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Discussion Starter · #1 ·
I can see why a less volatile fuel is so important in turbo charged cars, but in the naturally aspirated V6 engines (B308I) is it necessary to incur the extra cost of Optimax (98 Octane in Oz) or will regular unleaded (91 Octane) do the job just the same. I don't know if the B308I is a particularly high compression engine, given the 155kw output its about average in terms of power for given capacity. Anyone know if 98 Octane is worthwhile (I do know that there is a sensor that will retard ignition to prevent detonation - I don't want to suboptimise, just wondering if I am wasting money on the Optimax?).
 

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Without Trionic on the V6, I don't think there is any point in using 98 RON Optimax. I use Optimax in my 9000 Aero, and you can notice the difference, after Trionic had adapted, but, I can't say I noticed any appreciable difference when I tried Optimax in my 9000 V6. So, now, I just use the standard unleaded in the V6, but in the UK, standard unleaded is 95RON, not 91 as you seem to have....????
 

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My '96 handbook (which covers 4-cylinder and 6-cylinder engines) says:

Fuel: lead-free, recommended 95, minimum 91 RON
9000 Aero (manual gearbox): recommended 98, minimum 91 RON.

Sounds like Saab wouldn't expect any improvement on the V6 beyond 95 RON.

You could, of course, try some higher-octane fuel and see if it makes a noticeable difference.

Edit: Graham, I believe the V6 uses the Motronic system which does have a knock sensor and is adaptive.
 

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Discussion Starter · #6 ·
Unfortunately Australian regular unleaded is 91 Octane. We used to have a premium unleaded at 95 Octane but that has now been replaced with BP Gold which is 98 Octance and about 10% more expensive than regular undleaded - equally priced to Optimax. Interesting this morning when I started the car - now with a full tank of Shell unleaded 91, how the fumes smelt particularly less refined and the car appears to accelerate with marginally less urge in the upper rev range. Otherwise in the lower rev range all seems to be the same. Thanks for the advice gents
 

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Originally posted by trollfan:
[qb]Unfortunately Australian regular unleaded is 91 Octane.[/qb][/b]
I assume your octane rating is RON, as in Europe/UK, not AON, as in the US/Canada? I haven't seen "Regular" here in the UK for many years and the lowest grade stuff we get is "Premium", which is 95 RON.
 

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Basically any engine will run better with a higher octane fuel. It will show up more on a turbo because it is "boosted" performance. At the end of the day it's whether you want to pay for the extra octane if you don't really need/use the extra kick.
 

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Discussion Starter · #9 ·
Basically any engine will run better with a higher octane fuel  [/b]
Not exactly true kimosabi. Higher Octane doesn't burn more intensley or otherwise to create more power - NO. Higher octane fuel is less volatile which means that those engines that operate at high levels of compression, can do so without the risk of detonation (basically the fuel igniting prematurely during the piston up-stroke). Essentially higher octane fuel allows the engine management system to operate at maximum boost (for turbos) or compression, without retarding ignition (the knock sensor does this to prevent detonation). Some engines do not operate at high levels of compression by their design and therefore experience no gain in power if they use high octane fuel, in fact the opposite is true. Higher octane means your fuel is less volatile, your engine design will determine power output.
 

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Discussion Starter · #10 ·
I assume your octane rating is RON, as in Europe/UK, not AON,[/b]
Sadly the most commonly used fuel is 91RON. The only up side is that it costs about 36p per (around 0.9 AUD)litre here. Most of the Australian cars run pretty big engines (3.8 - 4.0 litre engines are average) which don't require much boost / compression. Interestingly many of these cars are returning very impressive fuel consumption figures that are not much different to bigger four cylinder engines. I suppose this is consistent with the theory that if you want to create more power (energy) you need to add more fuel to the chemical exothermic reaction that goes on in the bowels of the engine, so wether you're doing it in a 4 cycliner or a V12 is not important (although the later cars tend to be bigger heavier beasts that require more power to move). However, if your big engined car has higher levels of torque and taller gearing, then you start to get rather efficient motoring. I recently went on a trip to the snow with a friend in his 5.7 litre SS Commodore (regular piece of boof-head Aussie motoring) and on the highway he returned better economy than my V6 (the gearing on the 9000 V6 has the engine spinning at over 2500 rpm at 100km/h, whereas the push-rod 5.7 is barely ticking over 1000rpm at that speed)...interesting NO?
 

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Originally posted by trollfan:
[qb] QUOTE
Basically any engine will run better with a higher octane fuel  [/b]
Not exactly true kimosabi. Higher Octane doesn't burn more intensley or otherwise to create more power - NO. Higher octane fuel is less volatile which means that those engines that operate at high levels of compression, can do so without the risk of detonation (basically the fuel igniting prematurely during the piston up-stroke). Essentially higher octane fuel allows the engine management system to operate at maximum boost (for turbos) or compression, without retarding ignition (the knock sensor does this to prevent detonation). Some engines do not operate at high levels of compression by their design and therefore experience no gain in power if they use high octane fuel, in fact the opposite is true. Higher octane means your fuel is less volatile, your engine design will determine power output. [/qb][/b][/quote]Maybe so, but you put higher octane fuel into any engine and it will run better due to what you have listed. As it runs better, it is also more powerful due to this.
 

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Discussion Starter · #12 ·
Maybe so, but you put higher octane fuel into any engine and it will run better due to what you have listed. As it runs better, it is also more powerful due to this.  [/b]
Not to put too fine a point on the argument, and I am only responding in a friendly debate However, my prior point was that if your engine is not designed to operate at higher compression then less volatile fuel (ie. higher octane) is not required. Therefore there is no gain whatsoever to these engines, and in some naturally aspirated engines that operate at low compression - typically larger low-tech engines (ie. which have a low compression ratio) engine efficiency is reduced because the air-fuel mixture is not volatile enough, requiring higher levels of ignition, if this is avialable. You need to understand that some engines are designed to operate at their best using lower Octane fuel. As it turns out you only need to consult your user manual for your car to figure out the recommended Octane rating that your engine is designed to operate at. For SAABs turbos will always benefit from higher octane fuel, my intial enquiry concerned the naturally aspirated SAAB V6, which does operate at a fairly high level of compression (ie. compression ratio) and also requires at premium unleaded (95 Octane) to operate best.
 

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Originally posted by Neil(CS2.3T):
[qb]Basically any engine will run better with a higher octane fuel. It will show up more on a turbo because it is "boosted" performance. At the end of the day it's whether you want to pay for the extra octane if you don't really need/use the extra kick. [/qb][/b]
I stand by what I said at the start, it will run smoother, I didn't say it would be more powerful or efficient, just smoother.
 
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