Octane and Timing

Jeremy Wylie · Post by **Jeremy Wylie** » Wed Sep 17, 2003 3:50 pm

I understand there is little advantage in throwing 96 or 98 in your tank unless you advance the timing to take advantage of the higher octane.

I have a 1999 1.8 (import) - can the timing on this be adjusted in this way? Who could do it for me?

Or would it be just a waste of time as I'm not going to see much improvement?

Thanks

Jeremy
[...]

Eric West · Post by **Eric West** » Wed Sep 17, 2003 4:55 pm

Basically, there is no perceivable difference in performance between 91 and
96 in an engine designed for 91. It is a "anti-knock" rating, not a power
rating. It controls how much the mixture can be compressed before it will
detonate (explode uncontrolledly) on ignition, rather than burning smoothly,
which is the desired state. The higher the octane, the more compression it
will stand.

The petrol companies will be happy to take your extra money for 96, but they
will be the only ones to get something out of it.

Of course, if your engine is designed for 96, then 91 must not be used as it
will detonate, particularly under high load, and the pressure wave inside
your cylinders can, and probably will, damage your engine.

As far as spark advance goes, there appears to be some gain in changing the
advance from standard, (10 degrees?), to 14 degrees BTDC. There are other
people in the club better qualified than me to comment on this.

I would be interested in knowing the reason for this, apparently successful,
change, as I have not yet done it to my MX5, but would if there is a defined
advantage peculiar to the MX5. Advancing the timing beyond makers specs is
not something that I would normally recommend.

Any good mechanic should be able to set the spark advance for you

Regards

Eric

jeff · Post by **jeff** » Wed Sep 17, 2003 5:13 pm

If you have a '99 NB model (non-popup lights) then I
hear the crank angle sensor (CAS) housing needs to be
drilled to allow it to be adjusted like the NA models
could. There's a writeup in miata.net/garage about
this.

Also I'm pretty sure the pre-2001 engines only needed
91 ... have a look thru your Japanese manual near the
back and you should see something like "91 RON" in one
of the spec. tables. If so then it'll tolerate a 2-8
deg. timing advance (on 91) once you "hack" the CAS.

Personally I found it to be a subtle but noticeable
improvement. Being free (on my '96) it's good value

A Cold air intake is at least as beneficial though!

cheers,
Jeff
--- Jeremy Wylie <jeremywylie@mail.com> wrote:

I understand there is little advantage in throwing
96 or 98 in your tank unless you advance the timing
to take advantage of the higher octane.

I have a 1999 1.8 (import) - can the timing on this
be adjusted in this way? Who could do it for me?

Or would it be just a waste of time as I'm not going
to see much improvement?

Thanks

Jeremy
--

__________________________________________________________
[...]

Post by **zorruno** » Wed Sep 17, 2003 7:04 pm

98+ models (NBs) have sensors that can sense knock and adjust spark timing
on the fly. You cant make timing adjustments unless you drill holes in your
car to move the crank angle sensor like the miata.net writeup as jeff said.

You can put higher octane fuel in an NB and you might get better performance
as advance is automatically increased up to the point of knock (which will
be greater with higher octane fuel), then backed off. But then I don't know
how the computer calculates this (how does it know to add a bit more
advance?), or it may only advance to a certain point anyway, in which case
higher octane fuel would do little.

In the earlier models, adding higher octane fuel can definately make a
difference - if you advance the timing at the same time. If you increase
the advance to more than 14+ deg btdc you may need higher octane fuel. Gary
M had some dyno graphs from various timing settings I think?

Mine is set around 16 degrees, and I get some minor knock on heavy load on
91 - but my wife drives it and tends not to rev it as constantly as I do, so
I use 96 all the time to be sure/safe/pedantic. A clean out of my cooling
system (flush, less coolant etc) made a difference, but 98 is only a few
cents more so - hey...

So it comes down to playing with it and seeing if you can notice anything,
or spend $$$ with dyno testing, or just use 96 (and advance the timing on
earlier models) and hope it gives magic power like I do

cheers
zorruno

Gordon Tucker · Post by **Gordon Tucker** » Wed Sep 17, 2003 7:39 pm

My 94 1800 runs 96/98 octane with the advance at 14 degrees. Performance
definately has improved (feels more like the response of my 1600) I.E revs
better. I tried going up to 17 degrees in stages, but there wasn't much
improvement over 14 degrees and didn't like the occasional preignition. I
have also noticed an improvement in fuel economy during normal open road
driving negating the extra cost.
I have heard of a refinery worker who won't put 91 into any of his cars -
what does that tell you!
Gordon.

SLYDIT · Post by **SLYDIT** » Wed Sep 17, 2003 8:01 pm

Ive seen people putting 91 into thier New Turbo XR6
falcons and Gen3 holdens. I guess they must have lots
of money for engine rebuilds rather than good quality
fuel.(if there is such a thing in nz.)Put in 96 or
98,(98 is a cleaner and less additive filled fuel).
Working in the fuel industry, i too would never put 91
into a car i valued as i know what gets put into it.(IE, DIESEL!!!)
Glenn

my2cta · Post by **my2cta** » Thu Sep 18, 2003 9:35 am

I've found that I get consistently and measurably better fuel consumption
on my totally unmodified 2000 model using 96 octane rather than 91 (8%
improvement), presumably being able to take advantage of the NB engine's
automatic timing advance/retard feature that ensures no knock on lower
octane fuel. The improvement in fuel consumption is more than enough to
justify the extra few cents per litre. Any difference in performance is not
noticeable by the "seat of the pants" detection method though. I use 96
because it gives me a bigger range per tankful on long trips (over 600 kms
per tank is possible) and saves a few dollars in the process.

