octane booster

[kleenair at ix.netcom.com]

gofast at ix.netcom.com wrote:
> 
> Hi guys. I *think* I am running out of octane in my BMW 530i - I have high
> compression pistons, and the car is choking at high rpms. Have checked most of
> the obvious things like fuel pressure, filter, etc.
> 
> I read here that xylene or Toluene is the way to get more octane. How much
> should one add to each tank?
> 
> What is the minimum octane I would need with 9.5:1 pistons?
> 
> Thanks in advance.
> 
> Samir Shah
> gofast at ix.netcom.com
> Stamford, CT


That is a complex question to answer.  Minimum octane number requirement 
depends on the shape of the piston top, cylinder head, cooling jackets, 
etc.  Also, 9.5:1 is not that high.  I think there are several production 
engines that run 9.5 or even 10:1 from the gas pump.  The only problem 
low octane fuel poses is engine knock, not high speed misses.  In fact if 
your octane is too high (with a weak ignition system) it can cause a 
miss, since the fuel is harder to ignite.

One thing you may want to check is your ignition system.  The higher the 
compression ratio, the harder it is to ignite the charge.  If you can 
have the engine under load while checking the spark (with a timing light 
for instance), it will tell you if the plug you're checking was firing or 
not.

If spark is your problem, make sure all your ignition components are 
good.  A temporary solution might be to slightly REDUCE the plug gaps to 
make it easier to jump the gap, but this has adverse effects on emissions 
and I think low rpm / load operation.  Eventually you may have to go to a 
better coil or if possible increasing dwell to let the coil charge 
slightly longer.  You should be careful about increasing dwell 
(even if you can do it) because most factory stuff is already near the 
maximum value that the coil can handle.

Also, did you check your fuel pressure while the problem was occuring or 
at idle?  What does your O2 sensor read during the problem?

Best Regards,

Mazda