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Hi all, we have just bought our first MX-5. Its a 1998 NB8A. The nice young lady we bought it from said she always used high octane 93 or 95RON fuel as its better for the engine. When I looked in the owner's manual it only says to use over 90RON. With the price of petrol now I dont want to pay the extra but will if its a real advantage. Interested what long time MX5 drivers are using. Whats the go guys?

91Ron is whats in the Unleaded Pump if the car is stocko then it should run perfectly fine on Regular Unleaded

but if you love your car you'll give it that extra few rons and pay the price

I'm using Premium 98 Ron its about 10c above unleaded 95ron is about 5c

Quicksilver wrote:Hi all, we have just bought our first MX-5. Its a 1998 NB8A. The nice young lady we bought it from said she always used high octane 93 or 95RON fuel as its better for the engine. When I looked in the owner's manual it only says to use over 90RON. With the price of petrol now I dont want to pay the extra but will if its a real advantage. Interested what long time MX5 drivers are using. Whats the go guys?

Mine is 95 NA8, which I run on 98 octane BP all the time, as the timing has been adjusted. You should perhaps get yours checked to see whether yours has been adjusted as well.
If this is the case then you should use minimum 95 octane.
There are plenty of guys on here more knowledgable than I who can give you an idea on how to check this.

cshaks wrote:Mine is 95 NA8, which I run on 98 octane BP all the time, as the timing has been adjusted. You should perhaps get yours checked to see whether yours has been adjusted as well.
If this is the case then you should use minimum 95 octane.
There are plenty of guys on here more knowledgable than I who can give you an idea on how to check this.

I don't think his has been done, not as easy to do with an NB

and I always use 95 or 98 and only normal unleaded if it's a quick fill of say $5 until I can find some premium

Sorry I should have read the original post a little closer.

Chris

Unleaded Petrol (ULP)
ULP is the most common fuel type in Australia for passenger vehicles. Regular unleaded petrol was introduced in 1985 in anticipation of new Australian Design Rules initiated by the Federal Government to reduce Australia's air pollution. Central to the new design rules was the requirement for all vehicles to be fitted with a catalytic converter.

ULP was produced to allow the new vehicles to operate with the catalytic converter. Most vehicles built or imported since 1986 and a number of pre-1986 vehicles have been fitted with catalytic technology. If your car is a pre-1986 vehicle and you are not sure if it can run on ULP, check with your manufacturer first before using ULP.

ULP has a Research Octane Number (RON) of between 91 and 93.

Premium Unleaded Petrol (PULP)
PULP is a special blend of petrol designed to bring high octane, and hence high engine power, as well as knock- free performance to unleaded cars with a high-octane requirement. Most petrol companies have a specially named version of PULP (for example BP Ultimate and Caltex Vortex)

PULP has a Research Octane Number (RON) of 95.

98 RON
98 RON has a Research Octane Number (RON) of 98. It is a high-octane unleaded fuel that maximizes engine power and performance, as well as producing less pollution. It is more commonly used by imported and high performance vehicles

98 RON is promoted as providing excellent fuel economy. It has low levels of benzene, sulphur and lower aromatics and a sulphur content which is 10 times lower than the national standard for unleaded fuels.

From the WA Fuel Watch site.

ULP will be fine for your NB8A.....but you might be wise to avoid ethanol blends.

Endorse JBT's comments and add the rider- save yourself the 8cent or so penalty it is just wasting yr money imho.

As a suggestion, calculate your mileage on both 91RON and 98RON. On my NB8A I found 98RON gave a greater percentage increase in efficiency than the increase in price between 91 and 98.

That alone makes it worth the extra money, not to mention the better engine response.

marcusus- that calc has been done and reported . The nominal gain is reportedly well outweighed by the additional cost. I would also question whether it will perform any better as well if it designed to be optimum on 91, your gut feel is not always very accurate. But as you suggest and if one is thinking it might pay dividends then do the tests- multiiple fills one against the other.

Thanks for all your replies folks. As I expected, there is no cut and dry answer. Maybe those who are after competative performance will find an advantage in high octane fuel. For the rest of us the difference might not be worth the extra cost. I might mostly use 91RON and occasionally a high octane to see if I can feel a real difference. I believe high octane once in a while helps to clean injectors too but probably not necessary continuously for that purpose. Thanks again

i've had up to 30-35 kms MORE mileage from 91 to 98.. says a lot about the combustion of the petrol..

i always run pulp BP, but when i'm feeling rich i'll pour in bp ultimate.. i find BP gives really good performance for my car compared to other petrols.. maybe that's why you pay at least 1-2 cents more than other petrol types.....

sliq wrote:i've had up to 30-35 kms MORE mileage from 91 to 98.. says a lot about the combustion of the petrol..

i always run pulp BP, but when i'm feeling rich i'll pour in bp ultimate.. i find BP gives really good performance for my car compared to other petrols.. maybe that's why you pay at least 1-2 cents more than other petrol types.....

I run BP ultimate in my MX (NA8) all the time and BP ULP in my Ford Territory.
The strange thing that I find is that when I have to fill the Territory with something other than BP, I find that I use about 2-2.5 extra litres per 100 kms. Not sure whether this happens with the MX as I only use BP.
ANyway thats my 2cents... sorry for the slight hijack.
Chris

Does the MX-5 ECU recalibrate to optimise higher RON?

Our Subaru Forrester manual suggests the ECU takes a couple of tankfills to recalibrate for different RON and improved performance will not be felt straight away. it runs 10:1 comp ratio.

I do not have an owners manual for my import NA6 so dont know if its ECU takes a while to recalibrate or makes no changes??

If the timing/compression isn't raised to make use of the octane you'll lose power but it'll be smoother. Lower octane makes the petrol more volatile.

The only thing I would mention is that I've heard the detergents added to the premium fuels will leave your ports/injectors clearer of carbon. So over a long period of time the engine will retain more power/smoothness. I have not however ever seen actual scientific proof of this.

Personally I run ultimate 98 but I track my car an awful lot and I have an aftermarket ECU. I know my petrol filler on the NB8B also states only use 95 octane as I think the NB8B has 10:1 CR though. NB8A and before were 9.5:1 and lower.

Hellmun wrote:If the timing/compression isn't raised to make use of the octane you'll lose power but it'll be smoother. Lower octane makes the petrol more volatile.

Backing up the above comment:
Internal combustion petrol engines make the best power JUST on the verge of detonation. Using a higher RON fuel will raise the detonation threshold, thus you won't make as much power.

On an otherwise stock, naturally aspirated BP engine, you only need 91RON as the engine, all things being equal and it's in good nick, will not detonate/ping/pre-ignite/knock at even the highest RPMs.

The reason why high-powered turbo cars use 95RON or better is because the internal stresses and pressure are far greater than naturally aspirated engines, thus their detonation threshold is much narrower, thus necessitating higher RON fuel in order to minimise the chance of detonation.