, and dump it into my fuel tank (strained of
course). No problems, almost 300k. Prob not real good for the fuel
filter though

I believe "cetane" is the opposite of "octane" in that it is the measure of
a fuel's likelihood to detonate -- whereas "octane" is the measure of a
fuel's likelihood to *resist* detonation. I believe that detonation is the
normal means of combustion in a diesel engine, and that diesel engines are
built stronger than gasoline engines, in order to tolerate the higher
internal engine forces. If I'm wrong about this, *please* correct as
appropriate.

- Greg Reed

 

[Spudster]

OMG... I think you guys better read this, and fast especially if you are
modifying a WRX for more power:

http://www.streetrodstuff.com/Articles/Engine/Detonation/

Steve.
'03 Rally Blue WRX 5spd
~320 hp

 

[y_p_w]

Then tell us what he was wrong about.

1) Premium gas is universally considered to be 91+. In California
that's pretty much all that's available (it used to be 92 at
the vast majority of pumps). I haven't figured out where one
gets the 93 octane unleaded recommended for the WRX STi.

2) The ECU of any modern fuel injected engine should prevent running
lean or rich for any extended period of time as long as the
engine has reached full operating temps.

3) Technically octane rating is the resistance to "knock" compared
with a reference fuel composed of pure octane. Resistance to
knock is really just resistance to spontaneous combustion (before
ignition by a spark or flame) due to a combination of pressure
and temperature. The two ways this happens are called detonation
and preignition.

4) Detonation is one kind of spontaneous explosion. The initial
explosion from the spark plug causes a pressure increase in the
entire cyclinder as the flame expands. This pressure increase
may cause a part of the fuel/air mixture to spontaneously ignite
before the flame reaches it. So you end up with two seperate
explosions in the same cylinder. Very bad.

5) Preignition is another kind of spontaneous explosion. It's
usually caused by some residual heat source (carbon cinders
or a spark plug with an improper heat range) igniting the
fuel/air before the spark plug ignites. Also bad.

6) Knock sensors will detect mild knock before the driver can hear
it, and long before it gets to the point where it can damage an
engine. They work reliably enough that some manfacturers say
it's OK to use regular fuel in an engine designed to take
advantage of premium (like a Toyota Camry V6).

 

[Ragnar]

Thank you. But you're not "nospam".

 

[Ross]

I believe "cetane" is the opposite of "octane" in that it is the measure
of

a fuel's likelihood to detonate -- whereas "octane" is the measure of a
fuel's likelihood to *resist* detonation. I believe that detonation is the
normal means of combustion in a diesel engine, and that diesel engines are
built stronger than gasoline engines, in order to tolerate the higher
internal engine forces. If I'm wrong about this, *please* correct as
appropriate.

Modern direct injection diesels work by the injector spraying a very fine
mist of Fuel into the combustion chamber, this fuel ignites and burns when
it hits the very got air that has been compressed by the cylinder.

Ross

 

[Danny Russell]

OMG... I think you guys better read this, and fast especially if you are
modifying a WRX for more power:

http://www.streetrodstuff.com/Articles/Engine/Detonation/

Good info, thanks.

The long, needle-fine (.4mm) center electrodes of the Denso Iridium plugs can turn into glow plugs
and light the mixture off early. In this case you'd have pre-ignition which would most likely
result in detonation. This is one good reason why the NGK Iridiums with their shorter and wider
(.6mm) center electrodes are superior.

It's easy to understand why these two terms (detonation & pre-ignition) cause confusion. There are
a lot of subtle variations regarding abnormal ignition events, and many times it's not clearly one
or the other which is occuring, but a combination of the two. -Danny

 

[Edward Hayes]

Good info Danny. Thanks

Good info, thanks.

The long, needle-fine (.4mm) center electrodes of the Denso Iridium plugs can turn into glow plugs
and light the mixture off early. In this case you'd have pre-ignition which would most likely
result in detonation. This is one good reason why the NGK Iridiums with their shorter and wider
(.6mm) center electrodes are superior.

It's easy to understand why these two terms (detonation & pre-ignition) cause confusion. There are
a lot of subtle variations regarding abnormal ignition events, and many times it's not clearly one
or the other which is occuring, but a combination of the two. -Danny

 

[y_p_w]

Modern direct injection diesels work by the injector spraying a very fine
mist of Fuel into the combustion chamber, this fuel ignites and burns when
it hits the very got air that has been compressed by the cylinder.

Basically you're relying on this mixture to detonate. Also - don't
glow plugs help speed it up (almost an intentional preignition) when
the engine or air is cold?

 

[FNO]

measure
of is
the

Modern direct injection diesels work by the injector spraying a very fine
mist of Fuel into the combustion chamber, this fuel ignites and burns when
it hits the very got air that has been compressed by the cylinder.

Basically you're relying on this mixture to detonate. Also - don't
glow plugs help speed it up (almost an intentional preignition) when
the engine or air is cold?[/QUOTE]

They help make it happen when it is cold. When the engine is cold, the
pressure alone is not sufficient to cause ignition; the engine is designed
to run most efficiently when warmed up. So, you generally need the
glowplugs to get things happening when the engine is cold

 

[NitroTrike]

They help make it happen when it is cold. When the engine is cold, the
pressure alone is not sufficient to cause ignition; the engine is designed
to run most efficiently when warmed up. So, you generally need the
glowplugs to get things happening when the engine is cold

My cummins does not have glow plugs, their usually found in smaller 4 cylinder
and low output v8's (like the oldsmobile diesel and gm 6.2 "slug"). Mine does
however have a "grid" that glows like a toaster in the intake manifold, i'm
guessing to heat the incoming air. I never use it, besides I like the way it
pours out the white smoke when its 10 degrees. My neighbors like it too