Hybrid Car – More Fun with Less Gas

better mileage with higher octane? - Page 15

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Posted by clare at snyder.on.ca on January 4, 2007, 2:14 am
 


On Wed, 3 Jan 2007 20:22:52 -0500, "Solar Flare"


Dieseling is part of it - but you can totally eliminate dieselling by
totally closing the throttle on shut-down.Dieseling is more
pre-ignition (or auto ignition) than detonation. It is still a
"normal" slow burn. No air and/or no fuel means no dieseling. It is
DETONATION that is prevented by higher octane fuel. Detonation, if
unchecked, will almost invariably also cause pre-ignition. The
combination of the two ia almost always deadly. Detonation scrubs the
barrier layer off the pistons - the pistons absorb excessive heat and
start to melt down. The hot piston lights the fuel before the
prescribed ignition point and the pressure peaks while the piston is
still coming up. This blows the weakened overheatred piston.

Funny thing - chicken and egg type - pre-ignition can cause
detonation. Detonation can cause pre-ignition. Knowing which came
first is essential to prevent the problem repeating itself.

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Posted by y_p_w on January 4, 2007, 7:11 pm
 


Solar Flare wrote:

Or forced induction.  Pretty much anything that increases the
fuel/air pressure in the cylinders would require higher octane fuel.

My 2004 WRX with a 2.0L turbo four engine has a compression
ratio of 8.0:1.  It sounds low, but then you add the boost.


Posted by Steve on January 4, 2007, 7:31 pm
 

y_p_w wrote:


7:1 isn't uncommon when boost is anticipated. And the other cool thing
about boosted engines is that they have cam profiles with very minimal
valve overlap so they generally idle as smooth as a Packard v12. Whereas
a normally aspirated engine with similar performance would have 10:1
compression and a wild cam that would lope at idle like a '67 426 Hemi.

Posted by y_p_w on January 4, 2007, 8:05 pm
 Steve wrote:

Well - mine is a horizontally opposed four, so I do feel a bit of
shake at idle.  :-(


Posted by Solar Flare on January 4, 2007, 11:54 pm
 My '69 Cutlass 350 Olds engine ran a 13.8:1 compression ratio and
yup...it needed high octane.



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