Regular vs High Test

Does high octane gas generate more pollutants out the exhaust than regular
(lower octane gas)? Just curious.

Gary wrote:
> Does high octane gas generate more pollutants out the exhaust than regular
> (lower octane gas)? Just curious.
>
>
Yes and no. If your engine is timed for regular and you put super in,
it will make less efficient use of the fuel so you will use more of it
(it burns just as completely, but the peak cylinder pressure is too late
for best results; high octane fuel burns with a slower flame front).

If the engine is properly timed for premium, it *may* be more efficient
than on regular, depending on the engine. However, as the MX-5 engine
was designed for regular Japanese fuel of 89-90 octane, I expect it
works best with that fuel.

Incidentally, in the UK we only have premium (95 RON) or super (97 or 98
RON). Most enthusiasts advance their MX-5's timing from the stock 10
deg BTDC to 14 deg to allow for the slower burning of the 95 octane
fuel. This doesn't result in pinking but improves power and mpg.

Zog The Undeniable > wrote:

>Gary wrote:
>> Does high octane gas generate more pollutants out the exhaust than regular
>> (lower octane gas)? Just curious.
>>
>>
>Yes and no. If your engine is timed for regular and you put super in,
>it will make less efficient use of the fuel so you will use more of it
>(it burns just as completely, but the peak cylinder pressure is too late
>for best results; high octane fuel burns with a slower flame front).
>
>If the engine is properly timed for premium, it *may* be more efficient
>than on regular, depending on the engine. However, as the MX-5 engine
>was designed for regular Japanese fuel of 89-90 octane, I expect it
>works best with that fuel.
>
>Incidentally, in the UK we only have premium (95 RON) or super (97 or 98
>RON).

The US (and I assume Japanese) octane numbers cannot be compared to the
European ones. We average in the much lower MON.

> Most enthusiasts advance their MX-5's timing from the stock 10
>deg BTDC to 14 deg to allow for the slower burning of the 95 octane
>fuel.

Not true. Miata owners in the US do the same thing on our "87" octane
fuel. The Miata engine timing is set to avoid expensive engine damage
under warranty, rather than maximum engine performance and hopping up
timing on the NA would increase torque noticeably especially down
below (and void the warranty.)

Also, it is not clear to me that higher octane necessary burns slower
than lower octane. I seem to recall something about the combustion
proceeding in stages. Do you have a reference that there is a difference
in bottom line flame propagation speed?

> This doesn't result in pinking but improves power and mpg.

In the US, you can typically use standard gas to 14 degrees BTDC
but you will need premium at 18 BTDC (beyond which further advance
makes performance decrease, as well as engine risk sharply increase.)
This depends on individual engine and ambient conditions; some cars
need premium even at 14 degrees.

Leon
--
Leon van Dommelen Bozo, the White 96 Sebring Miata .)
http://www.dommelen.net/miata
EXIT THE INTERSTATES (Jamie Jensen)