MX5 1992 Roadster Petrol - 91 or 95

boeing747 · Post by **boeing747** » Fri Aug 31, 2007 8:09 pm

Hi, sorry, have another quick question, which petrol would the MX5 Roadster 1992 run on,91 or 95? Thanks,

Post by **zorruno** » Fri Aug 31, 2007 8:40 pm

boeing747 wrote:91 or 95?

Either.

95 is more expensive, but people report better mileage, and possibly better $/km overall. I've never done the math myself.

A cheap mod on an earlier MX5 is to advance the timing to get a change in the power curve. Sometimes in some cars when the timing is advanced, people say they need to use a higher octane fuel to prevent knock. If your timing is set to the Mazda spec (10 degrees btdc) it will easily be fine on 91.

boeing747 · Post by **boeing747** » Fri Aug 31, 2007 8:49 pm

Great, Thanks or that, we put 91 in, and thought i seemed to be running fine, but just wanted to double check!

Post by **zorruno** » Fri Aug 31, 2007 8:55 pm

on 31/08/2007 8:49 p.m. boeing747 wrote:

Great, Thanks or that, we put 91 in, and thought i seemed to be running
fine, but just wanted to double check!

no problem. Welcome to the world of happy MX5 ownership

[Posted via external email]

Grant · Post by **Grant** » Sat Sep 01, 2007 7:42 am

The difference in price of 91 to 95 is only 5 cents per litre and at about
$1.60 per litre you only need to inprove the consumption by a little over 3%
to break even. My 1992 auto MX5 improved by about 6% when I changed from 91
to 96 (at the time) fuel. The cost per km was therefore lower and the car
ran slightly better. That car has now done well over 200,000km. I keep
records of fuel consumption for all my vehicles so know the improvement is
not wishful thinking!
My Honda Odyssey (2.3l Vtec engine) improved consumption much more.
My brother noticed no improvement in consumption in his 1990 MX5 when he
experimented.
Apart from the improvement in fuel consumption, the engines ran slightly
smoother and quieter. Don't know about a performance improvement as that's
too hard to tell. I have not changed the timing on the MX5 so perhaps I
should try that.

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SLYDIT · Post by **SLYDIT** » Sat Sep 01, 2007 2:53 pm

95 is CRAP fuel...better to use either 91 or 98 octane...95 octane is a lower octane fuel that they add all sorts of crap to it to get it to 95... but all the stuff they add can make it worse than a "true" 95 octane in terms of fractioning....98 on the other hand isnt boosted to get to the octane target. My turbo car running on 98 has the SAME combined running fuel useage as it did when it ran 91 and a stock airbox and exhaust, except now i have double the power.... i think ill have my cake AND eat it thanks.

Post by **zorruno** » Mon Sep 03, 2007 5:15 pm

SLYDIT wrote:95 is CRAP fuel...better to use either 91 or 98 octane...95 octane is a lower octane fuel that they add all sorts of crap to it to get it to 95... but all the stuff they add can make it worse than a "true" 95 octane in terms of fractioning....98 on the other hand isnt boosted to get to the octane target. My turbo car running on 98 has the SAME combined running fuel useage as it did when it ran 91 and a stock airbox and exhaust, except now i have double the power....

I saw previously your background was in fuel, so I assume you have a bit of knowledge on this. However, I don't understand how they 'add crap' to get 95, but don't 'add crap' to get 98. Are they a completely different manufacturing process? Different additives? Do you have some links or white papers on this?

...yes... I'm genuinely interested

I assume also you are exaggerating a bit when you say you have double the power and the same fuel usage...?

cheers