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Old 03 January 2008, 15:29   #21
Country: UK - England
Boat name: Ribell
Length: no boat
Join Date: Oct 2006
Posts: 195
Ah, normal.

My above statement was based on tuning results of my petrol 'turbo' engine, is that not a normal engine?

Done a bit of google searching to try and expain where I am coming from and found this and

In an overly-simplistic nutshell (sorry JC!), chipping a motor will usually advance the ignition timing, adjust the fuel/air ratio, and modify several other highly proprietary and secret things to increase performance. Among the main factors in detonation are (1) heat; (2) compression ratio; (3) octane rating; (4) fuel/air ratio; (5) ignition timing. Modifying any of these factors can either increase or decrease the propensity to detonate. Advance the ignition timing and you increase the potential to detonate. Retard the timing and you decrease the potential to detonate. You can balance these factors by modifying some to offset others. If you increase the fuel/air ratio and advance the timing, you can raise the octane and end up with more-or-less the same propensity to detonate as the non-modified motor. Alternately, you could lower the compression ratio, but this would cause a loss of horsepower and efficiency and undo all the gains of modifying the mix and the timing.
And, no I do not have an Impreza.


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Old 03 January 2008, 15:58   #22
Country: France
Town: quimper
Boat name: kai 2
Make: capelli
Length: 5m +
Engine: yamaha 100
Join Date: Jul 2006
Posts: 105
Originally Posted by Polwart View Post
Except that in the USA the Octane rating used at the pump is not the RON number as it is in the UK. It is measured differently. UK standard road petrol (95 RON) would probably be sold in the US as 91 Octane (the exact number depends on the blend - they are not directly convertable like say US gals to UK gals). Similarly the standard road gasoline in the US which is sold as 87 octane would be sold in the Europe as something like 92 RON.
That's right; in the US, the quoted octane is RON + MON / 2, where MON is a tougher, more real life test and is nearly always lower than RON.

Therefore, US quoted octane figures at the pump will be lower than ours for the same fuel.

Nonetheless, the overall gasoline pool in the US has a lower octane than that of Europe, with the vast majority sold being 'regular' 87, (about 91 RON in Europe). This is doubtless due to the long US history of large, relatively 'basic' engines, eg. big cc V8's, etc. that don't make that much power for their size.

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Old 03 January 2008, 16:13   #23
Country: UK - Scotland
Make: HumberOceanOffshore
Length: 8m +
Engine: Volvo KAD300/DPX
Join Date: Oct 2002
Posts: 5,045
v666 your link is interesting. I'm aware of the interrelationship of ignition timing, octane rating etc. but the text in the link implies that the gain from having the maximum possible safe advance is always worth having. If this is so, then your statement above would indeed appear to be true and bernithebiker's statement about not using higher octane fuel is only correct provided no ignition timing adjustments are made.

Initially, I thought you'd got your reasoning wrong in that a higher octane fuel will REQUIRE an advanced ignition setting to ensure it had time to burn sufficiently to reach a desirable pressure by top dead centre, rather than ALLOWING extra ignition advance to be applied. From the text of your link, it would appear that it may be a bit of both. The higher octane fuel requires more advance but, because of it's octane rating, it will also allow even move advance so the pressure at TDC will also be higher than with a lesser octane fuel. However, the engine develops it's torque on the firing down stroke after TDC so we need more information on the conditions within the cylinder at that time to understand how the extra power is developed.
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Old 04 January 2008, 03:13   #24
Country: UK - Scotland
Boat name: Wildheart
Make: Humber/Delta Seasafe
Length: 5m +
Engine: Merc 60 Clamshell
MMSI: 235068449
Join Date: Apr 2007
Posts: 4,614
Most modern "brain" controlled engines will have a "knock sensor" (usually mounted on the cyl head) and will continuously try to advance the spark for max efficiency etc. until it detects a "bang" before the spark. Net result is that you can run any Octane (within the spec) and it will adjust timing to suit, even if you have a weird mix from three different pumps in your tank.

Premix musem pieces like the ones I tend to hang on my transom don't usually have such niceties, so I certainly won't be throwing my dosh at the higher octane stuff!

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