Inlets And exhausts

Archives of Posts to the NZ MX5 List back in 2001
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Mike Donovan

Inlets And exhausts

Post by Mike Donovan » Fri Dec 14, 2001 7:51 pm

Just looking at the Enthusiasts Manual and it looks to me as though there
should be a rubber 'snorkel' attached to the airbox and ending near the
firewall. My car has only the airbox with a flange which looks like it should
have something hanging off the end. Can anyone confirm this please?
Also as far as modifying the timing goes is the recomended setting of 14
degrees BTDC done at idle, with or without a jumper from the 10 terminal (in
the diagnostic connector) and ground?

My car has a sticker on the underside of the fuel filler lid saying to use
only 96 octane gas..which I've used since buying the car. If I change the
timing(provided it hasn't alreay been done) do I need to run the higher octane
gas?
Thanks
Mike

Gary Morrison
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Inlets And exhausts

Post by Gary Morrison » Fri Dec 14, 2001 8:44 pm

Mike,

Short TEN (Test ENgine) to gnd in diagnostics box, set timing and also adjust
idle bypass screw to keep idle at 850. Might have to go back and forth. With
my car when I did this I also added a free flow exhaust and dynoed each step.
Best was at 18 degrees, more torque, and the exhaust lifted the top end. I
have dyno graphs if you're interested. You'd need 96 octane.

Gary

MN
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Inlets And exhausts

Post by MN » Sat Dec 15, 2001 1:57 am

We've had our car for about 4 years and it had a sticker saying 91 octane. Its
been running on this and with the timing at 14 degrees without any engine
knock, pink, hesitation or anything else during that time. The Enthusiasts
Manual specifies 87 or higher for US spec and 90 for European spec cars (why
the difference?). Norman Garrett's performance handbook says 18 degrees
requires higher octane fuel so definately 96. For what its worth (the figures
can always be argued) he has a comparative table which shows the significant
increase in hp and torque from moving away from 10 degrees:

stock 14BTDC 18BTDC
max HP 90.7 99.5 98.0
max torque 82.6 89.7 87.9

My understanding is that this is without any other changes to the engine and
exhaust.

Mark
cheers,

Mark

Fletcher Blades

Inlets And exhausts

Post by Fletcher Blades » Sat Dec 15, 2001 9:19 am

It is quite conceivable that you've been running 14 degrees on 91 with no
knock/ping...

But some cars, due to manufacturing tolerances cant, and will need 96 to run
14... its a suck it and see situation... Sometimes, time of year comes
into it... a borderline car will run 91 in winter with cold air-temps, but as
it warms up, need 96 to avoid knock...

Most cars will need 96 to run 18 degrees, and its a good safety measure to
just tell every one to run it...

The reason for the different octane recommendations for US and Europe is not
because the cars are different, but because the Octane numbers are derived in
a different way, so the same fuel will have a different number...

Europe, Australia, NZ, and most of the world as far as I'm aware, reports RON
(research octane number) on the fuel you can buy. USA reports "pump octane"
or "(R+M)/2" , it is the average value of the RON and MON (motoring octane
number) for that fuel... MON is typically lower than RON, so for a given
fuel the (R+M)/2 will be lower than the RON.

We have 91,96, and now, in a few selected places, 98. In the USA 87-89 is
low octane, 90-92 are mid-grade, and 93-94 are high-test... Its my own
speculation (but I have no data or other evidence) that these fuels are
essentially the same as the ones we get... its just the "name" thats
different. Even if they are not the exact same formulations we get, they will
be much closer than the octane numbers on the pumps suggest...

Fletch.

zorruno
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Inlets And exhausts

Post by zorruno » Sat Dec 15, 2001 10:26 am

Hi Michael
should be a rubber 'snorkel' attached to the airbox
It should be there as stock - some people remove it to flow more air into
the airbox. Some duct it elsewhere e.g. to firewall, down below engine, or
above the radiator to supposedly get cooler air into intake. You can see a
good pic in that autospeed article.
Also as far as modifying the timing goes
I can't vouch for this article's quality (I haven't read it), but yes, you
need to jumper TEN (not 10) and GND when setting base timing.
http://www.miata.net/garage/ignition.html#timing
use only 96 octane gas..which I've used ... do I need to run the higher
octane gas?

The MX5 is designed to allow 91 (or lower probably) with stock 10* BTDC
timing. My timing is around 16* and running 91 gives some noticeable knock
on load when I tried a 3/4 tank once. This is not the case with all MXs
though, some engines seem to have higher limits than others. Best is to try
and see. The car only takes 40 or so litres, and it only gets filled up
every couple of weeks, an extra $4 per week hasn't bankrupted me yet...


PS all - I'm back - what'd I miss?? zorruno
(z)

Gary Morrison
Need, more, 5-ing, time....
Need, more, 5-ing, time....
Posts: 100
Joined: Sun Jun 04, 2006 3:24 pm
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Inlets And exhausts

Post by Gary Morrison » Sat Dec 15, 2001 5:01 pm

My results were different to Norm Garrets...

Mike Donovan

Inlets and Exhausts

Post by Mike Donovan » Sun Dec 16, 2001 5:23 pm

Once again thanks all for the advise and pointers re changing the ignition to
14 degrees. The car was already there..must have been done by a previous
owner. Just to make sure though..while standing in front of the car looking at
the markings (with ten grounded and idle at ~850 RPM) the marking 2 to the
left of the 10 marking is aligned with the notch on the crank pulley.
My air flow meter is a flapper valve type,
Thanks again,
Mike

Fletcher Blades

Inlets and Exhausts

Post by Fletcher Blades » Mon Dec 17, 2001 7:18 am

Mike,

Two marks to the left of 10 is exactly right... well done!

Fletch.

Mike Donovan

Inlets and Exhausts

Post by Mike Donovan » Tue Dec 18, 2001 10:11 pm

Hi Fletch,
thanks for the confirmation re the timing..it was already done though the idle
was quite a bit out.
Mike

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