Which fuel for NA MX5?

zacMX5aus · Postby **zacMX5aus** » Tue Jun 13, 2017 11:56 pm

Hi all,
I've just purchased a stock na6 MX5 and can't find anywhere a definitive answer to what octane fuel I should be choosing at the pump? The car didn't come with an owner's manual so if anyone could give some guidance I'd appreciate it!
Zac

Ekliptix · Postby **Ekliptix** » Wed Jun 14, 2017 12:15 am

I just bought an NA8 and the manual says to use a minimum of 90 octane unleaded. I'll be using 95 or 98.

jordan.k · Postby **jordan.k** » Wed Jun 14, 2017 12:30 am

I ran 95 when my na6 was stock ecu and it seemed to like it. I knew people that put 91 in with no issues.

If you have advanced the timing a bit maybe you will want 98.

StanTheMan · Postby **StanTheMan** » Wed Jun 14, 2017 5:43 am

I used to use 95. patchy just seemed to like it.
At the time I had 15deg advance.

I now run E85 but that's another story.

JBT · Postby **JBT** » Wed Jun 14, 2017 5:54 am

Minimum RON 91. E10 is not approved for pre-NC standard MX-5s

93_Clubman · Postby **93_Clubman** » Wed Jun 14, 2017 11:18 am

zacMX5aus wrote:car didn't come with an owner's manual

http://www.five-speed.dk/Download/NAOwnersManual.pdf

zacMX5aus · Postby **zacMX5aus** » Thu Jun 15, 2017 12:49 am

Thanks for the responses everyone. I think I'll go with the 95 for now and see how the car likes that. Would rather pay a little more at the pump than risk damaging anything

Mr Morlock · Postby **Mr Morlock** » Thu Jun 15, 2017 11:01 am

not a chance anything will be damaged. Mazda / engineers say 91 is all good and thats what the car engine was designed for. Run 95 and it cost more and sure it will run ok. The NBP2 and later are designed for 95 and the 2 engines are about a decade apart.

manga_blue · Postby **manga_blue** » Thu Jun 15, 2017 12:14 pm

There's no universal answer. Each engine is a little different in terms of its ignition tune and hot spots in the combustion chambers - both of which affect its likelihood of pre-ignition or "pinging".

My NA8 has high compression pistons, a significantly shaved head and advanced ignition timing yet still runs fine on 91, which is what it runs on day to day. Other NA motors will ping like mad on 91. I would say that if it doesn't ping on 91 then use it, otherwise go up in octane and/or retard your ignition until pinging stops.

bruce · Postby **bruce** » Thu Jun 15, 2017 4:38 pm

91 is what Mazda recommends. Everything else is marketing hype.

manga_blue · Postby **manga_blue** » Thu Jun 15, 2017 5:26 pm

I completely agree with you, Bruce. The trouble is though that Mazda made those recommendations for a stock motor in new condition 25 years ago. Since then so many NA6 and NA8 motors have been altered one way or the other: wear and tear, coke build-up, different pistons, machined heads, thinner gaskets, chamber polishing and re-shaping (all of which may leave new hot spots), etc, etc. It's no longer a case of running what was recommended but running what you're able to. As I said: I'd start with 91 and go up in octane or change the timing until it runs without pinging.