Tim Dutton
MY2CTA

Gary Morrison · Post by **Gary Morrison** » Thu Sep 18, 2003 3:24 pm

Yes, I have dyno graphs showing the effects of going to 14 then 18
degrees, and then adding a free flowing exhaust. I can email to anyone
who might be interested...

On Wed, 17 Sep 2003 19:04:50 +1200, zorruno wrote:

In the earlier models, adding higher octane fuel can definately make a
difference - if you advance the timing at the same time. If you increase
the advance to more than 14+ deg btdc you may need higher octane fuel.
Gary
M had some dyno graphs from various timing settings I think?

DBM58 · Post by **DBM58** » Thu Sep 18, 2003 3:31 pm

I would be interested
Thanks
David Moughtin

David & Judy Kerr · Post by **David & Judy Kerr** » Thu Sep 18, 2003 5:17 pm

Yes please Gary
dkerr@clear.net.nz

Thanks,
Dave.

Mikkels · Post by **Mikkels** » Thu Sep 18, 2003 6:08 pm

Hi Gary

I'd like a copy of the dyno graphs please.

Don

lou Girardin · Post by **lou Girardin** » Fri Sep 19, 2003 8:01 am

It doesn't actually advance timing to the point of detonation, it retards
timing from the ignition map curve when detonation is detected.
Lou

David & Judy Kerr · Post by **David & Judy Kerr** » Fri Sep 19, 2003 5:35 pm

Gary I got the file thanks.

If I am reading it right then there is a definite advantage by going to 18
deg, What if anything would this do to the engine? was there a marked increase
in combustion temp that would do damage?

Dave.

Gary Morrison · Post by **Gary Morrison** » Fri Sep 19, 2003 6:12 pm

Hi Dave,

Looking at the graphs the 18 deg one offers more torque, but slightly
less top end power than the 14 deg one. Only with the free flow exhaust
did the 18 deg one offer both more power and torque. We're talking about
a modest increase in comparison to what is possible with super and turbo
chargers, so the conditions inside the engine will be unlikely to cause
any damage. The only proviso is that the 18 deg advance needs 96 octane
fuel to avoid pre-ignition.

Gary

Post by **zorruno** » Fri Sep 19, 2003 6:44 pm

Hmmm... so I assume after knock sense it would slowly recover? This would
mean that using higher octane fuel wouldn't do anything at all to timing
correct?

David & Judy Kerr · Post by **David & Judy Kerr** » Fri Sep 19, 2003 8:17 pm

Thanks Gary, I've used 96 since burning a valve while driving the south island
a couple of years ago, the exhaust is next for me. You don't know where I
could pick-up a set of extractors do you?

Dave.

Gary Morrison · Post by **Gary Morrison** » Sat Sep 20, 2003 11:23 am

zorruno,

The Link ECU, I believe, pulls back the timing after knock is detected,
and in the zone it occurs, and that retard is not restored until the car
has been stopped, and turned off, and restarted. (correct Fletch, Jeff?).
I image the knock sensor in the standard ecu works similarly.

Gary

On Fri, 19 Sep 2003 18:44:58 +1200, zorruno wrote:

Hmmm... so I assume after knock sense it would slowly recover? This would
mean that using higher octane fuel wouldn't do anything at all to timing
correct?

Gary Morrison · Post by **Gary Morrison** » Sat Sep 20, 2003 11:26 am

The only extractors I can vouch for are the Stewart ones from Oz, and I
believe he is reluctant to make those because of some fitment problems
experienced in the past. I know that Colin had some locally made ones
that were p*ss poor, and when he put in Loch Stewarts there was a much
better result. I assume some of the US sources have done their homework
properly too.

On Fri, 19 Sep 2003 20:17:31 +1200, David & Judy Kerr wrote:

Thanks Gary, I've used 96 since burning a valve while driving the south
island a couple of years ago, the exhaust is next for me. You don't know
where I could pick-up a set of extractors do you?

Dave.

Fletch · Post by **Fletch** » Sat Sep 20, 2003 3:06 pm

Yes, thats how the Link works... whether the stock '98+ ECU follows the same
scheme? I have no idea...

Fletch.

lou Girardin · Post by **lou Girardin** » Mon Sep 22, 2003 8:31 am

That's right, and is also the reason some tuners don't like knock sensors,
because of the power loss. Knock sensors tend to over retard the timing till
knock ceases.
Lou

jeff · Post by **jeff** » Mon Sep 22, 2003 1:38 pm

The Link pulls 1.5 degrees of timing from the ignition
zone when it detects knock. As Gary/Fletch said, it
doesn't keep the setting (unless you "Store" it),
perhaps because the assumption is that the underlying
cause is due to bad gas/high temps etc - ie transient
knock-inducing conditions.

I'd guess that the stock ECU does the same thing, but
hey! who knows ...

David & Judy Kerr · Post by **David & Judy Kerr** » Mon Sep 22, 2003 10:01 pm

Thanks Gary I might look at Australia because quotes from the USA put the
freight as expensive as the headers.

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